Computer Science

and Technology

U.S. Department

of Commerce

National Bureau

of Standards

A 11 10 2 LS5ML1

JtSpr

PUBLICATIONS

NBSIR 87-3516

Workshop on

Factory Communications

March 17-18, 1987

Robert Rosenthal, Editor

2 g 87 sored by:

J

' > The National Bureau of Standards

Institute for Computer Sciences

and Technology

The Institute of Electrical and

Electronic Engineers

Industrial Electronics Society

Computer Science

and Technology

NBSIR 87-3516

Workshop on

Factory Communications

March 17-18, 1987

NBS

RESEARCH

INFORMATION CENTER

iJXUoo — '

mi

Report on

Recent Factory Communications Research

in Government, Industry and Academia

Including MAP Applications, Simulation

and Analytic Modeling Tools

Robert Rosenthal, Editor

Systems and Network Architecture Division

Institute for Computer Sciences and Technology

National Bureau of Standards

Gaithersburg, MD 20899

Cosponsored by the National Bureau of Standards

Institute for Computer Sciences and Technology

and the

Institute of Electrical and Electronics Engineers

Industrial Electronics Society

Issued March 1987

U.S. DEPARTMENT OF COMMERCE

Malcolm Baldrige, Secretary

National Bureau of Standards

Ernest Ambler, Director
-

-
TABLE OF CONTENTS

ACKNOWLEDGEMENTS iv

LETTER FROM THE EDITOR .......... 1

NON -MAP FACTORY NETWORKS . 2 Field Bus in The Hierarchy of Factory Communication: The

Limits of a Classical Approach — Decotignie, J.D. and

Pleinevaux, P. 3 A Communication Protocol for Advanced Automation LAN (AALAN)

Liang 17

Local Area Networks for an Industrial Automation System

Sintonen, L. 22

Real-Time MAP: Network Architecture Standard for Manufacturing

Cells -- Vijay Chauhan 35

APPLYING MAP NETWORKS 50

Open Systems Interconnection for Real-Time Factory

Communications: Performance Results — Franx, C. and

Mills, K 51

NBS IEEE 802.4 MAC-sublayer Conformance Testing — Chen, H. and

Mack, W 73

Clock Synchronization on the Factory Floor — Gora, W., Herzog,

U. and Tripathi, S 87

DESIGN AND SIMULATION TOOLS 109

Simulation of MAP Protocol for Use in Communications Studies

Kanadi, T. and Gruber, S 110

Implementing a Factory MAP Network Simulation Tool Using the

Ada Programming Language — Schultz, W.L., Bae , I,,

Chandna, A. and Khatibi, F 152

Server Networks for the Design and Analysis of Factory

Communication Systems — Zeidner, L 171

ANALYTIC AND SIMULATION MODELS 178

A Flow and Error Control Model with Phase Type Timeout Periods — Chakravarthy , S.; Pimentel, J.R. and Ravi, K.V. . . 179

Setting Target_Rotation_Times in an 802.4 Network -- Gorur, M. and Weaver, A 199

Performance Modeling of IEEE 802.4 Token Bus -- Jayasumana, A.

and Fisher, P 221

Are Priorities Useful in an 802.5 Token Ring? -- Peden, J. and

Weaver, A 253

Performance Analysis of an Integrated Traffic Slotted Ring--

Ali, M. and Helgert, H 271
Acknowledgements

A workshop is only as successful as the participants make it. The opportunity

for a successful workshop was made possible by a dedicated committee.

Workshop Chairman:

Dr. Alfred C. Weaver

Department of Computer Science

Thornton Hall

University of Virginia

Charlottesville, Virginia 22903

(713) 333-6792

Technical Program Chairman:

Dr. Juan Pimentel

Electrical and Computer Engineering Department

GMI Engineering and Management Institute

1700 West Third Avenue

Flint, Michigan 48502-2276

(313) 762-7992

Assistant Program Chairman:

Mr. Kevin Mills

Building 225, Room B-217

National Bureau of Standards

Gaithersburg, Maryland 20899

(301) 975-3627

NBS Liaison:

Mr. Robert Rosenthal

Building 225, Room B-217

National Bureau of Standards

Gaithersburg, Maryland 20899

(301) 975-3603

Registration Chairman and Treasurer:

Dr. Scott D. Carson

Computer Science Department

University of Maryland

College Park, Maryland 20742

(301) 454-2002

Special thanks are due to Ms. Mary Lou Fahey, Ms. Sharon Reeves, and Ms. Joan

Wyrwa whose secretarial skills and local arrangements expertise made the

workshop possible.
TABLE OF CONTENTS

ACKNOWLEDGEMENTS iv

LETTER FROM THE EDITOR 1

NON-MAP FACTORY NETWORKS 2 Field Bus in The Hierarchy of Factory Communication: The

Limits of a Classical Approach — Decotignie, J.D. and

Pleinevaux, P 3 A Communication Protocol for Advanced Automation LAN (AALAN)

Liang 17

Local Area Networks for an Industrial Automation System--

Sintonen, L 22

Real-Time MAP: Network Architecture Standard for Manufacturing

Cells — Vi jay Chauhan 35

APPLYING MAP NETWORKS 50

Open Systems Interconnection for Real-Time Factory

Communications: Performance Results — Franx, C. and

Mills, K 51

NBS IEEE 802.4 MAC-sublayer Conformance Testing — Chen, H. and

Mack, W 73

Clock Synchronization on the Factory Floor — Gora, W. , Herzog,

U. and Tripathi, S 87

DESIGN AND SIMULATION TOOLS 109

Simulation of MAP Protocol for Use in Communications Studies--

Kanadi, T. and Gruber, S 110

Implementing a Factory MAP Network Simulation Tool Using the

Ada Programming Language — Schultz, W.L., Bae , I.,

Chandna, A. and Khatibi, F 152

Server Networks for the Design and Analysis of Factory

Communication Systems — Zeidner, L 171

ANALYTIC AND SIMULATION MODELS 178

A Flow and Error Control Model with Phase Type Timeout Periods — Chakravarthy , S.; Pimentel, J.R. and Ravi, K.V. . . 179

Setting Target_Rotation_Times in an 802.4 Network — Gorur, M.

and Weaver, A 199

Performance Modeling of IEEE 802.4 Token Bus — Jayasumana, A.

and Fisher, P 221

Are Priorities Useful in an 802.5 Token Ring? — Peden, J. and

Weaver, A 253

Performance Analysis of an Integrated Traffic Slotted Ring

Ali, M. and Helgert, H 271
Acknowledgements

A workshop is only as successful as the participants make it. The opportunity

for a successful workshop was made possible by a dedicated committee.

Workshop Chairman:

Dr. Alfred C. Weaver

Department of Computer Science

Thornton Hall

University of Virginia

Charlottesville, Virginia 22903

(713) 333-6792

Technical Program Chairman:

Dr. Juan Pimentel

Electrical and Computer Engineering Department

GMI Engineering and Management Institute

1700 West Third Avenue

Flint, Michigan 48502-2276

(313) 762-7992

Assistant Program Chairman:

Mr. Kevin Mills

Building 225, Room B-217

National Bureau of Standards

Gaithersburg, Maryland 20899

(301) 975-3627

NBS Liaison:

Mr. Robert Rosenthal

Building 225, Room B-217

National Bureau of Standards

Gaithersburg, Maryland 20899

(301) 975-3603

Registration Chairman and Treasurer:

Dr. Scott D. Carson

Computer Science Department

University of Maryland

College Park, Maryland 20742

(301) 454-2002

Special thanks are due to Ms. Mary Lou Fahey, Ms. Sharon Reeves, and Ms. Joan

Wyrwa whose secretarial skills and local arrangements expertise made the

workshop possible.
LETTER FROM THE EDITOR

Factory and laboratory automation requires computer communication networks

capable of providing rich connectivity among diverse computer systems,

sensors, actuators, robot controllers and other devices. Protocol

specifications for networks capable of meeting manufacturing and technical

office requirements are evolving nationally and internationally. Networks

that implement these specifications enable distributed computer applications

within the factory; and, with these distributed computer applications,

automated design and manufacturing is possible.

The workshop provides a technical forum for the exchange of information,

research results and new applications for factory communication networks.

These proceedings report recent efforts of government, industry and academic

researchers in four major areas: the application of manufacturing automation

protocols (MAP), the application of non-MAP protocols, the use of design and

simulation tools and the use of analytic and simulation models.

The Institute for Computer Sciences and Technology at the National Bureau of

Standards is pleased to co-sponsor this Workshop on Factory Communications

with the Industrial Electronics Society of the Institute of Electrical and

Electronics Engineers. The papers presented at the workshop and the workshop

discussions represent the views of the authors and not necessarily the views

of the NBS or the IEEE.

r* - ---- - *•

Robert Rosenthal

Manager, Network Integration Program

Institute for Computer Sciences and Technology

National Bureau of Standards

Gaithersburg, Maryland, 20899

1
Non-MAP Factory Networks

2
Field Bus in the Hierarchy of Factory Communications:

the Limits of a Classical Approach

J.-D. Decotignie and P.Pleinevaux

Ecole Polytechnique Federate de Lausanne

Laboratoire d'Informatique Technique

16, ch. de Bellerive,

CH-1007 Lausanne

SWITZERLAND

Abstract

Fietd buses were introduced to replace the traditional wiring of sensors and actuators to programmable controllers. After a brief summary of the advantages presented by field

buses , we examine the features of the interconnected devices and the requirements that

these networks must fulfill. We then describe Phoebus, a prototype field bus developed for a machine tool. This implementation is compared to other proposals . In a second part, an architecture based on the OSI reference Model is presented with

elements of services and protocols which match the requirements for a machine tool field

bus.

Finally, we examine a series of problems which are introduced when several automation

equipments share the same field bus.

Introduction

Manufacturers have been offering industrial networks that interconnect control or management computers, CRT terminals and programmable controllers (PLC). The

problem with these implementations is that, in most cases, equipment from different

manufacturers cannot be directly interconnected. Specific hardware and software interfaces

had to be developed to get around the compatibilty problem, leading to tremendous

installation costs.

It has become obvious that costs could be drastically reduced if a standard existed for

interconnection of programmable devices. General Motors has encouraged this

standardization effort by introducing MAP [GM85].

The initial idea was to interconnect all intelligent devices on a single type of network.

However, it quickly became apparent that, at least two types of network were needed. A

backbone network that would carry information over the whole plant for management, and

a real time network to be used for supervision and coordination of equipment at shop floor or flexible cell level. Furthermore, it appeared that direct connections between sensors or actuators and the first level of automation (PLC, CNC) could be favourably replaced by a network, the Field

Bus.

Therefore, factory communications can be represented in a three-layer structure (fig.l).

\ 3
X.25

Sensors and Actuators

Figure 1 : Hierarchy of communications in the factory

The Advantages of Field bus

The replacement of direct connections by a network - the Field Bus - has the following

advantages:

- the cabling costs can be drastically reduced.

- installation and maintenance are made easier since these operations involve the

manipulation of a unique cable.

- detection and localization of cable faults are very difficult and time consuming in

traditional installations; with a network these operations are easily implemented.

- expansion is made easier because of the modular nature of a network.

Furthermore, the use of a network brings new functionalities which were not

previously available : - data consistency for a large number of inputs and outputs. Sampling of inputs or locking of outputs can be synchronized so that PLCs can work on an exact picture of

the process at a given time.

- improvement of analog signal quality (S/N ratio); this trend can already be seen as analog transmitters are becoming digital.

- filtering and pre-processing of inputs are possible since each node of the network has

processing capabilities.

4
Device and traffic Characteristics

The devices considered here are those that are traditionnally connected to the first level of

automation (PLC: Programmable Logic Controller, CNC: Computerized Numerical

Control). They are:

- Status indicators such as contact closures;

- On/off actuators for pumps, power supplies, lamps...;

- Process sensors, transmitters or transducers for measurement of physical parameters

such as temperature, flow, pressure, level... These are usually analog;

- Counters or totalizators;

- "Intelligent" devices such as motor controllers or heat flow computers.

Sensor and actuator characteristics are summarized in Table 1. Devices are rather

inexpensive. They transmit or receive small paquets of data (1 to 16 bits). The associated

response times range from less than 1ms for control loops, 10ms for events (alarms, end

of track...), 50ms for on/off functions, to a few seconds for physical parameters such as temperature, flow, level...

Value nb.bits response time price

Rapidly, varying analog values 1 2 =1 ms =$50

Slowly varying analog values 1 2 =1 s =$50

Event type logic inputs 1 =1 ms $1-5

State logic inputs 1 20-1 00ms $1-5

On/off actuators 1 20ms-1 s var. Counters/totalisators 16 = 1 ms $200

Table 1: Sensors and actuators characteristics

All these values are sampled or updated periodically by cyclic polling. Events which

require a short response time can be processed as interruptions or, more often, by short

cycles within normal polling cycles.

Transmission errors should be detected but correction is insured by cyclic repetition (report

by repetition). Retransmissions would alter the cycle time and should be avoided. In case of error, it is a good policy to assume that states have not changed since last cycle. On the

contrary, wrong informations can be very dangerous.

Besides this synchronous traffic, parameters such as set point data or scaling can be sent to "intelligent devices". Similarly, parameters such as reasons for alarms or statistics can be

read from some devices. Such messages are typically asynchronous and cannnot be

verified by cyclic repetition. This means that retransmission must be allowed since it is needed to insure a correct transmission of the information.

REQUIREMENTS FOR A FIELD BUS

Machine-tool control

For a CNC project under development at EPFL [Gre87], we needed a Field bus that could

replace the direct connections between the sensors and actuators usually controlled by the

PLC part of the NC system and the PLC itself. In our particular case, the requirements were:

- constant polling cycle;

- 30 meters in length;

- low attachment cost;

5
- small size so it could fit inside the machine-tool;

- less than 20 stations connecting less than 100 sensors and less than 100 actuators;

- response time below 20ms;

- no cross-communications;

- stations must be powered remotely;

- good noise immunity.

These requirements were drawn taking into account current characteristics of PLCs in NC

systems and correspond to medium applications of field bus as drawn by EEC [W0086].

IEC proposed requirements

As a comparison, the requirements drawn by the EEC can be summarized as follows:

- length vs speed

350m (medium distance version)

>150 messages/sec. (process control) < 10000 messages/sec. (manufacturing automation)

40m (short distance version)

>5000 messages/sec. (manufacturing automation)

maximum possible (robotics)

- 30 stations on the line

- 60 logical addresses + subaddresses

- Response time

5 ms (manufacturing automation)

20 ms (process control)

- No redundancy

- Provision for backup host with mastership

- Cross communication between nodes

- Power through signaling lines (option)

Phoebus

As mentioned above, Pheobus is a field bus developed for a project called "Numerical

Control for Machine Tools" [Gre87]. It is based on the INTEL 8344 microcontroller. It has a multidrop topology with a single master station and up to 27 secondary stations.

The primary (master) station is composed of an 8344 microcontroller communicating with

a PLC through a dual port RAM. This RAM contains the transmission and reception

buffers along with the secondary station's descriptors.

Secondary stations have a much simpler architecture: an Intel 8344 microcontroller with

program memory (EPROM + RAM) and input/output circuits. Programming a communications protocol on the 8344 requires the selection of the SIEJ's response mode. Two modes are available: in the AUTO mode, the SIU implements in hardware a subset of SDLC [EBM79]. In flexible mode, only address recognition and error checking are done in hardware; frame identification, response initiation and all other

functions are performed by software.

Phoebus uses the flexible mode and a subset of SDLC protocol for both the primary and

the secondary stations. In this mode, the protocol may be simplified. Frame numbering for

instance is not implemented in Phoebus since all frames sent by the master - command

frames - are implicitly acknowledged by the corresponding response frame. The

synchronization frame is the only exception to this rule. Furthermore, we cannot take

advantage of the SDLC window mechanism because information is always transfered in one frame.

It can be shown that HDLC and SDLC protocols have a Hamming distance of 1 [Fun82].

To avoid this problem we have chosen to fix the Information field length to 16 bits. One

may wonder if a 16 bits CRC is adequate to protect 16 bits of data.

6
The secondary station driver

The secondary station driver can be described by means of a finite state machine (fig.2).

Basically we distinguish two states :

Disconnected state: The machine enters this state after power up and initialization or after having received

a DISC frame. The station remains in this state until it receives an SNRM command

from the primary station; it enters transfer state after transmission of a UA response.

The station responds to any other frame - apart from UI as will be explained later - with a DM frame.

Transfer state: Under normal conditions, the secondary station remains in this state, waiting either

for an Information frame or a Synchronization frame, UI. Upon reception of a UI,

all secondary stations sample their inputs, store them in transmission buffers and

lock the new value of their outputs. This value has been received in a previous data

exchange in which the secondary station received the new value for its outputs and

returned the sampled value of its inputs.

Figure 3: Secondary station automaton

The primary station driver

The primary station is responsible for collecting and distributing data to the secondary

stations at regular intervals of time. It issues commands to the secondary stations and

receives responses from them. These commands are entirely determined by the state of the

secondary stations which is reflected in a data structure called station's descriptor. Such a descriptor contains the following information : (1) the 8 bit address of the station ; (2) the

station's status which determines the next command to be sent. Status information is the

current station's state and error indicators such as timeout and incorrect information field

length ; (3) the number of bytes sent and received from this particular station ; (4) two

pointers indicating the position of the sensor and actuator buffers in external RAM.

7
All this information constituting the secondary stations' descriptors is contained in a dual

port RAM which can be accessed at initialization by the PLC. This processor must

configure the network, i.e. indicate which stations are to be polled and in which order - reflected by the order of the descriptors in memory. Once initialized, the primary station

receives a command indicating that polling can start. It issues a Synchrionization frame to sample the data and starts polling each station according to its descriptor.

Performance tests Tests were performed to assess various parameters of Phoebus. A critical factor is the

station time that we define as the time necessary to load a station’s descriptor, exchange

two bytes of data in each direction with the secondary station, and update its descriptor.

For a transmission speed of 375 kbps, we measured a station time of 770fis. This time

corresponds to 470(is of transmission and 300|is of processing time. These figures show

that transmission at 2.4 Mbps - six times faster than in the test conditions - would only

reduce the station time by a factor of 2. The reason is to be found in the fixed processing

time of 300|is corresponding to the load and save operations performed on the station's

descriptor, the frame preparation and the response processing.

If we assume now that our network contains 20 secondary stations, each receiving two

bytes of information or transmitting two bytes to the primary station, the total cycle time

necessary to poll the network would amount to 15.4 milliseconds. This result fulfills the

requirements we have set for a machine tool.

EXISTING IMPLEMENTATIONS

The purpose of this section is not to be exhaustive but to present a few representative

implementations. Some are commercial products; others are still in the research or development laboratories and may evolve but their essence will remain the same. The first ideas for a field bus have emerged in the nuclear and military fields, giving birth

to CAMAC and MIL-STD 1553 [Hav86]. The instrumentation field contributed the HPIB

(IEEE 488, EEC625) bus. Among these three implementations, MIL-STD 1553 is the only one that uses a serial transmission and can meet the requirements for a field bus.

More recently, BITBUS [Int84] and several proposals such as FIP [Tho86], FB FT2

[Fun86-2], PROWAY C, and Phoebus have appeared. All these proposals have a centralized medium access control, except PROWAY C which uses a decentralized tokenpassing scheme.

Table 2 summarizes the main features of five implementations. The transmission efficiency

is measured as the ratio of the useful bits (here 16 bits) over the number of bits required to insure the transmission. The delay between command and response is assumed to be equal

to the transmission time of 25 bits [Fun86-1].

All proposals, except BITBUS and Phoebus, use a Manchester encoding technique.

BITBUS and Phoebus support two alternative bit signaling methods as mentioned above

(Phoebus hardware description).

Except for BITBUS, support is provided for broadcast data transmission without

acknowledgement but only MIL-STD 1553 has a built-in mechanism to verify the correct reception of a broadcast frame. This is indicated by a flag in the status field transmitted as a

result of a further poll. In PROWAY C, it is assumed that each active station transmits a single broadcast data

frame without acknowledge when it receives the token and then releases it. In table 2, only the first two solutions meet the requirements of the IEC for a field bus. The

other proposals are rather low end solutions that can handle continuous process control

with a medium response time. BITBUS suffers from two drawbacks; it cannot handle

8
broadcast messages (required for synchronization purposes) and it does not ensure correct information protection (Hamming distance 1 [Fun82]).

MIL

1553 FIP

PROWAY C

BITBUSPHOEBUS

SPEED(Mb/s) 1 3 1 0.375 0.375

LOGICAL ADDR 1024 65535 65535 65535 255

STATION NB. 32 100 100 28 32

EFFICIENCY 22% 16% 6.8% 7.4% 11.6%

HAMMING DIS. 2 3 3 1 3

ACCESS TIME

[ms]

Nx.065 Nx.035 20 7 Nx.77

BROADCAST YES YES YES NO YES

LLC Services

RDR

SDA

RDR

SDA.N

SDN

RDR

SDA

RDR

SDA

MAC Central Central Distr. Central Central

MULTIMASTER NO NO YES NO NO

CROSS COMM. YES YES YES yes" NO

PROC.CAP.AB. NO FUTUR EXTERN YES YES

Table 2: Comparison of several Field Bus Proposals

All solutions, except PROWAY C, implement SDA, RDR and SDN services (see next

section). In FIP, all responses can be considered as SDN services. PROWAY makes use of SDN LLC service.

The application area for field buses requires synchronization capabilities. This feature can only be insured by broadcast messages. Furthermore, correct reception of broadcast

frames should be provided. Among the proposals that allow broadcast messages, only

MIL-STD 1553 allows a verification of the correct reception of these messages.

Reading table 2 shows that the transmission efficiency is low even in the optimistic case presented here (16 bits of information). To keep this relative efficiency, the user is obliged

to group on/off sensors and actuators in which case the field bus looses its main

advantage. It is then desirable to design field buses that provide an efficiency larger than

1% for single on/off sensor or actuator.

9
COMMUNICATIONS ARCHITECTURE FOR THE FIELD BUS

The Open Systems Interconnection (OSI) Reference Model allows the interconnection of

heterogeneous computer systems by specifying a series of protocols which must be

supported in order to allow interoperation of these devices. Field bus as ordinary computer

networks should have a similar architecture, but the basic features of such networks

preclude at this moment the use of a full seven layer architecture:

- transmission of sensors's or actuators's states across higher level networks seems

unlikely because of the long access times of such networks. Therefore, the network

layer is not needed.

- the transport layer should be dropped since there is currently no protocol supporting

broadcast and multicast addressing at this layer.

- the session layer, which is intimately bound to the transport layer, is not interesting in

the context of field bus.

Consequently, a three layer architecture seems appropriate for a field bus (figure 3).

Figure 3: Field Bus Architecture

The physical layer

The following observations can be made on current implementations:

- all implementations have adopted the bus topology.

- Shielded twisted pair is the most commonly used transmission medium, although coaxial

cable (FIP, ProControl_P) and fiber optics (FIP) are allowed.

- The number of supported devices is 30 or 32.

- Manchester encoding is supported by MIL STD 1553 and FIP, NRZI by BITBUS and

Phoebus.

Medium access control

Classically, medium access control can be performed in a centralized or decentralized form.

Decentralized medium access control seems very attractive at first glance, but leads to several drawbacks:

- interfaces are more complex;

- performance is reduced due to token management;

- cycle time cannot be constant;

- each station must know information types and cycle lengths.

10
On other hand, centralized access control provides important advantages:

- minimum updating time when all process variables change simultaneously;

- high protection against loss of information by using cyclic repetition;

- combination of real time acquisition and restitution of process datas with real time

supervision of network topology;

- quantified constant cycle time;

- support of broadcast or multicast messages with no address or with a single address

field;

- simple channel access protocol in slave station leading to simpler slave stations.

A major argument against centralized access control is reduced system availability, since

the operation of the whole network depends upon a single point. This can be overcomed

by powerful redundant techniques for the master station. Furthermore, "democracy" is not

always an advantage in the whole system availability [Pow86].

Application layer

The integration of a field bus in a MAP network suggests that the application layer be

compatible. MAP specifies MMS (Manufacturing Automation Protocol) as the application

layer protocol for manufacturing applications. Interesting functionalities provided by MMS

are the remote variable access and event management functions. However, a complete

implementation of MMS on a field bus is not realistic because of the size of MMS.

Therefore a subset must be defined.

Services and protocol

Timing and addressing constraints impose that a field bus be based on Connectionless

services. Three data transfer services are generally introduced [PR083]: Send Data with

Acknowledge (SDA), Send Data with No acknowledge (SDN) and Request Data with

Reply (RDR).

The SDA service allows a user to send information to a single remote station

(L_DATA_ACK.request). The user receives a confirmation of receipt or non-receipt of the

data (L_DATA_ACK.confirm). At the remote station, the information is passed to the

remote user along with the address of the sender (L_DATA_ACK.indication). When

received by the remote user, this information is known to be correct and identical to the

information sent by the source station.

The SDN service allows a user to send information to a single remote station, to a group of

remote stations (multicast) or to all remote stations (broadcast). The user receives a confirmation of transmission completion (L_DATA.confirm) but not of data delivery. At

the remote station, the information is passed to the remote user if received correctly

(L_DATA.indication).

The RDR service allows a user to request information (L,_REPLY.request) previously

submitted by the user at a remote station (L_REPLY_UPDATE.request). The requesting user receives either the data or an indication that the data was not available

(L_REPLY.confirm). The remote user receives an indication that the submitted information

has been fetched by the initiator of the service (L_REPLY.indication).

When using a field bus in conjunction with a PLC, a number of constraints must be

respected:

1) The typical operation of a PLC is a cyclic one. This cycle can be broken into three

phases:

- acquisition of the sensors’ state.

- processing.

- update of the actuators' state.

11
Send Data with Acknowledge (SDA)

L_Data_Ack

Figure 4

Send Data with No Acknowledge (SDN)

Indication L_Data

Figure 5

Request Data with Response (RDR)

Figure 6

As a consequence, the network protocol is also cyclic, with a fixed period imposed by

the PLC. This implies that all data exchanges must be executed in one and only one

transaction; retransmissions must be forbidden, since their allowance doubles the

maximum cycle time. This does not mean that error detection is not done. Indeed, for a PLC it may be disastrous to work with erroneous data or to update an actuator with a value different from that computed by the PLC. Information must be protected - by a CRC or simple parity, the question is open - errors must be detected and signaled, but

their correction need not be immediate.

2) Data flows are from the sensors to the PLC and from the PLC to the actuators. Therefore, a master-slave protocol and a RDR service are a natural approach to this application. This consequence is due to the fact that state evaluations are centralized in the PLC and thus cross-communications between sensors and actuators are in general

not desired. However, should the PLC function be distributed among secondary

stations, then a peer-to-peer protocol would be a natural approach. A second case in which direct communication between a sensor and an actuator may be desirable is in the case of alarm notifications. An actuator can immediately react to a sensor state

12
change without waiting for a command coming from the PLC. However, both the

PLC and the actuator must be notified, justifying the need for multicast or broadcast

addressing.

3) A distinction must be made between synchronous and asynchronous traffic. The

synchronous traffic concerns the cyclic acquisition and update of inputs and outputs

and constitutes the main part of the traffic. The asynchronous traffic involves all other

information transfers: alarms, parameter setting, download of programs, network

management. Efficient use of the bandwidth and the cycle period impose that the

asynchronous part of the cycle be shorter than the synchronous part. Because of the

finite duration of the asynchronous sub-cycle, certain messages may be delayed for one or more cycles, and a natural selection of messages must be made by means of a priority mechanism. SDA and SDN services are appropriate for asynchronous traffic,

with the possibility of broadcasting and multicasting in the case of SDN.

4) In order to give the PLC a coherent picture of the process, inputs must be sampled

simultaneously. This synchronization is obtained by broadcasting a message on the

bus. Acknowledgement of this synchronization command may or may not be

grouped. It is obvious that in the case of a ring topology, a group acknowledgement is easy to implement. But for a bus topology, a satisfactory solution does not exist.

Therfore the most frequent solution is to acknowledge the command in a subsequent

response.

FUTHER PROBLEMS

A simple control system can be characterized by three parameters (figure 7):

- the sample period te or cycle time which is the time elapsed between the beginning of a process cycle and the next one;

- the processing time tp which includes the input state acquisition q, the algorithm

computation time q. and the output update time

- the response time tr which is the time between a change in an input state and the

corresponding change at ouptut as ordered by the control system (tr = t e + tp = te + t c +

'i +

0 )•

Figure 7: Simple Control System Timing

13
Replacing direct connections by a field bus will increase the relative importance of tj and t Q

over tc

. Let us look at some problems caused by this replacement.

A simple PLC

When a single automaton is controlling sensors and actuators through a field bus, it may or may not be the master of the network. However, its own sample period dictates the same

cycle time for the field bus.

The required time to refresh the sensor state table (q) and to update actuators (tQ) may be

far less than the sample period. If this is the case, the network is being used well below its capabilities.

On the other hand, if this time (tj + t0) is comparable with the computation time, the

network introduces a large delay in the response time. Several overlapping schemes can be

imagined to reduce the performance degradation [Des87]. The excess delay has generally no influence over the control algortithms.

A single motor control loop

In high performance machining applications and for high quality motor control,. a sample

period around 1ms or below is usually required. This means that q

+ t0

should be well

below 1ms and its effect should be taken into account by the control algorithms.

In order to efficiently use the field bus, other kinds of traffic should occur only during the

unused period (t s

- q

-

0).

Multiaxes control

In this case, several control loops are handled via the network. To use the network

efficiently, it is necessary to interleave (in time) the control loops (figure 8). In case of non-interpolated (independant) axes, this case is equivalent to a single axis

control loop. But, when axes are co-interpolated, interleaving leads to severe modifications

in the curve interpolation algorithms (as well as changes in the control equations) to insure

a good synchronization between axes.

Control Loop 1

Control Loop 2

Control Loop 3

|TnpT out 1 1 inp. out 1 field bus

computing computing control

processor

I inp. out | computing

1 inp. out 1 computing

inp. out j linD. out computing computing

sample period

[time^

Figure 8: Several control loops on a single field bus

14
Multi PLCs

When several reflex automation devices run a set of sensors and actuators over a single

field bus, their sample periods are generally different. If these periods are constant, the

network can be used optimally by externally synchronizing the different automatons.

Otherwise, the field bus will introduce another penalty due to simultaneous requests for

acquisition or update. The excess delay will not be constant. Comments

When a single automation device (PLC, control loop, CNC) is linked to its sensors and

actuators via a field bus, a single master/multiple slave configuration is an obvious choice.

When several reflex automatons are connected to the network, the configuration choice

becomes more difficult. However, the problems mentionned above show that, due to the

real time constraints, the field bus cannot be made transparent and all cycles have to be

designed carefully before implementing the network. In that sense, design and

management can be made much easier with a single master station.

CONCLUSION

A conventional master slave protocol like SDLC can be used to design a field bus for

medium speed applications, but entails to much unnecessary overhead for high

performance process control. This remark also applies to all other proposals when

connecting a single on/off sensor or actuator to the field bus.

Care should be taken to minimize the delays due to higher level software that may exceed

the transmission time.

It has been shown that the "old" single master/multiple slave configuration exhibits good

behaviour in case of hard real time constraints. Furthermore, this configuration is much

easier to handle when dealing with complex control applications. In these applications and

because of the real time constraints, a field bus cannot be made transparent unless used

inefficiently.

Finally, due to the complexity of handling mutliple scan cycles, it becomes necessary to provide CAD tools for building the network operations.

ACKNOWLEDGEMENT

This project was performed under grant of the EPFL inter-laboratory project "Commande

Numerique de machines". The authors would like to thank N.M.Greene for her help in

writing this paper.

References

[Des87] D.Desmons, J.-D.Decotignie, "Field Bus in the Numerical Control of MachineTools: How to Get the Best of It", presented at the International Workshop on

Industrial Automation Systems, Tokyo, Japan, February 4-6, 1987.

[Fun82] G.Funk, "Message Error Detecting Properties of HDLC Protocols", IEEE

Trans.Comm., COM-30, pp. 252-7, December 1982.

[Fun86-1] G.Funk, "FB-FT2", IEC SC65 (Funk) 19. [Fun86-2] G.Funk, "Alternative Field Bus Solutions", IEC SC65 (Funk) 20, March 1986.

[GM85] General Motors Co, "MAP specifications 2.1", March 1985.

[Gre87] J.C.Gregoire, "A New Perspective on the Architecture of Numerical Control",

presented at the International Workshop on Industrial Automation Systems,

Tokyo, Japan, February 4-6, 1987.

15
[Hav86] N.Haverty, "MIL-STD 1553 - a standard for data communications",

Communication & Broadcasting, Vol.10 (1), pp.29-33, January 1986.

[IBM79] EBM, "IBM Synchronous Data Link Control: General Information", document

GA27-3093-2, Research Triangle Park, North Carolina, USA, 1979.

[Int84] INTEL Corp.," The Bitbus Interconnect Serial Control Bus Specifications", in

Distributed Control Modules, 1984.

[Pow86] D.R.Powell, "Dependable Architectures for Real Time Local Area Networks",

presented at the Advanced Seminar on Real-Time Local Area Networks,

Bandol, France, April 16-18, 1986.

[PR083] PROWAY LAN Study Document Revision B, October 1983.

[Tho86] J.P.Thomesse et al., "An Industrial Instrumentation Local Area Network",

presented at IECON 86, Milwaukee, WI, September 29- October 3, 1986.

[W0086] G.C.Wood, "Field Bus, a Developing Low Level Industrial LAN Standard",

presented at EFOC/LAN, Amsterdam, Netherlands, June 23-27, 1986.

16
A Communication protocol for Advanced Automation LAN ( AALAN

)

Lung-Sing Liang

ISDN Group, Telecommunication Laboratories,

Directorate General of Telecommunications,

Ministry of Communications , PO Box 71*

Chung-Li, Taiwan

ABSTRACT

A local area network used tor advanced automation

functions is described. These functions include: office automation, computer aided design/computer aided manufacturing a robotic system, image processing remote teaching and expert systems. The design and

implementation of the layered architecture of AALAN and

its relationship with the existing protocols, such as the transmission control/intemet protocols (TCP/IP) and the Xerox Network System (XNS) are also described.

Xeywords: computer networks, local area networks, advanced automation, transmission control/intemet

protocols

1 . INTRODUCTION

With the advent of computer hardware technology,

multifunction workstations' and the multimicroprocessor based computing systems2 have evolved considerably, while the cost has correspondingly decreased. The

integration of these workstations and computers to perform advanced automation, i.e. OA, CAD/CAM,

robotics, image processing, remote teaching and expert

systems, are becoming more urgent day by day. The basic components of an advanced automation

system will include a message management, a data management and an activity management system. These

will be integrated together in a communication network 3

. A LAN differs from a conventional 'long-haul' communication network in its communication bandwidth, communication protocols and architecture. Usually, in long-haul networks the transmission bandwidth is an

expensive resource and network design emphasizes communication link efficiency with complicated routing,

flow control, network administration and momtonng

facilities. Unlike long-haul networks, the design philosophy of a LAN is quite different because of its high communication

bandwidth, low transmission delay and low transmission

error rate. In a local environment the LAN will connect all the terminals, host computers, intelligent workstations, computer aided engmeenng (CAE) stations and CAD/

CAM stations together. Therefore the LAN communication

protocols will differ considerably from those in a longhaul network, based upon the LAN's differing technology and application environment The application environment will indude an office information system, a robotic system, a distributed database management system (DDBMS) and a network

application system. The interconnection of these stations

will be difficult because of the heterogeneous hardware and software environment A homogeneous communication system must be developed to provide the communication service necessary for advanced auto- mation.

2 „ NETWORK CONFIGURATION

The proposed network architecture is shown in Figure 1 . It indudes PDP11/70. PDP11/34, AT&T 382s. Xerox 1108s.

Ritge and Intel Microcomputer Developing System

(MDS). Most of these devices will support the Ethernet

interface without the device driver. The Xerox 1108

workstation will be dominated by the Xerox Networking

System (XNS) protocol. The three VAX-1 1 computers are networked by DECnet running on top of an Ethernet The robotic network consists of five workstations, each

of which consists of a Z-1 00 personal computer and one

Hero robot It is a loop structure with one stapon as the master station connected to the Ethernet An AT&T PC

6300 is connected to one of the AT&T 3B2s via the kermit

file transfer protocol 4 allowing remote teaching activity There is an image processing system developed on the PDP11/70 and AT&T PC 6300. An office information system, IMMS5

, is developed on the PDP1 1 /70. 382s and

Xerox 1108s. The distributed database management

system (DDBMS) is being developed on the INTEL MDS.Due to its populanty and the existing hardware

consideration. Ethernet was chosen for the physical and

link layers

6

. By employing a repeater, this departmental LAN can be connected to the campus VAX DECnet/

Ethernet network approximately 1 km away

3 . LAYERED ARCHITECTURE

The network architecture consists of a set of layer definitions and their protocol implementation. There are

five individual layers 1-5. These are the link, network,

transport, presentation and application layers. The

definition and funcnon of each layer will be described

later. The interface between each layer will be known as the interpiocess communication. The transmission

control/internet protocols (TCP/IP)7- 8 and the XNS protocol have obtained wide acceptance from LAN and office automation vendors. The TCP/IP is a standard

communication protocol running under Unix BSD 4x. All Xerox office products are based upon PARC Universal Packet (PUP) or XNS protocols. Some other companies

have also implemented the XNS protocol on top ot Ethernet9

. figure 2 compares the XNS, TCP/IP and AALAN with

the corresponding ISO Open System Interconnection Reference Model Levels 1, 2 and part of level 3 (the

physical, data link and intranetwork layers), are equivalent

to the XNS level 0. The XNS level 1 corresponds to layer 3b

of the ISO network layer (the internetwork layer). The

internet protocol/intemet control message protocol (IP/ ICMP) or internet datagram protocol (IDP) defines the format of an internet datagram. The IDP addresses, routes and delivers internet packets inside the network. In most

17
datagram protocols, the IDP does not guarantee reliability.

It may lose a packet, deliver a packet which is out of sequence or even duplicate a (jacket. The IP and ICMP of the TCP/IP protocol is much more complicated than the corresponding IDP of the XNS protocol. In the advanced

automation application, the IDP is able to interface with

Ethernet. The XNS layer 2 or TCP is equivalent to ISO layer 4 — the transport layer. In the XNS layer 2

10

, there are five individual subprotocols, namely, Sequence Packet Protocol GPP). Packet Exchange Protocol (PXP). Error Handling

Protocol (ERP), Echo Protocol (ECHO) and Routing

Information Protocol (RIP). The XNS approach has been

adopted because of its modular design scheme. In layer

six, the Telnet”. FTP' 3

, MAIL13, TFIP3< protocols of the TCP/IP are used In layer 6 of the AALAN we have the

following functions: SND, RCV, LST, DEL, SHLand LOGIN

for AALAN

These six functions will be described in more detail

later. The following [unctions are based on the presentation

layer of AALAN: (Message Management System), ESMH

(Expert System for Message Handling) and some other

applications, such as, DDBMS. remote teaching and

videoconferencing systems in the application layer. The software structure is shown in figure 3. The

following modules are run under Unix: the Ethernet device driver, IDP handler, PXP handler, error packet

handler and time packet handler. This will briefly be

described in the following sections.

Ethernet device driver

The DEC 'Deuna' Ethernet and the AT&T 3BNET Ethernet

controller are used for the PDPi 1 /70 and 3B2 respectively. The Deuna device dnver (for the PDP1 1 /70) is wntten in C

1

. Control of the deuna driver is via the open and dose

system command (to initiate and terminate the network

operation), the wnte and read system command (to send and receive the data packet), the gtty and stty system command (to query and modify Ethernet mode and

status variables). The 3B2 supermicrocomputer allows the

user to develop high layer protocols, other than 38NET. to

interface with the Ethernet device driver16 The 3BNET

supports the direct access to the Ethernet data link layer

protocol. The application interface to the network is provided via a subset of the network interface (Nl), the

driver-user interface. The high layer protocol interface to 3BNET is also provided by the system commands such as open, dose, read, wnte and iocti. The locti command is used to get and set the parameters of the network control

block.

Internet datagram protocol handler

The function of the internet datagram protocol (IDP) is to address, route and deliver internet (jackets in the network. The IDP consists of two parts: control and data. The control part consists of transmission control, desti- nation address and source address. There are four fields in the transmission control: checksum, length, transport control and packet type. The checksum is a software checksum, and is in addition to the PCS error checking mechanism provided by the Ethernet link layer protocol. The checksum is calculated by add-and-left-rotate of all the 16-btt words of the internet packet except the checksum itself. It is assumed that the main function of this checksum is as an internal verification between the communication processor and the memory. Sources of the internet packets which do not want to generate the checksum and are willing to tolerate a lower communication reliability may fill the checksum field with all 'ones' (65533 in dearnaJ notation). This value indicates

that no checksum is calculated. The checksum will be

recalculated when any content of the (jacket is altered dunng the routing of this packet. One byte Is used for

transport control. The 0-3 bit should be zeroed and the 4-7 bit is the hop count The hop count is used by the

internet router and initially should be set to zero. Each time an internet packet is rerouted, this field will be

increased by one and the software checksum will be

recalculated. When the hop count exceeds 16, this packet will be discarded to avoid the infinite iooping of an

internet packet. An error packet will also be transmitted according to the error packet protocol. The length field is the total length of the internet packet measured in bytes

including the checksum field. In general, the maximum

length of the internet packet is S76 byte, which indudes a 30 byte header and 546 byte of data identification numbers are used for the network (32 bits), host (48 bits) and socket (16 bits) for source and destination station respectively. A socket identification number is the process number used for communication. Other types of socket number are well known. The packet type field is used to identify the format of the data field of the internet packet It is also the bridge to the higher layer protocols. There are six different types used in the Xerox network

system: routing information, echo, error, packet exchange,

sequenced packet and expenmem packet

Packet exchange protocol handler

The packet exchange protocol (PXP) is used to transmit a request (jacket and receive a response with a reliability higher than that obtained by directly transmitting a datagram, and lower than that by using sequence packet

protocol (SPP). The PXP is a simple one packet protocol. The PXP will retransmit a request packet without duplicate detection when the request station is unable to acquire the response packet The ID field in PXP is a 32-bit field used to cross cneck between the client and the server. If the ID is the same for both the requestor and replier, then

the request-response packet pair completes the trans- action. The number for the ID is a monotonicaily

increasing number. The dient type field contains the

registered value which identifies the dient of the

protocol.

Error protocol

The error packet handler (ERP) enables any station in the network to recognize the occurrence of an error condition. The ERP can also be used as a diagnostic tool and as a mechanism for improving the network performance. An

error protocol packet is returned from the destination socket of the host detecting the error. This (jacket is sent

to the source socket of the packet when an error occurs. The contents of the error packet are two 16-bit words,

indicating the error number and the error parameter

followed by at least 42 bvte of the offending packet. The

sending socket can use the received error packet to cany

out a check on itseff. The error number identifies the error type. e-g. checksum incorrect, specified socket nonexistent. packet too large to be forwarded etc The error parameter is only specified lor certain types of error. An

error protocol will not respond to a broadcast or muiticast packet to prevent the generation of a large number of unnecessary error packets.

Time packet handler

The time packet handler is used to form a ome request packet according to the format shown in figure 4. It also disassembles the time acknowledgement packet to obtain the time information.

k .AALAN HARDWARE

The hardware for AALAN is a microprocessor based

18
controller implementing the Ethernet protocol, for example: Deurva (Unibus). Excelan (Unibus, Multibus), Ungermann-6ass (Ridge), 3BNet controller (3B2 super- microcomputer) and Xerox Ethernet controller (Xerox 1108 workstation). Some of the stations allow the user to develop his own high layer protocol other than their standard protocol such as 3BNet, XNS, PUP etc The

device driver is developed with Deurva for PDP11/70

running PW8/Umx.

5 .AALAN PRESENTATION LAYER

The presentation and application layer protocols are implemented on a PDP11/70, a 3B2 and a Xerox 1108

(Lisp machine). There are six presentation layer functions. From the design view point of AALAN. each station has a

'general server' and a 'dient' as shown in Figure 5. The

general server consists of a file server, an echo server, a command execution server, time server etc The 'dient'

part inauoes the runcnon 'SND', 'RCV', 'DEL', LST', SHL

and 'LOCIN'. The server is always in the active state ready

to receive the service request from the 'dient' part of another station. Communication only occurs between

the dient and the remote station; there is no direct communication link between the 'dient' and the 'server"

in the same local station. The 'server" and 'dient' employ

the services of the lower layer communication protocols.

Six different patterns are used to identify the corres- ponding protocols. We can assume that each ‘dient" is an

application process or is controlled by another application process. Once the 'dient' issues a speafic call request, the

general server will classify the request according to the

request type, socket number and the presentation protocol pattern. Depending on the request dass, one of

five different actions may be taken. These are time server, echo server, login server, shell server and file server. The

file server will indude the 'SND', 'RCV', 'DEL' and 'LST

functions. Any incorrect format encountered dunng the request and acknowledgement phases between the dient" and

'server" will invoke the error protocol to issue an error packet. The time server and echo server will be described

in later papers. This paper will focus on the file server, shell server and the login server, which are written in c language and the interiisp D programming language. The corresponding functions will be described in the

following sections.

SND

syntax SND (station namei The 'SND' function can be used by an interactive user

directly or interface with an application program. The

following is an example of how to send a file to a remote

host (PDP11/70) station from a local 3B2 supermicro- computer. %SND PDP11-70 LOCIN: LIANG PASSWORD: ++ ++++++

PLEASE INPUT SOURCE FILENAME?

: LANMANUAL

PLEASE INPUT DESTINATION RLENAME?

: LANMANUAL1

FILE LANMANUAL1 COPIED MORE FILE TO SEND <Y OR N)?N

%

The whole directory can also be transferred using the pdpsndObsnd) shell language procedure. The login security check is also provided to ensure that the user has the

access right to the file.

RCV

syntax RCV (station name]

The 'RCV' function is used to receive a file from a remote

station. The 'LST' function may be used to examine the

directory at the remote station to make sure that the file you expea to receive actually exists. The operation of 'RCV' is very similar to the "SND'

funaion. The login secunty checking is also provided. The

'PDPRCV’ (3brcv) can also be used to carry out a remote

directory copy

DEL

syntax DEL (station namei A file in the remote station can be removed by the 'DEL' function. If file does not exist, then a 'no such file' error- message will be returned to the user.

In order to make the correct deletion, the 'LST' function may be used (see earlier).

SHL

syntax SHL (station namei 'command stnng' A local procedure may be submitted from the local

station. This function may easily be interfaced to some

application program. The ‘command stnng' can be anv command text or a senes of commands up to 51 2 byte. Regardless of whether the command is executed or not. the results will not be passed back to the station which

issued the command. The following is an example of how

to issue a 'mail' request and 'pnnt out" a hardcopy of a file, from the 3B2 station to the PDP11/70 station. SHL PDP11-70 'MAIL CHANG < LETl; PR LANMANUAL | OPR'. The first part is to mail a file named 'LETl ' to the login user 'CHANG' and the 2nd part is to copy the file 'LANMANUAL' to the line printer. The login secunty check is also implemented.

LST

syntax LST [station name]

This function is designed to display the contents of a direaory at a remote station. Before a file transfer or a file deletion, the 'LST command should be used to check the remote directory. This command can also be used to replace the 'SHL'

function. The execution results will be returned to the

station which issued the command originally eg. LST 3B2 'CAT LANMANUAL'. We can also check the users currently logged in on the 3B2 from the PDP11/70 by issuing the command LST 3B2 "WHO'.

In short, the '1ST function is the most general function and can be easily adapted to interface with many

application programs. As with the other functions security checking is also implemented.

Other software modules

There are some other software modules, such as a ome

server, an echo server, an SPP (Sequence Packet Protocol) and other transport layer protocols. (The time server and echo server will be described in other papers.) The SPP

provides the reliable transmission of successive internet packets with sequence control . window size flow control

19
ID of process and a scheme to avoid duplicated packets. The command patterns for 'SND'. 'RCV', ‘1ST’, 'DEL', 'SHL' and 'LOGIN' are shown as follows: REQUEST ACK NAK SND ssssssssss SSSSSSSSSS nnnnnnnnnn RCV nTTTTTTTT fTTTTTTTTT nnnnnnnnnn

LST iiiimni iniimii nnnnnnnnnn

DEL dddddddddd dddddddddd nnnnnnnnnn SHL hhhhhhhhhh hhhhhhhhhh nnnnnnnnnn LOGIN gggggggggg gggggggggg nnnnnnnnnn

The AALAN will support a user-friendly interface. The user can interface with AALAN using either an interactive mode or through program handshaking. For example, in MMS the query message is broadcast to multiple stations. The query message can be flexible enough to be in the form of a file or a remote executable command. The

respond message will be obtained via the 1ST or SHL

functions. Similarly, in a distnbuted ruled-based ESMH

system the user condition message will be processed by

the SND and RCV functions. From the design view point

of user applicanon systems, benefits can be obtamed from

the flexible presentation layer interface.

6 . CONCLUSION

The high-level protocols such as XNS, TCP/IP and AALAN

will be widely used on different kinds of LANs for vanous

applications The design considerations will be concentrated on modulanty, simplicity, portability, ease of

interface etc All these high layer protocols in AALAN are

written in C In the near future, these protocols will be built as utility software under a different operating system.

Further enhancement of extsong functions and the

introduction of new functions into the AALAN mav be

easily accomplished. This is due to its modular design.

It is possible to predict that portable, compact and

modulariy designed high level protocols for LANs will become more popular.

16 AT & T 3B2 Computer Unix System 5.2.3BNET

application interface manual AT & T Technologies, USA (1964)

ISO

reference

level

XNS XNS

protocol TCP/IP

protocol

z<£

< AALAN

7

4 PRINT, MAIL

FILE CLEARING HOUSE

COURIER

TIME SERVER

Telnet

FIP. MAIL

TFIP

5 008MS. MMS

ESMH

j 4

SNO. RCV, LST oelshl.

LOGIN

6

8

4 2

RIP ECHO

ERROR. PXP

SPP TCP/UDP 3

RIP. ECHO ERROR PXP

SPP

3b 1 iOP IP It ICMP 2 (OP

3*

0

.Ethernet X.25 oox

Tl .LEASE0

LINE

2 - 1 Ethernet

1

Figure 2. Protocol relationships comparing XNS. TCP/IP and AALAN with the corresponding ISO Reference Models

REFERENCES

1 AT&T Personal Computer 7300 user's manual AT & T

Technologies. USA (1984)

2 Liu, A CA testbed tor distnbuted processing (TED) ECE

Department. IIT, Chicago, USA (1985)

3 Chang, S K Office information system design ECE Department IIT, Chicago, USA (1965) 4 Cruz, F D Kermit protocol manual Columbia University Center for Computing Activities, New York, USA

(Apnl 1984)

5 Chang, S K ef al. An intelligent message management

system Global Telecom, New Orleans USA (December

1965) 6 Krueger, V Focus conference: opportunity in the US

telecommunication market Nielsen dazaquest (October

1985)

7 Postei, I DARPA internet protocol specification ISC. USC RFC-791 (September 1981) p 47-100

8 Postei, | 'DARPA transmission control protocol

specification' ISC USC RFC-793 (September 1981) p 131-221

9 Lien, M Experience in implementing XNS protocols

Bridge Communications, USA

10 Internet transport protocol Xerox system standard. Xerox Coro.. USA (December 1981)

11 Postei, I Telnet protocol specification ISC. USC RFC- 764 (June 1980) p 225-239

1 2 Postei, J File transfer protocol ISC. USC RFC-765 Oune

1980) p 243-312

13 Sluizer, K and Postei, | Mail transfer protocol ISC, USC RFC-785 (May 1981)

14 Sollins, K R Trivial file transfer protocol MIT, USA

(June 1981) p 379-396

15 Draizen, H The user's manual of DEUNA ISL ECE Department IIT, Chicago. USA (1985)

20
Figure 7. ,Verworx architecture o: AALAS

Destination address

Source address

Tvoe Checksum

Transport control PXP

Destination network

Destination host

Destination socket

Source network

Source host

Source socket

identification field

Client type

Figure 4. Format ot time request packet

21
LOCAL AREA NETWORKS FOR

AN INDUSTRIAL AUTOMATION SYSTEM

Leo Sintonen

Tampere University of Technology

Computer Systems Laboratory

P.O.B. 527, 33101 Tampere

Finland

Abstract

A modular robotics system is defined. The communication

structure is described and discussed. It is found that

by the use of modern high-speed local area network

technology together with a suitable system architecture

a modular system can be built and reasonable real-time

processing requirements can be met.

22
Contents

1. Introduction

2. System Architecture

3. Software Structure

4. Communication Architecture

5. Messages and Transfer Delays

6. Communication Subsystem

7. Conclusion

8. References

23
1 . INTRODUCTION

The purpose of the project is to study the design of a flexible and modular industrial automation system that

can be used for assembly, manipulation and production

automation. The communication architecture is defined

and described.

The system consists of modules and a central controlling

computer, which are connected together by a high-speed

bus. Modules which are programmable components for

production, manipulation or measurement, can have internal

communication subsystems. A part of this project is to

study the communication requirements and communication

architecture

This paper describes the basic system architecture in

chapters 2 and 3. Communication is described in chaptes

4 and 5. A module subsystem communication-protocol .is

suggested and described in chapter 6.

2. SYSTEM ARCHITECTURE

An industrial automation system in this context is a system used for automated manufacturing, assembly,

transportation etc. in a factory scale.

The system is modular, allowing change of manuf actoring

programs. The system is controlled by a central computing

facility. System modules are different kinds of machines

needed to perform the process, fig.l. Typically there

are numerically controlled machines (NC:s), robots,

servo systems, transducers, manipulators e.t.c. These

machines are connected to the central computing system

by the communication facility, which is a bus-type local

area network system.

The system of fig.l. performs a system process which

consists of the processes of the individual machines,

controlled by the central computer.

The computing structure is distributed. The system task

given to the system (from the outside world, from CAD/CAM

-system) is divided into subtasks distributed to the

computing submodules of the system. Computing submodules

are robot controllers, servo controllers or in general

controller modules existing in different machines, or

measurement on alarm data collection units.

In order to perform the distributed system task modules

communicate with other modules and with the central

computer via the system bus.

24
3. SOFTWARE STRUCTURE

The computing submodule can be of type controller or of

type interface, depending on the complexity of the task

it should handle.

Functions of the type controller are:

to implement the control algorithm needed to perform

the module subtask

to maintain communication to the system bus

to handle the communication inside the communication

subsystem of the module (internal communication)

All this is performed under the control of the submodule

executive

Computing submodules of type interface are used to collect

simple numerical data, e.g. measurement values (position,

velocity, force, e.t.c.) or more sophisticated data,

like picture data, and send it to a computing module via

the system bus. Also, simple commands and valves coming

from the system bus are transmitted through this interface

to the actuators. The function of the computing module of

the type interface are:

to implement the communication interface to the system

bus

to handle the data and command representation

to transmit data and/or commands from and to the

actual device

4. COMMUNICATION ARCHITECTURE

During the configuration phase, the central computer

downloads program to the memory of computing subsystems.

A part of this configuration phase may also be the

learning phase of robots. Information is transferred

mainly via the system bus from/to the central computer,

but also during the learning phase in the robot's

communication subsystem.

During the operational phase the basic communication is

within a module. This reduces the communication load on

the system bus.

25
Module - to - module communication is basicly

synchronization information during the operational phase

caused by different event conditions.

Status, alarm and command messages are exchanged between

the control computer and modules.

Numerical data, video, digitized picture and voice

transmission may be needed in some cases between the

central computer and modules.

Communication levels

The levels of the communication architecture with

reference to the OSI- model are depicted in fig. 2.

At the representation level there is an agreement of the

representation of data values, e.g. floating point format

and other, in order to make possible to use diverse

equipment in the system.

There is also a harmonization of the basic module command

set. This set includes commands to operate the system

from the central computer. Examples from this set are:

Run Initiate the predetermined sequence (machine

dependent )

Stop Stop the operation. Can be restarted only by

Run

Break Abort the sequence. Can be restarted by

Continue. For abnormal conditions.

Continue Continues the sequence from the previous

Break-point

Home Go to predefined settings.

Communication between communication software modules in

different computing subsystems is based on logical

channels between these modules. Logical channels are

established during the configuration phase and remain

fixed through the operational phase.

At the bus level, there is intention to use to some

standard industrial protocol, like IEEE 802.2 at the

LLC-level and IEEE 802.4 at the MAC-and physical level

/!/. The LLC-level is connectionless type in order to

avoid overlapping connections between same program

entities

26
This arrangement means that in many cases the

communicating entity can be identified by simple address,

which is the same as the bus station address, or by a bus address + a generic name. This arrangement is

different from the OSI- model, and the purpose is to

reduce the communication overhead.

At the physical level the choice of the protocol is

dictated by the factors of the deviced response time and

the environmental electromagnetic disturbance conditions.

Broadband communications should be, at least

theoretically, less sensitive to noise than levelband

communications. In addition broadband system makes

possible to use multiple channels in the same medium.

This feature can be used to reduce transfer delay under

heavy load conditions. There is no protocol standard yet

suggesting this type of operation.

5. MESSAGES AND TRANSFER DELAYS

Typical messages during the operational phase are quite

short. Long messages occur during the program downloading

phase, but the response time is not critical in that

operation.

The header and other overhead belonging to a link-level

frame is typically 10 to 20 bytes. It can be estimated

that the overhead of embedded higher-level protocols is

of the same magnitude. Thus, the total overhead in a message is 20 to 40 bytes.

Numerical data value e.g. in the floating-point format

occupies 4 bytes. A message containing e.g. values for

ten parameters would also take 40 bytes. It is obvious

that the average message length will be in the order of

one hundred bytes. Visual (picture) information messages

are considerably longer. A typical bit map memory has

size of 2 M bytes.

The transfer delay of the token-passing type of operation

has been analyzed widely, see e.g. references in /2/.

The transfer delay including queueing delays, is given

by the expression.

T = T n • w n • w • ( 1 —p ) p

• t

4- - + ..J 4- —

2 2 ( 1-p) 1-p

where x = average message transmission time,

n = number of stations, w = 'walking' time of token (per

station), p = n • X • x, X = messages/s per station.

27
Sample values for 1 Mbps, 5 Mbps and 10 Mbps bit rates

are given in fig. 3. Parameters selected are for a typical

task profile derived form the application of the system.

Results show, that e.g. for a 5 Mbps system with 20

messages/s per station the average delay is approximately

0,6 ms.

It is rather difficult to approximate the delay of higher

levels. It depends of the protocol level and of the

scheduling of the program and the processing time of the

computer. Practical experience and related simulation

results / 3 / have shown that the response time of higher

layers is considerable longer than that of the bus itself.

The time delay caused by the higher levels can be reduced

by designing the protocols such that the implementation

is rather simple. This can be done by rejecting some

services defined by the OSI-model but not relevant in

this type of application. This should result in simpler

and faster buffer management and reduced intertask

communication, thus reducing the total running time of

the program.

If the central computing unit is used to sample the

values of some modules of the systems, it is obvious, that

the maximum workload will be in this processor. The

maximum usable sampling rate is thus dependent of the

processing capacity of the central processor, and not of

the communication bus. With the approximation above the

sampling rate will be in the order of 10 to 100 samples/s.

Transmission of picture information takes considerably

longer time. E.g. transmission of contents of 1 Mbyte

picture memory takes 1,6 seconds at the 5 Mbps bit rate.

By the use of compression techniques this time can be

shortened. It is obvious, that in order not to lengthen

the bus response time considerably the picture information

should be transmitted in a separate channel whether in

digital or in analog form.

6. COMMUNICATION SUBSYSTEM

The task of the communication subsystem is to connect

different devices of the cell, transducers, intelligent

sensors and actuators e.t.c. to the controlling processor.

Communication is typically between the control processor

and a device (= substation). Typically there is no need

for communication between substations. The control

processor inputs are measurement values, positional data

e.t.c. and the processor sends basically commands, and/or

numerical data.

28
The number of different devices in a cell is in the order

of ten, and the distance between a device and the control

processor is in the range of 0,1 to 10 meters.

The communication protocol was designed to be as simple

as possible but still effective and reliable. The

implementation is based on existing microprocessor serial

communication VLSI which makes the solution also costeffective . The architecture of the communication subsystem is a bus, consisting of a single pair, double pair (Rx/Tx),

or a coaxial cable, fig 4. The length of the cable can

be up to 400 meters, and the bit rate can be several

hundreds of kilobits per second (< 0,5 Mbps).

The communication method is polling. CPU gives permission

to send to a substation in order. This substation sends

it's data or an acknowledgement to the CPU. There is

only one message on the bus at a time. Communication is

asynchronous and the length of the message is 62 byte (one

line) maximum but can be variable otherwise.

Control processor (CPU) sends polling messages and data

messages. The format of these messages is the following:

CPU sends:

SA Type = Poll Pollir

8 8

A B C D

SA Type Length Data

n x 8

message to a substation + poll

Fields B and C can be combined to a single 8-bit field.

SA = substation address

29
Substations send always a response to the message from

the CPU. The message types are the followings

Substation sends:

0000 ACK/NACK SA

8 8

Polling message

received, nothing

to send.

A B C D E

0000 Type a data

byte

count

data SA

8 8 8 n x 8 8

Data

message

from a substation

Poll to the next station is the acknowledgement, that

CPU has received the message correctly.

If the number of stations is less than 2 = 128, CPU

messages (polling messages) and substation responses can

be separated by one bit:

^

Poll-bit

SA

1 7

0 SA

1 SA

CPU sends

Substation sends

Address and type fields A and B are combined to one byte

in the CPU message as well as the fields A and E in the

substation message.

Error control is based on the error detecting by parity

checking, and repeated transmissions. This should -give a rather good performance in normal conditions. More

powerful error checking method may be necessary in

difficult environmental conditions and can be easily

added to this protocol. In very severe electrical

disturbance cases shielded cables or optical fiber cables

have been used with good results.

30
The time of the polling period on the response time of

this system can be approximated with constant message

lenghts. Let the number of substations be 10, the average

message length be 5 x 2 + 1 = 11 bytes with 10 bits/byte,

and let the bit rate be 0,5 Mbps. Then

CPU sending time 0,02 ms

substation sending time 0,22 ms

CPU latency time 0,02 ms.

Thus the time of the total polling period is 2,5 ms.

7. CONCLUSIONS

A communication system architecture for a distributed

industrial robotics automation system is outlined. The

system is represented in a layered harmonised form.

Protocols do not exist at the moment to implement all

the levels.

Broadband multichannel token-passing LAN is shown to be

a good communication solution in regard to reliability,

efficiency and expandability. The response time of the

LAN-level communication system is in the order of

milliseconds. The total response time will depend largely

on the capacity of the processors. A protocol for

subsystem communication is represented.

8 . REFERENCES

111 IEEE 802.x series of draft standards for LAN:s

111 J.C. Hammond - P.J.P. O'Reilly: 'Performance

Analysis of Local Computer Networks', Addison-

' Wesley 1986

/ 3

/

W. Hoffman - T. Kersting: 'Simulation of Ethernet

under Real-Time Conditions', Process Automation

1984 .

31
Fig.

1.

Modular

industrial

automation

system.

32

Workstations

Sensors

NC:s
7 Application Application

6 Presentation Data and Cormand

5 Session * presentation

A Transport

Process-to-process

3 Network links

2 Data link - - . IEEE 802.2, connectionless

1 physical Broadband multichannel (IEEE802.A)

Fig. 2. System communication layers and their relation to

OSI-laye£.

Dtlau

lud Number of stations n = 50

Average message lenght = 100 bytes

Walking time v = 40*bit time

Fig. 3. Delay curves of a token-passing system.

33
Fig. 4a. The local LAN

Fig. 4b. Block diagram of the local LAN interface.

34
REAL-TIME MAP:

A NETWORK ARCHITECTURE STANDARD FOR MANUFACTURING CELLS

By

V ij ay Chauhan

Honeywell Corporate Systems Development

MN63-C060

1000 Boone Avenue North

Golden Valley, Minnesota 55427

Division

Workshop on Factory Communications

Co-sponsored by

IEEE Industrial Electronics Society and

National Bureau of Standards

to be held March 17-18, 1987 at

National Bureau of Standards

Gaithersburg, Maryland

35
i

ABSTRACT

In the wake of the Manufacturing Automation Protocol (MAP) development,,

efforts to establish network architecture standards in the area of

manufacturing automation have reached a frenetic pace. Market opportunity is

the basic driver for this feverish activity by all vendors of industrial

automation products. In 1984, the U. S. market for process and programmable

controllers was $1.1 billion and is expected to increase to $2.6 billion by

1990-— a growth rate of 15.9 percent. This paper addresses the architecture

and communication needs/standards of a segment of this market that is relevant

to HoneywellTM real-time control at the manufacturing cell level.

36
1-1

SECTION 1 INTRODUCTION

The manufacturing industry is going through a rapid standardization phase in

the area of factory-floor device communications. The lack of standards has

driven up device interconnection costs because each interconnect is

custom-made for a given pair of devices. This trend limited the gains

realized by increased automation and caused General Motors to look into ways

devices could interoperate at the "plug-campatible" level, similar to that

achieved by the telephone industry. The result was the MAP specification.

This specification is a suite of communication protocols derived from IEEE,

ANSI, ISO, etc., standards and is based on the seven-layer reference model for

Open Systems Interconnection (OSI). MAP is intended to provide standard

interconnect solutions on a plant-wide basis; however, for real-time

communications, the MAP architecture is considered too cumbersome to meet

timing requirements for manufacturing-cell-level communications. This is due

largely to the seven layers of communications overhead present in the MAP

architecture. To solve this, a collapsed architecture that bypasses four

layers of protocol and has increased network responsiveness has been designed

to provide standardized interconnect solutions at the cell Pevel for real-time

control. This architecture is termed Real-Time (RT) MAP. The collapsed

architecture, however, is reduced in functionality when compared to the

seven-layer MAP architecture.

Section 2 describes the RT MAP architecture, the protocols associated with the

architecture, and the status of the standards. Since the architecture is

based on draft proposals that are evolving toward international standard

status, the architecture is a moving target.

Section 3 presents some of the research work done within Honeywell with regard

to the performance study of the Real-Time MAP architecture. A prototype

network has been built for performance modeling; the status of the network is

presented.

37
2-1

SECTION 2 REAL-TIME MAP ARCHITECTURE AND STANDARDS

ARCHITECTURE

The discrete manufacturing architecture can be hierarchically divided into six

levels, starting from the enterprise level and going down to the sensor level

(Figure 2-1) . At the enterprise level, whole plants distributed

geographically over large distances are connected via wide-area networks

(WANs). At the sensor level, a varied collection of sensors are linked

together via potentially simple proprietary networks. Located within this

spectrum of network architectures are the interconnect solutions offered by

MAP and RT MAP. The MAP architecture is ideal for a backbone network that is

dispersed throughout the entire plant. This architecture is designed to carry

high volumes of data of a heterogeneous nature (data, voice, facsimile,

etc.). In an attempt to be "everything to everybody," the MAP architecture

resulted in a hefty set of network protocols that were loaded with

functionality. However, the one functionality that had to be sacrificed (to

provide for the rest) was network responsiveness (i.e., the ability to get

things across to the other host quickly). At the level of the manufacturing

cell, the maximum tolerable end-to-end response time is on the order of 20 to

100 milliseconds. At this level of the manufacturing architecture hierarchy,

network responsiveness takes priority over any other functionality. A

standard network architecture that provided the required network

responsiveness was needed; the RT MAP architecture was designed to fulfill

this need. Layers of protocols present in the MAP architecture whose

functionality was not a priority were eliminated. RT MAP is a collapsed

version of the MAP architecture with a smaller set of protocols that results

in a smaller set of code/ hardware and, hence, lower networking overhead.

Figure 2-2 contrasts the full MAP and RT MAP architectures. The MAP

architecture, which is based on the reference model for Open Systems

Interconnection (OSI) provided by the International Standards Organization

(ISO), specifies a network protocol for each layer of the reference model

(currently the presentation layer protocol is not specified). The RT MAP

architecture was derived from the MAP architecture by removing protocols that

provided functions thought to be dispensible.

Real-Time MAP goals include:

• Minimal machine instruction cycles to deliver protocol data units to

the application interface;

• Immediate message acknowledgement;

• Broadcast capability;

• PRCWAY and 802 compatibility.

38
FOREIGN

CAO

SYSTEMS

AND

WORKSTATIONS

2-2

39

Figure

2-1.

Discrete

Manufacturing

Architecture
2-3

PULL MAP RIAL-TIMi MAP

RS-511 CASE KERNAL

APPLICATION AS-511

PRESENTATION

ISO SESSION KERNAL SESSION

ISO TRANSPORT

CLASS 4 TRANSPORT

ISO CLNS NETWORK

LLC TYPE 1 802.4 DATA LINK LLC TYPE III 802.4 WITH PROWAY

EXTENTIONS BROAD BAND PHYSICAL CARRIER BAND

Fil* T601 73-074 1M

Figure 2-2. Layer Specification

The following are Real-Time MAP design criteria:

Access time

Signalling rate

Signalling method

No. of stations

Distance (perimeter)

Typical message length

Maximum message length

25 ms

5 Mbps

Phase coherent

32

1 km (3300 ft)

16 to 20 octets

<1K octets

To provide real-time communications capability, the following two requirements

must be met: (1) a facility to provide immediate acknowledgement for every

message transmitted by a sending station, and (2) a decrease from the MAP

standard in the time for a message to travel from the physical layer to the

application layer.

The first of these requirements is provided for by including PRCWAY's

extension to the MAC layer specified by the IEEE 802.4 standard. The second

requirement is met by bypassing some of the intermediate layers of the OSI

reference model. This bypassing of layers can only be achieved by sacrificing

some of the functionality normally provided by the bypassed layers. The

layers that have been bypassed to provide a real-time architecture are network, transport, session and presentation. Figure 2-3 represents the RT

MAP architecture.

40
2-4

The notable differences between the RT MAP protocols and their counterparts in

the MAP architecture are:

• Physical Layer—In the MAP architecture, a broadband coaxial cable

based on the specifications laid down by the IEEE 802.4 standards

committee is identified as the physical layer. This specification is

based on the standard CATV cable used by the cable TV industry. The

bandwidth is divided into channels, each approximately 6 MHz wide.

Thus it is possible to pass information of various kinds (voice/data/

facsimile, etc.), each on a different channel. This requires modems

that are fairly intelligent and frequency-agile; it also requires

active components such as head-end remodulators, etc. RT MAP specifies

a single-channel, frequency-shift-keying (FSK), carrierband,

phase-coherent signalling method. Here the modems are relatively

simple and therefore are expected to be cheaper (a requirement for the

RT MAP architecture).

• Data Link Layer—At the MAC sublayer of the data link layer, the MAP

architecture specifies the IEEE 802.4 standard, and the RT MAP

architecture specifies the IEEE 802.4 standard with some extensions for

implementing the immediate acknowledgment functions. At the LLC

sublayer, the MAP architecture specifies a simple datagram TYPE 1 service. The RT MAP architecture, on the other hand, specifies a Class

Fil* T601 73-0742M

Figure 2-3. RT MAP Architecture

41
2-5

3 service (a union of TYPE 1 and TYPE 3). The TYPE 3 service provides

an acknowledged connectionless service that results in reduced

transport layer functionality.

• Network, Transport, Session, Presentation Layers—The MAP architecture

specifies protocols as shown Tn Figure Z-"2, while the RT MAP

architecture has null protocols for these layers.

• Application Layer—At the application layer, the MAP architecture

specifies a CASE protocol, and the RT MAP architecture does not. In

addition, the MAP architecture specifies a host of application-specific

service elements such as RS-511, Vertical Terminal Protocol (VTP),

FTAM, etc., while the RT MAP architecture specifies only RS-511 as the

application layer.

In meeting the RT MAP goals, the following functionality has been compromised:

• High-quality guaranteed delivery;

• Session services, e.g., data synchronization;

• Real-time global message delivery (off-segment);

• Indefinite message length;

• Reduced flow control;

e Concept of connection.

The RT MAP cell level architecture connects to the plantwide network (which is

potentially based on the MAP architecture) via a gateway, as shown in Figure

2-4. However, a gateway is not the only type of device that can be used to

interconnect MAP and RT MAP networks. Routers and bridges are alternative

interconnect devices (Figures 2-5 and 2-6, respectively). A router provides

connection at the network layer, while the bridge provides connection at the

data link layer. The trade-offs in selecting a bridge, router, or gateway

involve issues related to cost, performance, dissimilarity of networks,

security, etc.

Two types of node architectures have been identified for an RT MAP network

(Figure 2-4). One type of node is expected to have collapsed RT MAP

architecture, as specified in Figure 2-3 and shown in Figure 2-4. The second

type of node is expected to have a dual architecture. One part of the dual

architecture exhibits the RT MAP architecture and the other part exhibits the

MAP architecture. This dual-architecture node is also known as a MAP/ RT MAP

node. The purpose of the MAP/RT MAP node is to provide a way for MAP nodes to

obtain faster response times on control and time-critical networks.

NOTE: The RT MAP architecture node is also known as a Mini-MAP node. The

dual- architecture (MAP/RT MAP) node is also known as an Enhanced Performance

Architecture (EPA) node. These two are the only accurate names to date. All

other names should be ignored to reduce confusion.

42
MAP

Network

2-6

43

Figure

2-4.

RT

MAP/HAP

Network

Interconnect
2-7

INTERNET

LLC 1 LLC 2

MAC 1 MAC 2 PHY 1 PHY 2

LAYER 3

LAYER 2

UYER 1

Figure 2-5. Router Architecture

INTERLINK

LAYER 2 MAC 1 MAC 2 PHY 1 PHY 2 LAYER 1

Figure 2-6. Bridge Architecture

Standards

The real force behind the development of RT MAP is its conformance to

standards that are defined by recognized standards bodies and committees. Not

until these standards are officially established, whether they are interim

standards or final, will the full capability for interconnection of devices

from different vendors be realized. Therefore, tracking and driving the

standards must parallel their implementation. This is accomplished by active

participation at appropriate standards meetings and through establishment of a corporate strategy on the level of participation.

All protocols associated with the RT MAP architecture are at various stages of

becoming either international standards or IEEE standards. Table 2-1 shows

the currently identified work items that relate to the RT MAP architecture.

The standardization process within MAP is as follows. Architectures/Items for

Standardization (IFS) are defined and designed by various MAP subcommittees.

The subcommittee passes its work on to the MAP Technical Review Committee

(TRC). When the TRC approves the work item, it must decide in which revision

of the MAP specification the approved work item should be introduced. It also

must decide what changes to the entire MAP specification are required to keep

it consistent with the recent addition. Typically, the MAP subcommittees

develop architectures based on protocols that are being standardized by bodies

such as IEEE, EIA, ANSI and ISO. Thus, the proposed architectures are based

on either existing or evolving standards.

44
2-8

All protocols associated with RT MAP are emerging standards. No

implementation of the RT MAP architecture exists in its entirety; thus,

questions related to performance, throughput, size of code, protocol overhead,

reliability, etc. , have previously had no implementation-based answers. Section 3 describes efforts within Honeywell to answer these questions.

Table 2-1. Status of Standards

Standard 1986 1987 1988

LLC Draft ProposallQ86 IEEE Standard 1087 IS*

RS-511 EIA Draft Proposal EIA Standard 1087 IS 1088

Network

Management

Draft Proposal in IEEEIEEE Standard 2087? IS

Directory

Services

Draft Proposal

in MAP Subcommittee

and ANSI

ANSI Standard 2087? IS?

User Interlace MAP Standard 2087

* International Standard

60551-3437-1

45
3 = 1

SECTION 3 REAL-TIME MAP PROTOTYPE NETWORK

In early 1986, a research program was put in place within Honeywell with a specific goal of understanding the performance, protocol and service offered

by the Real-Time MAP architecture. The goals of the program ares

« Build a real-time MAP network by the end of 1986;

• Integrate typical factory floor, cell level devices (WCC 1250, IPC

620) ;

• Obtain performance measures for the prototype network such as end-to-end delay, throughput, etc. (See Figure 3-1);

• Integrate performance measurement hooks for layer-by-layer analysis of

the architecture;

• Build a generalized multi-node performance model for Real-Time MAP;

• Validate performance model against prototype network.

The prototype three-node network (Figure 3-2) consists of two WCC 1250

microcomputers and an IPC 620 programmable controller. The WCC 1250 is a 68000-based, multiuser, multitasking supermicro manufactured by Digital

Datacom, Inc., a Honeywell subsidiary. The IPC 620 is a programmable

controller manufactured by IPCD. Each of the hosts will interface to the RT

MAP network via a Network Interface Unit (NIU). The major task of the program

is to build an NIU that plugs into the backplane of the host. The NIU will be

68000 based, with a Token Bus Controller (TBC) (supplied by Motorola) that

will implement the MAC sublayer and a carrierband modem (supplied by Control

Data Sy stsems/Computrol) . The NIU will implement the entire RT MAP

architecture, ieaving the host to perform the application-oriented tasks.

Currently the three-node network with RS-511 and LLC Class 3 protocol

implemented in software is up and running. The network is going through an oplimization phase. Simultaneously an evaluation effort for

petri-net/queueing network performance modeling tools is underway.

46
3-2

Node i Node 2

Performance Measures for Layer N:

• Response Time: Til - TO

• End-to-End Delay: T5 - TO

• Queueing Delay: Ti - TO (for a requesl)

• Service Time (T2 - Ti) + (TlO - Til) for Node 1

60551 -341 7MD1

Figure 3-1. Performance Measurements

JV- F.le T601 72-07&3M

ure 3-2. Prototype Network for Real-Time MAP

47
4-1

SECTION 4

CONCLUSION

Increased automation implies an increase in the amount of control and

functionality distributed to devices that are closer to the shop floor,,

Together with the need for peer-to-peer, high-performance communications,

Real-Time MAP with its lower protocol overhead and its connectionless mode of

service has been designed to provide for these needs. Studies along the lines

described in this paper will help ultimately in designing better communication

solutions for the manufacturing automation industry.

48
5-1

SECTION 5 REFERENCES

1. ANSI/IEEE Std 802.2-1985

2. ANSI/IEEE Std 802.4-1985

3. ISA SP72.01 - PROWAY LAN

4. General Motors MANUFACTURING AUTOMATION PROTOCOL ver 2.1

5. General Motors MAP/Enhanced Performance Architecture Overview,

1985

November

49
Applying MAP Networks

50
Open System Interconnection for Real-Time Factory

Communications: Performance Results

Cornells Franx Kevin Mills

Philips National Bureau of Standards

Eindhoven, The Netherlands Gaithersburg, MD USA

This paper considers the applicability of Open System Interconnection (OSI)

protocols, as now defined and implemented, for use in real-time factory

communications. Factory communications requirements are described by

outlining the hierarchial nature of the factory network architecture and by

defining the nature of real-time at the lowest level of hierarchy. Two

possible solutions for real-time factory communications are described: 1) the

full MAP seven layer architecture and 2) the MAP enhanced performance

architecture (EPA). Measured performance of a five layer OSI protocol

implementation is described with special emphasis on one-way delays.

Measurement results are also given for throughput. The ability of present OSI

standards to guarantee real-time performance is evaluated. A flow control

problem is identified concerning use of an OSI transport protocol over a type

1 class 1 logical link control protocol.

51
I. Introduction

Data communications within a factory must meet a hierarchy of performance

requirements including real-time at the lowest levels and time-critical within

a workcell. The Manufacturing Automation Protocol (MAP) standard, requiring

all seven OSI protocol layers, was thought inadequate for real-time and time

critical applications. Thus, a subset of protocols defining an Enhanced

Performance Architecture (EPA) has been added to the MAP standard. The

purpose of this paper is threefold: 1) to describe the hierarchical

performance requirements within a typical automated factory, 2) to explain the

differences between the full seven layer MAP standard and the three layer EPA,

and 3) to provide measured performance results for applications using the

lower four layers of the MAP standard. Each of these topics is covered in a separate section below. Some conclusions are drawn with respect to the

performance possibilities of EPA and the full MAP protocols and areas of

further research are indicated.

II. Factory Communications Requirements

Communication requirements in a factory depend on the type of production

process carried on in the factory. So the production process and the related

production control architecture have to be described before the communication

requirements can be stated.

In Philips there are many factories with discrete assembly lines, for instance

to produce radio sets. The control structure of these lines will be based on the National Bureau of Standards hierarchical model for production control

that was adopted by Philips (see Figure II-l) [ALB81]. The controller

processes at different levels in the hierarchy will be implemented on separate

computer systems. In the automation module and device control layers

individual systems control robots, positioners and local workpiece transport

within an assembly station. The workstation controller sequences the

activities of the automation module controllers to execute assembly tasks with

elapsed time in the two to five second range. The workcell controller routes

incoming parts to available stations, coordinating the area transport and the

assembly workstations. Data communication between the computer systems is

required when their controller processes interact.

^Certain commercial equipment is identified in this paper in order to

adequately specify the experimental procedure. Such identification does not

imply recommendation or endorsement by the National Bureau of Standards, nor does it imply that the equipment identified is necessarily the best available

for the purpose.

52
FIGURE 1

1 -1 . LOGICAL MODEL OF A PRODUCTION CONTROL HIERARCHY

53
The time requirements of a controller process become less stringent as the

process is positioned higher in the model. Controllers at the top of the

model are concerned with long term planning and control, at the bottom of the

model they do direct real-time control. At the upper layers large batches of

data (> 1Mb) have to be transported at one time, for instance a daily

production plan. This batch data must be transported within several minutes.

Going down in the model time requirement become shorter and more stringent,

but the amount of data to be transported at one time decreases as well.

In the Philips environment, from the workcell controller down, communication

times on the order of one second can reduce production throughput by 10-20%.

Consider, for example, the communication between workcell controller and

workstation controller. At this level work orders and status reports are exchanged with message lengths of 100 - 500 bytes. A message must be sent and

delivered within 100 to 200 ms.

Communication at the next lower level between automation module controllers

and workstation controller is more critical, with 100 ms being a maximum time

for communications. Communication within the workstation becomes highly time

critical, especially between device controllers and automation modules.

Control loops at this level require small amounts of data (< 20 bytes) to be

transmitted very fast (within 10 ms) with a repetition frequency of up to once every 10 ms. All figures presented here are meant to give a global

indication of the communication requirements and they only concern the

delivery of production control information.

When there are tight time limits like those at the bottom level, the traffic

is often called "real-time" generally without defining the meaning of "realtime". In this paper it has the following meaning:

Communication is real-time when a message must be passed from

one process to another within a previously specified time

limit in order for the process to correctly perform its

function

The data communication network has to guarantee that except in the case of

system or component failure the message will be transferred within that time

limit. To prevent messages with a more relaxed time limit from interfering

with urgent messages and delaying them, priorities have to be allocated to

messages. Higher priority messages are handled before messages with a lower

priority

Another important aspect of real-time communication is its error behavior. In case of a link failure only limited time should be spent on error recovery

procedures, then the application processes must be warned to enable them tc

take appropriate action.

As mentioned eariler, real-time communication can be found at the bottom of

the model, however, it is important to realize that not all communication is

for the purpose of production control. Communication for software downloading

54
and reporting of production statistics has different requirements. Generally

the time limits are not very tight in these cases but reliability is more

important, requiring more extensive error recovery efforts.

The different communication requirements at the bottom two layers of the

factory model demonstrate the need for a network that offers a real-time

communication service as well as a more reliable, higher level communication

service. Without a standardized network offering both types of service,

proprietary networking solutions are inevitable.

III. OSI and MAP Enhanced Performance Architecture

Two possible architectures for real-time factory communications will be

discussed, the "full" MAP architecture based on the seven layer OSI reference

model and the three layer Enhanced Performance Architecture (EPA). In MAP

version 2.1 a selection of protocols and options for six layers of the OSI

model (Fig. II-2) has been made. The missing protocol for the presentation

layer will be supplied later. Based on this selection of protocols, vendors

have started to make interoperable data communication products for the

factory.

Almost from the beginning of MAP there were doubts from the process control

industry that seven layer MAP (called "full" MAP) could meet the performance

requirements for real-time applications. This led to the introduction of a

new, three layer architecture called the enhanced performance architecture

(EPA) (Figure II-3). Many of the ideas behind EPA were adopted from Proway

which is a local area network for the industrial environment defined by ISA SP

72 [ ISA72 ]

Both full MAP and EPA use at their bottom layers the IEEE 802.4 token bus

[IEE85]. The first important difference between the two is at the data link

layer where EPA uses logical link control (LLC) type 3, acknowledged

connectionless service, instead of LLC type 1, unacknowledged connectionless

service [IEE84]. LLC type 3 confirms the arrival of data at the destination

LLC layer and retransmits data in case of errors. More precisely when a station has the token it sends an LLC type 3 frame and then waits for a returning LLC type 3 frame that is in fact an acknowledgement. Without this

response, a timeout will occur and the LLC frame will be retransmitted. Only

after the acknowledgement is received or the maximum number of retransmissions

is exhausted can the token be passed to the next station. It is notable that

this LLC protocol interacts with the operation of the token passing protocol

at the MAC layer.

The time the station can hold the token (token-hold time) is limited;

therefore, the LLC type 3 acknowledgement and retransmission scheme has to

work very fast, and only three retransmissions are allowed. Without a tight

limit on the token-hold time the total performance of the token bus could

degrade severely when there are repeated LLC type 3 failures.

The LLC type 3 timing characteristics make it impossible to transit a bridge

because a bridge introduces unacceptably large message delays when it passes

LLC frames from one network segment to another. As a result the use of LLC

type 3 is limited to a single' segment.

55
uurae mr va.i

HCTOCOlf

MAP/1PA

PROTOCOLS

Layer 7 190 mu (DP; 1571 lUIMIt 111 Appliestisn Pile Transtar Protocol. nawfecturint Natsava Mumtacturlav huimi Ponat

Standard (Mart) and

Coven Application Aerviee

Klaoatt (CASK)

Service

Layer 9 nu WC0KM ItiMnutlen (ASCII and linary tneodino)

um s 130 Session (IS) 8327 tost ItlUOl Basle Cceomed Subset and

Session Kernel. Pull Buplae

Layer 4 :io Transport (IS) 1073 won

transport Class «

bajrv ) ISO Internet (BIS) 1473 son

Seteork Connectionless network Service

(CUTS)

Layer 2 ISO Loflcal Link Control (DXS) 8802/2 Bata Link (ISIS 802.2)

Clast i, Type i Class 3, Type 3

Layer i ISO Token Pestlae sue (DIS) 8802/4

Physical Cm 802.4)

Broadband Carriarband

FIGURE 1 1-2. MAP V 2.1 AMD FUTl^c V AP/E?A PROTOCOL STACK

KM drriirkHtf lapvt

FIGURE 11-3. WDES ON AN EPA SEGMENT

56
The use of LLC 3 to provide real-time message delivery is not without

potential problems. When a token-holding station is awaiting an immediate

acknowledgement several events can occur. For example, the token-holding

station may retransmit the message or the token-holding station may give up

and pass the token. Either of these messages may collide with the

acknowledgement. Collision with the retransmitted message will cause another

retransmission. Collision with the token will invoke token recovery

procedures

It is very likely that EPA, in addition to the bottom two layers with IEEE

802.4 token bus and LLC type 3, will ultimately have a third protocol layer

containing Manufacturing Message Service (MMS) which runs directly on top of

the LLC. The mapping between MMS and LLC has already been described in a document prepared by MAP's Programmable Devices Committee.

In the EPA specification layers 3 - 6 of the OSI model are absent. Because of

the missing layers EPA offers reduced functionality when compared to "full"

MAP. The consequences of skipping four layers will be discussed briefly,

starting at the network layer.

Because there is no network layer it is impossible to traverse intermediate

systems (routers). But, since intermediate systems introduce relatively large

and unpredictable message delays, this limitation is necessary to achieve

real-time performance. Without a network layer it is possible to leave out

the integrity assurance protocol of the transport layer as well because LLC

type 3 already offers end-to-end acknowledgment. Some of the missing

transport functions such as, resequencing, flow control, and multiple

associations, are covered by the mapping between LLC type 3 and MMS.

The next higher layer, the session layer, offers dialogue control and

resynchronization which are not suited for real-time applications. Therefore,

the session layer can be skipped as well. Finally EPA has a presentation

syntax (X.409) [CCI84] and an application layer protocol when the layer seven protocol, MMS [EIA85], is added to EPA.

The use of EPA and "full" MAP can be combined on a single EPA segment (Figure

III-3). On the segment there can be MAP/EPA nodes with the three and seven layer stack and Mini-MAP nodes with the three layer stack only. The choice

between "full" MAP or EPA should be based on the communication requirements of

the applications. Applications in a MAP/EPA node select the seven layer stack

to get the functionality of "full" MAP and then are able to communicate with

all other MAP/EPA nodes on the segment and with "full" MAP nodes on the

broadband backbone. Real-time applications select the EPA stack to get faster

responses and then are able to communicate with other MAP/EPA nodes and MiniMAP nodes on the same segment. Real-time applications must be aware of the

reduced functionality of EPA.

The main differences between "full" MAP and EPA have been pointed out. Now

the suitability of both architectures to handle real-time communication will

be discussed. The functionality of "full" MAP is not always required, and the

associated protocol activities are sometimes undesirable for real-time

communication (i.e., message routing, extensive retransmissions and

resynchronization). When the seven layer OSI stack is used, message delays

57
are unpredictable and can not be controlled. One of the main reasons for this

is the lack of control over the allocation of system resources making it

impossible to give real-time communication priority over other communication

and activities like network management (see Section IV). Without priorities

all communication is treated in the same way regardless of its urgency and it

is possible that large file transfers will interfere with short real-time

messages and delay them.

EPA is in a better position to provide real-time communication than "full"

MAP. Because it has only three protocol layers, processing delays can be much

smaller. Moreover, it has a priority mechanism at the LLC layer that, when

MMS honors priorities as well, permits full application-controlled use of

priorities. Message delays are predictable when the number of nodes in the

segment is known and the segment is in a stable operating condition. It

should be realized that this is only true for messges with the highest

priority -- lower priority messages are delayed by higher priority messages

making it more difficult to predict their delay. LLC type 3 has a favorable error recovery mechanism because it performs retransmissions in a short time

and then warns the application. Other failures delaying the transmission of a message should be reported as well (i.e., the collapse of the token ring).

For real-time communications the EPA stack seems to have attractive

properties, but Philips is concerned about the cost to produce error-free

application software for EPA and about the portability and flexibility of this

software. Also, conformance and interoperability testing will be difficult

for EPA because there are many options, such as connectionless or connectionoriented use of MMS, MMS subsets, and non-token stations.

IV. OSI Measured Performance

In order to assess the suitability of OSI protocol standards for factory

applications, the National Bureau of Standards ( NBS ) and Philips conducted a cooperative project to measure the communication performance in a small

testbed network using OSI protocol implementations. Intel Corporation donated

hardware and software for the project. A brief description of the testbed

network is given, followed by a discussion of the performance results.

58
A. The Testbed Network

The local area network testbed implemented at the NBS is illustrated in Figure

IV-1. Four Intel 310 nodes and a passive, real-time monitor are connected to a CSMA/CD local network. A global clock circuit is connected to each Intel

310 node to provide a synchronized measurement clock. The internal

architecture of each node is shown in Figure IV-2

OSI communication services are provided by Intel's iNA-960 software [INT84A]

running on a 186/51 COMMputer

TM

board [INT84B]. The 186/51 contains two

processing elements: an 80186 (transport, network, and logical link control)

and an 82586 (media access control). Traffic generation and measurement

software runs on a separate host board based on an 80286 processor [INT83].

Communication between the host and COMMputer board is via message passing

using the Multibus Interprocessor Protocol (MIP) [INT84A, Appendix E]

Figure IV-3 shows how the global clock board provides a synchronized 100 us pulse to each Intel 80286 CPU board. The clock pulse is connected to a 16-bit

programmable interval timer (PIT). The PIT overflows every 6.5 seconds

causing a 16-bit software clock to be updated. The entire 32-bit clock is

available to user software.

Figure IV-4 illustrates the time delay measurements made within the

measurement software. A user task requests communications services by issuing

a request block ( RB ) to iNA-960 via a system call. The time required to

return from the system call is measured as Tl. Once the iNA-960 has provided

a requested service, the RB is returned to the user program. T3 measures the

time elapsed between issuance of the RB and its return. An RB normally

contains a user message within it. T2 measures one-way delay for user

messages. T4 measures the duration of an experiment.

The variables controlled by the traffic generation software are listed below

(Table IV-1). Another set of variables, such as retransmission timer values

and transport message sizes, are controlled on a connection-by-connection

basis using iNA-960 network management services. The network management

services are also used to monitor lower level measures such as collision

counts, count of packets sent and received, and number of packets dropped due

to buffer overrun. A passive, real-time monitor enables unobtrusive

evaluation of experiment progress -- indicating number of connections, number

of retransmissions, protocol efficiency, and total data sent [MIL85].

The experiments divide naturally into three sets: 1) throughput profile, 2)

delay profile, and 3) multi-application profile. Measured results for each

set are discussed in the following sections.

*Although MAP requires use of a token passing bus media access control

technique, the object of this study is transport layer performance on an unloaded local network, and so the use of a CSMA/CD local network does not

invalidate the study.

59
INA-960

CSMA/CD

SUN

FIGURE IV-1. EXPERIMENT NETWORK TESTBED

Traffic MIP iNA-960 CSMA/CD

Generation &

Measurement

Figure IV-2 Architecture of an Intel 310 Network Node

60
CLOCK BOARD

Tl: SB accept delay

T2: User massage delay

T3: SB return delay

T4: Experiment duration

FIGURE l V - 4 . JGCR ;ei ays measured

61
Table IV- 1. Traffic Generation Variables

Application Priority

Inter-message Delay

Duplex or Simplex Data Flow

User Message Size

Total Data Transferred

Number of Connections

Number of User Buffers

62
B. Throughput Profile

The throughput profile shows total measured throughput under a variety of

conditions. Simplex and full-duplex data flows are considered and a flow

control problem is discussed. User message sizes are always 10K octets. To

achieve the best throughput the value for the adaptable retransmission timer

parameters had to be increased as the number of connections increased at each

node. This adjustment is required because the apparent round-trip time

increases as the load increases in each node.

Simplex Transfer

Figure IV-5 shows the total throughput measured during simplex data transfer,

between two Intel 310 systems, as the number of buffers per connection is

varied. Measures are shown for one, two, three and four transport

connections. The minimum values for the adaptable retransmission timers used

for each experiment are given below (Table IV-2). Throughput ranged between

60 Kcps and 108 Kcps. With only two buffers per connection end-point

available, throughput is increased (Figure IV-5) by adding connections because

unused CPU capacity is available within the system. Once four buffers are available per connection, the unused capacity is reduced and the overhead

associated with connection scheduling becomes evident. Little throughput

difference was observed between three and four connections.

Full-Duplex Transfer

Figure IV-6 shows throughput measured when the experiments were repeated using

full-duplex data transfer. Retransmission timer values used are shown in

Table IV-3. Throughput ranged between 90 Kcps and 104 Kcps.

Flow Control Problem

During the throughput experiments a problem was found with the combination of

the OSI transport protocol operating over a type 1 class 1 logical link

control protocol. The problem is illustrated using the two throughput curves

shown in Figure IV-7. One curve (single sender) shows a pair of identical

machines engaged in a two-connection simplex data transfer. As the number of

transport buffers per connection increases, the throughput increases toward

104 Kcps. No matter how many transport receiver buffers are offered, the

receiver's link buffers cannot be overrun because only two machines are involved and both machines have identical processing capabilities.

The second curve on Figure IV-7 (two senders), indicates what happens if the

sending machine is faster than the receiver or if two machines are sending to one receiver. As the number of transport receive buffers per connection

increases, the throughput decreases toward 35 Kcps. The iost throughput

occurs because the transport flow control window, a direct reflection of the

number of receive buffers, allows the link level buffers of the receiver to be

overrun, invoking transport layer retransmission procedures. Use of link

layer flow control would solve this problem; otherwise, transport layer buffer

decisions must be made by accounting for link layer buffer conditions.

63
Table IV-2. Retransmission Timer Values for Simplex Throughput

Connections Minimum( Secs „

)

Starting'

1 .256 .512

2 .512 1.024

3 .819 1.638

4 1.024 2.048

1

«- +

55-1 1 1 1 1 1 1 1 1 —I 1 1 1 1 1 1 1 1 1 f— 10 15 2 0 25 36 3.5 40 45 50 55 60 65 70 75 80 85 90 95 10 0 10 5 mjrrFRS /roNNrcT ion

FIGURE I V - 5 . SIMPLEX THROUGHPUT PROFILE

ii 0

64
Table IV-3. Retransmission Timer Values for Full-Duplex Throughput

Connections

1

2

3

4

Minimum ( Secs .

)

.512

1.024

1.638

2.048

Starting ( Secs .

)

1.024

2.048

3.276

4.096

88 *—

1 « 15

-t--—4—

2 8 2 5

(. 4 4 1 4 1 1

(

1 1—

3 8 35 48 45 58 55 88 85 78 75 88

nuriTRS/CONNrCTION/niRI CTION

—I—

8 5 —I 1 19 8 9 5 18 8

FIGURE IV-6. DUPLEX THROUGHPUT PROFILE

&

+

--1 185 118

65
iC's

FIGURE l V - 7 . RECEIVER OVERRUN

•rfi ssAc.r i r*<;-ni (nYTrs)

FIG 1 I V - S . ONE-WAY DELAY PROFILE

66
C. One-Way Delay Profile

This section presents a profile of one-way user message delays (T2 in Figure

IV-4) measured under a variety of conditions. In all of the delay

experiments, the user message size is varied between 100 and 10,000 octets.

However, when a user message is large, protocol segmenting is required

because each link packet will hold only 1500 data octets. The sending user on each connection submits one message and waits for an acknowledgement

indication before submitting the next message. This stop-and-wait operation

limits the overall load on iNA-960 during the delay experiments.

Single Connection Delays

Figure IV-8 presents measured one-way delays with and without checksum

enabled. The message transfers occur over a single connection in a single

direction. The receiver allocates three transport receive buffers so that no delay is incurred for closing and reopening the transport flow control window.

The lowest delays obtained occur with 100-octet user messages and no checksum,

33.5 ms average and 70.5 ms maximum. The addition of the checksum raises the

lowest delays to 38.4 ms average and 78.8 ms maximum. As expected, the effect

of the checksum on delay is more significant as the message size increases.

Multi-connection Delays

Figures IV-9 and IV-10 illustrate the effect of multi-connection traffic on one-way delays. For the results in Figure IV-9 the receive buffers are limited to one per connection. Thus, the effect of closing and reopening the

transport flow control window is evident. The lowest one-way delays are obtained with a single connection and 100-octet messages, 45.3 ms average and

88.6 ms maximum. This means that, on the average, 11.8 ms is required to

handle reopening of the transport flow control window (comparing Figure IV-S

with Figure IV-10). In the maximum case, 18.1 ms is required. While this

overhead increases delay at small message sizes, it serves to reduce the one- way delay as the message size increases. Forcing the extra delay to reopen

the window on each connection reduces the increase in traffic intensity

normally associated with larger user message sizes.

Figure IV-10 illustrates the same experiment with three transport receive

buffers allocated to each connection. As the load increases for two, three,

and four connections, the average and maximum one-way delays increase

significantly. The upper bounds on one-way delay in the previous case were

443.2 ms average and 904.5 ms maximum. The upper bounds in this experiment

are 1395.3 ms average and 2076.0 ms maximum.

An increase of this magnitude is almost certainly due to a queuing delay

incurred at the receiving user program. The user program submits empty

receive buffers to and accepts filled received buffers from iNA-960. As

configured, iNA-960 gives a higher priority to processing of transport

operations, including passing filled receive buffers to the user, than to

accepting empty receive buffers from the user. Therefore, a user’s receive

queue grows during periods when the user program is blocked waiting for iNA960 to accept an empty receive buffer. This effect is demonstrated by an

(\

67
MESSAGE SIZE (OCTETS)

FIGURE IV-9. MULTI -CONNECTION DELAY PROFILE WITH TAUT FLOW CONTROL

68
increasing request block accept delay (T1 in Figure IV-4) as iNA-960 traffic

intensity increases. This effect might be reduced if the MIP task on the

186/51 is run at a higher priority than the iNA-960 task.

D. Multi-Application Profile

The next set of experiments involves a pair of traffic generation tasks on each of two Intel 310 systems. The first pair of tasks is generating bulk

data traffic. The second pair of tasks simulates a status report application,

submitting messages at a rate sustainable by the system so that no queuing

delay is included. The load caused by the bulk data transfer is controlled by

varying, between 400 octets and 40K octets, the size of transmit and receive

buffers. Status report messages are fixed at 100 octets. The operating

system priority of the status reporting task is higher than that of the bulk

data task.

Figure IV-11 shows the experiment results when the data flow for both

connections is in the same direction. The abscissa plots throughput of the

bulk data transfer. The ordinate plots the average one-way delay for status

report messages. Ideally the status report message delays (average and

maximum) will be kept near the lowest delays available from the system. These

target delays are superimposed on the graph with dashed lines.

The results show that the status report delays increase in a pattern similar

to that seen when message sizes increase (Figure IV-10, two connections)

though the status report messages do not increase in size. Also note that the

lowest average and maximum delays are twice the ideal. These results

represent unacceptable behavior for applications requiring real-time response.

The only control mechanism available in the OSI transport standard is the

allocation of buffer space. Therefore, iNA-960 does not provide priority

scheduling for transport layer connections and the MIP implementation contains

no multi-queue mechanism. Thus, the operating system task priority is not

complemented by necessary control mechanisms in the communication system.

Figure IV- 12 gives the results of the same multi-application experiment except

that status reports and bulk data flow in opposite directions. These results

show the lowest average delay is five times the ideal, while the lowest

maximum delay is three times the ideal. Although these results are much worse

than for the simplex case, the delays do not rise much as the bulk data

throughput increases.

V. Conclusions

OSI protocol standards, as now specified, do not provide adequate mechanisms

for guaranteeing real-time performance for selected connections or messages.

This weakness in the standards is demonstrated by the iNA-960 multiapplication profile where high throughput transport connections dominate the

available resources forcing up the delay on all connections.

The performance measurements reported suggest limits to the traffic that can

be served by first generation OSI protocol implementations. For iNA-960, a typical OSI transport layer product, an upper bound on throughput of 108 Kcps

69
<5

70
and a lower bound on delay of 33.5 ms were measured. The measured performance

decreased as the number of connections increased. A flow control problem may

occur when an OSI transport protocol is used over a type 1 class 1 logical

link control protocol.

As long as "full" MAP cannot meet the complete hierarchy of performance

requirements for factory communications solutions like EPA are inevitable.

The EPA, with fewer protocol layers and a potential for application access to

priority queues, has better properties for achieving real-time communication.

Although the real-time performance of EPA is expected to be much better than

with "full" MAP, this has not been demonstrated. The LLC 3, with inherent

conflicts between token passing and contention, may prove difficult to

implement successfully and may not provide, deterministically, fast data

transfer between devices from multiple vendors. MMS may possibly turn out to

be an important performance bottleneck. The performance of EPA needs to be

investigated in a manner similar to that reported here for OSI protocols.

71
References

[ALB81]

[ CCI84

]

[EIA85]

[ IEE84

[ IEE85

[ INT83

[ INT84A]

[ INT84B

]

[ ISA72

]

[MIL85

J. Albus, et al., "Theory and Practice of Hierarchical Control,"

Proceedings of IEEE 1981 Fall Compcon , National Bureau of

Standards, 1981.

Presentation Syntax , CCITT Recommendation X.409,

1984.

Manufacturing Message Format Standard , EIA Standard 1393A, 1985.

Logical Link Control , IEEE Standard 802.2, 1984.

Token-Passing Bus Access Method , IEEE Standard 802.3, 1985.

Guide to Using the iSBC 286/10 Single Board Computer , Intel

Corporation, 14627-001, Santa Clara, CA, 1983.

iNA 960 Programmer's Reference Manual , Intel Corporation, 122193-

001, Santa Clara, CA, 1984.

iSBC 136/51 COMMputer Board Hardware Reference Manual , Intel

Corporation, 122136-002, Santa Clara, CA, 1984.

Proway Local Area Network Standard , ISA dS 72.01.

K. Mills, J. Gura, and C.M. Chernick, Performance Measurement of

OSI Class 4 Transport Implementations , NBSIR 85-3104, January

1985.

72
IEEE 802.4 Coni ormance Test Project

January 15, 1987

Hsin-Hvvai Chen

General Motors

Advanced Engineering Stall

GM Technical Center

Manul acturing Building A/MD-39

30300 Mound Road

Warren, Michigan 48090-9040

(313) 947-0558

Wayne A. Mack

VANCE Systems, Inc.

Dulles Business Park

3901 -V Bonanza Boulevard

Chantilly, Virginia 22021

(703) 471-9402

73
ABSTRACT

Coniormance testing of OSI network protocols provides end users with a level of

assurance that multi vendor products will interoperate. The token passing bus

protocols delined by the IEEE 802.4 standard, layers one and two of the OSI

protocol relerence model, requires a specialized media access control (MAC) test

facility to adequately test vendor implementations. The test facility includes a programmable MAC machine, a data collector, a test controller, a headend

remodulator, and the vendor supplied interface mechanism. This paper describes

the test system and the test methodology and scenarios used with the test system

to demonstrate MAC conformance.

Test suite generation, lor conducting tests on the test system, is based on a hierarchical design structure. Using the IEEE 802.4 protocol standard and the test

philosophy discussed in this paper, a suite of tests, known as the conformance test

scenario, are developed. The test scenario encompasses the entire test system,

providing individual test scripts lor each of the components of the test system.

The test scripts are implemented in the test languages of the test system

components and translated into executable binary codes by the appropriate

compilers.

Within the coniormance test scenario, four testing phases are identified: 1) frame

formation testing, 2) data transfer testing, 3) access control machine (ACM) state

transversal testing, and 4) parameter adherence testing. The frame formation tests verily the functionality of the MAC transmit machine. The data transfer tests

verify the operation of the MAC sublayer services which support the logical link

control (LLC) sublayer. The ACM state transversal tests verify the functionality

of the medium access control mechanism including token passing, token use, ring

maintenance, error detection, and error recovery. The parameter adherence tests

verily that all programmable variables can be set by the station management entity

and manipulated by the ACM state machine properly.

A methodology lor developing the ACM state transversal test scenarios is also

described in this paper. To achieve completeness, the ACM state machine function

is categorized into seven areas. In each area, all transitions and all possible

combinations of inputs and internal variables which can cause state transitions are covered. All changes of the ACM state variables caused by the execution of state

transitions are verified by the tests.

Additionally, this paper describes the test procedures that are used to execute the

test scenario using the test system.

74
Introduction

IEEE Standard 802.4 speciiies a communications protocol using a token passing

algorithm and a broadcast bus medium. The standard speciiies the physical layer

and part o i the data link layer, known as the medium access control (MAC)

sublayer, oi the International Standards Organization (ISO) Open Systems

Interconnection (OSI) seven layer reierence model. The remainder oi data link

layer, known as the logical link control (LLC) sublayer, is speciiied by a separate

standard - IEEE Standard 802.2.

General Motors has selected the IEEE 802.4 protocol ior use in its Manui acturing

Automation Protocol (MAP) because oi its deterministic characteristics. Since the

token passing bus protocol is both a complex and a new and developing technology.

GM has sponsored a cooperative research associate program at the National Bureau

oi Standards (NBS) since 1984. This program is known as the IEEE 802.4

Coniormance Test Project. The goal oi this program is to develop coniidence and

competence in the IEEE 802.4 token bus technology prior to its use in large

iactory installations.

One oi the key activities involved with achieving this goal is IEEE 802.4 MACSublaver coniormance testing. These coniormance tests will be used to verily that

the protocol implementations oi various local area network vendors* coni orm with

the specii ication. This provides assurance that various vendors' equipment will

interoperate within a network and that the network will operate in the expected

manner. The intent oi the MAC-Sublayer Coniormance Test Project is to develop

the test system, methodology, scenarios and procedures required to demonstrate

IEEE 802.4 MAC-Sublayer coniormance.

Objectives

There are several interrelated objectives ior MAC-Sublayer coniormance testing.

These objectives establish a philosophy which is carried throughout the test system

and procedures ior coniormance testing. These objectives are:

1) All tests shall be periormed in a manner which is non-intrusive to the

vendor's implementation. The vendor implementation being tested will be

allowed to operate as a stand-alone entity, reducing the risk oi errors being induced by the test equipment.

2) No special interlaces tc the vendor's device will be required. The

control oi parameters, LLC data requests, and LLC data interpretation

shall only be achieved through a vendor supplied mechanism. The

results oi tests may only be determined through the observation oi IUT

outputs under the iniluence oi an emulated bus test scenario.

3) The coniormance tests shall be kept independent oi the adjacent layers.

Only the MAC-Sublayer is being tested and not the LLC Sublayer,

Physical layer, or Station Management.

4) The test methodology and procedures shall be described through a high

level language such that the descriptions can remain independent oi the

test tools, yet provide suiiicient detail to implement the test bed

without ambiguity.

5) The tests shall be easily upgradable to support iuture revisions oi the

IEEE 802.4 standard.

6) Coniormance testing will be developed wdth interoperability as a goal.

75
MAC-Sublayer Architecture

The IEEE 802.4 MAC-Sublayer provides the mechanism lor accessing and moving

data, reliably, over the physical medium of the network. The MAC-Sublayer

supports interlaces to the Physical layer, the Logical Link Control sublayer, and

Station Management. The Physical layer resides below the MAC-Sublayer and

provides a broadcast communications medium in a bus topology. The Logical Link

sublayer resides above and is supported by the MAC-Sublayer. Station Management

supports local administrative services between the MAC-Sublayer and its manager,

providing the means and methods for initialization, network parameter reading and

setting, address validation and setting, network membership control, monitoring and

reporting of changes to the MAC entity's parameters, and supporting the exchange

of MAC service data units lor station management peer communications.

Internally, the architecture of the MAC-Sublayer is partitioned into five machines

as illustrated in Figure 1. These machines are: 1) the Receive Machine, 2) the

Transmit Machine, 3) the Interface Machine, 4) the Access Control Machine, and 5)

the Regenerative Repeater Machine.

The Receive Machine accepts MAC-symbols from the physical layer, integrates the

MAC-symbols into MAC-lrames, checks the validity of the frames, passes valid data

frames to the Interlace Machine and valid control frames to the Access Control

Machine, and indicates the current bus condition.

The Transmit Machine accepts MAC-frames from the Access Control Machine,

appends the appropriate amount ol preamble, the frame delimiters and frame check

sequence, decomposes the MAC-lrames into individual MAC-symbols, and passes the

MAC-symbols to the physical layer.

The Interface Machine acts as an intermediary between the MAC-sublayer machines

and the Logical Link Control (LLC) sublayer, and between the MAC-Sublayer

machines and Station Management. It interprets and generates service primitives

for incoming and outgoing MAC-lrames. It also performs address recognition on data frames passed to it by the Transmit Machine.

The Access Control Machine coordinates access to the shared bus with the other

stations on the network and limits its own access to the bus to guarantee

deterministic operation. It performs the functions of token passing, token use. ring maintenance, ring initialization, ring fault detection, and ring fault recovery.

It also accepts data frames from the Interface Machine and passes the frames to

the transmit machine at the appropriate times.

The Regenerative Repeater Machine is an optional component of the MAC-Sublayer

architecture which is only present in special repeater stations. Its function is to

connect electrically or frequency separate bus segments into an extended logical

bus netw'ork. A broadband headend is a special case ol a regenerative repeater.

MAC-Sublayer Test System

The conformance test system architecture defined by the MAC-Sublayer Research

Project for broadband testing is illustrated in Figure 2. This test system

architecure describes the structural relationship among the individual components

ol the test system. These components are: 1) the Implementation Under Test

76
(IUT), 2) the Test Controller, 3) the Programmable Media Access Controller

(FMAC), 4) the Data Collector, 5) the Modems, and 6) the Headend Remodulator.

The I UT is the vendor's product which is to be tested. To remain non-intrusive,

the interlace to the IUT is limited to the Physical layer interlace and a vendor

interl ace . The vendor interlace is a vendor supplied mechanism which 1) allows

the MAC-Sub layer programmable parameters to be controlled in the IUT, 2) invokes

an IUT send by issuing a MA_DATA. request to the MAC-Sublayer, and 3) detects

data reception by the IUT via a MA_DATA. indication issued by the MAC-Sublayer.

The Test Controller coordinates the actions ol the PMAC, Data Collector, and IUT.

Additionally, the Test Controller perlorms the analysis ol the various tests and

determines the results.

The PMAC emulates various token bus scenarios which may occur within a token

bus network. The PMAC is primarily used as a stimulus, exercising the IUT and

causing it to output various MAC-lrames.

The Data Collector provides the main observation point lor MAC-Sublayer

conlormance testing by monitoring the activities ol the bus, storing all or select

series ol MAC-symbols occurring on the bus, and time stamping each ol the stored

symbols. This provides a log ol the bus activity during conlormance testing. ®

The Modems and Headend Remodulator provide the Physical layer lor the test

system. The Modem associated with the IUT is vendor supplied such that the

testing is non-intrusive to the implementation.

Test Suite Generation

Having delined the test system architecture lor conlormance testing, the next step

is to deline the methodology 1 or deve loping and implementing the conlormance test suite. The hierarchical design structure that was adopted by the IEEE 802.4

Conlormance Test Project lor the conlormance test suite is illustrated in Figure 4. This architecture identities three levels ol test implementation: 1) test scenarios, 2)

test scripts, and 3) test code. At the top level ol this architecture are the test

scenarios.

The test scenario is a set ol abstract test definitions and is generated using

various sections ol the IEEE 802.4 standard and applying the test objectives and

philosophy previously stated. The test scenario encompass the entire MACSublayer conlormance test system and is written in a lormal high level language.

Using the test scenario and the architectural description ol the MAC-Sublayer test

system, individual test scripts are created lor each component ol the test system;

the test controller, the PMAC, the Data Collector, and the IUT. Each ol the test

scripts contains multiple tests, which correspond on a one to one basis with the

tests delined in the test scenario. The test scripts are written in languages

specilic to each ol the test system modules. Where multiple test modules are required to allow a component to perlorm an entire test, the modules are relerred

to as test script segments.

The test scripts are translated into test codes specilic to, and executable by the

individual MAC-Sublayer test system components. There is a one to one

relationship between test script segments and test code segments.

77
Test Phases

Based on the architecture and the test generation methodology described above,

live test phases have been identilied lor the conlormance test suite: l)Frame

Formation Testing, 2) Noise Burst Detection Testing. 3) ACM State Transversal

Testing, 4) LLC Data Trans! er Testing, and 5) Parameter Adherence Testing.

Frame Formation Testing verilies the Transmit Machine. Frame lormation testing

requires that the MAC-1 rames generated by the IUT contain the lollowing Helds:

1) A preamble which consists ol a series ol pad_idle MAC-svmbols. The

preamble is primarily used by the receiving modem to acquire signal

level and phase lock.

2) A Start Delimiter which indicates the end ol preamble and the start ol

the Irame.

3) A Frame Control lield which delines the Irame type (i.e TOKEN, DATA,

etc.)

4) A Destination Address.

5) A Source Address which must be the same as the address ol the

transmitting MAC entity.

6) A Data Unit containing 0 to 8191 octets ol LLC data.

7) A Frame Check Sequence lield octets containing the CRC-32 value

calculated lor the Frame Control, Destination Address, Source Address

and Data Unit lields.

8) An End Delimiter which indicates the end ol the Irame.

Noise Burst Detection Testing verilies the Receive Machine. This test requires that

the IUT reject invalid Irames. The IEEE 802.4 speci 1 ication delines a MAC-lrame

which meets any ol the lollowing conditions as being invalid:

1) It is identilied as such by the physical layer.

2) It is not an integral number ol octets in length.

3) It does not consist ol a Start Delimiter, one Frame Control lield, two

properly 1 ormed address lields, one Data Unit lield ol appropriate length

(dependent on the bit pattern specilied in the Frame Control lield), one

Frame Check Sequence lield, and an End Delimiter, in that order.

4) A CRC-32 computation, when applied to all octets between the Start

Delimiter and End Delimiter, tails to yield the correct remainder.

ACM State Transversal Testing verities the Access Control Machine. The IEEE

802.4 standard delines the Access Control Machine (ACM) as a linite state machine

as depicted in Figure 5. The 1 unctionality ol this linite state machine is actually

much more complicated than what is represented in this diagram, because the same

input may trigger diilerent transitions depending upon the current values ol ACM

variables. Also, transitions w'hich change variable values will allect later state

transitions. A list ol the inputs to the ACM. the parameters used by the ACM and

the internal variables used by the ACM is given in ligure 6. The methodology

used in developing the ACM state transversal tests is as lollows:

1) The ACM is divided into eight lunctional areas: the CLAIM TOKEN

algorithm, the PASS TOKEN algorithm, the USE TOKEN algorithm, the

RING MAINTENANCE algorithm, the RING ENTRY algorithm, the RING

EXIT algorithm, the RING COLLAPSE RECOVERY algorithm, and the

78
OTHER_HEARD transition.

2) Each transition emanating irorr. the idle state is assigned to one of the

seven areas, depending upon the context required to cause that

transition.

3) Every transition is to be tested.

4) Every possible combination oi inputs and variables that can cause a transition are to be tested.

5) In some cases, because not all oi the state variables are deiined for a state, it is possible that the ACM enters the same state with diiierent

values oi state variables. It is not necessary to emulate all possible

legitimate variable variations.

6) For those transitions that may have iterative occurrences such as ring

entry, ring maintenance and claim token processes, three cases are

tested: the minimum case, the maximum case and one intermediate case.

7) The change oi some variable inside the ACM state machine may not be

immediately observable alter the transition that causes the change

occurs. To verily that the variable is changed correctly, the IUT will

be induced to some state where that variable will cause a subsequent

transition.

8) To emulate a particular token bus environment, the test system may

need to send out a sequence oi invalid MAC-irames (noise_bursts).

9) The minimum delay beiore a station can receive a irame alter passing a irame such as token, who iollows, so li ci t_suc c ess or , or

request_with_response data irame is zero.

10) The minimum delay i or two consecutive irames is 2 microseconds.

11) Only legitimate events can occur.

LLC Data Transler Testing verities the services provided by the Interlace Machine

to the LLC-Sublayer. This test verities the iollowing capabilities:

1) the ability to send a data irame containing 516 octets oi data,

2) the ability to receive a data irame containing 516 octets oi data,

3) the ability to send a irame and insert the appropriate value in the SA

i i e Id

,

4) the ability to receive a broadcast irame,

5) the ability to receive a group addressed irame, and

6) the ability to send and receive a irame addressed to one's sell.

Parameter Adherence Testing verities the services provided by the Interlace

Machine to Station Management. These services include the setting oi various

parameters. This test verities that the iollowing parameters can be set by Station

Management:

1) the station address,

2) the set oi group addresses accepted by the station,

3) the slot_time,

4) the hi_pri_token_rotation_time,

5) target_rotation_time(4)

6) target_rotation_time(2),

7) target_rotation~time(0)

8) target_rotation_time(ring_maintenance),

9) the ring_maintenance_timer_initial_value,

10) the min_post_silence_preamble_length,

11) the in_ring desired i lag.

79
12) the max_retry_limit, and

13) the max__inter_solicit_count.

Conclusion

A conformance testbed has been implemented at the Institute for Computer

Sciences and Technology of National Bureau of Standards to demonstrate the test

methodology described here. This testbed has shown that not only is conformance

testing at the MAC-Sublayer feasible, but that the testing methodology is similar

to that which is used at the higher protocol layers. Thus the wealth of experience

that has been gained in conformance testing at higher layers (such as the

transport layer) can be utilized during the conformance testing of the MACsub layer in order to reduce test development time, effort and risk.

Rel erences

IEEE Standards for Local Area Networks: Token-Passing Bus Access Method and

Physical Layer Specifications, ANSI/IEEE Std 802.4-1985, October, 1985.

80
MAC-sublayer Functioning Partitioning

5

T

A

T

I0

N

N

A

N

A

6

E

M

E

N

T

FIGURE 1

81
Test Harness

FIGURE 2

82
PMAC Functional Partitioning

test controller

PHYSICAL LAYER

FIGURE 3

83
T est Code Generation

FIGURE 4

DESIGN

SPECIFICATION

ik.

TEST

CODE

PMAC

TC

DC

DA

84
ACM Finite State Machine Diagram

O-OFFLINE 6-AWAIT_IFM_RESP0NSE

1 -IDLE 7-CHECK_ACC£SS_CL ASS

2-

DEMAND_IN 8-PASS_T0KEN

3-

DEMAND_DEL AY 9- CHECK_T QKEN_PASS

4-

CLA IM_TOKEN 1 0- AWA IT_RESPGNSE

5-

USE_T0KEN FIGURE 5

85
Inputs provided by the receive machine:

rx_protocol_i rame rx_data_f rame

noise_burst bus quiet

Inputs provided by the Interlace Machine:

any_send_pending signal__received

Inputs provided by Station Management:

power_ok SM_ACTION. initialize

Parameters programmable by Station Management:

station address slot_time

hi_pri_token_rotation_time target_rotation_time(access class)

ring_maintenance_timer_init_value min_post_silence_preamble_length

in_ring_desired max_retry_limit

max_inter_solicit_count

Internal ACM Timers:

bus_id le_timer

contention_timer

response window_timer

token rotation timer(access class)

claim timer

token_pass_timer

token holdtimer

Internal ACM variables:

NS

access_class

sole_active_staticn

just_transmitted

contention pass count

inter_solicit_count

transmitter_l ault_count

echoes expected

NS known

in ring

lowest address

claim_pass_count

resolution pass count

re maining_re tries

\ irst_time

heard successor

Figure 6

86
Clock Synchronization on the Factory Floor

Walter Cora and Ulrich Herzog

Chair for Computer Architecture and Performance Evaluation

University of Erlangen-Nuremberg, West Germany

Satish K. Tripathi*

System Research Center,

UM1ACS, and

Department of Computer Science

University of Maryland

College Park, MD 20742

Abstract

One important issue for the coordination of flexible manufacturing systems (FMS) in an automated

factory is the synchronization among the manufacturing processes based on a common clock. This

paper describes the synchronization requirements on the factory floor and discusses several clock

synchronization algorithms, their theoretical bounds, and the results of joint work at Maryland and

Erlangen. The measurement results based on the implementations of the synchronization algorithms on

local area networks at Maryland and Erlangen are presented. For hierarchical LANs an algorithm is

developed and its behaviour is simulated.

1. Scope - The PAP model factory

Reduction in costs, diversification and customization of products, and shortening downtime of

manufacturing and assembly equipment are some of the basic requirements for the "factory of the

* This research was supported in part by National Science Foundation under grant 01R -85-00108 and by Alexander von Humbolat-Stiftung.

87
future". Many companies have installed new technologies, such as CAD (computer aided design).

CAM (computer aided manufacturing) or CAP (computer aided planing). However, normally

connection between different computer equipment and the automated devices is very limited.

The flexible fabrication of customized products requires a continuous exchange of information

and materials in the whole system. Therefore, the computer intergrated manufacturing (CIM) concepts

can be understood as the combination of highly flexible manufacturing and assembly cells together

with specific computer hosts to a whole working system. Characteristic for such a system is the

extensive use of the computer aided technologies for planing, design, testing, and controlling.

Furthermore, the versability of the all components in the whole system must be stressed [Dupo82],

At the University of Erlangen-Nuremberg project PAP ("Project for Flexible Automated

Production Systems") was initiated in 1985 as a common effort of both the industrial engineering and

computer science departments, supported by government and industry. The goal of the PAP research

project is to build a highly flexible model factory during the next five years at the campus of the

university (Figure 1).

Figure 1. The model factory of the PAP project

88
Important issues for the communication among different devices are a unique communication

architecture and standardized protocols. For the PAP model factory the MAP protocols ( Manufacturing

Automation Protocols ) [GM85, GM86] are selected and being implemented.

In 1980 Generals Motors initiated the MAP project, a common effort of various manufacturers

and users. The goal of the MAP project, still under development, is to establish commmunication and

application protocols on the factory floor based on the International Standards Organization’s (ISO)

seven layer model for Open Systems Interconnection (OSI).

The requirements for the PAP communication system (Figure 1) are derived from different fields

and conditions:

(1) Between several CAD working stations the exchange of graphic information must be provided

in many formats.

(2) For the mechanical engineering part of the PAP project, the communication system must

connect the automation equipment as programmable logic controllers (PLC), robots, and

numerically controlled machines (NC) directed by a computer to a flexible manufacturing cell

(FMC).

(3) It is also essential that information about a product can be stored, transferred, and used by

different machines at various places in a manufacturing and production system. Therefore, a

complex network between the automation equipment and special purpose computers like print

servers, database hosts, programming stations, and planning systems must be provided.

(4) Distributed control systems must support realtime response, when a part of the production

system is down. Thus the control systems must have an immediate ' access to the

manufacturing cells for their status.

(5) Several tasks must be provided by network management. One task is controlling and

supervising the whole communication system. Also statistics about events in the distributed

system and performance analysis are requirements for the network management.

In an error free environment the production can easily be planed and synchronized. But in

reality, inherent system uncertainties and unforeseen situations must be handled [Hatv84], One

important problem on the factory floor is the electronic noise generated by the automation equipment,

which can interfere with network transmission. If the exchange of information between the

89
manufacturing cells is delayed or interfered with, the whole production can be slowed down or

interrupted.

An extensive cooperation and synchronization between the flexible manufacturing cells,

therefore, leads to an improved overall system performance. E.g., in case of a failure, the control

system must determine, in real-time, which part has to move to which cell, when, how, and to what

manufacturing process. Therefore synchronization in an automated factory environment is essential

for organizing and distributing shared resources such as material, tools, and information.

The synchronization between different manufacturing and information processes depends on an

accurate synchronization of the individual clocks. Precise clock synchronization is also of importance

for performance measurement and evaluation in automated production systems. In addition, the

network management tasks need a global time base for the correlation of events, for the testing of

communication protocols and for the diagnosis of failures in a distributed system.

In the MAP architecture, which is the basic communication concept for the PAP project, clock

synchronization is handled by network management, but only a granularity of one second for the

individual clocks is provided. Indeed, the synchronization of the different manufacturing systems

depends on the specific application.

For process control the exchange of information must be provided under real-time conditions

and therefore the necessary accuracy must be in the range of a few milliseconds. Applications like

hardware measurements in a distributed system need a higher precision of few nanoseconds and can

only be realized with specific hardware solutions.

In the following we first overview useful algorithms on clock synchronization, the theoretical

bounds, and the results of the common work at Maryland and Erlangen. "Useful" means reasonable in

both, implementation and communication costs in a heterogeneous and hierarchical system like an

automated plant. In section 3 we present the results of the experiments in detail.

2. Clock Synchronization Algorithms

Before stating and analyzing the chosen algorithms in detail, we first give an classification scheme and

criteria for the comparision of different clock synchronization algorithms.

90
2.1 Classification

Since Lamport’s article "Time, Clocks, and the Ordering of Event in a Distributed Environment"

[Lamp78] various methods for time synchronization are discussed in literature. The algorithms for

clock synchronization can adopt centralized or distributed schemes which can be classified in the

following order:

(1) Central-dictatoric: A central controller directs participating processes to synchronize their

clocks. Fault tolerance and redundancy can be provided by election mechanism for the

central controller [Rica81], but normally failures of the central component cause a breakdown

of the clock synchronization. One example of this class of schemes is the GPS (Global

Positioning System) receiver [Borc85] which is directed by four navigation satellites. The

achievable clock accuracy over the entire world is in the range of nanoseconds, but the cost

of one receiver is about $ 5000.

(2) Central-democratic: A dedicated processor ( time master) periodically computes the general

network time by a voting mechanism with the individual controllers ( time slaves). Such a

scheme must provide some kind of mechanism to pass on the leadership to other processes in

case the master node fails. The TEMPO algorithm developed by Gusella and Zatti [G useS3]

is an example for such a scheme.

(3) Decentral-democratic: All of the time-processes execute an identical procedure to achieve

clock synchronization. Based on local time and the received timestamps from the other timeprocesses the clock will periodically be computed and, if necessary, set. Most existing clock

synchronization algorithms use a distributed approach for fault-tolerance [Lamp82] [Halp83]

[Marz83] [Lund84] [Srik 86], These algorithms, in general, are more difficult to implement

because more coordinations among the processes are required. Agreement protocols may also

be used to achieve better synchronization [Schn86].

Central dictatoric schemes are mostly realized by special hardware and therefore often ca not be

used in a heterogeneous factory environment because of the installation costs. The algorithms presented

here are based on software mechanisms and can be realized inside of the automation equipment and in

a heterogeneous computer network environment.

9
2.2 Basic Requirements

For a comparison of the clock synchronization algorithms the following criteria are used:

Accuracy: What clock accuracy can be achieved after the resynchronization? The necessary

accuracy depends strongly on the application and ranges from nanoseconds to seconds (see

previous section). If the clock synchronization requirements are only in the range of seconds or

hundred of milliseconds "low cost" algorithms should be used.

Communication costs: How many messages must be sent per resynchronization? Another

important question is, what relative load is caused by the rsynchronization algorithm onto the

communication system?

Computing costs: How much processor load results from the clock synchronization algorithms?

One important issue for this criteria can be that the time-processes should not disturb or block

other application oriented tasks.

e Complexity of the algorithm: How sophisticated is the algorithm to implement in a

heterogeneous network. It is significant that the amount of time for the implementation of clock

synchronization depends strongly on the complexity of the algorithm.

Fault tolerance: In which manner will failures of the communication system, the controller or the

time-process be managed? It is desirable that the abort or the controlled ending of a timeprocess has no or a only a minimal influence on other time-processes. The reconnection or

reloading of one time-process must also be supported by the chosen mechanism.

Flexibility: Algorithm for clock synchronization should be able to adapt to different

configurations of the network and the machines. E.g., different granularities of the individual

clocks must not have a strong influence on the achievable clock accuracy in the whole system.

Furthermore, integration of new systems into the whole process of clock synchronization should

be easy.

Of course there can be more criteria depending on the field of application. At the end of this section a

comparison of the following algorithms is presented.

92
2.3 TEMPO

The TEMPO algorithm [Guse83] uses a central-democratic scheme in which a dedicated process

( master) polls each slave process to measure the clock difference between the hosts on which the two

processes are running. After the master process has evaluated the time offsets between its clock and all

the slaves’ clocks, it computes a network average clock skew using a fault tolerant averaging function.

Described in a simplified manner, the algorithms works as follows:

For getting the difference between each slave clock and the master clock, the master transmits a

message with its timestamp to the slave process. Upon receiving this message, the slave process

records the time of reception and computes d vas difference between the reception time and the

transmitted master timestamp. Now the slave repeats the same procedure by sending the first

difference d ! and its actual timestamp. The master receives this message and computes analogous

d 2

. The difference between the two clock can now be obtained by {d x -d 2 )/2. The basic

method is discussed in [Elli73] and assumes that the transmission time is constant and has a

symmetric distribution function. In the case that such a symmetric distribution is not appropriate

a systematic error will appear.

The transmission time in a local area network is usually variable and therefore the above

described step of the TEMPO algorithm will be repeated several times. To eliminate high

variation in communication delays, a minimum-delay message is chosen on each direction from

the N-times a slave is polled.

The global net time will be computed based on the differences of the individual clocks. For the

computation of the global net time the mean value, the median, the mean value without extreme

values, and the minimum values can be used.

Each processor receives this global net time from the time master process and sets its individual

clock within a given minimum interval.

The accuracy of the TEMPO algorithm depends mainly on the estimation of the transmission time

and on the assumption of an unique distribution function in both transmission directions.

Detailed implementation and simulation results for the TEMPO algorithm conducted at Erlangen

and Maryland will be presented in section 3.2. For hierarchical local area networks, as in the factory

environment, an extended version of TEMPO (ETEMPO) was developed at Maryland and is discussed

in the following section.

93
2.4 ETEMPO

A hierarchical LAN such as in automated factories can be divided in several "sub-LANs". To see how

the TEMPO algorithm can be extended to hierarchical LAN, let us first consider the case of a twolevel hierarchical LAN where several sub-LANs are connected through a second-level LAN (E.g.

figure 1).

In this two-level hierarchical LAN, the clocks in each of the low-level LANs can be

synchronized with the TEMPO algorithm. Since each low-level LAN has a master process to direct

local synchronization, these "low-level masters" can be considered spokespersons for their networks and

may elect a "master of masters" among themselves to be in charge of synchronization of their clocks,

again using the TEMPO algorithm. Once the clocks of the low-level masters are synchronized through

this "high-level synchronization", local clocks of a low-level LAN may be asked to synchronize with

their master’s clock.

There is Ijttle procedural difference between the high-level and the low-level synchronization

except that a low-level master may wish to synchronize local clocks regularly during the idle period

between the two high-level synchronizations. For the high-level synchronization, the high-level master

polls the low-level masters (they are now the slave processes of the hierarchical LAN) to estimate the

clock skews the same way as done for a single LAN. The time offsets are again averaged, and

correction for e3ch low-level master is computed.

In case a low-level master process fails, the slave processs elect a new master for the low-level

LAN. The new master then joins the other low-level masters to synchronize their clocks. If the failed

process is the high-level master process, the other low-level masters will elect a new high-level master

to be in charge of the high-level synchronization. However, if a failure occurs in the communication

lines and a low'-level network is disconnected, the clocks in that network will not be synchronized

until repair to the communication lines is completed.

When a low-level master corrects its clock due to the high-level synchronization, it has three

options as to whether to propagate this adjustment to other clocks in its network. First, it can ask all

local clocks to immediately reset to their master’s time. Secondly, it may choose not to propagate the

adjustment to local clocks but waits until the next low-level synchronization round to synchronize the

local clocks. Thirdly, it can ask the other clocks to adjust by the same amount.

It is likely that each low-level LAN may synchronize their clocks more frequently than the

high-level synchronization. The clock synchronization model for a two-level hierarchical LAN can

theoretically be extended to a hierarchical network of several layers. The basic principle is that at each

94
layer up, one synchronizes the master clocks of the subnetworks of the current layer, and propagates

the clock correction down to its subordinate networks.

2.5 Interactive Convergence Algorithm CNV

CNV [Lamp85] is a decentral-democratic algorithm, where the global time is divided into intervals. At

the end of such an interval the time-processes compute the value of the clock using the following

algorithm:

(1) The differences between all clocks will be investigated. The chosen method has a maximal

error c for the computed differences.

(2) The extreme values of the computed differences will be set to zero and the mean value of all

differences is added to the individual clocks.

The accuracy after the resynchronization is determinable by the formula (6 x/ + 2)*e, under the

assumption that 2/3 of the / processors are working correct. If the difference of the clocks can be

kept with an error of 40 milliseconds and two of seven processors are working wrong, the maximal

achievable accuracy is equal to 560 milliseconds.

The major advantage of CNV is its fault tolerance against failures of nodes. In a factory

communication environment it can hardly be used, because of the enormous communication traffic

which will conflict with the information flow for the manufacturing processes. Another disadvantage

is the assumption that the individual clocks are initially synchronized to approximately the same

value. Under this restrictive assumption clock synchronization can hardly be achieved.

2.6 Modified Optimal Clock Synchronization mOCS

In a complex heterogeneous network like an automated plant it is often necessary to synchronize

various devices with different clock granularities. One problem with the TEMPO algorithm is that the

computable differences of the individual clocks are restricted by the granularity of the "worst" clock.

With OCS (Optimal Clock Synchronization) [Srik85] not only clocks are synchronized, their rates

of drift from the real time can also be minimized to the drift rates of the underlying hardware clocks,

thus "optimal accuracy" is achieved. The main advantage of the Optimal Clock Synchronization

algorithm for a heterogeneous network is that no computation of clock skew's or differences between

95
two clocks is needed. A modified OCS algorithm mOCS was developed at Erlangen, for the use in

broadcast networks. This type of network (e.g. Ethernet) or other bus systems are normally used as

backbone network in office or factory communication.

Figure 2. The mOCS algorithm

The time on each time-process is divided into continously numbered resynchronization intervals

of length p . The algorithm synchronizes the clocks in "rounds", where the number of the interval and

the round is the same. If one clock reaches the beginning of the k.ih interval, a broadcast message

will be sent to all time-processes, including itself (Fig. 2: P2 at tO, P3 at tl).

The messages contain a sender signature and the interval number. Every time-process counts the

arriving round ^-messages. If a time-process has enough messages received it knows that a sufficient

number of other processes are ready to begin a new time interval (Fig. 2: P3 at tl, PI at t2, P2 at t3).

Subsequent to this, it sets its clock to the value of k xp and the other time-processes do the same.

Because of the switching of the first "correct" process to a new' interval it is clear that the other

"correct" processes do the same after the maximum transmission delay and begin the kth time interval.

Under the assumption of n time-processes in the network the treshold for beginning a new

ti 2 interval should be — + 1 messages, in the case of signed, — n + 1 messages, in the case of unsigned.

2 3 The theoretical background in the case of faulty processors and the mimimum knowledge which is

96
necessary for achieving clock synchronization is discussed in [DoIe84, Halp84],

The accuracy of OCS and mOCS depends on the maximum transmission time. In [Srik85], it is

proved that the maximum difference between two clocks after resynchronization is not greater than

the maximum transmission time. The advantage of this algorithm is the low system load, only counting

of messages is necessary. In mOCS the number of messages is reduced even further. Each process has

to sent only one message per resynchronization interval. Under the assumption that after a sufficient

amount of messages are received, no new message is sent, the number of messages can be reduced to

n / 2+1.

The mOCS algorithm can be extended towards a general scheme. The processes are ordered into

two classes, where the members of the first class are running the original algorithm. The time*

processes of the second class accept only the impulses of switching to a new interval, but do not send

messages. This separation into two classes can be seen as a fault tolerant version of a central clock, if

the members of the first class can constitute a sufficient number of correct running clocks. If there is

only one "correct running" clock, then this extended version of mOCS is the same as a central

controller.

The main advantage of the algorithm is that no offsets or clock skews computations are needed.

However, it may lead to a substantial error in the case of large transmission times. If the estimation of

the clock skews results in an smaller error than the maximum transmission time, algorithms like TEMPO should be used.

2.7 Comparison

The accuracy of the algorithms discussed above are hard to compare theoretically and advantages are

application dependent. Note that methods with a central clock (radio sender, central "tick" by satellite)

provide a accuracy in the range of milliseconds to nanoseconds, but need a hardware integration into

the processor.

Algorithms that compute the differences between the clocks (e.g. TEMPO ) have to estimate the

transmission times or delays. Methods like mOCS or a central clock that need no clock skews

computation achive an accuracy of nearly the maximum deviation between the real and the estimated

transmission time. If the central clock is realized by a radio sender, then the influence of the

transmission time and so the inaccuracy can be reduced..

The number of messages per resynchronization is another criteria for comparing the clock

synchronization algorithms. A central clock needs only one message while mOCS requires between

97
n /?+ 1 and n messages. For the TEMPO algorithm the amount of messages depends on the number

of processors, n , and the number of polling, p , by the master process. This can be expressed by

(2xp+l)x«. The fault tolerant algorithm CNV transmits 2xp Xn x{n - 1) messages to achieve a

global agreement. Table 1 shows the number of messages for realistic networks produced per

resynchronization interval. In the case of TEMPO and CNV a polling rate of 10 is assumed.

number of

nodes

number of messages

TEMPO mOCS CNV

10 210 5-10 900

50 1050 25-50 24,500

100 2,100 50-100 99,000

1000 21,000 500-1000 9.99 xlO 6

TABLE 1. Messages per resynchronization interval

0 The fault tolerant aspect is supported by distributed algorithms like mOCS and CNV , which

recognize failures or breakdown of time-processes. But even in the case of faulty processes in a

system, CNV has no influence on practical clock synchronization, because of the achievable low

accuracy. Methods that use a central controller tolerate faults of the "slaves", but if the central

component breaks down, special mechanisms must be provided to elect a new "time master".

Based on our experiences with theoretical and experimental results at Erlangen and Maryland we

feel that:

mechanism that use a central dictatoric controller achieve good results, if the central clock has a

high accuracy and the timestamps can be transmitted in a fast way to the "slaves". mOCS should be chosen, if the desired accuracy is not better than the maximum transmission

time and where a granularity of one second or more (either in reading or setting the clocks) is

acceptable.

TEMPO can be used in "middle sized" networks, but it assumes a high granularity lor the

computing of the clock skews and for correcting the clocks. Networks with unpredictable

transmission times or a high variance can also be synchronized by TEMPO quite good.

For hierarchical LANs, ETEMPO is a good candidate for clock synchronization, but further

study has to be done for configurations of this type. General configurations need also basic

research and new algorithms with respect to defined criteria have to be found.

98
3. Experiments

3.1 Environment of the Implementation

The experiments described here are performed on 10 MB-Ethernets at Erlangen and Maryland. At the

time of the measurements the PAP model factory was not completed, therefore for the practical results

in Erlangen the University’s education network is chosen.

In Erlangen, the TEMPO algorithm is implemented on five UNIX V.2 hosts (3 Perkin Elmer

3205, 1 Perkin Elmer 3210, 1 CADMUS 9230). Measurements are made with the intention to get

realistic parameters for a general simulation model. The private communication protocol uses the data

link layer of the ISO/'OSI reference model. The simulation program was runs on an INTEL 310 under

XENIX for systematic study of the TEMPO and the mOCS algorithms under different conditions

(distribution time, fault rate, drift rate, etc.).

The implementation in Maryland is on three hosts running UNIX 4.3 BSD (2 VAX 1 1/750, 1 VAX 1 1/780). The measurements made in Maryland are taken for a program which emulates on each

of the hosts a sub-LAN. On the UNIX 4.3 BSD machines the datagram socket is used because the

communication protocol supporting the other socket type, the stream socket, would usually incur long

communication delay, especially when messages are lost and retransmission are necessary.

3.2 TEMPO

The implementation of the TEMPO algorithm on the local networks in Erlangen and Maryland allows

us to make measurements over several parameters. The results presented here confirm the

measurements in [Guse83]. The parameters experimented are:

(1) the interval between resynchronization,

(2) the number of times a master polls its slaves in estimating clock skews,

(3) the maximum round-trip communication delay allowed,

(4) the system load during experiment period,

(5) the initial clock difference.

99
3.2.1 Measurements at Maryland

At Maryland we study the clock skews measured at each synchronization round to determine the

effectiveness of the algorithm in keeping clocks synchronized. In this experiments, clock adjustment is

always rounded up/down to the nearest multiple of five milliseconds.

Among these parameters, only the interval between resynchronizations shows a significant

influence on the distribution of clock skews. For example, in three experiments where

synchronization takes place every two, four, and eight minutes, respectively, the distributions of clock

skews between the hosts Tove (VAX 11/750) and Gyre (VAX 11/750) are shown in Fig. 3. In these

three experiments, the process in Tove is the master, and each experiment makes 20 synchronization

attempts. The positive clock skews (Fig. 3) indicate that Gyre’s clock runs faster than Tove’s. It is

interesting to note that with two-minute interval, the most frequently occurring clock difference is

five milliseconds, whereas it is 10 milliseconds for the four-minute interval, and 15 milliseconds for

the eight-minute interval. When the experiments are repeated wieth the process on Gyre as the master,

the resulting distributions of clock skews (Fig. 4) are almost a mirror image of Fig. 3. This suggests that

the synchronization algorithm is independent of the placement of the master process.

Figure 3. Histograms of clock skews (in ms.) between hosts Tove (Master) and Gyre at synchronization interval of two, four, and eight minutes. The abscissa is the frequency of occurence.

100
FR

E0

u

EN

C

Y

-20 -15 -10 -5 0 CLOCK SKEW

Figure 4. Same as Figure 3 except for hosts Gyre (Master) and Tove

To measure clock skews accurately, there should be little variation in the communication delays.

Though the variation in the message delays is difficult to measure, we suspect it is not high in our

system, because most round-trip delays are shown to be under 20 milliseconds (Table 4.1) even if the maximum round-trip delay is set to 40 milliseconds.

Round-trip communication delay and its frequency count

Hosts Limit

Round-trip Delays (in ms.)

10 20 30 40

Mimsy/Tove 20 7 68 Mimsy/Tove 40 19 48 6 2 Mimsy/Gvre 20 16 59 Mimsy/Gyre 40 14 55 4 2

TABLE 2. Round-trip communication delay and its frequency count

On the local network at Maryland, clock adjustment is performed gradually; during every tick interval

of 10 milliseconds, a correction of one millisecond is made, if necessary. Therefore, the time it takes to

establish initial synchronization depends on how clock correction is actually implemented. It may seem

surprising that the measured clock skews are not sensitive to different system loads. However, because

i high-priority Internet ICMP time-stamping facility is used by processes to read the clocks of the other

hosts directly, delays in awaking idle slave processes do not interfere with the timestamps transmitted,

101
thus, clock skews computed from these timestamps do not seem to be affected by different system

loads.

3.2.2 Measurements at Erlangen

Between the "time master" and its "slaves" the following mean values and variances of transmission time

are observed on the local area network of the Erlangen University (Table 3).

slave hosts transmission time meart value variance [msec] [msec]

slave 1 (fauiOl) 67 64

slave 2 (faui02) 54 54

slave 3 (faui05) 55 30

slave 4 (faui50) 97 25

total 68 50

TABLE 3. Transmission times on the LAN at Erlangen

As an initial "time master", a Perkin Elmer 3210 was chosen because this host was not very heavily

loaded. The slave hosts fauiOl, faui02 und faui05 are of the same type (Perkin Elmer 3205), but

slightly different transmission times are observed. In the case of faui05, the transmission times with the

lowest variance are observed, about 90% of the messages were transmitted in an interval of 45 to 65

milliseconds,

in x90

80

70

60

50 -

40

30

20

10

0

r

£

0

a

40

ffl

0

50 m60

Master— Slave 2

Master — Slave 3

70 80 90 100 110 120 130 imsecl

Figure 5. Distribution of the transmission times at Erlangen

102
Host fauiOl also shows transmission time around 50 milliseconds, but also quite a few delayed

messages (9%) with a transmission time over 120 milliseconds were registred. This may be caused

repeated transmissions and high system loads. Host faui50 is a CADMUS 9230 machine with a 68010

processor. This node has significant higher transmission times, the messages are normally (86%)

measured in an interval of 90 to 100 milliseconds.

3.2.3 Simulation of TEMPO

The simulation is performed with 5 processors (clocks). It is assumed that no communication errors

occur. When a message arrives and the receiver is not ready, the message is lost.

The independent parameter considered for the simulation is the process-to-process transmission

time. In a local area network like the Ethernet, the transmission time has a standard distribution

function. Object of study was the effect of changing the mean and the variance of the transmission

time distribution.

Table 4 shows the achievable accuracy after the resynchronization in respect of the simulated

parameters*. For the scenarios S1-S5 the received minimum clock offsets were taken for the

estimation of the clock skews.

scenario polling

rate

transmission time

[msec]

accuracy mean value variance

[msec] [msec]

SI 3 N(10,2,5,15) 2.3 0.87 S2 3 N( 100,40,20,2000) 42 18 S3 10 N( 100,40,20,2000) 25 8.0 S4 10 N( 100, 100,20,2000) 22 11 S5 SI with additional 20 msec 11 1.0 receiving time for one host (unsymmetric transmission time)

S6 S2, but mean value taken for 78 18 computing the clock skews

S7 S3, but mean value taken for 50 18 computing the clock skews

TABLE 4. Accuracy of TEMP

O

(simulation model)

* N(a,b,c,d) means standard distribution (mean value, variance, minimum, maximum)

103
It is significant that if the mean value of all computed clock skews is used, the achievable

accuracy decreases. Using the minimum clock skews for the estimation of the transmission times there

is only a minimal error for this parameter.

An unsymmetric distributed transmission time leads to a systematic error. This error is nearly

U half of the difference of the transmission times for both direction and can easily arise in a

heterogenous network of processors with different interrupt structures, scheduling strategies or system

loads.

By increasing the number of polls a higher accuracy is achieved. Figure 6 shows the accuracy in

respect to the variance for the transmission time and the number of polls.

160

140 —

120

P

S, ioo

>>o

reu3

Oo<

80

60

40

20

0

P 1

Px: x "polls"

P 5

P 1 O

P20

50 100 variance of the transmission times [msec]

Figure 6. Influence of variance and polling rate ( TEMPO )

Indeed, the accuracy increases with a high polling rate, but it is not linear. Normally, the number of

polls should be between 5 and 15, only in the case of an extreme variance a higher rate should be

chosen.

3.3 ETEMPO

For the hierarchical network, the experiments are organized into two groups. In one group, the highlevel synchronization interval is fixed and it is experimented with different combinations of low-level

synchronization intervals. In the second group, the low-level synchronization interval is fixed and the

high-level synchronization interval is varied. Table 5 summarizes the set-ups oi these experiments.

104
Experiment High-Level Low-level Interval (min.) Max. Sync. Length

Number Interval (in min.) Gyre Mirrtsy Tove Count (in hrs.) HI 10 2 2 2 12 2 H2 10 4 4 4 12 2 H3 10 8 8 8 12 2 H4 10 2 8 4 12 2 H5 10 2 2 4 12 2 H6 10 2 2 8 12 2 H7 40 4 4 4 20 13 H8 60 4 4 4 20 20

H9 80 4 4 4 20 26

TABLE 5. Experiment Set-ups for High-level Synchronization

3.3.1 XI easurements on Low-level Synchronization Intervals

In a hierarchical network, each low-level LAN may use different synchronization interval for lowlevel synchronizations. In Experiments H1-H6 (Table 5), the intervals of the high-level

synchronization are kept at ten minutes while the low-level synchronization intervals are allowed to

vary from two to eight minutes. For experiments H1-H3, the distributions of clock skews between

low-level masters Gyre and Tove are shown in Figure 7.

Figure 7. Histograms of clock skews between low-level masters Gyre and Tove of

Experiments HI, H2, H3

The distributions are difficult to compare because there does not exist a characteristic pattern that car

be identified with each of the three experiments. These results are not surprising considering that after

a high-level synchronization, there are two factors causing clocks to deviate. One factor is the drifts

205
between clocks over the ten-minute interval between high-level resynchronizations. The other is the

total corrections made on each of the two clocks during the same period from low-level

synchronization within the individual LAN. More results are presented in [Chan86],

3,3.2 Measurements on High-level Synchronization Intervals

In Experiments H7-H9, the low-level synchronization interval is kept at four minutes for all low-level

LANs, while the high-level synchronization interval varies from 40 to 60 to 80 minutes. The

distributions of clock skews between low-level masters Tove and Gyre are shown in Fig. 8. It is

observed that longer interval between resynchronization does in fact cause wider clock seperation.

However, due to the fact that the high-level synchronization interval is significantly longer than the

low-level synchronization interval, the drift factor in this case thus plays a more important role, and

results in a clearer distribution pattern than earlier measurements.

FR

EQ

u

EN

C

Y

Figure 8. Histograms of clock skews between low-level masters Tove and Gyre of

Experiments H"L H8, and H9

3.4 mOCS

The achievable accuracy of mOCS depends only on the transmission time, and therefore, in the

simulation model for mOCS the distribution function of the transmission times is examined. Table 6

shows that for the education network at the University of Erlangen (Ethernet, UNIX hosts) mOCS is

106
not a good choice as TEMPO. The TEMPO algorithm provides a better accuracy in this case

[Herb86].

scenario transmission time

[msec]

accuracy mean value variance

[msec] [msec]

01 N( 10,2,5, 15) 9.4 3.1 02 N( 100,40,20,2000) 69 27

03 N( 100, 100,20,2000) 104 52

TABLE 6. Accuracy of mOCS (simulation model)

4. Final remarks »

Synchronization of clocks on a factory floor is an important problem. Hardware solutions for such

synchronization may be too expensive. In this paper we have presented software solutions to these

problem. A comparative evaluation of various algorithms has been presented. Some of the algorithms

have been implemented at Erlangen and at maryland. Experimental results have also been discussed.

The next step of the research will be the implementation of the studied algorithm in the PAP

model factory, so that precise measurements for a heterogeneous automation environment can be done.

Combining mOCS and TEMPO may give a better synchronization algorithm 3nd this needs further

investigation.

5. References

[Borc85] Borcherding, K.; Bultmann, D.; Franck, H.; Rehmert, R: "NAVSTAR GPS-Empfaenger,

Entwicklungsprogramm: Phase F, (NAVSTAR GPS-receiver, development program). Report:

BMFT-FB-W85-002, 1985

[Chan86] Chang, S.H.; "Clock Synchronization in a Hierarchical Network", MS. thesis. University of Maryland, Aug. 1986

[Dole84] Dolev, D.; Halpem, J.: "On the possibility and impossibility of achieving Gock Synchronization",

Proceedings of the Sixteenth Annual ACM, Symposium on theory of computation, Washington.

April 30-Mai 2, 1984

107
[Dupo82]

[E1U73]

[GM85]

[GM86]

[Guse83]

[Halp83]

[Halp84]

[Hatv84]

[Herb86]

[Lamp78]

[Lamp82]

[Lamp85]

[Marz83]

[Morg84]

[Rica81]

[Schn86]

[Srik85]

[Srik86]

Dupont-Gateimand, C.: "A survey of flexible manufacturing systems”, J. Manufacturing Syst., vol.

1, no. 1, 1982, pp. 1-16

Ellingson, C; Kulpinski, R.J.: "Dissemination of System Time", IEEE Trans, on Commun., no 21, pp. 605-624, May 1973

General Motors: MAP Specification, Version 2.1, March 1985

General Motors: MAP Specification, Version 2.1. A and 2.2, August 1986

Gusella, R.; Zatti, S.: "TEMPO - Time Services for the Berkeley Local Network", Report No. 4CB/CSD 83/163, University of California, Berkeley, December 1983

Halpern, J.; Simons, B.; Strong, R.: "An efficient fault-tolerant algorithm for clock synchronization", 1BMR RJ4094, 1983

Halpern, J.; Moses, Y.: "Knowledge and Common Knowledge in a Distributed Environment",

Proceedings Third Annual ACM, Vancouver, Canada, August 1984

Hatvani, J.: "Intelligence and cooperation in heterarchic manufacturing systems" in Proc. 16th CIRP

Int. Seminar Manufact. Syst., 1984, pp. 1-4

Herberth, H.: "Zeitsynchronisation in lokalen Netzen", (Gock Synchronization in Local Area

Networks), Diploma-Thesis, University of Erlangen-Nuremberg, October 1986

Lamport, L.: "Time, Cocks, and the Ordering of Events in a Distibuted Environment", Comm, of

the ACM, vol 21, no 7, pp. 558-565, July 1978

Lamport, L.; Shostak, R.; Pease, M.: "The Byzantine Generals Problem", ACM Trans. Program

Lang. Syst., vol 4, no 3, 1982

Lamport, L.; Melliar-Smith, P.M.: "Synchronizing Clocks in the Presence of Faults", Journal of the ACM, vol 32, no 1, pp. 52-78, January 1985

Marzullo, K.; Owicki, S.: "Maintaining the Time in a Distributed System", Proceedings of the Second ACM Symp. on Princ. of Distr. Computing, 1983

Morgan, C.: "Global and Logical Time in Distributed Algorithms", Information Processing Letters, Volume 20, Number 4, 1984

Ricart, G.; Agrawala, A.K.: "An Optimal Algorithm for Mutual Exclusion in Computer Networks",

Communications of the ACM, vol 24, no 1, pp 9-17, January 1981

Schneider, F.B.: "A paradigm for reliable clock synchronization", Technical Report 86-735, Cornell University, Dec. 1984

Srikanth, T.K.; Toueg, S.: "Optimal Clock Synchronization"; Proceedings of the Fourth Annual

ACM, Symposium on Principles of Distributed Computing, August 5-7, 1985

Srikanth, T.K.: "Designing fault-tolerant algorithms for distributed systems using communication

primitives", Ph.D. dissertation, Cornell University, TR 86-739, Feb. 1986

108
Design and Simulation Tools

109
SIMULATION OF MAP PROTOCOL FOR USE IN

COMMUNICATIONS STUDIES

by

TONY KANADI, STUDENT MEMBER

and

SHELDON GRUBER e SENIOR MEMBER

of

CASE WESTERN RESERVE UNIVERSITY

CLEVELAND, OHIO 44106

no
Abstract

A CAD software tool has been implemented for studies of communication capabilities of large networks

which use the Manufacturing Automation Protocol (MAP).

The first four layers of MAP have been simulated using

the C language in a UNIX environment to ensure a large

degree of portability. The response time statistics of

any two stations (transmitter-receiver pair) in the network can be examined in the presence of the communication load created by any number of other stations. Messages are sent randomly with statistics such as mean

number of messages per hour that are specified by the

user for each station in the network. The program also

creates an environment in which the token bus protocol

can be learned.

1 . Introduction

1.1. General Considerations

MAP is a network standard spe

aid the manufacturer in the quest

network that can utilize the produ

without constantly incurring th

incompatible protocols. The intent

tively easily, a large scale c network to handle the varied requi

floor. (The term computer used he

able controllers, robots and other

control systems as well as what is

nized as a computer.)

cifically created to

for a factory computer

cts of many companies

e costs of adapting

is to provide, relaomputer communications

rements on the factory

re includes programmmicroprocessor based

conventionally recogIt has been recognized that this environment is

somewhat hostile in that the considerable noise introduced in the network increases the error rate. Studies

have been performed and technical solutions found.

One area that has not received much consideration

is the communications bottlenecks that can occur when a large number of stations compete for the existing facilities even when high data rate equipment is used.

Perhaps the most important reason for this oversight is

that it is much too expensive to wait for the existence

of a large installation in order to study the effect.

Furthermore, message delays are likely to be network

dependent. The cost of reworking hardware connections

would be substantial, and moreover there would be no

guidance on how to design the new configurations. Such a trial and error process is clearly impractical. To pei

form such studies, one may create the network in a special purpose hardware, or simulate the required protocols in software using a general purpose computer. The

latter, while slower, is certainly the less expensive

111
and more flexible route. While the dominant effort was

to produce a simulation of a network of stations adhering to the lower four MAP layers, a further consideration was to create this in a portable manner. C language

in a UNIX environment seems to be the natural choice to

support portability.

2. Simulation of the Token Bus Protocol

2.1. Discussion

To simulate a token bus protocol, multiple stations

have to be created and run simultaneously. Each station

has its own variables and shares some global variables

with other stations. Several methods which were tried

and abandoned are briefly mentioned. The final method

is discussed in detail.

2.2. Method 1

Thi

specif ic

that fun

created

,

inf inite

deleted

correspo

correspo

The

1 . Eve

ces

pro

are

s t a ica

is

2 . As

the

hav

s method uses a progr am which CO nt ai ns a function. St ations a re s imul at ed by c ailing

ct ion in the pro gram, and ea ch time a st at ion is

the func t i o n is called. The f uncton runs in an

1 oop man ner unt i 1 its co rre spond i

g s t a t i on is

from the ne twork. D e 1 e t ion o f a s tat ion

nd s to te r eturn to the ma in pro gram o f its

nd ing fun ct i on

re are se ver al d isadvant

ges with this app roach

ry t ime a st at i o n is crea t ed , a si mu 1 at ed pros has t o b e created and a stack a rea h as to be

v i ded for th at simulated process . Th e s tack

a has to be par tit ioned t o accommo dat e ea ch new

t i on . Si nee the actual s t ac k size is no t dynam11 y dete rm i ned

,

the numb er of stat ion s po ss ib 1

li mi ted

t h e progr am hops from one s t at i on to an other

,

stack po i nter as well as the pr ogr am c ounter

e to be a d ju s t ed accordin gly •

3. The two functions above, parti

stack pointer & program count

assembly language, thus limiti

the program.

tioning and adjusting

er have to be done in

ng the portability of

Most ly

abandoned

because of the latter, this approach was

112
2.3. Method 2

Another method could use the multi-tasking capability of the UNIX system. Each station is simulated by an

operating system process. This provides every station

with enough stack area and partitioning is not needed

here. A parent process exists to keep track of all the

stations (processes) that have been created. Synchronizing all stations is done by signaling back and forth

between parent process and each station process sequentially. Global variables are stored in files, and

interprocess communication is done by using UNIX facilities. This approach was tried successfully on an IRIS

UNIX operating system, but carries these disadvantages:

1. Creation of a station means creating a process

which runs a certain program. Even with a relatively small program, the creation of many stations

will use so much memory space that it becomes

impossible to have reasonably large number of stations for the simulation.

2. Addition of a station considerably slows down the

s imu 1 a t i on

3. A significant number of interprocess communication

results in the loss of some of the signals sent

between processes which, forces the sending process

to resend the signals. This kind of real operating

system signaling is both complicated and not fast

enough for this simulation.

These drawbacks caused a search for a better method

and culminated in the final choice below.

2.4. Chosen Method

This method is based on the following observations:

1. Even though all stations run "simultaneously”, they

can be simulated by running them sequentially which

results that only one station is active at any

time. A single process is therefore sufficient

since no two processes ever run simultaneously.

2. Each station is represented by its parameters in

the process. An element of a linked-list which contains all the variables of a station can therefore

represent the station itself.

3. The protocol is implemented by having only one

set of functions which processes all elements of

the linked-list (stations) sequentially. All the

stations use the same protocol; in other words all

113
the stations call the same functions which

represents the protocol. Processing one station

means that the function processes the variables of

that station (an element of the 1 inked-list ) . This

process involves changing the values of some of the

variables in the station. A time signal for all

stations corresponds to the function processing the

variables of all the stations in the network

sequentially. So every time variables of station

one is processed, station one is said to be time

signalled. The time itself is updated after one

pass of processing all stations in the network. The

fact that the same function processes all the

stations sequentially implies that the function

shall never pause in the middle of processing a station. If that situation were to occur, the

program would become deadlocked. This leads to the

final observation below:

4. If a function of the protocol contains a pause

situation, the function can always be divided into

two or more separate functions. For example the

function ”a()" below can be divided into two

separate functions in addition to updating function

"statesO" as illustrated below:

114
states (ptr)

stat_ptr ptr;

{

boolean x;

if (x) a( ) ; /* if x is true then calls a */

}

a( )

{

p = i;

sleep ( 10 )

q = 3;

}

/* */

states (ptr)

statptr ptr;

{

boolean x; static boolean start=TRUE;

if (sleept imer ) {

sleeptimer —

if ( ! sleept imer) start = FALSE;

/* sleep timer equals zero */

>else if (x && start) a();

else if (x) after_a();

/* timer just goes off, execute the second half*/

>

a()

{

p = l;

sleeptimer = 10;

}

af t er a ( ) {

start = TRUE;

q = 3;

}

115
By using the above observations, a hierarchical

link list can be used to represents multiple stations

that might exist in the network. The hierarchical

structure conforms to the addressing structure by having

regions which contain segments which in turn contain

stations. Each station is represented by an element of

statstruct type. A segment represents a logical ring

while a region groups several segments together. This

hierarchical structure is illustrated in Figure 1. It

can be seen from the figure that stations which reside

on the same segment share the same bus variable. This

bus simulates the physical medium of the network.

The simulation of the token bus protocol requires

two major operations:

1. Configuring the network which includes adding or

deleting of stations or modifying existing stations.

2. Running the simulation by employing an infinite

loop function that traverses every station sequentially, and pass the station pointer to the protocol function. Each pass of all stations in the network corresponds to the smallest unit of time in

this simulation, which is an octettime. The program can be ended by having a time limit to the

simulation, say 100,000 octettimes.

This simulation, however, is not very helpful since

nothing appears on the screen during the execution, and

there is no way to check that the simulation program

runs correctly. In order to make sure that the simulation program runs correctly, ten windows are created on

the screen to monitor dynamically the ACM state of any

stations. Since at most ten stations can be monitored

at any time, commands to move the windows from a set of

stations to another are provided. Each station in the

network has a boolean variable have wind which is set to

TRUE when the station is being monitored, and. FALSE otherwise. If a station’s havewind is TRUE, the station

has an access to a window on the screen and it uses that

window to show the ACM state as well as some other

information about the station. As time progresses, the

state of the station might change and the screen is

updated accordingly. As the monitoring windows move

within a segment, between segments or between regions,

the variable havewind of affected stations are adjusted

accordingly. These windows also serve as a learning

tool for people who want to understand the transitions

of the ACM states dynamically. Figure 2 shows a screen

dump of the screen windows. Screen manipulation uses

the "curses" utility package which is common to all UNIX

systems. This package is not terminal dependent and

116
therefore maintains the portability feature of the simulation program.

This method eliminates

previous two methods and has

1. The program can run vir

desired. Addition of

to an addition of a lin

work configuration.

2. The program is relative

process with respect

runs hundreds times fas

method discussed.

the disadvantages of the

the following advantages:

tually as many stations as

a station merely corresponds

ked-list element in the netly fast since it is a single

to the operating system. It

ter than that of the second

3. The program is portable to any UNIX system. It can

run in any UNIX system with little or no modification. The only requirement is that the UNIX system

support the "curses" utility package, a standard

feature of UNIX systems.

4. It can be ported to any system which supports the C

language by modifying functions which relate to

screen manipulation. These functions do the most

basic things such as cursor positioning and reverse

video

The only inevitable weakness of thi

appears when considering the real transmissi

the broadband coaxial cable. The transmis

might be as fast as 10 Mbits/second. Comp

real system, the simulation runs much slower,

lation, nevertheless, produces response t

t ime un its.

s pro gram

on s pee d of

s ion s peed

ared to the

The s iiiiuime in real

117
2.5. Implementation of MAC Layer "Machines”

MAC layer consists of four Bain "machines" and an

optional one. The four machines are Interface Machine

(IFM), Receive Machine (RxM) , Transmit Machine (TxM) and

Access Control Machine (ACM). The optional machine is

Regenerative Repeater Machine (RRM) and is not implemented in this simulation. The implement at ion of the

four main machines is the focus of this sub-section.

2.5.1. Interface Machine (IFM)

IFM interfaces MAC sublayer with its upper layer.

It provides the upper layer with two main services,

accepting data to be sent to other stations and acknowledging incoming data destined for the station. The

IFM uses two primitives to support those services,

MADATA. request and MADT. indication. There is another

primitive provided by the MAC sublayer,

MADATA . conf i rmat i on which is not important to and omitted from this simulation. This primitive confirms to the

upper layer the success of the last MADATA . request

primit ive

2.5. 1.1. MADATA. request

This primitive abbreviated as madtreq in the

simulation uses four queues (sendq) to store data from

the upper layer. Each queue stores data with a certain

priority. Data with priority 0 and 1 are stored in

send_q[l], data with priority 2 and 3 in send_qf2],

priority 4 and 5 in send_q[3], and priority 6 and 7 in

send q[4]. Each sendq is a first-in-first-out (FIFO)

queue. The flow chart of this primitive is presented in

F i gure 3

2.5. 1.2. MA_D ATA . indi cat ion

This primitive abbreviated as madtindic in the

simulation acknowledges incoming data by passing the

pointer to the data to the upper layer. The incoming

data is received from the bus the Receive Machine (RxM)

discussed later.

2.5.2. Access Control Machine (ACM)

This machine is discussed at length in [3J. It is

the "brain" of the MAC layer by communicating with other

ACMs of other stations which share the bus. The function

of this machine is to keep the token moving in the logical ring. When the station has the token, the ACM

machine checks the sendq queues to see if there are any

data to be transmitted. If data exist and time is still

available, the data will be removed from the

118
corresponding sendq and transmitted

tion by calling primitives provi

Machine. When time expires, the ACM

to its successor by sending the

The simulation follows the document

menting all the functions specified

to destination staded by the Transmit

transmits the token

token control frame.

closely by implein the document.

Each station in the network has a set of timers

which is created and freed as a station is created and

deleted from the network. All timers created are

inserted in a timer queue. Each timer in the queue is

updated periodically by reducing the values left on the

timer. If the value becomes zero, the boolean field

''expired" is set to TRUE. Timers can either have

octettime or slot_time time interval. If the interval

is octettime, the timer value is decremented every time

signal; otherwise the timer value is decremented every

slottime time signals. If slot_time equals three

octettimes, a timer with slottime interval is decremented every three time signals.

Once a logical ring is established, a "steadystate" operation of ACM is presented in Fig. 4.

2.5.3. Transmit Machine (TxM)

This machine accepts the MAC pdu from the ACM and

transmits it to the bus. In real implementation, each

bit of data is converted to corresponding MACsymbols.

The transmit machine is implemented in this simulation

by using three primitives: sendfrm, sendcompletef rm and remfrm. Before discussing these three primitives, a closer look to the bus structure and its relationship

with stations transmitting data into it is presented

below:

2. 5. 3:1. Bus Structure

The type declaration of the bus is taken unchanged

from the program shown below:

119
typedef struct {

/* No of stats currently sending */

int no_of_ senders

/* next state of the bus, one octet after */

buscond nextcond;

/* current state of the bus */

buscond cond;

/* length of data, if not collided */

int length;

/* pointer to control type data */

char * data;

/* pointer to upper layer data */

npdutyp * lay3_data;

/* source address of sender */

maddr SA;

/* destination address */

maddr DA;

/* frame check sequence, not implemented*/

fcstyp fcs;

/* 11c data or control type data */

frmtyp FT;

/* priority if FT = 11c, otherwise

type of control type data */

frm subtyp FC

/transmission error, not implemented*/

boolean erroccurr ed

} busstruct, * busptr;

The buscond refers to one of the following conditions:

quietbus, busy, collidedbus and completefrm. Quiet

bus refers to the fact that no station is transmitting.

Busy means only one station is transmitting since the

last quietbus condition whereas collidedbus means more

than one station are transmitting since the last

quietbus condition. Comp let e_f rm is a condition which

refers to the fact that one station has successfully

transmitted its complete data into the bus. The conditions which its transitions are shown in Fig. 5.

2.5. 3. 2. Transmit Machines conditions

Each station has four conditions with respect to

the TxM: notsending, sending, complete_send and

aftersending. Fig. 6. shows the transitions of these

condi t ions

120
2. 5. 3. 3. Send_ f rm

In addition to all the timers which exist for the

ACM, an additional (sendtimer) is provided for this

simulation with octettime interval. As with any other

timer, it is updated when a time signal arrives.

Sendfrm first checks the condition of the bus. If the

condition is not quiet_bus, it means that some other

station is transmitting and the function cancels the

transmission of data and gives up the token, since some

other station must also have a token (multiple tokens);

otherwise send_frm sends the MAC pdu, updates the bus

nextcond and starts the count down sendtimer with the

length of the data transmitted. Notice that the

nextcond field of the bus is the one updated. This

enables stations to transmit data simultaneously within

a timing signal pass. At the end of the pass, the bus’

"cond" is set to bus’ "nextcond".

This primitive does not put the data it is sending

on the bus. It just marks the bus to indicate to other

stations that a station is transmitting. This is an

efficient scheme since it avoids putting data onto the

bus only to have it discarded because of collision.

2.5. 3.4. Send_complet e_f rm When the sendtimer’s value becomes one, function

sendcompletefrm is called. This function checks the

state of the bus. If the bus is busy, it means that no

collision condition exists and this function will put

the station’s data on the bus and change the state of

the bus to completefrm. If the bus is in co 1 1 i dedbus

,

the state of the bus is not changed. After this function is called, the station’s condition is updated to

completesend as shown in Figure 6. Since it is possible

to send data to itself, this function also checks the

destination address of the data. If the destination

address matches station’s own address, the IFM is

activated by calling the function madtindic to indicate arrival of incoming data.

2.5. 3.5.

This function is called when the sendtimer changes

from non-zero value to zero, thus showing that the sending period is over and the station should not transmit

any longer. Remfrm decrements the number of senders in

the bus. If the number becomes zero, Rem_frm set the bus

condition to quietbus. Remfrm also frees the maclayer

pdu and its corresponding data unit which are created by

the madtreq function in the Interface Machine.

121
2. 5. 3. 6. Addition and deletion of stations

Addition of a station to the network is done simply

by creating a station element, initialize all variables,

creating and initializing all necessary timers, and

inserting the new element into the linked-list structure . Deletion of a station then corresponds to deleting

the station element from the network as well as freeing

all the timers associated with it. Since deletion can

be done dynamically, it is possible that a station might

be deleted when it is actually transmitting data to the

bus. Function "aftstatrem" is therefore provided to

adjust the bus in case of a deletion of an actively

sending station. This function also frees timers.

2.5.4. Receive Machine ( RxM

)

This machine is represented by function checkbus.

This function checks the condition of the bus at every

timing signal ( octett ime ) . It sets four boolean variables busquiet, noiseburst, rxdatafrm and

rxprotfrm. If the bus condition is either busy or

col 1 ided bus , the station’s bus quiet is set to FALSE to

indicate that transmission is occurring in the bus. If

the bus condition is comp 1 et e_ f rm , checkbus copies

incoming data from the bus to the station’s incoming

buffer, sets station’s busquiet variable back to TRUE

and sets either rx_data_frm or rxprotfrm to TRUE

depending on the type of the data. All four boolean

variables are used by the ACM machine and three of them

can only be set back to FALSE by the ACM machine. The

rxdatafrm is read by both ACM and IFM machines but can

only be set to FALSE by the IFM machine.

Finally, if the bus shows a quietbus condition,

variable busquiet of the station being visited is

checked. A value of FALSE of busquiet variable implies

that the station has previously heard either a busy or

collided bus. This shows that some other station(s) have

transmitted data to the bus, but the data is either

never completed or a collision has occurred since the

bus never reaches the completefrm state. A summary of

the operation of RxM is presented in Fig. 7.

2.5.5. Miscellaneous

Since this simulation runs dynamically, user input

from the keyboard is checked continuously. The keyboard

input is checked by polling the input buffer every time

a timing signal pass to all stations was generated.

Function checkinput does the checking of the input

buffer. Polling the input buffer after every pass,

122
however, slows down the simulation substantially. This

polling and the display can be disabled by pressing a signal interrupt on the keyboard.

When a command such as addition of a station is

generated from the keyboard. The program might request

more information from the user consequently. The user

might also want to cancel the command in the middle of

interaction with the program. The program provides this

service by providing a user interrupt signal on the keyboard.

Segments can have different transmission speed, the

default is 1 0Mb i t s / second . Segments which have the

default speed are timed every signal pass, segments

which have lower speed are timed less often. For example, stations which reside on 5 Mbits/second segments

are timed every two signal passes; thus lowering the

transmission speed to half of those stations which

operate on default speed.

3. Simulation of the Network Layer

3.1. Discussion:

Since LLC sublayer is not implemented in this simulation, the next upper layer after the MAC sublayer is

the network layer. Having implemented the MAC sublayer,

the simulation is capable of transmitting data between

stations which reside on the same segment. This chapter

deals with the transmission of data between stations

which reside in different segments. This layer accepts

layer_4 pdu from transport layer, divides the layer4 pdu

into some smaller layer3 pdu’ s if necessary, and

provides routing algorithm so that the layer_4 pdu can

be sent from any station to any other station in the

network. It also indicates to transport layer the

arrival of layer_4 pdu’ s from other stations.

This layer provides a primitive (function) to transport layer. This function, n un i t d t _ r eq , accepts

layer_4 pdu, creates as many layer 3 pdu ’ s as necessary,

and sends each of layer_3 pdu ’ s by calling function

sendpdu. Send pdu routes the layer_3 pdu to destination station by calling the routing function and

madtreq function provided by the MAC sublayer. The

flow-chart of n unitdt req and send pdu combined

together is shown in Fig. 8.

Transmission of data between stations which reside

on two different segments is done through a bridge. The

bridge acts as a pair of stations which reside on the

two segments. If a station in segment one wants to

transmit data to a station in segment two, it sends the

123
data to bridge's station on segment one. When the

bridge’s station on segment one receives the data, it is

automatically assumed that data have been received by

bridge’s station which reside on segment two. Every

region contains a bridge queue to keep track of all the

bridges which connect segments within that region.

Regions are stored in a queue of type region q. Besides

storing regions, regionq also contain a bridge queue to

keep track of all the bridges which connect segments of

different regions. (Inter-region bridges).

Fig. 9. illustrates bridges which connect segments

within a region. The bridge q is a field of the

region struct. Another bridge q which queues all bridges

connecting segments of different regions, is also a field of the regionq. Bridges within a region are

sorted by the segment numbers of the two segments being

bridged in ascending order; so bridge x which connects

segments 1 and 2 is in front of bridge y which connects

segments 1 and 3, and bridge y is in front of bridge z which connects segments 2 and 3. Inter-region bridges

are sorted in the same manner, this time by using region

numbers of regions being bridged instead of segment

numbers. Since connecting every pair of segments which

exist in the network with a bridge would be very expensive and unnecessary, only selected segments are connected through bridges. The main function of the network layer is to route data from sending station to destination station through intermediate bridges. A routing algorithm is therefore essential for this layer.

3.2. Characteristics of Bridges

1. Bridges are assumed to have infinite buffer space.

2. Every Bridge acts as a pair of stations which tune

in to two different logical rings. Each bridgestation therefore contends for token before

transmitting data in the medium.

3. Incoming data from a certain priority from one end

of the bridge is queued to the other end on the

same priori ty

4. Incoming data is treated in FIFO basis on both

d i rect ions.

3.3. Royting Algorithm

Since no routing algorithm currently exists for a network connected by bridges, a hierarchical algorithm

was devised for this simulation. The algorithm uses a tree structure with variable number of children. Segments residing in a region are configured as a tree

124
structure in which segment one acts as

Regions are also configured as another

with region one acting as the root

structure of segments and regions differ

one structure contains segment_no while

tains regionno. The structure of the

below:

typedef struct sm {

struct sm * next;

int

} seg lvl st ruct

segmentno

the root node,

tree structure

node. The tree

only in that

the other contree is shown

typedef struct segtree {

int segmentno; / node level in the tree, root => 1 /

short level;

struct segtree * first_child;

struct segtree * siblings;

/pointer to older child /

struct segtree leftsibling;

struct segtree parent; / parent node /

/same level connect i ons /

seglvlstruct smlvl;

} segtreestruct;

The dec 1 arat io n abo ve is ill us t r at ed i n Fi g . 10 •

A menu f or the tree s t rue tur e i s P rov i ded by this

p ro gr am . It enables user s t o V i ew the t ree st ru c ture and

m ov e from on e node to an ot h er wi th re 1 at i ve e ase

It

a 1 s o shows the r el at i ons hip s b et we en the cur r ent node

( th is node i s highl ighted) t o an y o th er node on the

s cr een . A s creen dump of the me nu i s pres ent e d in Fig.

11 . Each

declarat io

ture. The

s t ruct ures

child rela

tree, eve

should hav

communicat

create a t The level

the tree s the same 1 Conne

of two se

segment on

mun i cat ion

region to

segment contain s thi s struc t u re in its

n an d e ach reg i on CO ntains a s im i lar s t rucuser dec ides the CO n f i gurat i on of the t ree

. A br i dge exi sts be tween a pa i r of pa r entt ions hip in the t ree s t ructure

S in ce i t i s a ry n ode in the t ree st ructure ex ce pt the root

e one par ent. Ch ild ren of the same par en t can

e th rough the par ent nodeTh e use r can

r ee s true ture wit h a s m any leve 1 s a s des i red

of the root nod e i s d efined t o be oneOnce

t ruct ure is confi gur ed

,

nodes w hi ch res id e on

e ve 1 can also be connec ted by b r i dg es .

c t ion of two regi ons is actual

y a connec t ion

gmen t s of the two re gions. In thi s s irnul

a

t ion

,

e of each region is des ignated to b e the commed i a for all s t a tions which r es ide in that

s t at i ons of other re gions

125
The routing algorithm uses these two tree structures to decide which segment it will route data to.

This routing algorithim is simple and yet fairly general. It enables users to configure arbitrary configurations with relative ease.

Example 1

Assume that there are seven segments (logical

rings) in region one, and a ring structure is desired.

The tree structure is then configured as shown in Fig.

3.1.

This configuration forces all communications

between segments to travel through segment one. Let's

then assume that after running the simulation it is

realized that transmission time between segment two and

segment six needs to be faster. A natural enhancement is

to add a brige between segment two and six. This is possible because segment two and segment six are in the

same region and both are of level two. This addition of

a bridge causes segments two and six to communicate

directly while leaving all other routes unchanged. This

updated configuration is shown in Fig. 3.2.

126
easily conf igFig. 3.2.

Example 2

A straight chain of segment can be

ured as shown in Fig. 3.3.

Fig. 3.3.

127
Example 3

Sosetiaes a bridge between two segments having different parent is desired. This is allowed as long as

the two segments are in the same region and both of them

are of the same level. Figure 3.4. shows such a configuration

128
Example 4

This last example, shown in Fig. 3.5. illustrates a relatively large network. Routes for several segmentpairs inside a region as well as between regions are

shown in Fig. 3.6. The routes from one region to another

are analogous to routes between segments in a region.

Fig. 3.5.

129
FROM TO ROUTES

region segment region segment region segme:

1 11 1 14 1 11

1 6

1 2

1 5

1 9

1 14

1 11 1 13 1 11

1 13

1 7 3 3 1 7

1 2

1 1 3 1

3 3

2 10 2 15 2 10

2 5

2 9

2 15

2 6 2 8 2 6

2 2

2 1 2 4

2 8

Fig. 2.6.

The flow-chart of this algorithm is presented in

Fig. 12.

130
4 Transport Layer

4.1. Discussion

This layer is supposed to provide an

coanunication between two stations; but sin

lation program is used specifically for

among ground floor devices, the function

is tailored to serve a more specific task:

input messages.

end to end

ce this simucommunicat ion

of this layer

generation of

This chapter also discusses

to store and retrieve network

physical storage.

briefly the

configuration

mechanism

into/from

4.2. Generation of Input Messages

Input messages on

address, destination

of transmission and pr

source address belong

deletion of messages a through the data menu,

of number depending on

higher the frequency

number. This range o numbers generated

described next. Keep

are actually templat

messages. The actual m dynamical ly

the network have specific source

address, message length, frequency

iority. Messages with the same

to the same station. Addition and

re done dynamically by the user

Each message is assigned a range

the frequency of the message. The

, the higher the range of the

f number is used to match to

by the random number generator

in mind that the messages above

es of messages instead of actual

essages are created and deleted

Having set up all message templates in the network,

a mechanism to actually send the message is needed.

This message generation is done by utilizing a congruential random number generator. This random generator produces uniformly distributed random numbers. When a station is timed, it calls the random number generator

which returns a number. If the range of number of any

message in the station covers the number generated by

the random number generator, the message is transmitted;

otherwise no message is sent in that time signal. The

random nature of the generator makes the simulation more

real ist ic

If a message is transmitted, a copy of the message

is stored in layer4_queue until the message is acknowledged by its destination station, when the message

is acknowledged, it is freed from the queue and response

time statistics of that particular message is updated.

At the end of the simulation, user can get statistics of

all the messages in the network. This statistics

includes the number of messages sent during the simulation time, the worst response time and the average

132
response time of the message. The

also provides a histogram of response

sage in the network. This histogram

ASCII characters on the screen which,

fancy but maintains the portability

gram

simulation program

time of any mesis done by printing

even though not

feature of the pro4.3. Save and Retrieve Configurations

Once a network configuration is established, a mechanism to save the configuration into physical

storage is a must. Saving a configuration requires saving all regions in the network and their bridge connections, all segments inside a region and their connections, and all stations inside a segment and their variables. The algorithm uses the multi-children tree

structure which exists for both region level and segment

level. For the segment level, each segment corresponds

to a node in the tree structure. The segments are stored

by traversing the tree in preorder walk; that is root is

stored first, then leftchild node and finally the

right_child node. In addition to storing all the segments which is a node in the tree structure, the algorithm also saves segments which are not connected to any

other segments through bridges (free segments). For a more detail discussion of tree walk, consult chapter 3

of [4].

Ret r i evi ng a configu r at

mo re work. The program w il

sa ve the curr ent configu r at

Be f o re 1 oad i ng a configu r a

t i on has to b e comple t el

requ ires f ree ing of all 1 i nk

wo rk . Thi s de let ion funct i on

to guar an tee t ha t no poin t er

cu r r ent c onf i gur at i on is d e 1 by a 1 gor i thm di s cussed in [4

5. User Inte r f a ce

5.1. D i scuss i on

ion from a file requires

1 first ask if user wants to

ion and acts accordingly,

tion, the current configuray deleted. This deletion

ed-list elements in the netis intricate since it has

is left dangling. Once the

eted, the file can be loaded

].

User inte rf ac

The man ipulat ion

"fri endly" t o the

inte rf ace prog ram

s t an ding o f any ki

tings and th e cu

rel i es heavily on

input buffer and

numb er and charact

user The us er

programmer . It is

e plays a big role in a CAD program.

of the screen to make the program

users is discussed here. The user

does not require any conceptual under

nd, but a knowledge of terminal setrses utility is necessary. The program

function check_input which checks the

decides whether number, character,

er or arrow key is pressed by the

interface is helpful to both users and

helpful for users since without the

132
user interface, the program can not be used at all and,

it is helpful to programmers for it shows any unexpected

results instantly on the screen. The user interface

fakes the most time to program since it requires many

nitty-gritty functions and is being used to make sure

that the program runs as expected. Many extra precautions were taken in the development of this program to

minimize possibility of fatal error. The philosophy

behind the simulation program is to make the program

simple, easy to understand, modular and adaptable to

changes. The simulation program does not provide any

analysis of the token-bus protocol, but it has provided

a foundation for further development by providing a working token bus protocol, a simple routing algorithm

and input-output capability which will be discussed in

the next chapter.

This simulation program uses some rudimentary functions of the "curses" facility listed below:

1. move: This function moves the cursor to a certain

position on the screen, for example "move (5,20)"

moves the cursor to position y = 5 and x = 20.

2. printw: This function works like printf and starts

printing from the current position of the cursor.

3. standout: This function starts a reverse video output on the screen.

4. standend: This function ends a reverse video output

on the screen.

5. refresh: All the functions above do not actually

change the screen appearance until a refresh command is encountered.

Curses initializes the screen by using function

initscr and uses function endwin to reinitialize the

screen before getting out from the program. Curses uses

terminal database which is usually stored in a file

called termcap. Every terminal has its own "termcap

database" in that file and curses just needs to know the

terminal type to access the correct database. Curses is

therefore independent from any terminal type.

133
5 .

* Implementation of Segment Menu

Even though there are several menus in this simulation, the screen manipulation of each menu uses basically the same technique. The implementation of segment

menu is discussed briefly here. The source code of this

function ( segdat a_conn ) is stored in file seg_conn„c

The segment menu shows bridges which exist within a region. It enables user to position the cursor to any

intersection of two segments. If a bridge exists in the

intersection, the intersection is marked, otherwise it

is left blank. A screen dump of the segment menu is

presented in Fig. 13. Notice that some intersections

might not be used since intersection of row-segment 2

and col-segment 1 is identical to intersection of rowsegment 1 and col-segment 2. In this simulation the

lower half of the screen is used. Implementation of

segment menu uses the following data structure:

/*

typedef struct

*/

short y_st , x_s

short ylength

short x_length

bridge_ptr br_

ll_struct

typedef struct {

cell_struct mat r ix [ 1 1 ]f 7

]

cell_struct horheader [ 7

cellstruct verheader [ 1

} screen_ type

/* */

The screen is represent

type screentype above,

horheader of screentype co

are used to display the co

Each element of the twocorresponds to an intersecti

tains pointer to correspondi

between the two segments or

Some variables are cruc

discussed below:

ed by var i abl e "scr een n of

The fields verheader and

rresp ond to windows wh i ch

rresp ondin g S egaent numbe rs . dimen siona 1 array mat r ix

on of two segments

,

and conng br idge if a bridg e ex is t NULL otherwi

e

ial t o the program and are

1 . Constants ROWNO and COL_

vertical windows and h

screen respectively. ROW_

is set to 7.

NO ref ers t o the number of

ori zont al w indows on the

NO is set t o 10 and COL NO

134
Seg_cur_cell of type cell_struct points

current window position with respect

dimensional array "matrix". An example

ment to this variable is seg_ci

mat rix [5 ][ 2 ] . The window which seg_cur_cell

to is the intersection highlighted on the screen.

s to the

to twof ass ignce 11 = &

11 po ints

3. Find_cur_cel 1 of type boolean. This variable indicates whether seg_cur_cell already points to an

intersection or not. A value of FALSE corresponds

to seg_cur_cell equals to NULL and no intersection

is being highlighted on the screen.

4. Segcurhor of type segmentptr points to the

col_segment of the intersection being highlighted.

5. Seg_cur_ver of type segment_ptr points to the rowsegment of the intersection being highlighted.

6. Seg_cur_x of type integer refers to the position of

the highlighted intersection with respect to the

left-most intersection. When left-most intersection

is being highlighted, the seg curx equals to 0.

The right-most intersection to (C0L_N0 - 1) which

is 6

7. Segcury of type integer is equivalent to

seg_cur_x but refers to the y axis. Seg_cur_y

ranges between 0 to (R0W_N0 - 1).

8 Hor_bridge_st art and verbridgestart are fields of

the regionstruct and are used to point to the

start of row-segments and col-segments being

displayed on the screen.

9. Cur_pos points to current command being highlighted

on the screen.

The previous screen dump with its comments should

suffice to clear up any ambiguity. The flow chart of

this function is shown in Fig. 14. In all, the simulation program provides seven interactive menus which are

configured in a hierarchical way as shown in Fig. 15.

135
6. Conclusion

This simulation closely follows the token-bus protocol of IEEE-802.4; hence its resulting response time

is expected to be close to the real network. By simulating the protocol very closely, however, the program

runs much slower than its simulated network. Two things

can be done relating to this simulation program. One is

to improve the technique in simulating the time and the

set of timers affected by the progression of time.

Specifically when a station is transmitting a large

number of bytes of data into the bus, say 10,000 octets,

the simulation actually traverses the linked- list elements 10,000 times, updating the time after every pass,

eventhough no stations change its ACM state within the

10 , 000-octet-t ime period.

This time-simulation can be easily enhanced within

a single logical ring (segment) by increasing the time

value directly by 10,000 as well as updating all

affected timers; but since many segments, each transmitting different size of messages, are simulated simultaneously with the same set of timers, this enhancement

is becoming rather complicated. The pay-off nevertheless, is very tempting, especially for a network which

transmits mostly large chunks of data.

Another field of interest is to design a replacement communication model based on communication theories

by statistical model, and use the simulation program as

a benchmark to test the model. When this model is completed, it would be a powerful tool to analyze any arbitrary network without having to run the simulation.

136
Reference

1. "Network Layer Principles", European Computer

Manufacturing Association, Final Draft

#ECMA/TC24/82/18 (April, 1982)

2. "GM MAP SPECIFICATION", Version 2.1 (March, 1985)

3. "IEEE Standard 802.4 - 1984 and ISO Draft International Standard 8802/3", Draft F, IEEE P802. 4/84/30

(July, 1984)

4. "Data Structure Techniques", Standish A.T. (1980)

137
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depending on data type

received.

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then set both bus_puiet

and noiseJburst to TRUE

Fig. 7

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149
150
MENU SYSTEM

151
Implementing a Factory MAP Network Simulation Tool Using the

Ada 1 Programming Language

W. L. Schultz, Senior Member IEEE

In Don Bae, Student Member IEEE

Asheem Chandna, Student Member IEEE

Farrokh Khatibi, Student Member IEEE

Center for Automation and Intelligent Systems Research

Case Western Reserve University

Cleveland, Ohio 44106

Abstract

A methodology for the implementation of a factory scale MAP

network simulation tool is presented. The methodology is

centered on the task type replication facility and the task

rendezvous facilities of the Ada programming language. The

result allows physical network entities to be directly

represented as program code modules in a form that is highly

readable and easy to understand.

Introduction

Stations on a MAP 2 network are autonomous. This requirement

associates a great deal of complexity in each station's inherent

operational capabilities. Despite the deterministic nature of

this complexity, the interactions among a large number of these

stations is difficult to describe due to the sheer magnitude of

their interaction possibilities. Thus, simulation offers a means

for evaluating different MAP architectures, operating modes, and

topologies prior to physically installing an actual network.

In order to simulate a large factory network, issues of:

(1) topology, (2) interconnection of separate physical segments,

(3) station mode of operation (Mini-MAP or Full MAP), and

(4) traffic patterns (size and frequency of messages) must be

taken into account. The representation and specification of

these issues to a simulation program are problems in their own

right. It is our intent to utilize the capabilities of an

engineering graphics workstation for the convenient specification

1. Ada is a trademark of the Ada Joint Project Office, United

States Department of Defense.

2 . Manufacturing Automation Protocol (MAP) Standard , Version 2.2, General Motors Corporation, 1986.

152
Schultz, Bae, Chandna and Khatibl MAP Network Simulation

and display of network simulation inputs, status during running,

and results. However, this paper deals with the use of Ada for

representing and simulating autonomous network entities, that is,

components of the network.

Thus, we have split the job of factory MAP network

simulation into components of: (1) user interface and

(2) protocol implementation and simulation. This paper presents

an approach to the latter problems using the several special

facilities, and specifically the tasking mechanism, of the Ada

programming language. In addition, we propose an alternative

interim user interface method — direct use of the Ada language.

In implementing the MAP protocol we have started with the

lowest layers and are working toward higher layers following the

incremental modeling approach described by Yeh 3 and further

discussed by Rahimi and Jelatis 4 . In Ada cultural terms this

approach would be called "stepwise refinement."

Our result is targeted primarily for use in the simulation

of large factory networks. Our goal is to provide network

designers and researchers with a new, more complete tool.

Newcomers to the field should find the structure and details of

our implementation helpful in furthering their understanding of

MAP network issues.

Design Goals

A complete Factory Network Planning Tool must have a "toolkit" of network components including interconnection

devices, message generators, and measurement tools accessible to

the user. The toolkit is required to be compact in the sense

that the simulation code necessary to represent a factory network

must be reasonable -- it must fit in a computer available to the

user. Speed is also an issue for practical simulators and is

incorporated in these design goals as a call for good coding

practices

Using the packaging and tasking concepts and mechanisms of

the Ada programming language 5 , - the toolkit can be realized as a

3. Yeh, J. W. , "Simulations of Local Computer Networks," Proc.

4th Conf . on Local Computer Networks, Oct. 22, 1979,

pp . 56-66

4. Rahimi, S. K., and Jelatis, G. D., "LAN Protocol Validation

and Evaluation," IEEE Journal on SAC, Vol . 1, No. 5,

Nov. 1983.

5. Gehani , N., Ada, An Advanced Introduction , Prentice-Hall,

Inc., Englewood Cliffs, New Jersey, 1983.

253
Schultz, Bae , Chandna and Khatlbl MAP Network Simulation

convenient set of devices that can be connected together to form

factory networks. The goal then is to realize each network

entity as a Ada data structure, procedure, function, or task in

such a way as can be easily replicated and interconnected by

means of straight forward Ada program statements. The highly

readable nature of the Ada Language will facilitate the checking

and explanation of Factory MAP Network configurations so

specified. The possibility to produce a highly readable

implementation of the toolkit modules invites their careful and

clear presentation to serve as a benchmark for other

implementations

Simulation Approach

Our approach relies heavily on the ability of the Ada

Language to replicate MAP stations represented as concurrent

tasks. This approach is similar to that taken by Powers and Nute 6

in their simulation of a CSMA/CD Network. The major entities of

our system are (1) stations and (2) buses. We have kept our code

nearly parallel to the procedures and data objects described in

the appropriate standards documents. Where possible, we have

used exactly the same names given to these entities by those

documents. Careful use of reentrant procedures has minimized the

additional memory required when a task is replicated. The main

Ada program serves to specify the number of stations and buses,

their particular interconnection, and the nature and type of

communications among them.

Figure 1 illustrates a straight forward factory network plan

composed of two different types of buses: the broadband backbone

bus segment and several broadband or carrierband bus segments

bridged to the backbone bus. Each bus, the backbone segment and

each of the several bridged bus segments, has a separate logical

token ring. It is expected that the number of stations on each

bridged bus segment will be between 20 and 30, that the number of

such segments connected -to the backbone segment will be

approximately 30, and that 20 additional stations (main frame

computers, etc.) will be connected directly to the backbone

segment. This yields an approximate size for systems to be

simulated of roughly 750 stations and 30 buses.

Figure 2 illustrates the four main types of station present

within a factory network. Each of these station entities has

many functions in common with other stations; in particular,

layer 2 is common to all and layer 1 chooses 1 of 2 possible

media choices. In a similar fashion, layers 3, 4, and 5 supply

common functionality to all stations using them. This leaves

6. Powers, W. S., and Nute, T., "Implementing a Simulator as a

Set of Ada Tasks," Proc . of the Eastern Simulation Conference

on Simulation in Ada, Norfolk, Virgina, March 3-8, 1985.

154
CO j

00

To Foreign Network

Key

essssssss^ Broadband

=i : Carrierband

S : Station

B : Bridge Application

H.E. : Head End

G/R : Gateway or Router Station

Figure 1. A Typical Factory Network

(Station and Bus Entities)

155
Figure 2a. MAP Station Entities

Full MAP Station

Layer 7

Layer 2

Layer 1

'Figure 2b. MAP Station Entities

Mini-MAP Station

156
Layer 7 Layer 7

Layer 6

Layer 5

Layer 4

1

1 Layer 3

1 1

\\s \!/

Layer 2 Layer 2

Layer 1 Layer 1

Figure 2c. MAP Station Entities

EPA - MAP Station

Bridge Application With Consistent Link

Address And Frame Size Restrictions

Layer 2 Layer 2

Layer 1 Layer 1

Figure 2d. MAP Station Entities

Two Segment Bridge

157
Schultz, Bae, Chandna and Khatlbl MAP Network Simulation

layers 6 and 7 as the only services that may have functional

dependencies on the operating system or machine architecture.

Figure 3 shows the layers of a station implemented as

"packages" of code connecting to its higher and lower layer

neighbor by means of well defined procedural and data object

"entries." In this representation the procedural entries are

"sockets" while the adjoining layer procedure calls are "plugs"

into the procedure entries represented by the "sockets." Data

objects are represented by the formal parameters supplied by the

calling layer. This representation, called a structure diagram , is fully described by Buhr 7

; we use it throughout the remainder

of this paper to indicate the organization of the code in our

implementation

Figure 4 (a through c) shows the detailed interconnections

among the physical 8 (layer 1), MAC 8 sublayer (layer 2), LLC 9

sublayer (layer 2), and the station management functions. The

connections shown are all that should be visible to other layer

entities; they need not and should not access structures internal

to these layers directly. Thus, by refining each layer, as shown

for the MAC sublayer in Figure 5, attention for design and

implementation can be focused on the issues within a particular

layer

Once implemented, a station must be replicated and connected

to its copies. The vehicle for this connection is an entity, the

bus, which serves to schedule, synchronize, and pass information

among the several stations to be connected to that bus. In large

factory systems, of course, there are many buses. Thus, the bus

entity must be replicated in order to represent a large network;

scheduling and synchronizing buses is accomplished by means of a global clock. Since buses do not pass information directly to

other buses, the interconnection of buses must be done explicitly

among stations connected to different buses. That is, a bridge

station must be used to interconnect 2 buses.

In summary, our approach to the simulation of a factory MAP

network is to implement a template for each of the network

entities. These templates are then replicated and interconnected

as needed to represent the configuration required. This

technique rests on the Ada task typing facility to replicate the

required network entities as autonomous, concurrent, yet

7. Buhr , R. J. A., System Design with Ada , Prentice-Hall,

Englewood Cliffs, New Jersey, 1984.

8 . ANSI/IEEE Sta 802.4-1985, Token-Passing Bus Access Method and

Physical Layer Specifications , IEEE, New York, 1985.

9. ANSI /IEEE Std 802.2-1985, Logical Link Control , IEEE,

New York , 1984

158
TRANSMIT

Q

Figure 3. Simplified Structure Diagram for a

Full MAP Station

o

RECEIVE

U

0

159
Figure 4a. Detailed Structure Diagram

Lower MAP Layers

160
p

H

Y

(parameterless)

PHY-RESET.request

LAN_topology_type « > phy_role <, > PHY-RESET.confirmation

mode_class < » PHY-MODAL_CAPABI L I TY-QUERY .request

mode_class > PHY-MODAL_CAPABI L I TY-QUERY

confirmation capability-report ° »

mode_class < ° PHY-MODE-QUERY.request

mode_class o > current-mode > PHY-MODE-QUERY.confirmation

mode_class < » nev_mode < o PHY_MODE_SELECT.request

mode_class » status > PHY-MODE-SELECT. confirmation

(parameterless)

PHY_MODE_CHANGE.indication

Figure 4b. Detailed Structure Diagram

Physical Layer to Station Management

161
M

A

C

desired-protocol 4- MA-INITIALIZEJROTOCOL.request

MA_I NI TI AL I ZE_PROTOCO L

confirmation status

variable-name

desired_value

MA_SET_VALUE.request

status

MA_SET_VAlUE.confirmation

4

variable_name <- MA_READ_VALUE.request

current-value *- status o- MA_READ_VALUE.confirmation

event

MA_EVENT.indication

fault-type *- MA_FAULT_REPORT.indication

set of group address 4

- MA-GROUP-ADDRESS.request

status o- MA_GROUP_ADDRESS.confirmation

da 4- m_sdu < MA_CDATA.request desired-quality

desired-quality * MA_CDATA.indication

MAjCDATA.confirmation

quality o- status o-

-)

M

A

c

S

T

A

T

I0

N

M

A

N

A

G

E

M

E

N

T

Figure 4c. Detailed Structure Diagram

MAC Sublayer to Station Management

162
Figure 5. Structure Diagram For

MAC Sub-layer Internals

163
Schultz, Bae , Chandna and Khatibi MAP Network Simulation

communicating tasks. Ada is the only presently available

computer programming language in wide-spread use that supports

this methodology.

Specifics in the Use of Ada

Each of the network entities simulated, the bus and the

station, were implemented via Ada's task type facility.

Replication of these entities is done by simply providing a set

of unique names for the purpose of declaring tasks of type bus or

type station in the same way that one might declare data

variables in another language.

The replication of a task has some side effects, however.

As might be expected, the memory consumed by 10 replicated tasks

is about 10 times that consumed by one task -- a problem for a system intent on supporting 750 such tasks. Fortunately, Ada

provides facilities for the coding of reentrant procedures. By

coding the procedural parts of the station and bus tasks as

reentrant procedures, one copy of these procedures serves all of

the replicated tasks. With this programming refinement, only the

variables local to the task and its minimum of program statements

are replicated. This, at the present state of development, saves

about 2500 lines of Ada code per station.

The needs for scheduling, synchronization, and mutual

exclusion during data exchange are supported by means of Ada's

rendezvous mechanism. In simple terms, this mechanism is the

syntactical equivalent of a procedure call whereby a calling task

names the required entry on a receiving task and provides access

to its data areas for the exchange by specifying them as

parameters of this "entry" call. The receiving task accepts a call at its entry using the syntactical equivalent of a procedure

definition. For the time that the receiving task remains within

the accept statement, the formal parameters of the accept

statement provide direct access to the caller's data area -- the

receiving task is free to read or modify the caller's data area

as required. After completing the accept statement, both tasks

are free to resume their activities in concurrent fashion. While

the calling task was waiting for the receiving task to accept and

process its call, however, it was prohibited from further

activity. Thus, this rendezvous mechanism provides

synchronization and mutual exclusion during data exchange

implicitly. It also provides a convenient means for scheduling.

The convenient graphical technique provided by Buhr 7 is

used to illustrate the relationship among tasks provided by the

rendezvous mechanism. Figure 6 shows the relationship between

bus tasks and station tasks. While some detail has been omitted

for clarity, the diagram clearly indicates which tasks initiate

an interaction with another task. In this illustration, the data

to be received (or transmitted) by each station task is the

current bus state and data on a particular bus.

164
:

cnD

CQ

I a

z

<

QC

0C

<

o

>]

Figure 6. Structure Diagram of a Typical

Factory Network.

165

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Schultz, Bae , Chandna and Khatibi MAP Network Simulation

Figure 7 presents a detailed view of the modular components

used within a station. Figure 8 illustrates the use of this

reentrant package of MAC sublayer components to implement one

station including the connection to its bus task. As mentioned

earlier, an important benefit of the reentrant package structure

is memory savings when the package is shared among many station

tasks

In conclusion, the Ada language provides the facilities

required for replicating concurrent tasks and for synchronizing

communications between them. In addition, other Ada facilities,

which permit the visibility or scope of a data or procedural

entity to be limited to only those portions of code which need to

use it, help to improve one's confidence in the reliability of

the simulation. The fact that entities in the physical network

are directly represented by code entities make the description

and discussion of a network so simulated more easy to understand.

Partial Verification of the Simu 1 a t o

Verification of each significant coded module proceeded as

described in the following steps:

Step 1: The minimal test cases required to completely verify

the module were deduced and documented.

Step 2: The expected results for each of the test cases were

calculated and documented.

Step 3: Each of the test cases were simulated and the results

compared with the calculated results.

To date we have verified all of the major simulator modules

except for the ACM of the MAC layer which has been only partially

verified due to its many states used only during error

conditions. Partial verification of the ACM has been

accomplished using the sequence of transitions and state changes

required for the network to initialize the logical token ring.

For this case, a varying number (from 2 to 20) of stations were

switched on at exactly the same time in the state of "desiring to

be in the ring" or "having frames to send." The results obtained

from this simulation were the same as those expected (see step 2

above )

Figure 9 shows a graph of the time, in octet times, required

for the logical ring to become fully established. Two cases are

presented: (1) each station only desires to be in the ring but

has no messages to transmit and (2) each station desires to be in

the ring and has a short message to transmit. As expected, the

transmission of a message by each station increases the time

required to initialize the ring.

166
package MAC

| procedure STATION_MANAGER

1 procedure RxM

| procedure ACM

package IFM

1 function ANY_SEND_PENDING

“1 function SEND PENDING

package RXMJTXMJJTILITIES

| function ASCII_TO_TEXT

“ function TEXT_TO_ASCII

procedure TXM_SEND

procedure TXM_DISASSEMBLE

procedure GET_PENDING_FRAME

procedure PASS_DATA_FRAME

procedure ACM

package ACM UTILITIES

function ADDRESS TO DATA

function DATA_TO_ADDRESS

function MAX_BUS_IDLE

function CONTENTION_DELAY

function RESPONSE_DELAY

function CLAIM_DATA_UNIT

function ADDR_BIT_MAP

function RANDOM_4

package ACM TIMERS

package ACM_TIMERS

package BUS_IDLE_TIMER

procedure START

function VALUE

function EXPIRED

. the other nine ACM timers

. are also included here

Figure 7 Structure Diagram of Station Support Package

(reentrant code shared among stations)

167
task STATION

task STATION is local declaration

begin

-- instantiate

STATION_PARAMETERS

--accept

STATION_ADDRESS

MAC.STATION_MANAGER

loop

MAC.RxM

MAC.ACM

MAC.IFM.PASS DATA FRAME

end loop

end STATION

package MAC

Figure 8. Station-Bus Interaction Diagram

168
Number of

Figure 9. Ring Initialization Time

169
Schultz, Bae , Chandna and Khatlbl MAP Network Simulation

Conclusions

This presentation has focused on the Ada programming

language as a vehicle for simulating factory scale MAP networks.

Our simulation tool, though incomplete, shows that physical

entities such as network stations, buses, bridges, and head ends

as well as the processing software within a station can be

represented as Ada package and task modules. Further, task

modules for different network entities may be interconnected in a straight forward fashion by means of the task rendezvous

mechanism to represent the network to be simulated.

Ada can serve not only to specify the modular network

entities but also as the breadboard on which modules are

interconnected, message traffic is generated, and network

performance in measured. Ada is readable. Ada can be written in

a clear fashion to promote confidence in simulation results.

The Ada programming language was designed for the production

programming of large embedded computer systems. Such systems

typically range upwards in size from 100,000 lines of code. To

date, the size of the code produced by this project is an order

of magnitude below this lower bound. In short, there is ample

room to grow in the Ada environment and in the utilization of

this methodology.

Acknowledgements

The authors gratefully acknowledge the General Motors

Corporation, C-P-C Group, for the their support of this work.

170
SERVER NETWORKS FOR THE DESIGN AND ANALYSIS

OF FACTORY COMMUNICATION SYSTEMS

Lawrence Zeidner

Dept, of Manufacturing Engineering

Boston University

110 Cummington St. Boston, MA 02215

ABSTRACT

Server networks are generalized networks of

asynchronous processes communicating along

software-programmable communication links

according to specific protocols either within

the confines of a single mainframe or across several physical machines. The advantages of server networks in simulation, modeling, and

production systems stem from the modularity of

each server as a separate data-driven software

element within the entire system. As modular

elements whose communication connections with

the rest of the network are clearly defined,

servers can be exchanged for direct comparison.

Their modularity streamlines the software

development effort. In the field of

manufacturing, server networks can be applied

to the scheduling and routing problems posed in

flexible manufacturing systems if technology

transfer occurs from the field of computer

operating systems. In much the same way that

computer operating systems, such as VM/370,

present the user with an apparent virtual

machine through time slicing and scheduling,

these operating systems for the factory floor

present each workpiece with an apparent virtual

factory through the management of routing and

scheduling. The advent of inter-process shared

variables in the APL2 programming language

presents a working environment well-suited to the construction of server networks. Our

research pertains to the development and

application of a software methodology with

which to construct server networks.

Key words: Server Networks; Software Engineering; Large-System Integration

Tools; Software Modularity; Co- operative Processing; Data-Driven

Control Flow ( DDCF )

INTRODUCTION

Server networks offer the flexibility to model components of large systems at various

levels of complexity and to choose the level of complexity most appropriate for any particular

study. The tradeoff in choosing an appropriate

component model is between the advantages of

detailed emulation on one hand and associated

computational costs and time required on the

other. Because the software that models each

system component is localized in its own

server, it can be replaced by an alternative

software model residing in a different server. The data-driven control-flow methodology,

described in Reference 1 and applied to server networks in Reference 2, makes manual

construction of small networks (i.e., those

consisting of only a few servers) a simple,

straightforward exercise. However, the

construction of large server networks is best

accomplished through the use of large-system

integration tools and concepts. Accordingly, a minimal set of parametrically tunable generic

"utility" servers has been identified that can

be combined to form a powerful server-network

infrastructure

Each server has a kernal of generic

software that enables it to function as a member of the server network aside from its

application software. This kernal of software

will be identified.

A powerful large-system integration tool

designed for server-network configuration has

been built. It is an expert support system

that is used to specify the servers in the

network, their interconnection and protocol,

and their resource allocation. It enables the

system designer to examine, merge, archive, and

retrieve entire subnetworks and to employ

server macros through parametric specification

and design rules.

INFRASTRUCTURE

In an effort to simplify the construction

and use of server networks, we have developed a set of generic "utility servers" that are described individually in Reference 2. A minimal set of parametrically tunable servers was sought rather than an exhaustive set containing every possible variation. To date, seven utility servers have been identified.

They fall into five subnetwork categories:

dispatching, command, status, logging, and

timing.

The Dispatching Subnetwo rk

A server network is built by customizing a set of generic virtual machines. Although each

machine has no "personality," it does have a specific set of computing resources at its

disposal. The dispatcher allocates generic

virtual machines from this pool to participate

in specific server networks. When such an allocation occurs, the dispatcher communicates

with virtual machines from the pool, telling

each one what its role will be and, thus, which

set of predefined software to adopt. In addition, based on the pattern it receives from

the configuration console and on a set of design rules, the dispatcher builds the

appropriate drive-data matrix and downloads it to the virtual machine. The drive data informs

the virtual machine of the communication

variables it shares with the rest of the

network, of its communication protocols, and of

the software it must execute upon interrupt by

171
each armed variable. A virtual machine

connected to other virtual machines through

shared variables with defined protocols,

complete with a set of software that is

executed in response to specific stimuli,

constitutes a server . Virtual machines in the

generic pool can be thought of as servers although they are connected only to the

dispatcher, and have only enough software to be

dispatchable. The dispatcher not only

customizes the server by giving it a "personality," it also maintains contact with

it so that the server can be: dynamically recustomized whenever necessary, and

restored to the generic server pool when

it is no longer needed for the server network.

Figure 1 illustrates the servers and shared

variables that constitute the dispatching

subnetwork. The configuration console shares

three variables with the dispatcher.

PATT: A pattern matrix, armed by the

dispatcher, that indicates sharedvariable connections between the pairs

of servers and that specifies the

protocol (Table 1). INST: An instruction variable, armed by the

dispatcher and executed upon receipt

from the configuration console.

WHOS: An unarmed allocation matrix that

indicates the computational resource re- quirements associated with each server. The system designer specifies this

information from the configuration

console. The dispatcher uses this information to select server assignments.

The dispatcher shares four variables with

each server that it controls.

EXSV: A drive-data matrix, armed by the server, that is used upon receipt to establish communication connections,

protocols, and a pattern of behavior in

response to each possible external

stimulus. The EXSV matrix, together

with the software it calls, constitutes

the server's "personality" (Table 2). INST: An instruction variable armed by the server and executed upon receipt to load

a specific set of software or to return

to the generic server pool when no longer needed.

RPLY: A reply variable, armed by the dispatcher, that informs the dispatcher of

the disposition of the instructions that

it has sent to the server.

WHOS: An unarmed allocation matrix that

indicates the resources allocated to each virtual machine and the current

server assignments. The servers use the

server-assignment information to make

decisions regarding their environment.

Figure 1. The Dispatching Subnetwork

MED SERVER A VAR 1 ABLE SERVER D ARMED

O cnsT PATT dspT 1 ~~ 0 CNS 1 INST DSP 1 l

1 EXC 1 EXSV DSP 1 0 o CNS 1 INST EXC I 1 0 DSP 1 EXSV LOG I 1 0 LOG 1 LOGO DSP 1 G o EXC 1 INST LOG 1 1

1 EXC 1 RPLY LOG 1 G

0 EXC ) LOGD LOG 1 0

0 CNS 1 LOGO LOG 1 G

G MON 1 ACTV EXC 1 ! 0 MON 1 STAT EXC 1 0

Table 1. The PATT Pattern Matrix. Each row corresponds to a shared variable.

The third column indicates the

variable's surrogate name. The

second and fourth columns indicate

the two partners who share this

variable The first and fifth

columns indicate the protocol; they

contain a 1 if the variable is armed

on that partner's side and a 0 otherwise

VAR I ABLE partner ARMED

INST CNS I 1 EXSV DSP 1 1 INST LOG 1 G

RPLY LOG 1 t LOGD LOG 1 G

INST STA 1 G

RPLY STA 1 1 ACTV MON 1 I STAT MON 1 G

INST MON 1 G

RPLY MON 1 1

• « •

Table 2. The EXSV Drive-Data Matrix. Each row corresponds to a shared variable that

is shared by the server that owns this matrix. The first column

indicates the variable's surrogate

name. The second column indicates

the partner. The third column

indicates the protocol; a 1 indicates that the variable is armed for

this server.

The system designer specifies resource allocation constraints and objectives at the

configuration console. They are passed to the

dispatcher through the WHOS allocation matrix,

where an assignment algorithm chooses the

appropriate virtual machine for each server. Specific assignments can be guaranteed through

appropriate instructions from the configuration

console, thus, overriding the assignment

algorithm entirely or in part.

Multiple hierarchical dispatchers are rele- vant for cooperative processing among separate

physical machines. While local dispatchers

preside over the servers in a single physical

machine, overall dispatchers group together

whole machines full of servers. The advantage

of local dispatchers is that they can respond

to local reconfiguration needs without the

necessity for inter-machine communication.

172
The Command Subnetwork

Human interaction with a server network

involves both command and status subnetworks.

The command subnetwork provides not only the

exDert support system, with which commands can

be generated, it also provides the servers that

issue and monitor the performance of these

commands. In a production environment,

different expert support systems would be

provided for such users as production planners;

purchasing, sales, and marketing *9ents;

design, manufacturing, and quality-control

engineers; machinists; and repairmen. Each

expert support system must enable the user, within his scope of authority, to issue sets of

commands to the production system even though

he need not be cognizant of the form of these

commands, their sequence, or the precise set of servers involved. For example, when a salesman

enters a new order using his expert support

system, commands must be issued to adjust

production scheduling, stock levels, tooling

requirements, and a variety of °ther values

within the system, few or none of which he need

be aware of

The command subnetwork consists of console,

executor, and router servers. The console

server presents an expert support system with

which to generate tasks to be performed by

other servers in the network. These tasks must

be constructed, issued, and monitored for

completion, often a lengthy process. The

console server must remain responsive;

therefore, it cannot devote itself to such

time-consuming activities. Instead, the

console merely communicates with the executor

servers, constructing tasks for them to issue

and monitor.

A console server can control any number of

executor servers, each of which may serve any

number of other consoles. Conflicts are avoided through the use of a router server, which gets a request for an executor's service

from a console and returns the name of an available executor. This request might include

the nature of the work to be done. The router

would make its choice accordingly. For instance, the task might include specification

of a set of servers to be notified of an event. Not all executors are connected directly to all

other servers. Therefore, the router's choice

might be governed by the connectivity of the

available executors to the servers that will be

needed to perform the task.

Figure 2 illustrates the basic connectivity

of the command subnetwork. The console server

shares one variable with the executor.

INST: An instruction variable armed by the server and executed upon receipt.

The executor shares two variables with each

server that it controls.

INST: An instruction variable armed by the server and executed upon receipt.

RPLY: A reply variable, armed by the executor,

that describes the disposition of the

instructions sent earlier.

Figure 2. The Command Subnetwork

Figure 3 illustrates the connectivity of a multi-console, multi-executor command subnetwork containing a router server. The router

server shares one variable with each console

and another with each executor.

RQST: A request variable, armed by the router,

that is used by the console to request

and specify executor service and that is

used by the router to indicate to the

console a suitable available executor.

BUSY: An unarmed variable that is used by the

executor to indicate its availability to the router and that is used by the

router to reserve executor service when

assigning a console to the executor.

Figure 3. A Multi-Console, Multi-Executor Com- mand Subnetwork

The Status Subnetwork

The status subnetwork provides output

describing the status of the server network,

the engineering system being controlled or simulated, and the progress of the simulation

itself. Responses and completion of console

requests and commands are displayed. Comprised

of status-display servers and monitor servers,

the status subnetwork is in many ways the

complement of the command subnetwork.

The status-display server spends its time

in one of two states: formatting the relevant

data for display on its textual and graphical

displays or, in a dormant state, waiting either

for the arrival of some urgent message or for a regular timing interrupt to signal another

output formatting cycle. The time interval

between successive output cycles can be

regulated to correspond to server-network

activity so that excessive computation does not result from the unnecessary reformatting of

unchanged data, and so that the output refresh

does not cycle so slowly as to skip meaningful

data changes.

The monitor server provides a service for

the status display that is analogous to the

service provided for the console by the executor. The monitor is used to assist the status display so that it can refresh its data

quickly enough. It provides two general

categories of assistance: activity queries and

computational assistance.

173
Figure 4 illustrates the connectivity of three

the status subnetwork. The monitor shares two variables with each server that it monitors.

ACTV : A variable, armed by the monitored

server, sent by the monitor and checked

shortly thereafter for modification.

STAT: An unarmed variable that the monitored

server uses to indicate the nature of

its work to the monitor.

Figure 4. The Status Subnetwork

One useful type of status display data is

the activity or communication status of the servers in the network. An easy way to implement this check is to send out a query to each server along a communication variable

(ACTV) that triggers a trivial response

function. Thus, the monitor can send out all

of the queries, wait momentarily, and then

gather all of the responses. If a server is

busy with one function or is inactive, there

will be no response. (Placing the activity

variable highest in the search order for the servers assures a response from even the

busiest server, unless such a server is busy

responding to just one variable.) The other

type of status display data gathered by the

monitor is the status of each server (STAT).

This is gathered by means of an unarmed

variable shared between the monitor and each

server it monitors. It is the responsibility

of the server to keep the status variable

updated. For example, in a manufacturing

server network, a server controlling a numerical control f^C) machine might fill its

status variable with information regarding the

active workpiece, the active tooling, the

progress of the present manufacturing process,

or the expected time until completion.

The monitor shares one variable with the

status display.

STAO : A variable, armed by the status display,

that contains the status-display output

information that is ready to be displayed . Many forms of status displays that are extremely helpful to tne engineer require an excessive amount of computation. The monitor

server can be used to perform such computation.

Furthermore, if such computation is

particularly well-suited to a special-purpose

computing facility, the monitor server can reside in such a machine while the status

display resides elsewhere.

The Logging Subnetwork

In a typical server network, most servers

are neither consoles nor status displays.

Instead, they are tightly coupled networks of

automatic, remotely triggered software pro- cesses. For the purposes of result validation

and justification, software verification, error

isolation, and computational load balancing,

in-process logging is essential.

Events to be logged fall into categories

- engineering-application events,

- server-network communication events, and

7

s °ftware-implementation events. Within each of these categories events may be divided along another axis into:

- error reports,

- appropriate-event reports, and

- tuning-delay reports.

From the console, it is possible to selectively enable either specific logging events or whole categories of events for particular durations,

as needed. Each server has embedded logging calls in its software, and the disposition of these calls depends on the particular selection °f active logging events at the moment, as cnosen from the console server. Log messages accumulate locally and are only gathered and sorted by the logging server when requested. This procedure keeps the logging server from becoming overwhelmed with

an enormous number of individual events and creating a bottleneck for the entire server

network.

Figure 5 illustrates the connectivity of

the logging subnetwork. The logging server

shares one variable with each server to which

it provides logging services.

LOGO : An unarmed variable in which log data

accumulates locally. This variable is a FIFO stack that is filled by the server

and drained by the logging server asynchronously without any need for

interlocks

Figure 5. The Logging Subnetwork

The Timing Subnetwork

While servers are essentially asynchronous

software processes, engineering and management

applications frequently require that subsets of

the entire server network by synchronized.

Clock servers provide this synchronization

while offering control through a console and

visibility through a status display.

Figure 6 illustrates the connectivity of

the timing subnetwork. The clock server shares

one variable with each server that requires

timing services.

TIME: A doubly armed variable that is used by

the clock server to signal the start of

each timing interval and that is used by

the other servers in a simulation

environment to indicate their completion

of work for that interval. In a production environment, the clock sets the TIME variable in real time and the

variable need not be armed by the clock

because no information is passed back to

it by the servers.

174
Figure 6. The Timing Subnetwork

Both discrete-time and discrete-event

clocking can be provided for simulations.

Discrete time clocking can be provided by a straightforward incremental clock that sends

out a value over a set of doubly armed

variables (TIME). These variables trigger the servers into action. When each server has

finished its simulation of the interval, it

sends back a value to indicate completion. The

clock waits until all servers have responded,

then it begins the next interval. The clock is

in an excellent position to offer good,

computational, load-balancing logging messages

based on the order of the responses.

Discrete-event clocking can be provided by

a slight variation to the discrete-time

approach. When each server has finished the

interval, it calculates the expected time of

its next event and returns this information to the clock as its response. The clock chooses

the earliest response time as the next event

time. This approach, while appropriate for

simulation, is inappropriate for real-time

applications, which require access to a realtime clock and rely upon the server network's

computational ability to keep up with the rate

of actual events.

A SERVER KERNAL

As a member of a server network, each

server requires a kernal of software just to allow it to respond to stimuli, to communicate

with its assigned neighbors, and to be

dispatchable , commandable, monitorable,

loggable and dockable. An effort has been

made to isolate, refine, and standardize this

kernal of software so that other server software can be reliably built upon it.

Response to Stimuli

Each server has two fundamental states of

operation. It spends most of its existence in an idle state waiting for one of its armed

variables to awaken it when modified by a partner. In this idle state, a server consumes negligible computing resources. Whenever a server is not in the idle state it is either

checking which variable awakened it or, upon

isolating this variable, it is executing the

functionality associated with the variable.

Naming conventions are used to simplify the

association between shared variable names, their surrogate names, and the associated

functionality on each partner's side. APL2

allows the partners to refer to a shared

variable by two different names and to associate the two names with a third and

possibly different name called a surrogate, we have chosen to use the generic name (e.g.,EXSV,

PATT, or INST) as the common surrogate, since a server can have several variables with the same

surrogate name shared with different partners,

but it can only have one variable with each

actual name. We build the shared variable's

actual name by appending the partner's name onto the generic name. For example, if the

clock ( CLK1 ) shares a TIME variable with the

executor (EXC5), the clock calls the variable

lTMEEXC5 , the surrogate name is TIME, and the

executor calls the variable TIMECLK1. These

names are generated automatically from the EXSV

matrix.

The associated function calls are generated

in a similar fashion. The function executed

upon being awakened by an armed variable is

given the same name as the surrogate and is

called with the partner's name as the argument.

Continuing the example, if the TIME variable is

doubly armed, the clock executes the expression, TIME 'EXC5', upon being awakened, and the

executor executes the expression, TIME 'CLK1'.

By reducing the proliferation of the nearly

identical software that occurs when each armed,

shared variable has its own devoted function,

this use of the surrogate name as the function

name simplifies software development and

maintenance considerably. In addition, the

EXSV matrices can be based solely upon network

connectivity and arming protocols, with no need

to include function calls explicitly. Therefore, the PATT matrix can be constructed auto- matically from the connectivity and arming

information, and the EXSV matrices can be built » automatically from the PATT matrix. Thus, with

appropriately named sets of software, the server network can be sketched at the configuration console using the configuration ESS,

and automatically a functional server network

is created and begins processing. Likewise, at any time it can be halted, reconfigured, and

restarted.

Communication

Whenever a new or modified EXSV matrix is sent by the dispatcher, or when a new set of

software is loaded, the server must use the

EXSV matrix to establish communication

connections and protocols. First, however, it

must sever its old connections since they are

no longer applicable. A RETRACT function

retracts all of the previously shared variables

as governed by the old EXSV matrix. A SHARE

function shares all of the new shared variables

and arms them appropriately as governed by the new EXSV matrix.

Dispatchability

The dispatcher must always be able to command a server to load a new set of software

when it takes on a new role and to adopt a new

set of communication connections when the

network configuration changes. Therefore, each

server must be equipped with an EXSV function

that calls RETRACT to sever old connections and

that calls SHARE to establish new connections

and protocols. Each server also needs an INST

function that takes an instruction passed in

along the INST variable and executes it,

sending a suitable reply back to the

dispatcher

Commandability

The INST function that is used to execute

the dispatcher's commands is also used for the

executor's commands. In this case, the reply

is sent to the executor, but no change of

software is necessary because the INST function

receives an argument (as indicated above in the

175
section on "Response to Stimuli") and this

argument indicates which partner is used as the

destination for the reply. For example, if the

dispatcher (DSP2) sent the instruction, the

reply would be sent back through RPLYDSP2 . If the executor ( EXC7 ) had sent it, the reply

would automatically be sent through RPLYEXC7.

Monitor ability

The monitor uses the ACTV variable to check

whether each server is still functional. The

ACTV function merely modifies the value of the

variable (e.g., ACTVM0N4) to verify its operation. Whenever the server's software, performs

a computation that takes a significant period

of time, it modifies the unarmed status (STAT)

variable, since it will appear nonfunctional by

not responding to ACTV queries during that

period. If the server encounters a software

error and halts during the process, the value

of the status variable can be helpful in

locating the error.Logability

The LOGD variable is unarmed so there is no LOGD function in each server; however, each

server has calls throughout its software to a LOG function. The log function appends new log

messages onto the FIFO LOGD stack variable.

Clockability

Each server that participates in simulation

timing needs a TIME function, which notes that

the clock cycle has begun. In most cases, however, it merely toggles a bit in the

PREREQUISITES vector, a Boolean vector used to monitor the events that must occur before local

action can be taken. When the appropriate set

of bits has been set, the local action is

initiated and the bits are reset.

THE CONFIGURATION EXPERT SUPPORT SYSTEM

The configuration expert support system is

used by the network designer at the

configuration console to graphically specify

the servers in the network, the sets of

software they will use, their computational

resource requirements, and their communication

connections and protocols. This information is

used to build the variables WHOS and PATT,

which are shared with the dispatcher. The server network, which runs using existing sets

of software, is dispatched automatically based

upon information in these two variables. Four

facilities constitute the configuration expert

support system: the Network Layout Facility,

the Server Requirements Facility, the Server

Software Facility, and the Archival Facility.

The Network Layout Facility

This textual and graphical facility enables

the network designer to sketch a server network

by indicating servers and their communication

connections and protocols. Textually, the servers receive names (e.g., DSP2, EXC5 ) , and

the shared variables are identified by name

(e.g., TIME, EXSV ) . This facility provides

everything necessary to build the PATT matrix.

The Server Requirements Facility

This textual facility is used to indicate

the specific computing resource needs

associated with specific servers. For example,

loggers should be given sufficient disk storage

to maintain voluminous files containing past

logged events. Status-display servers should

be dispatched to virtual machines already

connected to workstations equipped with the

appropriate textual and graphical accessories.

During the software-development cycle, servers under careful scrutiny should be dispatched to virtual machines already connected to workstations so that their software can be

examined and possibly modified; other more

stable computational servers should be

dispatched to disconnected virtual machines.

In a multiple-computer server network that

spans two or more physical machines, care should be taken to assign servers to virtual

machines according to computational load. Certain servers, such as those doing detailed

modeling of system components, may require

considerable computational power during each

clock cycle. If possible, these servers should

be dispatched to run on the fastest machine

available, and the load on such machines should

be balanced so that they are not overloaded and

do not perform worse than other slower machines.

These requirements are used to form the

preliminary version of the WHOS matrix that is sent to the dispatcher. The dispatcher assigns

servers accordingly to available virtual

machines and fills in the actual assignments to form the completed version of the WHOS matrix,

which it shares with the dispatched servers.

The Server Software Facility

This simple facility enables the network

designer to graphically associate sets of server software with servers in the network.

This allows great flexibility since alternative

software sets can be compared in side-by-side

tests, and software modifications can often be

made and carefully integrated once a server network is already running, without necessarily

restarting the entire network. The software

data coupled with the computing needs

constitute the preliminary WHOS matrix.

The Archival Facility

This facility provides the ability to archive subnetworks or entire networks,

retrieve, copy, merge, and manipulate server networks at a macroscopic level. This is

extremely useful since few server networks are built completely from scratch. It is much more common to begin either with a "starter set" consisting of an appropriate infrastructure

configuration of utility servers already

connected, or with a previous server network to which slight modifications are to be made.

Often the only change is the version of

software set associations that need to be

updated to try a new experiment.

176
CONCLUSIONS

As engineering systems continue to increase

in scope and complexity, outgrowing existing

design and development tools', server networks

offer significant advantages as a software

methodology capable of providing large-system

integration tools. Their data-driven control

flow, their modularity, and their ability to be

dynamically reconfigured automatically based

upon graphical respecification, combine to make

server networks a promising mechanism with

which to address the challenges of cooperative

processing among the multitude of separate

elements of computing hardware that are making

their way onto today's factory floor. [3-5]

REFERENCES

1. Zeidner, L.E. and Y. Hazony, "Data-Driven

Control Flow of Interactive Engineering

Applications," submitted for publication to IEEE Transactions on Software Engineering .

2. Zeidner, L. , "Server Networks for

Engineering and Manufacturing," CIMTECH , Boston, MA, 1986, No: MS86-238.

3. Zeidner, L.E., et al., "A Dynamic

Simulation Manager for Robotics, ComputerIntegrated Manufacturing and Communications

Conference , Anaheim, CA, April 1985, No. MS85-336

4. Zeidner, L. , Hazony, Y. and A. Williams,

"An Expert System Generator," submitted

for publication to IASTED Control and

Computers , 1986.

5. APL2 Programming: Language Reference, IBM ISH20-9229 , Copyright 1985, International

Business Machines.

ACKNOWLEDGMENTS

The development of server networks enjoys

continual support and encouragement from Y. Hazony, CIDAM Director. Isolation and

refinement of the server kernal software was performed using implementations by M. Abt, A. Williams, and w. Labossier. G. D'Angelo

provided invaluable technical editing of this

paper

177
Analytic and Simula Lion Models

178
A Flow and Error Control Model with Phase Type Timeout Periods

by

S. Chakravarthy* , J. R. Pimentel 5*01

* and K. V. Ravi 5*0** GMI Engineering and Management Institute

Flint, Michigan 48502-2276

ABSTRACT

In this paper, we present a finite queueing model to study the

transmission process at a network station. This queueing model is

appropriate for modeling the flow and error control mechanisms at

the higher layers of the OSI reference model such as the transport

layer. The basic: assumptions of the queueing model under study are

as follows. Messages (packets) arrive at the source station

according to a Poisson process with rate X. At any time only N+l

new messages may be present in the system; one in the transmitting

unit (service) and at most N waiting in the primary buffer for

service. Any new arrivals are considered to be lost. The

transmission times of the messages are independent and identically

distributed random variables having a common exponential

distribution with parameter . A copy of the transmitted message

is kept in the secondary buffer and is removed once an

acknowledgement for it is received from the destination station.

The secondary buffer can store at most W messages. It is assumed

that the arrival of acknowledgements at the source station follows

a Poisson process with rate pt2 A transmitted message may require

a subsequent transmission if no acknowledgement for it is received

within a specified timeout period. We assume that the timeout

period follows a phase type distribution. The class of phase

type distributions includes many well known distributions such as

the generalized Erlang, hyperexponential, etc.

,

as special cases

and has a number of interesting closure properties. Using Markov

chain theroy, we discuss numerically stable algorithms to compute

various steady state system performance measures such as

throughput, expected numbers of messages waiting for

transmissions/retransmissions, acknowledgements. A number of

numerical examples are presented.

He Department of Science and Mathematics

Department of Electrical and Computer Engineering.

To be presented at the IEEE workshop on Factory communications,

to be held during March 17—18, 1987 at Maryland.

179
1. INTRODUCTION

In this paper, we consider a finite capacity queueing model

in which the messages (packets) arrive at the source station

according to a Poisson process with rate X. At any time at most

N+l new messages may be present; one in the transmitting

unit(service) and at most N waiting in the primary buffer. Any new

arrivals are considered to be lost. The transmission times of the

messages are independent and identically distributed random

vaiables having a common exponential distribution with parameter

. A copy of the transmitted message is kept in a secondary

buffer and is removed once an acknowledgement for it is received

from the destination station. The secondary buffer can store at

most W (window size) messages. It is assumed that the arrival of

acknowledgements at the source station follows a Poisson process

with rate /j2- A transmitted message may require a subsequent

transmission if no acknowledgement for it is received within a specified timeout period, which is assumed to be random.

This queueing model is well suited for modeling the flow and

error control mechanisms at the higher layers of the OSI reference

model. For example, at the transport layer message delays are

caused due to one or more of the following :

- variable size of the messages.

- different paths used by the messages.

- congestion in the network.

- availability of the protocol resources.

Hence, the model presented in this paper can be used to study

the performance of the transport layer protocols in which

retransmission timeout periods cannot be considered to be

constants. For more details on the transport layer, we refer the

reader to Stallings [7] or Tanenbaum [8].

180
We assume that the timeout period follows a phase type

distribution C PH-distribution) with representation Cgi,T) of order

m. PH-distributions and PH-renewal processes were introduced by

Neuts [4]. The class of PH—distributions includes many well known

distributions such as generalized Erlang, hyperexponential, etc.,

as special cases and has a number of interesting closure

properties. Also the class of PH-distributions is dense in the

class of all distributions on t0,oo). A detailed discussion of the

properties of PH-distributions and their uses in stochastic

modeling may be found in Neuts [5].

Using Erlangian timeout periods, Sethi and Ghosal [6]

considered a finite queueing model to study the transmission

process at a layer, such as data link layer, in which the timeout

periods are constants. While it is possible to have more than one

acknowledgement during a timeout period in their model, we assume

in our model that each message has its own timeout clock and is

triggered only when it is at the head of the queue in the

secondary buffer. However, with minor modifications our model can

be used to study various schemes of the acknowledgement process.

Their analysis of the model is completely different from ours. It

should be noted that the coefficient of variation for Erlang

distributions is less than 1 and hence their model is appropriate,

with higher order Erlangs, in cases where the timeout periods are

almost constants.

Thus, the system under study consists of a transmitting unit,

a primary buffer to store the incoming new messages; a secondary

buffer to store the transmitted messages waiting for an

acknowledgement and an auxiliary buffer to store the messages

that are to be retransmitted. The messages are transmitted on a first come first served basis. However, a message that requires a retransmission has a non-preempt ive priority over the messages

waiting in the primary buffer. The timeout period is active as

long as there is at least one message in the secondary buffer.

This system is described pictorically in Figure 1 below.

181
timeout clock

Primary Transmitting Secondary Acknowledgement

buffer unit buffer Crate

C size N ) C rate ) C size W )

Figure 1

The paper is organized as follows. In Section 2, we briefly

review the basic: definitions and preliminaries of PH-distributions

that are needed in the paper. The description of the Markov

process governing the system under study and the generator of the

Markov process for a specific case of the model are given in

Section 3. The algorithmic procedure for computing the varioussteady state performance measures of the model is discussed in

Section 4 and some numerical examples are given in Section 5.

182
2 PHASE TYPE DISTRIBUTIONS

In this section, we review the basic definitions and

preliminaries of PH—distributions that are needed in the paper.

We consider a continuous time parameter Markov chain CMC) on

the states < 1, 2, . .

.

,

m, m+1 > with infinitesimal generator Q

given by

ryO

0

Cl )

where the m x m matrix T satisfies < 0, for 1 < i < m. and T- • ij > 0 , for i ^ j. The column vector yO is such that

Te + T°= 0, and the initial probabi 1 i ty vector of 0 is g 1 veri

by C Q, om+i 5 , with q e+ am+l = 1 • We assume that T~1

exists soi that the states 1,2,..., m are all transient and

absorption into the state m+1, from any initial state, is certain.

The probability distribution FC.) of the time until absorption in

the state m+1, corresponding to the initial probability vector

C a, am+1 ) is given by

FCx) = 1 — q exp CTx) e, for x 2 0- C2)

Definition: A probability distribution FC.) on [ 0

,

co ) is a PH-distribution if and only if it is the distribution of the time

until absorption in a finite MC of the type defined in Cl). The

pair (.Q ,T) is called a representation of FC.).

The noncentral moments iX of FX . ) are all finite and given

by

vl = C-l) S T for i > 1 C 3 )

183
To avoid uninteresting assume that.

PH-distribution used in

complications, we assume that the

this work do not have a jump at the origin

so that F(Q) = cxm+:j = 0.

We conclude this section with some examples of

PH-distri butions.

Examples 1. The exponential distribution with parameter X is a PH-distribution with representation q

= 1 , T = - X.

2. The (generalized) Erlang distribution of order m with parameters X^ , X 2

,

. .., X m , has the representation

q = (1,0,. ..,0) and

-X 1

-XT =X2

-Xm-1

3. The hyperexponential distribution

F(x) = otj (1 — e J ) has the representation

J = 1

a = ( ,

. . . , om) and T = diag ( -Xj , . . . , —X m ^ > a diagonal

matrix

3. THE MARKOV PROCESS

The system outlined in Section 1 can now be described by a Markov process with [1 + 2W(N + 1) + mNV'(W-l)+ m(W^+N)] states. The

state space may be partitioned into N+4 sets of states and one

state. These are denoted by Aj , A2

, 0, 1, . .. , N, N+l and * and

their descriptions are given in Table 1.

184
Table 1

State Description

* The system is idle.

A1 = {(i '

, j) : 1 < i < N+l

,

0 < j < V-l >

There are i new messages in the system

including the one that is currently being

transmitted and j messages are waiting

for retransmission.

A2 = <Ci,j') : 0 < i < N,

1 < j < V >

There are i new messages in the system

and j messages including the one in

service are to be retransmitted.

0 = <(0, j',r,k) : 0 < j < V-l,

1 < r < V-j,

1 < k < m >

There is no new message in the system,

j messages including the one in service

are to be retransmitted, r messages are

waiting for acknowledgements and the

phase of the timeout period is in k .

i = <Ci ’ , j , r, k> : 0 < j £ V—2

,

1 < r < W—1—

j

1 < k < m > U

< < i , 0 , V , k ) : 1 < k < m > U

< C i , j

?

, r , k ) : 1 < j < V-l , 1 < r < V-j,

1 < k < m >

There are i new messages in the system,

j messages are waiting for retransmissions, r messages are waiting for

acknow 1 edgements and the phase of the

timeout period is in k. Either the

transmitter is blocked C because the*

secondary buffer is full) or a new

message or the one that needs a retransmission is in the transmitting

unit, for 1 < i < N.

N+l = < C N+l 7

, j, r, k) : 0 < j < V—2

,

1 < r < W-l-j,

1 < k < m >

There are N+l new messages, j messages

are waiting for retransmission, r messages are waiting for acknowledgements and the phase of the timeout

period is in k.

We have found it, somewhat convenient to order the sets of states

lexicographical ly

In the sequel the notation e stands for a column vector of

1

’ s , a unit column vector with 1 in the i—th position and 0

elsewhere and I an identity matrix of appropriate dimensions. The

symbol ® will denote the Kronecker product of two matrices.

Specifically L®M stands for the matrix made up of the block L^jM.

For more details on the Kronecker products, we refer the reader to

185
Bellman [11 or Marcus and Mine [31.

The Markov process with the infinitesimal generator Q is very

sparse. To display Q for the general case requires setting up a number of notations. However, we give below in Table 2 the

generator for the case N = 4 and V = 3. The general form of 0 ,t is

given in Chakravarthy [21. In the sequel, we shall discuss the

solution of the steady state equations and the algorithmic

procedures for this case only. The discussions for the general

case will be presented elsewhere.

4. ALGORITHMIC PROCEDURES

4.1 The Stationary Distribution

The stationary probability vector x of the generator Q is the

unique solution to the system of equations

xQ = 0 , xe = 1 ( 4 >

In view of the high order of the matrix Q, it is essential to use

its special structure to evaluate the components of x. We first

partition the vector x as x = ( x , , £2 > xo

» xi

' *2 ’

' ' ' ’ Xc

5 ^ according to the states A^ , A2

, 0, 1, 2, . .

.

,

5. Further

partitioning the components of x as

£

= c £-11 ’ **1 2

’

• •

• , l15 3 is a vector of order 3 ,

£

= c **20 ” *-21 * • • • ’ ^24 3 £2J is a vector of order 3 ,

x0 = c X01 ’ x02

• * • ’ x06 3 - x0j is a vector of order m,

xi = < y i’ z i5 ’ for 1 < i < 4

,

with

i2yi3' y i4 , y^j is a vector of order m.

il » z i2 ' z i3 , z ij is a vector of order m ,

and

186
Xg = ( Xg^ , Xg 2

y Xgg ) , Xg • is a vector of order m ,

we obtain from equation (4)

-X x* + ^2 xoi e = 0, (5)

X x + y 11 S1 + ^14^2 = C6)

x

j +1 + yj+i^s-t + y J+i ,4 s2 = °> C7)

for 1 < j < 3,

X £14 “ V± ^ig + X51 S1 + X53 S2 = °’ <8:>

-CX+^1 )i2 0

+ X01 R + X04 S1 + X06 S2 = °» C9)

X £2 j - (X+t»1 )S2> j+1 + z j+l,l Sl + z j+l,3 S2 = °’ (10)

for 0 < j < 2

,

X ^23 ~ ^1 ^-24 + Z41 S1 + Z4 3 S2 = °' (11>

( ^11 + ^20'>f1 l + x01 A3 + /J2 x02 eG = °' C12)

x02 A3 + ^2 x03 e ^ + ^l Cx04 + yil 5 = 0 (13)

X03 A3 +

^l Cx05 + y 12 ) = °’ C14)

C ^ll + ^20 )M2 + x02 T

°

q + x04 A1 + ^2x0Sefi = 0 - (15)

^03

^ + x05 A1 + ^l Cx06 + *14? = °> C16)

C ^ll + ^20 )M3 + *05 ^® + x06 A1 = (17)

187
C ^12 + + Xx01 + yil Al + ^yi2es = °’

Xxo2 + yi2 Ai + ^yi3eG + ^i Czn + y2i ) = °>

X x03 + y 1

3

A3 + aj1

<- z12 + y 22 ) ~

y 12 T

°a + y 1

A1 = °'

C ^12 + ^21 )M2 + Xx04 + Z11 A1 + ^2z12 es = °>

Xx05 + yiS^^ + Z1 2 A1 + ^l (z13 + y24 ) = °>

C ^12 + ^21')M 3 + Xx06 + z12 T°° + Z13 A1 = °>

C ^i3 + ^22 )M i + x yn + y21 Al + V2 V 2 2 BQ = °’

Xy 1 2 + y 22 A1 + ^2 y 23 ec + ^l Cz21 + y31 5 = °>

X y l 3 + y 23 A3

+ ^1 Cz22 + y 32 :> = °»

x yi4 + y22 T°^ + y24 Ai = 0

<i-13 +i22 :>M 2

+ Xzll + Z21 A1 + ^2 z22 e^

= °»

Xzl 2 + y 23^°^ + z22 A1 +

^l Cz23 + y 34 ) = °»

C ^1

3

+ ^22 )M3 + Xzl 3 + z22 T

°

s + Z23 A1 = °>

(£.14 + ^23 )M 1 + Xy 21 + y 31 A1 + jU2 y 32 e£! = °>

X ?22 + y32 Al + V2 y33 e® + ^l Cz31 + y41> = °>

( 18 )

( 19 )

( 20 )

(21 )

( 22 )

(23)

(24 )

( 25 )

(26)

(27)

(28)

(29)

(30)

(31 )

(32)

(33)

183
X y C 34 ) 23 + y 33 A3

+ ^1

z32 + y 42 ) = °’

Xy24 + y 32 T

°

£! + y 34 Al = °’

( ^14 + ^23 )M2 + Xz21 + z31 A1 + *J2 z32e£s = °'

Xz22 + + Z32 A1 +

^l Cz33 + y 44 ) = °’

C£.1 4 + £2 3 )M3 + Xz23 + z32 T

°

£3 + Z33 A1 = °'

C ^15 + ^24 )M1 + Xy 31 + y 41 Al + ^2 y 42 GG = °’

Xy 32 + y42 Al + ^2 y 43 ec +

^l (z41 + = °’

X y 33 + y 4

3

A4 + jUl (z42 + x52 ) = °’

Xy 34 + y 42 T°^ + y 44 A1 = °»

C ^15 + ^24 )M2 + Xz31 + z4

A2 + ^2 z42 es = °»

Xz32 + y 43 T°- + z42 A2 + ^l (z43 + x53 ) =

C ^1S + ^24 )M3

+ Xz33 + z42 T

°

£! + Z43 A2 = 0

Xy41 + *51 A2 + ^2 x52 ec = °’

Xy42 + x52 A2 = 0

Xy 44 + x52 T

°

£t5 + xS3 A2 = °’

( 35 )

(36)

(37)

(38)

(39)

(40)

(41 )

(42)

(43)

(44 )

(45)

(46)

(47)

(48)

with the normalizing equation

189
( 49 )

5

x* + Lj e + £2 e + ^ x

j

e = 1 • j=0

The coefficient, matrices appearing in the preceding equations are

given by

= T - (A+a^+^^I >

A2 = T — (/j^+^2^I >

= /u^(e^«>£t) , for 1 < i < 3 ,

R = e^®T° , (SO)

A^ = T — (A+yj2^"^ A^ — T 1 ,

= fj2 (ej«>e) + (ej +1 ®T°). for 1 < i < 2.

By using the special structure of the Kronecker products, which

arise as coefficient matrices in equations (6) through (11), we

can express and £2 in terms of quantities of smaller

dimensions. The final expressions are given as

Theorem 1 : Ve have.

-1

£•11 = (X+'Li i

) (Ax* + (J 2 y 1e ’ ^ll 70 + ^2 y 14 e ’ y \i T

° :>

'

-1

£l,j+l = CX+^1 5 CX ^1

j

+ (^2 y j+l ,l e ' y j+l,l T

°+^2 y j+l ,4 e ’ y j-H,4 T

° )]

for 1 < j < 3,

£1 S = C X

£*l 4 + C M2 xSl e ’ ^2 ^53^ + ^l 1^ ' *53

^

) ]/^l '

-1

£*20 ~ (A + a^) ( xoi T

° + ^2 x04 e > X04 T

° + ^2x06 e ’ X06 T° ^ > CS1 5

-1

£*2 , j+1 “ C

^2

j

+C ^2 z j+l ,l e ’ z j+l , l T

°+ ^2 z

j + l ,3 e > z

j + 1,3 T°^ ]

for 0 < j < 2,

1 90
1 X ^23 + ( ^2 z41 e * ^2 z43 e + *11^° ’ 5 3 ^1 - **24 “

The equations (5) — (48) will now be recast into a form which

makes them appropriate for solution by block Gauss-Seidel

iteration. Ve evaluate the three inverses

-1

E*2 = C <^+^2)1 ~ T ]

B4 = [ v2

I - T ] 1

,

that are needed for the iterative procedure. In Theorem 3. 2.1 [5]

it is shown that the above inverses do indeed exist and are

nonnegative. Below, we shall give a few sample equations that are

used for computational purposes. The remaining equations are not

listed due to space constraint and the interested reader can get

these by similar steps. From equations (3) and (12), we obtain

respectively

,

“I

B^ = t 2^1 — T 3 ,

-1

B3 = t (\+^2 :>I “ T ]

x* = ( 4i 2 x01 e)/X »

X01 = C ^2 x02ec + ( ^11 + ^•20 :>M

1 3B3

•

The purpose of this is to obtain numerically stable recursive

scheme. It should be noted that the quantities that appear on the

right-hand side of the above equations are all nonnegative and

thus lead to stable algorithms. Ve solved the above (recast)

equations by block Gauss-Seidel iteration. The successive solution

vectors are kept within a compact polytope by forcing them to

satisfy the normalizing equation (49).

191
4 , 2 Measures of System Performance

In this section we list a number of system performance

measures. The throghput Cy') of the system is defined as the rate

at which the messages are successfully transmitted. On noting that

y

- A P( primary buffer is not full ) or 7 = ^ P ( at least one

message in the secondary buffer ) , we see that

y = X [1- £15 e - £2 4 e -z4 e- y43e-x5

e 3= ^2 [ 1 ' x * “ e “ t2 e 3

The blocking probability

given by

4

<5=2 y J3 e j=i

of the transmitting unit is

The system idle probability is given by x . The other system

performance measures that are discussed in this paper are as

follows : Means and standard deviations of the numbers of messages

waiting in the primary, secondary and auxiliary buffers. These

quantities are easily computed from the stationary probability

vector x. For example, the mean number ( 77 ) of messages waiting in

the primary buffer is given by

3 ” = 2 k r k+l e - *- 2 k e - ^k3 e + zk e + ^+1 ® - >^+1.3® 1 k=l + 4 [ ^15 e + ^24 e + *43° + z4 e + Xge 3.

4 . 3 Accuracy Checks

Algorithms for general PH-distributions have a powerful

accuracy check, which is based on the following property. If T is

an irreducible, stable matrix with eigenvalue of maximum real part

-c < 0, and if a is chosen to be the corresponding left

eigenvector, normalized by ge = 1 , then q is a positive vector and

the PH-dist.ri bution with the representation (q,T) is the

exponential distribution with parameter c.

192
First, we obtained the numerical solution for the exponential

distributions in their simple form. Next, we implemented the

general algorithm, but chose the representation of the

PH—distribution so that it was in fact exponential. The general

algorithm does not utilize this fact in any manner, but the two

sets of numerical results agreed very much.

A number of other accuracy checks are also available. For

example, the result of Theorem 2 below may be used to check

whether the computed quantities satisfy it. Ve conclude this

section with the following theorem.

Theorem 2: Ve have,

X [ ^lje + £2,j-l e + xj-l e 1

= vi< I yjke > + tj2r yji e + >'j4 e ] +

’'ji T° + >'j4 T°- 1 5 J 5 4 - k=l

k^3

X[ y<il e + ^42 e + y44 e 1 = ^l xS e + ^2 [x51 e + ] + + ^-sT0 -

Proof : We shall outline the proof for the first two equations as

the others are similar. Postmul tiply ing each one of the equations

C12) through (17) by e and adding the resulting equations and

using equations (6) and (9), we get the equation for j=l . Now

postmultiply ing each one of the equations (18) through (24) by e and by adding the resulting equations and using the equation

obtained for j=l , and equations (7) and (10), we get the equation

for j=2.

5. NUMERICAL EXAMPLES

In order to test the feasibility of the algorithm proposed in

this paper, a FORTRAN code was developed and tested on a large

number of examples. In this section we shall discuss a number of

examples.

193
The purpose of the first example is to show how the system

performance measures are affected due to a change in the

probability distribution of the timeout period.

Example 1 . Here, = ^2 = ^ 00 » and X = 50. We consider four

PH-distributions whose representations Ca,T) are given as follows:

T

I) Erlang of order 10 with parameter 100.

II) Exponential 1 -10

III) General PH- CO. 3, 0.7)

distribution

IV) Hyperexponential CO. 99,0. 01)

-10 7.4

5.75 -25

-25 0

0 -1/6.04

While each of these distributions have the same mean 0.1, the

distributions are qualitatively very different. The variances of

—3 —3 —3 these distributions are respectively 10 , 10x10 , 14x10 , and

Q 722.8x10 . The performance measures: throughput ( , blocking

probability C6), idle probability Cx ) , expected number of

messages in the primary, secondary, and auxiliary buffers denoted

respectively by EPB, ESB, and EAB and their standard deviations

denoted respectively by SPB, SSB, and SAB are given in table 3.

TABLE 3

Performance measures for four systems with different timeout

distributions all having the same mean.

I II III IV

r 48. 6259228 48. 2128816 48. 0443538 47. 4467966

<5 0. 0567970 0. 0554344 0. 0551878 0. 0530324

*

X 0. 2486512 0. 2254771 0. 2172642 0. 1932486

EPB 0. 5881190 0. 6819047 0. 7178059 0. 8346006

SPB 1 . 0273865 1 . 1007878 1 . 1265698 1 . 2032027

ESB 0. 7880407 0. 7756025 0. 7714101 0.7543386

SSB 0. 9652667 0. 9567162 0. 9543451 0. 9427258

EAB 0. 0000646 0. 0242191 0. 03406010 0. 0668052

SAB 0. 0080358 0. 1564650 0. 18576732 0. 2603967

194
Examining table 3 we observe that as the variance of the

timeout period increases, the performance measures throughput, the

blocking probability and the idle probability appear to decrease,

as is to be expected.

The objective of Examples 2 and 3 is to see how the various

performance measures are affected by considering two

PH-distributions having different means and variances but with the

same coefficient of variation and by varying X.

Example 2. Here = ^ — 100. The timeout period has the

PH-representation Ca,T) given by

a = CO. 999, 0.001), T =

- 100

0

0

-1/90. 01

—

The mean and coefficient of variation for this PHdistribution are respectively 0.1 and 40.241521.

Exanple 3. Here = y^ = 100. The timeout has the PH

representation Ca,T) given by

q = CO. 999,0. 001)

,

T =

-1 . 0000003446

-1 . 1109876637x10-4

The mean and coefficient of variation for this

PH—distribution are respectively 10 and 40.241521.

The throughput, idle probability, and blocking probability

are given in table 4.

While the distribution considered in Example 2 can be

visualized as producing timer expirations of short periods with

occational bursts of timer expirations of intermediate periods,

the distribution in Example 3 can be visualized as producing timer

expirations of relatively large periods with occational bursts of

intermediate periods.

195
TABLE 4

Performance Measures for Examples 2 & 3 having

with different, means and variances.

PH—distributions

Throughput Idle prob. Blocking prob.

X/^ Ex. 2 Ex. 3 Ex. 2 Ex. 3 Ex. 2 Ex. 3

0. 1 9. 9991353 9. 9999041 . 7200927 . 8090761 . 0002287 . 0001778

0.

19. 9236208 19. 9938591 . 4801913 . 6380620 . 0030376 . 0024916

0.

36. 4542506 39. 6053605 . 1531751 . 3554163 . 0247622 . 0287528

0.

6

43.7547136 56. 3212742 . 0368445 . 1616858 . 0477109 . 0917169

0.

8

45. 8197577 66.7187442 . 0093176 . 0610520 . 0585978 . 1592081

1 .46. 3950986 71 . 4762545 . 0027565 . 0213765 . 0630283 . 2028236

1 .46. 5799154 73. 3532804 . 0009464 . 0076382 . 0649292 . 2250017

1 .46.6479653 74. 0819515 . 0003670 . 0029046 . 0658163 . 2355487

1 .

6

46. 6761243 74. 3775309 . 0001570 . 0011868 . 0662634 . 2406199

1 .

8

46. 6888927 74 . 5050929 . 0000726 . 0005195 . 0665036 . 2431535

2. 0 46. 6952100 74 . 5637168 . 0000359 . 0002419 . 0666398 . 2444774

5. 0 46. 7306207 74. 6258908 . 0000000 . 0000001 . 0668685 . 2462757

Examining Table 4, we observe that

- by considering PH-distributions having the same coefficient of

variation and increasing the mean significantly from 0.1 to 10,

the throughput and idle probability appear to increase

accordingly

- the blocking probability however appears to decrease for values

of X up to about 38 and appears to increase for X greater than

38. The result seems to be obvious for large values of X.

However, for small values of X, the result is not that obvious.

- in general performance measures follow expected behavior with

respect to message arrival rate. For example, increasing X, the

throughput increases asymptotically towards its saturation

value.

296
6 . REFERENCES

[1] Bellman, R. (1960). " Introduction to Matrix Analysis”,

McGraw Hill, New York, NY.

[23 Chakravarthy , S. (1986). " A Stochastic Model for a Computer

Communications Network”, working paper. Department of Science

and Mathematics, GMI Engineering and Management Institute.

[3] Marcus, M. and Mine, H. (1964). "A Survey of Matrix Theory

and Matrix Inequalities”, Allyn and Bacon, Boston, MA.

[43 Neuts, M. F. (1975). “Probability Distributions of Phase Type”

Liber Amicorum Prof. Emeritus H. F»lorin, Department of Mathematics, University of Louvain, Belgium, 173-206.

[53 Neuts, M.F. (1981). "Matrix-Geometric Solutions in Stochastic

models- An Algorithmic Approach”, The Johns Hopkins University Press, Baltimore, MD.

[63 Sethi, A. S. and Ghosal, R. (1986)." An analytical model for

window mechanisms with error control", F’roc . INF0C0M86, pp

162-171

[73 Stallings, V/. (1985). ” Data and Computer Communications ”,

MacMillan Publishing Co. , New York.

[83 Tanenbaum, A. S. (1981)." Computer Networks ", Prentice Hall,

Englewood cliffs. New Jersey.

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TABLE
Setting Target_Rotation_Times in an 802.4 Network

Mangala Gorur

Alfred C. Weaver

Dept, of Computer Science

University of Virginia

Charlottesville, Va 22903

The IEEE 802.4 token bus protocol for local area networks is emerging as a popular standard for fac- tory automation applications. The enormous interest in this protocol is due to its various distinguishing

features, one of which is the provision of a prioritized mechanism to access the transmission medium. The

implementation of the priority scheme by any station in an 802.4 network is optional. If a station imple- ments the priority scheme, then the objective is to allocate bandwidth to lower priority frames only after transmission of higher priority frames. Each of the lower three access_classes is assigned a target token

rotation time. This goal rotation time is different for each access_class and is known as the Target_Rotation_Time (TRT) of an access_class. The TRT setting at each access_class decides the amount

of time available to serve messages of that priority. The IEEE 802.4 token bus standard specifies only the maximum values for the Target_Rotation_Times. Hence it is essential to know how to set the TRTs to achieve a desired priority scheme.

In this paper we present an analytic model which can be used to solve for the TRT settings which

will implement a user-defined priority scheme. For example, suppose that the user wants normal service at Time_Available access_class until network throughput rises to a, with 0< a< 1. Then letting N be the number of stations in the network, XT be the transmission time of a token, XM be the transmission time of a message, and \s , , and be the arrival rate in packets per second at the Synchronous,

Urgent_Asynchronous, Normal_Asynchronous, and Time_Available access_classes respectively, and

using the analytic model, the effective value of the TRT^ is.

eff (TRTla ) = N-Xt

l - N XM W +\ia +^na + Ka )

The effective value can be higher than the actual TRT setting by up to one message transmission time. This

is according to the specifications of the standard that the transmission of a message once started will go on

to completion even if it runs past the expiration of the timers. The analytic predictions show close agreement with the simulation results. Thus, given a user- defined performance target (eg., “Time_Available access_class to receive normal service until network

throughput exceeds a”), we can calculate the TRT settings which will achieve that goal.

199
1. Introduction

The 802.4 token passing access method offers four levels of service called access_classes and messages can be transmitted at any of the four priorities. The four access_classes in descending order of

priority are Synchronous access_class, Urgent_Asynchronous access_ class, Normal_Asynchronous

access_class, and Time_Available access_class. The implementation of the priority scheme by any station is optional. By suitably setting the variables associated with the implementation of the priority

scheme, it can be ensured that this scheme gives preference to frames of higher priority. The amount of

time available to transmit messages from the Synchronous access_class is decided by the value of the

variable High_Priority_Token_Hold_Time (HPTHT). The amount of time available to transmit messages from each of the lower three access_classes is decided by the value of the Target_Rotation_Time

(TRT) setting at each access_class. This setting is different for each access_class.

The TRT setting at each access_class limits the token cycle time at each access _class. In this paper

we present an analytic model that can be used to calculate the values of TRT settings to obtain optimum

service at an access_class until a user-defined throughput is reached.

1.1. An analytic model for determining TRTs

A problem faced by token bus network designers and operators is the selection of

Target_Rotation_Times which will implement a desired priority scheme. It is possible to determine the

individual TRT setting of an access_class so as to obtain maximum possible service at an access_class

(that is, on the average a message gets transmitted within one token cycle) at an access_class until network throughput reaches or exceeds a user-defined threshold. For purposes of analysis presented in this

paper, throughput has been defined as the number of data bits transmitted (including all address and framing bits) per bit time expressed as a fraction of the bus capacity.

200
1.1.1. Definitions

An active access_class at a station is termed a server. The following notation is used

XT = duration of a token transmission (seconds/token transmission).

s = the Synchronous access_class.

ua = the Urgent_Asynchronous access_class.

na = the Normal_Asynchronous access_class.

ta = the Time_Available access_class.

Xac = the mean message arrival rate at each server of an access_class,

where ac is s, ua, na or ta.

S s the set of all Synchronous access_class servers.

UA = the set of all Urgent^Asynchronous access_class servers. NA = the set of all Normal_Asynchronous access_class servers.

TA = the set of all Time_Available access_class servers.

R = set of all distinct servers on the logical ring

= (S, UA, NA, TA).

Xx = the mean message arrival rate at server x zR (messages/second).

HPTTIT = the High_Priority_Token_Hold_Time.

TRTX = the Target_Rotation_Time at server x z(UA , NA , TA ).

TRT = the Target_Rotation_Time at each server of an access_class,

where asy is ua, na, or ta.

TCTx i = the time from the end of the service period

until the beginning of the i

,h service period (seconds)

= token circulation time as seen by server x on the i

lh token cycle,

i.e., the time interval for which the token is away from x.

TCX I s time from the end of the (/-l)*' service period until

the end of the i

th service period (seconds).

= token cycle time as seen by server x on the i

!k token cycle.

201
AXI = the number of message arrivals at* during the interval from the end of

the service period until the end of the i th service period.

Qxi s the number of messages enqueued at server x

after the (z-l)f* service period.

/ (n ) = time required to transmit n messages (octet_times).

t s residual time in the token_hold_timer (octet_times).

eff (t) = the effective amount of time which a server can transmit messages.

= max(0, t +/( 1) - one octetjime) octet_times.

TSZ i = duration of the i

,h service period.

TSac s average service time available to every server of an access_class,

where ac is s, ua, na or ta.

202
1.1.2. Assumptions

In the following section, an analytic expression for the token cycle time of an 802.4 network is

derived. The derivation is based on the following assumptions about the characteristics of the network:

1) The protocol management overhead is assumed to be negligible

2) Stations are permanent members of the logical ring.

3) Each station has traffic at all four access_classes.

4) Message arrival at each station follows a Poisson process.

5) Messages from all the servers are of constant length lM bits and take XM seconds for transmission.

6) Token is of length 1 12 bits for a 10 Mbps bus and takes XT seconds for transmission.

7) All Synchronous access_class servers have the same HPTHT setting.

8) Each station has HPTHT set to the maximum allowed value (52.43 msec for a 10 Mbps bus). Hence

messages from the Synchronous access_class normally are transmitted on the same token cycle as they

arrive.

9) All servers of the same priority have identical traffic.

10) All servers of the same priority have the same TRT settings.

11) The TRTs are set in the order TRT^ > TRTM > TRT,a.

12) Each active access class has N servers.

203
1.1.3. Service time

The amount of time available to any server on any token cycle is limited. A station starts transmitting messages when there is residual time in the token_hold_timer. If x is a synchronous server then the

average service time of x can be expressed as

TSX = min(eff ( HPTHT ), f (Qx + Ax )). (1)

where Qxis the average queue length at x, and Ax is the average number of arrivals at x during an average token cycle. If x is an asynchronous server then the average service time of server x can be

expressed as

TSr =< min(ejf(TRTx -TCTx ), f (Qx + AX )) 0

for TCTX < TRTX

for TCTX > TRTX

( 2)

where TCTX xzTA is the average token circulation time at every server at the Time_Available class, Qx

is the average queue length at x, and Ax

is the average number of arrivals at x. Refer to [Gorur 86] for a

derivation of token cycle time and token circulation time. Assuming an average arrival rate of messages

Xx at server x, the number of messages that have arrived while

a) the token is circulating around the ring and

b) x is being served on the i !h token cycle is

= \x TCTX ' i +\x -TSZti = Xz TCX I

. (3)

Hence the average number of messages that have arrived at any server x, x e (,S , UA, NA,TA ), during an

average token cycle can be expressed as

Ax = Xx TC. (4)

The time taken to transmit the average number of messages that have arrived at a server x can be

expressed as XM -Xx TC

,

from equation (4). As long as the time to transmit all the messages that have

arrived is less than or equal to HPTHT (if x is a synchronous server), or less than or equal to

(TRTiXSy - TCT) if x is an asynchronous server, the average service time at server x can be expressed as

204
TSx =f(Qx +Ax ). (5)

This is true for TC <HPTHT and TC <TRT^ . In the region TC < HPTHT it can be assumed that all the

messages from the Synchronous access_class get transmitted during the same token cycle. Also in the

region TC < TRT^ it can be assumed that all the messages from that access_class and from

access_classes of higher priority that have arrived during a particular token cycle get transmitted during

the same token cycle. Hence Qx can be considered to be negligible. Eliminating Qx from equation (5)

and substituting the value of Ax from equation (4), the average service time can be expressed as

TSx =fSac =Xm

-\

ac fC. (6)

Traffic from each access_class gets served in the order of priority, starting from the Synchronous

access_class, and if time is available, lower access_classes also get service. The average service time at

each of the lower three access_classes can increase with an increase in offered load from that

access_class until the average token cycle time equals the TRT of that access_class. Thus the throughput

which results in a token cycle time of TRT for an access_class corresponds to the throughput until which

maximum possible service can be obtained at that access_class.

1.1.4. Determining TRTs

If the designer determines that the Time_Available access_class should receive maximum possible

service until network throughput reaches a, 0<a<l, then

N 'Aa-lM

a= =—

CTCa

(7)

where Aa

is the mean number of messages transmitted per station per token cycle at a throughput of a,

and TCa is the average token cycle time at a throughput of a. Solving for Aa,

a C TCa n-im

( 8 )

From the discussion in section 1.2.3, it follows that maximum possible service at an access_class can be

obtained in the region where TC < TRT^y

. Hence the average number of messages that arrive during the

time interval TCa (region where TCa < TRTta ) should equal the mean number of messages transmitted in

205
this time interval, i.e., Aa. Hence

Aqj (^y ^ua ^ruz ) TC

Substituting for Aa in equation (8) we obtain the sum of Xs to be

a C

( 9)

Xs ^nn a AM,M

(10)

Many combinations of individual message arrival rates will yield the unique sum satisfying the above

equation. Since the token cycle time can be expressed as

TCa = N -Xr +N -XM

- A0 (ID

substituting for A,* from equation (9) yields

TC a - N XT + N -XM iK + ^-ua + ^na + Ka) 'TC a. ( 12)

Since the Time_Available access_class should receive maximum possible service until throughput

reaches a, it follows from the discussion of service time in the previous section that the token cycle time

at this throughput decides the TRT of the TimeAvailable access_class. Hence solving for

eff (TRTla ) = TCa ,

eff (TRTta ) = NXt

l - N XM -(ks + X^ + Xw + Xta ) (13)

which is the effective value of the Time_Available Target_Rotation_Time.

As the offered load increases, increasing the network throughput beyond a, the token cycle dme

increases beyond the TRT of the Time_Available access_class. Hence service to the Time_Available

access_class class gets reduced and eventually drops to zero when TCT[a equals TRTta as given by equation (2). Hence the traffic from the Time_Available access_class does not contribute to the network

throughput anymore. Hence the load carried by the network is contributed by the upper three classes

alone.

206
If the designer determines that the service at the Normal_Asynchronous access_class should reach

a maximum when the network throughput reaches p, 0 < a < P < 1, then

N 'Ap -lM

c-rcp

(14)

where Ap is the mean total number of messages transmitted per station per token cycle at a throughput of

p. Solving for Ap,

ApP-C TCp

N'lM

(15)

The mean number of message arrivals during the time interval TCp should equal the mean number of

messages transmitted during that time interval, i.e., Ap. Hence

Ap - + "Kna )’ TCp. (16)

Substituting for Ap in equation (15) we obtain the sum of message arrival rates of the upper three

access classes to be

"H 4- A _l_c_

N -lu ' (17)

Of course, many combinations of individual message arrival rates will yield the unique sum in equation

(17). The token cycle time in this region can be expressed as

TC$ = N-Xt +N Xm -Ap. (18)

Substituting for Ap from equation (16) yields

TCp = A -XT + N XM ( Xs + Am + A^) TCp. (19)

Since service at the Normal_Asynchronous access_class should be maximum at a throughput of p, it follows from the discussion of service time in section 1.2.3 that the token cycle time at this carried load

decides the TRT of the Normal_Asynchronous access_class. Solving for eff ( TRTna )~ TCp ,

20 7
eff{TRTna ) = N-Xt

l - N XM (Xs +kua + ‘kna )

( 20)

which is the effective value of the Normal_Asynchronous Target_Rotation_Time.

As the offered load increases, increasing the network throughput beyond (3, the token cycle time

increases beyond the TRT of the Normal_Asynchronous access_class. Hence service to the

Normal_Asynchronous class gets reduced and eventually drops to zero when TCTw equals TRT^ as

given by equation (2). At this point traffic from the Normal_Asynchronous access_class does not contribute to the network throughput anymore, so the load earned by the network is contributed only by the

upper two classes.

If a designer determines that the service to the Urgent_Asynchronous access_class should be maximized when the network throughput is y where 0<a <

P < y< 1, then

Y = N

C TC

M

( 21 )

Using logic similar to the previous discussion, the number of messages transmitted during the time interval TCy

is

yC TC,

A7 =

' N L

( 22)

and

Ay— (Xs + -TCr

(23)

Then

K + ^ua - J f

• (24) N -Im and

TCy = N -XT + N -XM ‘Ay, (25)

and

208
TCy = N -XT + N XM iK+Ka) TCr ( 26)

Now solving for eff ( TRT^ ) = TCy

,

eff(TRTua ) = N-Xt

\ — N XM -(Xj + X^

)

(27)

which is the effective value of the Urgent_Asynchronous Target_Rotation_Time.

As the offered load increases, increasing the network throughput beyond y, the token cycle time

increases beyond the TRT of the Urgent_Asynchronous access_class. Hence service to the

Urgent_Asynchronous class gets reduced and finally drops to zero when TCT^ equals TRT^ as given by

equation (2).

209
1.2. Results

In order to verify the analytic predictions, a 32 station network with each station offering service at

all four access_classes, with identical traffic at all servers of an access_class, was simulated using the

simulation package reported in [Summers 85] with parameters:

Bus capacity = 10 Mbps,

Size of data frames = 272 bits including framing,

Token =112 bits, XT = 16.2 microseconds (including 50 bit times of propagation delay),

N = 32,

XM = .0000272 seconds = 27.2 microseconds, 0

XT = .0000162 seconds = 16.2 microseconds.

The individual message arrival rates were assumed such that they satisfied the conditions derived in

the previous section. From the effective values of TRTs calculated using the equations derived in the

previous section, the actual values of TRTs are given by (eff (TRT^y ) -/ (1) + 1 octetjirne ), where f(l)

is equal to 34 octet_times, eff ( TRTla ) is 790 octet_times, eff (TRT^) is 852 octet_times, eff (TRTlia ) is

1157 octet_times, and TRTta is 0.6056 msecs, TRTW is 0.655 msecs, and TRT^ is 0.8992 msecs.

The TRTs were set to the actual values shown above. A series of network configurations were

simulated to show the variation of different parameters with throughput. The individual message arrival

rates at each access_class have been varied from one simulation to another to increase the total offered

load.

It can be observed from Figure 1 that average service time at the Time_Available access_class

increases until a throughput of about 0.18. As the throughput increases further, the average service time

starts falling and eventually drops to zero. From Figure 2 it can be observed that the average delivery

times are reasonably low until the network throughput exceeds 0.18. This shows that most messages are

getting transmitted on the same token cycle as they arrive. Beyond a throughput of 0.18 the token cycle

time has exceeded TRT,a

. This can be observed from Figure 7. The Time_Available class is getting

reduced service and messages are suffering higher queueing delays, so the delivery times are increasing

210
exponentially in this region. This can be observed from Figure 2.

The TRTta value was calculated so as to achieve maximum possible service until a throughput of

0.18. The results of the simulation agree with the expected values very closely.

It can be observed from Figure 3 that the average service time at the Normal_Asynchronous

access_class increases until a throughput of about 0.24. As the throughput increases further, the average

service time starts decreasing and drops to zero. From Figure 4 it can be observed that the average

delivery times are reasonably low until the network throughput is 0.24. This indicates that most messages

are getting transmitted on the same token cycle as they arrive. Beyond a throughput of 0.24 the token

cycle time has exceeded TRTw . This can be seen from Figure 7. The Normal_Asynchronous class is getting reduced service leading to higher queueing delays, and the delivery times rise exponentially in this

region. This can be observed from Figure 4.

The TRTW value was calculated so as to achieve maximum possible service until a throughput of

0.24. From the above discussion it follows that the results of the simulation agree with the expected

values very closely.

It can be observed from Figure 5 that the average service time at the Urgent_Asynchronous

access_class increases until a throughput of about 0.44. As the throughput increases further, the average

service time starts decreasing. From Figure 6 it can be observed that the average delivery times are reasonably low until the network throughput is 0.42. This indicates that most messages are getting transmitted on the same token cycle as they arrive. Beyond a throughput of 0.42 the token cycle tune has

exceeded TRT^ and hence the Urgent_Asynchronous class is getting reduced service. This can be

observed from Figure 7. Messages are suffering higher queueing delays and delivery times rise exponentially in this region, as seen in Figure 6.

The calculations showed that maximum possible service can be obtained at the

Urgent_Asynchronous access_class until a throughput of 0.44. The same is illustrated by the simulation

results.

211
Average

service

time

(microsecs)

Throughput

Figure 1 Average service time for TimeAvailable class

212
Average

delivery

time

(msecs)

Throughput

Figure 2 Average delivery time for TimeAvailable class

213
Average

service

time

(microsecs)

Throughput

Figure 3 Average service time for NormalAsynchronous class

214
Average

delivery

time

(msecs)

Throughput

Figure 4

Average delivery time for Normal_Asynchronous class

215
Average

service

time

(microsecs)

Throughput

Figure 5 Average service time for Urgent Asynchronous class

216
Average

delivery

time

(msecs)

Throughput

Figure 6 Average delivery time for Urgent_Asynchronous class

217
Token

cycle

time

Throughput

Figure 7 Token cycle time

218
1.3. Concluding Remarks

A static token passing bus has been studied and it has been shown that TRT settings play a crucial

role in deciding the service time at the lower three access_classes. Optimum service can be obtained at an

access_class until network throughput rises to the point that token cycle time equals the

Target_Rotation_Time of that access_class. The analytic model developed in this study can be used to

determine TRTs for any desired throughput range for a network configuration.

In this study all participating stations were assumed to be active and with identical message mterarrival rates at all stations at an access_class. However the designer of a local area network has to take into

consideration the communication traffic at each station at each access_class as it may be specific to each

station and each access_class. Also the traffic from each station can be very bursty in nature. Nevertheless this study helped us gain a better understanding of the impact of certain parameters critical to the

implementation of the priority feature.

219
Bibliography

[IEEE 1985]

Token-Passing Bus Access Method and Physical Layer Specifications, EEEE Standard 802.4-1985. [Gorur 86]

Mangala Gorur and Alfred C. Weaver, Priorities and Dynamic Ring Membership in an 802.4 Net- work, Master’s thesis, DAMACS Report No. 86-19, University of Virginia, Department of Computer Science, July 86. [Summers 85]

Catherine F. Summers and Alfred C. Weaver, Performance Studies of the IEEE Token Bus, Master’s thesis, DAMACS Report No. 85-06, University of Virginia, Department pf Computer Sci- ence, May 1985. [Weaver 85]

Catherine F. Summers and Alfred C. Weaver, The IEEE 802.4 Token Bus - An Introduction and

Performance Analysis, Computer Science Report No. TR-85-19, University of Virginia, August 26, 1985.

220
PERFORMANCE MODELING OF IEEE 802.4 TOKEN BOS*

by

Anura P. Jayasumana, Member, IEEE

Department of Electrical Engineering

Colorado State University

Fort Collins, CO 80523

and

P. David Fisher, Senior Member, IEEE

Department of Electrical Engineering and Systems Science

Michigan State University

East Lansing, MI 48824

ABSTRACT

The priority mechanism of IEEE 802.4 token bus allocates the channel

bandwidth among different priority classes of messages by means of a set of

timers at each station. An analytical model is presented, which relates the

throughput of each priority class of messages to the traffic intensities of

different classes, the target token-rotation times and the high-priority

token-hold time. Network is assumed to be symmetric with respect to the

parameters and traffic distribution among nodes. Simulation results are used

to evaluate the accuracy of the model. The model can accurately predict the

behavior of token bus networks with a large number of nodes.

This research was supported in part by NSF under Grant No. ECS8515824.

221
I. INTRODUCTION

The broadband token-passing bus scheme of IEEE 802.4 has emerged as the

standard for factory Local-Area Networks due to several major reasons such as

its deterministic nature, reliability at peak loads, and its support of a

message based priority mechanism. The priority mechanism of the IEEE 802.4

token-bus standard allocates the channel bandwidth among different priority

classes of messages by means of a set of timers at each station [1]

Typically, token-bus networks are used to connect a large number of devices

together. Simulation of the IEEE 802.4 priority mechanism under such

conditions requires a large amount of computer time and storage. Therefore,

it is desirable to develop analytical models to predict different

characteristics of this scheme. The complexity associated with the tokenpassing bus priority scheme makes it extremely difficult to model it using an

analytical approach. But once such a model is developed, it usually offers

results for a variety of load conditions and parameters at a low cost.

Furthermore, analytical models show qualitative relationships between

parameters and variables more clearly than do models developed by other

approaches. Analytical approaches have successfully been used to model the

IEEE 802.4 token bus, only with a single class of messages [2,8,9,10]. These

models have been used to investigate performance of bus networks with respect

to variation of parameters such as offered load, message length, signal

propagation delay, token size and token holding strategies. Analytic models

have also been used to address starvation and stability issues related to the

IEEE 802.4 priority scheme [6,7]. This paper presents an analytical model,

which predicts the throughput characteristics of token-passing networks that

implement the IEEE 802.4 priority mechanism.

222
The model is based on the law of large numbers, which can be stated as

follows [5]: A large population of demands will present a total load to a

system of resources, which is equal to the sum of the average requirements of

each individual demand, and further this load is a highly predictable

quantity, A model developed using this law is thus capable of predicting the

performance of networks with a large number of nodes.

Next section reviews the priority option of IEEE 802.4 standard. In

Section III we consider the behavior of the priority scheme under specific

input conditions. Section IV describes the analytical model. Simulation

results are used to verify the analytical model in Section V. Limitations of

the model as well as further work currently underway in this area are

outlined.

II. IEEE 802.4 Token-Passing Bus Access Scheme

In the token-passing access method, a token controls the right to access

to the physical medium. When a station receives the token, it becomes

temporary master of the network and has the right to transmit one or more

messages subject to the constraints imposed by the priority scheme discussed

below. If the node does not have any messages to send, or when its tokenholding time expires, the token is passed to the next station. This creates a

logical ring around which the token passes. Hence, in the steady state, there

will be alternate data and token transfer phases. The logical ring maintenance

responsibilities such as ring initialisation and lost token recovery are

distributed among all token using stations of the network.

The IEEE 802.4 standard [1] provides an optional priority mechanism.

The priority of each frame is indicated when the LLC sublayer requests the MAC

sublayer to send a data frame. The MAC sublayer offers four levels of priority

223
classes, called access classes. The access classes are named 0, 2, 4 and 6,

with 6 corresponding to the highest priority and 0 to the lowest. We use the

term 'class H' to denote the highest priority class, and class 1 to denote any

of the lower three priority classes. There is a separate queue for frames in

each class to wait pending transmission. Any station not having the priority

option transmits every data frame with the highest priority value. The object

of the priority scheme is to use the network bandwidth to transmit the low

priority frames when there is sufficient bandwidth available. Each access

class acts as a virtual substation in that the token is passed internally from

the highest access class downward, through all access classes before being

passed to successor.

The priority scheme works as follows: Any station, which receives the

token, initiates transmission of frames of the highest priority class in a

time not exceeding some maximum time called the high-priority token-hold time

(HPTHT) . This high-priority token-hold time prevents any single station from

monopolizing the network. After sending the high-priority frames, it starts

servicing the queue of the next access class. Each of the three lower access

classes at a node is assigned a target token-rotation time TTRT(l). For these

three access classes, the station measures the time it takes for the token to

circulate around the logical ring. If the token returns to the queue in less

than the target token-rotation time, the station is allowed to send frames of

that particular access class until the target token-rotation time has expired.

If the target token-rotation time has expired by the time the token returns,

the station is not allowed to send frames of that access class. The fraction

of bandwidth that will be allocated to various classes is controlled by the

target token-rotation time of each access class. The responsibility of

setting these values lies with the station management.

224
III. PERFORMANCE OF THE PRIORITY SCHEME

In this section, we consider the behavior of the priority scheme under

several specific input conditions. First, several terms used in performance

analysis of local-area networks are defined. In the description below,

utilization U of a network refers to the fraction of time the network spends

carrying data packets. Throughput of a network S, is defined as the total

number of data bits received at the destination per second ( expressed as a

fraction of the bandwidth) . As a data frame includes header information,

preamble etc., in addition to the data itself, the throughput of a network is

less than its utilization. The relationship between S and U is given by

S = a U , (1)

where

a

Tm

Tm

+ Toh

( 2 )

TQh is the length of overhead portion of the data frame. It includes such

components as the preamble, destination and source address fields, frame check

sequence, frame control field, and delimiters. The mean length of messages

(data component) of each priority class is assumed to be T . The components

m of the utilization and throughput of class i are denoted by U,. and

respectively. Equation (1) also holds separately for each component of the

throughput. The offered load (G) is defined as the number of data bits

generated by all active stations per second, expressed as a fraction of

channel bandwidth. The offered load of class i is denoted by G^.

225
Consider a token bus network with N nodes. Each node consists of four

queues, one for each priority class of messages. Frames arrive at these

queues with independent Poisson distributions. The frame lengths are assumed

to be exponentially distributed. Assume that the total traffic in a given

class is equally distributed among the N nodes. The high-priority token-hold

time at each station is set to a constant value HPTHT. Also assume that the

target token-rotation time of class 1 at each station is set to a constant

value TTRT(l)

The token-circulation time T

q is defined to be the time required for the

token to circulate among the N stations, in the absence of any traffic in the

network. Let T be the mean token-passing time between two stations. This

includes the station delay, the head end delay, the transmitter and receiver

modem delays, the token transmission time as well as the propagation time.

Bit time, the time required for the transmission of a single bit, is used as

the basic time unit. All other time intervals are expressed in bit times.

For a network with N nodes.

T = NT.. (3)

G t

The token-rotation time is the time elapsed from the instant an access class

at a station receives the token till the next instant the same access class at

the same station receives the token. Let TRT denote the mean token-rotation

time averaged over all the queues. The utilization of the network is then

given by.

U

[TRT - N T ] TRT

(4)

226
From Equation (1), the throughput is given by,

[TUT - N T,]

(5) S a TRT

This assumes error free operation and neglects the overhead associated with

ring maintenance functions such as nodes switching on to and off the logical

ring, and lost token recovery. In this paper, we consider the steady state

operation of the network. A static logical ring is considered, in which the

number of stations remains unchanged. The overhead associated with logical

ring maintenance functions is neglected, i.e., error free operation of the

network is assumed.

LEMMA 1: When a queue of class H is heavily loaded, its mean token-holding

time is given by (HPTHT+T ) . When class H is heavily loaded, (HPTHT+T )N time m m units per token rotation are used to transmit class H messages.

Proof: When a station receives the token, it loads the 'token hold timer'

with HPTHT, the high-priority token-hold time. The station then transmits

high-priority (class H) frames until either the token hold . timer expires or

the frames in the queue are exhausted. When a queue is heavily loaded, i.e.,

when it has more frames than it is allowed to transmit, the token hold time of

a queue is limited by the first condition. In this case, the station

attempts to transmit one more frame, provided the token hold timer has not

expired. If the token hold timer expires during the transmission of a message,

the node will still complete message transmission before it transfers control

to the next access class. Thus the actual token hold time of class H exceeds

HPTHT. The memoryless property of exponential message length distribution

227
ensures that the mean length of message portion left to be transmitted after

token hold timer expires is also where is the mean message length. Thus

under very high loads, when the station tries to send as many class H

messages as possible in a given cycle, the mean token-hold time for class H

is given by (HPTHT+T ) . Due to the assumption that the traffic is m symmetrically distributed among the queues, when class H is heavily loaded,

each of the queues can be assumed to be heavily loaded. Therefore, each queue

will hold the token (HPTHT+T ) time units. Thus in each token rotation,

m (HPTHT+T

m

)N time units will be spent transmitting class H messages. Note that

this result is not applicable to the case where HPTHT is zero. In that case,

the token-hold time, and hence the throughput of class H are zero.

It is assumed that the message length includes overhead such as address

and preamble. However, in practice only the data portion of a message is

exponentially distributed. This could cause the mean token-hold time to differ

from the above value. However, when the length of overhead segment of the

message is small compared to the data portion, this result is still valid.

When class H is heavily loaded, the time required by the token to

circulate among the stations and to transmit high-priority messages is given

by TH, where

TH = [ HPTHT + T + TJ N . (6) m t

Therefore, if the target token-rotation time of a low priority access class is

larger than this value, it is possible that the messages of the lower priority

class will also be transmitted, even though class H is heavily loaded [3].

228
LEMMA 2: The throughput of class H is related to the mean token-rotation time

as follows:

S

H = Gr

if TRT< TH

S

R

= Min

Proof: When class H is heavily loaded, (HPTHT+T )N time units per rotation are

m spent transmitting class H messages. In addition, NT time units per rotation

are required for token transmission. Thus if TRT is less than (HPTHT+T +T. )N, m t class H is not heavily loaded, i.e., message transmission of class H is not

limited due to HPTHT. Thus all the messages of class H get transmitted, and

hence the throughput is equal to the offered load. When the token-rotation

time exceeds (HPTHT+T +T.)N, up to (HPTHT+T )N time units per rotation may be m t m spent transmitting messages of class H. Therefore, the maximum throughput of

class H is given by,

[HPTHT+T ] N

q _ _ S (8)

H(max) TRT

This is the maximum throughput of class H, when the mean token-rotation time

is TRT. Throughput of class H reaches this value only if the total load in

class H exceeds this value. When the throughput of class H has reached the

value given by Equation (8), we say that the throughput of class H is limited

by its high-priority token-hold time. When the offered load of class H is less

than STT/ ,, all the messages of class H get transmitted. Thus, the

throughput of class H is given by Equation (7).

a [HPTHT+T ]N __ m TRT

if TRT> TH

(7)

229
LEMMA. 3: When all the network traffic belongs to class 1 (1 - 0, 2, or 4), and

class 1 is heavily loaded, the mean token-rotation time is given by

TRT = -^~y[TTRT(l)+T

ffi+T

t ],

(9)

Notes This result is valid only when tTRT( 1) is greater than the token

circulation time NT^. When this condition is not satisfied, no messages of

class 1 are transmitted.

Proof: A queue is heavily loaded if it has more messages than it is allowed to

transmit. Let be the mean token-passing time of a network with N nodes.

Consider the activity of this network when class 1 is heavily loaded. For

convenience, assume that no messages are transmitted in the initial rotation.

The result however is valid for any other initial token rotation. When the

queue of class 1 of station 1 receives the token, x time units are left in its

token rotation timer, where

x = [TTRT(l) - N T ]

'

Since the queue is heavily loaded, it will transmit class 1 messages until the

token rotation timer expires. When it expires, the transmission of current

frame is completed, before the token is passed to the next station. Due to the

memoryless property of the exponential distribution, the mean length of the

message portion left to be transmitted after the timer expires is also T .

Hence, the first station holds the token on average for (x+T ) time units as m shown in Table 1. During this rotation, no other station is allowed to send

messages. During the next rotation, station 2 uses (x+T

ffl ) time units. This

continues for n rotations, during each of which, only a single station is

230
allowed to send messages. The mean token-hold time of the station transmitting

messages is (x+T^) time units. In the (N+l)st rotation, the priority scheme

does not allow any messages to be transmitted. It can be seen that this (N+l)

token rotation pattern repeats itself. Therefore, the mean token rotation time

is given by

TRT

[ (x+T +NT, )N + N T

J

m t [N+l]

( 11 )

Substituting the value of x from Equation (10), we get the result of Equation

(9).

For large networks, if TTRT(l) is large compared to the token passing

time and the mean message transmission time, the mean token-rotation time when

class 1 is heavily loaded can be approximated by

( 12 ) TRT = TTRT(l) .

To keep the explanation simple, we use this value in deriving the analytical

model

LEMMA 4: When the mean token-rotation time of a network is less than the

target token-rotation time of class 1 (1 = 0, 2, or 4), the throughput of

class 1 is equal to its offered load, i.e.,

S

1 = G1

when TRT < TTRT(l) . (13)

Proof: When the mean token-rotation time is less than l'TRT(l), the class 1

queues receive the token back within TTRT(l) time units with a high

probability. So they are allowed to transmit the messages of class 1. If there

231
are queues with class 1 frames waiting to be transmitted, these queues will

utilize the time left to transmit the frames, thus increasing the value of

TRT. When the load in class 1 becomes heavy, the token-rotation time will

approach TTRT(l) as given by Lemma 3.

LEMMA, 5: Consider the case when traffic is present only in class i, i-0,2,4,6„

Then the maximum mean token-rotation time is given by

TRT = TTRT(i) for i = 0,2,4, max

( I d) « [HPTHT+T +TJN for i = 6.

v ' m t

The maximum throughput of class i is given by

’Kmax)

= a

[ TRT , . - NT ] (max) t TRT

max

(15)

Proof: Follows directly from Lemma 1 and Lemma 3 (Equation 12). From this

lemma it is evident that the traffic in class 1 cannot drive the mean tokenrotation time above the value TTRT(l). This is true even when there is

traffic in other classes.

Define P (i = 0,2, 4, 6) as

[ TTRT(i) - N T

fc ] P

i = a TTRT(i)

[ TH - N T 3

i = 0,2,4

i = 6 . (16)

P^ is the throughput of the network when its mean token-rotation time is equal

to the target token-rotation time of class i, for the lower priority classes.

P (P ) is the throughput of the network when the mean token-rotation time is

equal to TH. From Lemma 5, it is seen that P^ is also the peak throughput of

232
the network when all the messages belong to class i. These descriptions are

valid provided HPTHT is not zero and each TTRT is greater than the token

circulation time T . A more complete definition for Pu will take care of this

condition by setting to zero when the value of HPTHT is zero. Similarly,

P, can be defined to be zero, when TTRT(l) is less than T . l c

Lemma 4 specifies the throughput only when the mean token-rotation time is

less than the target token-rotation time of a given class. When the tokenrotation time is much larger than the target token-rotation time of a given

class, the probability of these queues receiving the token before the

expiration of corresponding timer approaches zero. Therefore, the throughput

of that class becomes negligible, i.e.,

S

1 = 0 when TRT >> TTRT(l) . (17)

Equations (13) and (17) specify the throughput of class 1 (1 = 0,2,4), when

the mean token-rotation time is not in the vicinity of TTRT(l). Since these

results were obtained using mean value analysis, they cannot accurately

predict the throughput when the value of TRT is close to TTRT(l). In

developing the analytical model, we make an approximation for the throughput

of class 1 in this region. This approximation can best be illustrated by an

example.

Example 1: Consider a network in which TTRT(O) is less than TTRT(2) by several

packet times. Assume that initially the network load consists entirely of

class 0 traffic, and that this traffic is handled by the network with a mean

token- rotation time TRT, which is less than TTRT(O). Now consider what

happens when the load in class 2 is gradually increased. Initially, the mean

token- rotation time will increase to accommodate this new traffic in class 2.

233
As long as TRT does not exceed TTRT(O), all the load in class 0 is handled by

the network. But when TRT exceeds TTRT(O), the throughput in class 0 begins

to decrease and eventually goes to zero. In developing the model, we

approximate the variation of the token-rotation time by that given in Figure

1. When the mean token-rotation time reaches TTRT(O), we assume that it

remains at that value until the throughput of class 0 is completely captured

by the load of class 2. The corresponding throughput characteristics are also

shown in Figure 1. In the region b-c, the mean token-rotation time, and hence

the total throughput are assumed to be constant. In practice however, a

gradual increase of TRT occurs in this region, resulting . in a higher

throughput for class 0.

Let I, J and K be the three lower priority classes of messages ordered

according to their target token-rotation times. The TTRT's are assumed to be

greater than the token circulation time T . If this is not the case, then no

c messages of that class will be transmitted and hence its throughput is zero.

The model described in this paper can be used to evaluate the throughput of

each class only when the target token-rotation times differ by several packet

times, i.e., when

( 18 ) NT < TTRT(I) < TTRT(J) < TTRT(K) .

L>

The special case where two or more of these have approximately the same value

is considered at the end of Section IV.

In general, when the load in classes J, K, ( and H, when throughput is

not limited by HPTHT), is increased, the token-rotation time increases to

accommodate this load. When TRT reaches the value TTRT(I), it is assumed to

remain at this value until the throughput of class I is captured by classes J,

234
K (and H) . A further increase in offered load in classes J, K (and H, when

throughput of class H is not limited by HPTHT) results in increase of TRT

beyond TTRT(I). A similar approximation is made for the throughputs of

classes J and K when TRT is in the vicinity of their target token-rotation

times. To summarize, we approximate the throughput of class 1 (1 = 0,2,4) by,

S

1 = G

1

when TRT < TTRT(l)

S = 0 when TRT > TTRT(l) . (19)

When TRT = TTRT(l), the throughput depends on the loads in other classes and

their target token-rotation times as follows:

S

K

= [Pj, - Sr I + when TRT = TTRT(K)

Sj = [ Pj - S

R

- S

K ] + when TRT = TTRT(J)

S T = [ P. - S- - Sv - S. ] + when TRT = TTRT(I)

(20)

1 J n K. J

where [X ] + is defined by,

[ X ] +

= 0

= X

In this section, the behavior of the IEEE 802.4 priority scheme was

investigated under a constrained set of input load values. In Section IV, we

use these results to develop a more general analytical model for the tokenbus priority scheme.

if X < 0

if X>0. ( 21 )

235
IV. MODEL DESCRIPTION

An analytical model is presented, which relates the throughput of each

priority class of messages to the traffic intensities of different classes,

the target token-rotation times, and the high-priority token-hold time. The

model is based on the following assumptions: The traffic is symmetrically

distributed among the stations. Messages arrive at nodes with independent

Poisson distributions. Message lengths are assumed to be exponentially

distributed. The target token-rotation time of a given class is set to the

same constant value at each of the stations. The high-priority token-hold

time at each station is also set to a constant value.

The behavior of the IEEE 802.4 priority scheme was considered in Section

III, under a limited set of input load conditions. When there are messages

in more than one class, the throughput characteristics depend on the target

token rotation times, the high-priority token-hold time, and their

relationship to each other. A general model has to consider the following

cases separately:

(1) TTRT(I) < TTRT(J) < TTRT(K) < TH ,

(2) TTRT(I) < TTRT(J) < TH < TTRT(K) ,

(3) TTRT(I) < TH < TTRT(J) < TTRT(K) , and

(4) TH < TTRT(I) < TTRT(J) < TTRT(K) .

The model described below can be used to evaluate the throughput of each class

only when TTRT's and TH differ from each other by several packet times. The

special case where two or more of them have approximately the same value is

considered later. In this paper, we present an analytical model for Case 1,

i.e., when

( 22 ) TTRT(I) < TTRT(J) < TTRT(K) < TH .

236
Similar models have been developed for other cases [4] . Consider the mean

token-rotation time of the network, when traffic is present in all the

classes. The maximum possible value of the token-rotation time is TH, and

this occurs when class H is heavily loaded. The condition of Equation (22)

ensures that the throughput of class H is not limited by the high-priority

token-hold time, except when the mean token rotation time (TRT) is equal to

TH. In order to derive the throughput characteristics, we have to consider

the different values for the mean token-rotation time TRT.

(i) TRT = TH

When TRT is equal to TH, the mean token-rotation time is greater than any

of the target token-rotation times, i.e.,

TRT > TTRT(l) 1 = 0,2,4 . (23)

Using Equation (19), we get

S

I - SJ - SK

=0 • (24)

This condition would occur only if there is sufficient load in class H to

sustain the mean token-rotation time at TH, i.e., only if

From Equations (5) and (16), the throughput of the network is given by

Since the throughputs of the lower priority classes are zero, is given by.

( 26 )

237
(ii) TTRT(K) < TRT < TH

This condition occurs when the total load in classes K and H exceeds P_. P & S

is the peak throughput of the network when TRT is equal to TTRT(K), and is

given by Equation (16). Since TRT is less than TH, from Equation (7), the

throughput of class H is given by.

From the inequality of Equation (22), the mean token-rotation time is greater

than the target token-rotation times of classes I and J. Using Equation (19),

the throughputs of classes I and J are given by

S

I - S

J

• 0

(28)

According to the approximation for TRT described in Section III, when TRT is

greater than TTRT(K), the throughput of class K approaches zero. When TRT is

equal to TTRT(K), the throughput of class K is captured by class H. Using

Equation (20), the throughput of class K is given by.

S

K - 1 P K

- S

H

1+ ' (29)

This case would occur only if there is sufficient traffic in classes H and K

to drive the token-rotation time to a value above TTRT(K), i.e., when

°H + 2 P K

' (30)

When using the model to calculate throughput characteristics, we first check

to see whether TRT is equal to TH. If this condition is not satisfied, then

we check to see whether TRT lies between TTRT(K) and TH. Therefore, we need

238
not include an upper bound in the above equation

(iii) TTRT(J) £ TRT < TTRT(K)

From Equation (22), the mean token-rotation time TRT is greater than

TTRT(I). Therefore, using Equation (19), we have

Since TRT is less than TH, the throughput of class H is not limited by HPTHT.

Therefore, from Equation (7) , the throughput of class H is given by

We also have the condition that TRT is less than TTRT(K). Therefore, from

Equation (19),

(33)

The throughput of class J is thus given by,

S

J

[ PJ ' S

H

- SK

I+

(34)

This condition can occur only if tnere is sufficient traffic in classes J, K

and H to drive the mean token-rotation time above TTRT(J), i.e., when

G

H

+ G

K

+ G

J 1 P

J

* (35)

(iv) TTRT(I) 1 TRT < TTRT(J)

This condition occurs when none of the first three conditions occur, but

there is sufficient traffic in classes I, J, K and H to drive the token

239
rotation time to a value higher than TTRT(I), i.e.„

g

h

+ G

K

" °J+ G

I * p

x

•

( 36)

The throughputs for this case can also be obtained in a similar manner:

S

I

[ P I

" S

H

S

K

“ S

J

] * (37)

(v) TRT < TTRT(I)

Since the mean token-rotation time is less than all the target tokenrotation times and TH, all the messages offered to the network are

transmitted. Therefore, the throughputs are given by.

(38)

The analytical model for this case, i.e., when the inequality of

Equation (22) is satisfied, is summarized in Figure 2. This model however can

be further simplified as follows. From Equations (16) and (22), we get the

following relationship for P's:

P

I

< PJ

<P

K

< P

H

• <39>

240
This property reduces the model given in Figure 2 into the following set of

equations

SR

= Min ( G

H

, PH )

S

K = Min ( G

k

, [ PK

- S

H

] +

)

S

J " Min ( Gj , [ P, - S

H

- S

K ] +

)

S

x = Min ( G

I

, [ P

I - SH

- - S

J

] +

) . (40)

In deriving the above model, it was assumed that the target tokenrotation times of different classes differ at least by few packet times. The

reason for this is the fact that we look at the average performance of the

priority scheme. If the values of TTRT's are fairly close to each other, the

distribution of the token-rotation time has to be taken in to account in order

to evaluate the throughput. For example, consider the case where,

TTRT(O) = TTRT( 2) = TTRT( 4)

Here, although the three target token-rotation times are identical, at a given

node the access classes are served in the order of their priorities. This

results in a more favorable treatment to a higher priority class, compared to

a lower class. In this case, we need to look at the actual distribution of

TRT in order to find the throughput of each class. However, the model can

still be used to predict the total throughput of the classes for which the

TTRT's are approximately the same. For example, consider the case where,

TTRT(O) = TTRT(2) < TTRT(4)

In this case, the model can be used to evaluate the individual throughputs of

classes 4, 6, and the sum of the throughputs of classes 0 and 2.

241
A model similar to the one shown in Figure 2 can be obtained for cases

2,3 and 4 as well. However, in these cases, the throughput of class H may

become limited by the high-priority token-hold time even before the mean

token-rotation time reaches its peak value [4]

V. MODEL VERIFICATION

In this section, we use performance characteristics of the IEEE 802.4

priority scheme, obtained via simulations, to verify the analytical model. A

brief description of the simulator is provided in [3]

The network parameters

used are shown in Table 2. The station delay in the table refers to the

minimum time interval required between the end of reception of a frame by a

station and beginning of a message transmission by the same station. The

traffic is assumed to be symmetrically distributed among all the queues, with

packet arrivals following a Poisson process. The nodes are distributed

randomly along the bus. The mean token-passing time is the sum of mean

propagation time, transmitter modem delay, head end delay, receiver modem

delay, and the station delay. Simulations were carried out until the

performance results appeared to have reached steady values. The length of a

typical simulation run was about 10s. Below, we compare the results from the

analytical and simulation models. -

Three sets of throughput vs. offered load characteristics are used to

verify the model. In each case, the total offered load is changed, while

keeping the percentage of total load in each class constant. The fraction of

the total offered load in each class is shown in Table 3. In Set 2 for

example, each class generates 25% of the network load, i.e., the network

traffic is symmetrically distributed among the four classes. The throughput

vs. offered load characteristics corresponding to Sets 1, 2 and 3 are shown in

242
Figures 3, 4 and 5 respectively. It is seen that the values predicted by the

analytical model agree very closely with those obtained using simulations.

When the throughput of a class is decreasing with the offered load however,

the model tends to under estimate the throughput of that class. This can be

attributed to an approximation that was used in developing the model. The

mean token-rotation time in this case was assumed to remain at TTRT(l), until

other eligible classes capture its throughput. As shown in Figure 1, this

however, is different from what happens in the actual case. Means to overcome

this limitation are currently under investigation.

VI. CONCLUSION

In this paper, we investigated the performance of the priority option of

IEEE 802.4 token-passing channel-access protocol. First we considered the

behavior of the scheme under constrained input load conditions. Next these

results were used to develop an analytical model for the token-passing

priority scheme when

[ HPTHT + T + T. ] > TTRT(l), for all 1, 1 = 0, 2 , 4 , m t

where, HPTHT is the high-priority token-hold time, TTRT(l) the target tokenrotation time of class 1, T the mean message length, and T. the mean tokenm t passing time. N is the number of stations in the network. A symmetric network

is assumed, in which the traffic of each class is symmetrically distributed

among the nodes, and each node has the same set of timer values. The results

given in Section III can be used to obtain throughput characteristics for any

other relationship between TTRT's and HPTHT [4]

In case where two or more

classes have the same target token-rotation time, the model is capable of

evaluating the total throughput of these classes rather than the individual

throughput of each class. The model uses mean values of network variables to

predict the behavior of the network. Therefore, its accuracy depends on the

243
number of stations in the network. Simulations show this model to be very

accurate even for networks with as few as 10 nodes [4]

Simulation of large networks, to obtain the bandwidth allocated by the

priority scheme to different access classes, requires a significant amount of

computer resources. Models like the one described here, can be used to

evaluate such characteristics accurately, with a minimal amount of

computations. Although the model is based on several approximations, it

provides very accurate values for throughputs of different classes. This

model can be used to determine the timer values for IEEE 802.4 scheme, to meet

throughput requirements of different classes of traffic present in the

network.

The model is based on the assumption that all the nodes in the network

have the same high-priority token-hold time, and the same target token

rotation time for each access class. This however, may not be the case in many

practical networks. Extension of the model to evaluate such networks is

presently under investigation. Further investigation is also necessary to

model the case, where traffic is not symmetrically distributed among the

nodes. The model presented here does not address the issue of predicting the

delays of different classes of messages. However, it provides a starting point

toward developing such models.

244
REFERENCES

1. ANSI/IEEE std. 802.4 - 1985: Token-Passing Bus Access Method and Physical

Layer Specifications, IEEE, 1985.

2. Cherukuri R., Lin L. and Louis L., "Evaluation of token-passing

Schemes in LANs," Proc. Computer Networking Symposium, 1982.

3. Jayasumana A. P., "Performance Analysis of a Token Bus Priority

Scheme," To be presented at IEEE INF0C0M, March, 1987.

4. Jayasumana A. P., "A Performance Model for the IEEE 802.4 Token Bus

Networks," Technical Report (Under Preparation), Department of

Electrical Engineering, Colorado State University.

5. ELeinrock L., "A Decade of Network Development," Journal of

Telecommunication Networks, Comp. Sci. Press, Spring 1982.

6 . Nakassis A., "On the Stability of a Token Passing Network," Proc.

Workshop on Analytic and Simulation Modeling of IEEE 802.4 Token Bus LAN,

NBS, April, 1985.

7. Nakassis A., "Token Passing Networks and Starvation Issues," Proc.

Workshop on Analytic and Simulation Modeling of IEEE 802.4 Token Bus LAN,

NBS, April, 1985.

8. Sachs S. R., Kan K. and Silvester J. A., "Performance Analysis of a Token-Bus Protocol and Comparison with other LAN Protocols, " Proc.

10th Conf. on Local Computer Networks, 1985.

9. Ulug, M.E., "Comparison of Token Holding Strategies for a Static Token

Passing Bus, " Proc. Computer Networking Symposium, 1984.

10. Ulug, M.E., "Calculation of Waiting Times for a Dynamic Token Passing

Bus, " Proc. Computer Networking Symposium, 1984.

245
Table 1. Mean token-holding time of class 1 queues when

class 1 is heavily loaded.

Rotation 1 2 3

Station

4 • • • N

0 0 0 0 0 • e c 0

1 x+T 0 0 0 0

2 o

ffi x+T 0 0 0

3 0 om

x+T

m

0 © « « 0

« • • • © • • •

« « • • e •

• • c e • • « • «

• • « e « • • • e N 0 0 0 0 x+T

om

N+l 0 0 0 0

N+2 x+T

m

0 0 0 • • • 0

• • • • • • « •

« « • • • * e •

246
Table 2 Network parameters used for the performance analysis

Number of stations N 50

Mean data frame length T 1024 bits

Length of token 184 bits

Overhead per message T^

Bandwidth of the bus

184 bits

10 Mbps

Length of the bus 1.0 km

One way propagation delay 5 .0 flS Head end delay 7.0 ns Rx modem delay 3.0 fis Tx modem delay 0.5 fis Station delay 50.0 (IS High-priority token-hold time

Target token-rotation times

102.4 ns

Class 4 9000 ns Class 2 8000 ns Class 0 7000 ns Mean token-passing time T 83 .5 ns (HPTHT + T + T. )N

Z m t 14415.0 ns

Table 3. Percentage of offered load in class i, i = 0, 2, 4 and 6.

Set # Class 0 Class 2 Class 4 Class 6

1 10 20 30 40

2 25 25 25 25

3 40 30 20 10

247
OFFERED IQAO OF CUSS 2 G

2

OFFERED LOAD OF CUSS 2 G>>

Figure 1. Variation of mean token-rotation time [THT] and the

throughput characteristics for Example 1.

248
C ANALYTICAL MODEL FOR THE CASE

C WHEN TTST(I) < TTST(J) < TTBT(E) < TH

C IT (G(H).GE.P(H)) THEN

C T3T * TH

S(H) - ?(H)

S(E) - 0 S(J) - 0 3 ( 1 ) - 0 c ELSE IF ( (G(H)+G(E) ) .GE. ?<E) ) THEN

C TH > TRT 2 TTRT(E)

SO) - G(H)

S(E) - PLUS(P(S)-SO))

S(J) - 0 S(I) - 0 C ELSE IF ( (G(H)-KJ(E)+G(J)) .GE. ?(J) ) THEN

C TTHT(S) > TRT 2 TTST(J)

S(H) - GO)

S(K) - G(E)

S(J) - PLOS(P(J)-S(H)-S(E))

S(I) - 0 c ELSE IF ( (GO)*H3(E)+G( J)*KI(I) ) .GE. ?(I) ) THEN

C TTHT(J) > TFT 2 TT3TCI)

3(H) - GO)

S(E) - G(E)

S(J) - G(J)

3(1) - PL3S(P(I)-S(H)-S(E)-S(J))

C ELSE

C TTST(I) 2 TRT SO) - GO)

3(E) - G(E)

SO) - G(J)

S(I) - G(I) ENDIF

C

Figure 2. Analytical model for token-bus priority scheme when

TTHT(I) < TTHT(J) < TTRT(I) < TH .

249
THROUGHPUT

OF

CLASS

I

OFFERED LOAD G

Figure 3. Throughput vs. offered load characteristics corresponding

to load distribution Set 1.

250
THROUGHPUT

OF

CLASS

1

OFFERED LOAD G

Figure 4. Throughput vs. offered load characteristics corresponding

to load distribution Set 2.

251
THROUGHPUT

OF

CLASS

1

Figure 5. Throughput vs. offered load characteristics corresponding

to load distribution Set 3

252
Are Priorities Useful in an 802.5 Token Ring?

Jeffery H. Peden

Alfred C. Weaver

Department of Computer Science

University of Virginia

Charlottesville, Virginia 22903

The EEEE 802.5 token ring defines eight message priorities. The intent is that “high priority” messages should be delivered prior to “low priority” messages. Through a series of simulations we show that

this expected behavior only occurs under a unique set of circumstances: when there are very few network

stations, very short data packets, very short token hold times, and very high network load.

In the general case, we found that priorities did not markedly influence the delivery time of priori- tized messages. Use of the priority system generally resulted in more overhead and longer average message delays than when all messages were carried as a single priority.

253
Are Priorities Useful in an 802,5 Token Ring?

1. Introduction

The 802.5 token ring protocol defines eight message priorities. These priorities are intended to

ensure that higher priority messages receive better service than lower priority messages (i.e., are

delivered with less delay). In this paper we examine the usefulness of the priority scheme under varying

circumstances.

The priority scheme operates as follows: Every message is assigned a priority between 0 (lowest)

and 7 (highest). When a station receives a token (the special bit sequence used to confer the right of

transmission on a station) the station transmits any enqueued messages that are at or above the priority of

the token, higher priority messages first. Enqueued messages that have a priority less than that of the

token must wait until a lower priority token is received by the station.

As messages are repeated by non-transmitting stations, these stations may reserve the priority of

the token by setting the reservation bits in the packet header so that they equal the highest priority message that they have waiting (however, the value of the token reservation may not be lowered by a station). When a station has completed its transmission, it retransmits the token at the greater of the value of

the current token priority and the reservation. If a station raises the priority of the token, it is responsible

for eventually lowering it to its previous value.

2. General Performance

In order to establish baseline performance for the token ring, a reference configuration was established consisting of 40 active stations, a station repeater latency of 1 bit time, a data rate of 1 Mbps, constant packet lengths of 256 bits (including framing), and an exponential arrival distribution. Studies were

then made of network performance in both the single priority case and the multiple priority case (in

which network load was divided evenly among the eight priorities).

Figure 1 shows the total packet delay for both the single and multiple priority modes of operation.

Note the single curve for the multiple priority delay. The actual delay values shown in Figure 2a clearly

254
5.0E-03

. curve with circles: single priority operation

1.0E-04

Offered Load

0.05 0.15 0.25 0.35 0.45 0.55 0.65 0.75 0.85 0.95

Configuration:

40 Stations 256 Bit Packets including framing

1 Bit Time Station Latency

Exponential Arrivals 1Mbps Medium

Figure 1 — Service Delay

255
indicate that there is no significant or consistent difference between the delay experienced by a high

priority packet and that of a low priority packet Because of this, the delay for the multiple priority case

is given as the averaged delay of all eight priorities.

Also note that the delay for the multiple priority case is greater in the medium and upper load

ranges than the delay for the single priority case. The actual delay values for the single priority case are

shown in Figure 2b. The increase in overall delay for multiple priority operation is due to the added

overhead incurred by the protocol when handling more than one priority.

3. Effects of Parameter Variation

3.1. Varying the Number of Offered Priorities

To ensure that the observed delay using multiple priorities was not an artifact of the chosen parameters, simulations were run in which the number of message priorities was varied from two to seven, and

the same effect was seen. We therefore reduced the number of message priorities to only two, and varied

the load distribution between them.

Simulations were run such that the fraction of load given to the low and high priorities was 0. 1 and

0.9 respectively, and then varied in increments of 0.1 until the distribution of load between low and high

priorities was reversed to 0.9 and 0.1 respectively. The results of these simulations indicated that the

number of priorities offered to the ring, as well as the relative fraction of bandwidth assigned to each

priority, had no significant impact on the delay experienced by packets. We therefore concluded that the

effect was independent of the relative leading of priorities.

3.2. Varying the Number of Ring Stations and Packet Sizes

It was shown in [PEDE87] that for any particular offered load, achieved throughput for the token

ring decreases as packet sizes become shorter, and that this loss of throughput is exaggerated when the

number of ring stations is small. It was observed that the effects of multiple priority operation only had

significant impact when the number of ring stations was less than 40. It was also shown that delay varied

in inverse proportion to packet size. We therefore dealt with only those cases in which the number of sta256
tions was 40 or less, and where packets were short (on the order of 20 bytes of data).

4. Priority Effects

In this section we present and discuss the actual effects that the priority operation has on delay.

Figure 3 shows that even for only 10 ring stations and packet sizes of 256 bits, there is minimal difference between the delays experienced by high and low priority packets. Figures 4 and 5 show the priority

delays for ring configurations of 5 and 3 stations respectively, again with 256 bit packets. Note that even

for 3 stations, where the difference between delays for different priorities is the highest, it is less than a

factor of two from highest to lowest, and this does not occur until an offered load of 95% is reached (a

loading difficult to achieve with only 3 stations).

To get the maximum effect from the operation of the priorities, simulations were run in which the

ring was significantly overloaded for several seconds. Figure 6 shows the results observed. Note that

service shutoff does not occur for the lowest priority until an offered load of over 99% is reached. Also

note that the priority operation is very well behaved, in that service shutoff occurs in order of increasing

priority, and that higher priorities continue to receive guaranteed delay until their service shutoff.

4.1. Effect of the Token Hold Time

The amount of time that a station may transmit is governed by the token holding timer in that station. The default value for this timer is given in the standard as 10 ms [IEEE85]. Up to this point, the

effects of the priority scheme have been studied with the ring operating under the condition of the token

holding timer being set to the default value. (This is a reasonable assumption to make, as it is anticipated

that few users of this network will make changes in the default values of the various network parameters

defined in the standard.)

It was shown in [PEDE87] that the default setting of the token holding timer usually results in

exhaustive service of the message queues. Therefore, simulations were run in which the token holding

timer was set to less than the default value, in order to check the effects of the priority scheme under the

condition of non-exhaustive service. Again it was seen that as the number of ring stations increases to 40

and above, the effects of the priority operation rapidly become insignificant, even when service is limited

257
Offered Load

Delay by Priority 0 1 2 3 4 5 6 7

0.05 0.320 ms 0.320 ms 0.318 ms 0.320 ms 0.318 ms 0.319 ms 0.321 ms 0.319 ms

0.15 0.342 ms 0.341 ms 0.339 ms 0.339 ms 0.340 ms 0.339 ms 0.339 ms 0.337 ms

0.25 0.364 ms 0.362 ms 0.364 ms 0.363 ms 0.365 ms 0.364 ms 0.365 ms 0.365 ms

0.35 0.403 ms 0.398 ms 0.398 ms 0.397 ms 0.396 ms 0.399 ms 0.396 ms 0.400 ms

0.45 0.461 ms 0.460 ms 0.464 ms 0.461 ms 0.457 ms 0.458 ms 0.465 ms 0.462 ms

0.55 0.581 ms 0.571 ms 0.567 ms 0.560 ms 0.570 ms 0.571 ms 0.587 ms 0.564 ms

0.65 0.825 ms 0.831 ms 0.829 ms 0.822 ms 0.823 ms 0.831 ms 0.820 ms 0.816 ms

0.75 1.17 ms 1.15 ms 1.17 ms 1.16 ms 1.16 ms 1.16 ms 1.16 ms 1.13 ms

0.85 1.80 ms 1.83 ms 1.77 ms 1.80 ms 1.79 ms 1.76 ms 1.78 ms 1.74 ms

0.95 3.76 ms 3.76 ms 3.81 ms 3.80 ms 3.77 ms 3.72 ms 3.64 ms 3.63 ms

Figure 2a —Priority Delay Table

Offered

Load Delay

Offered

Load Delay

0.05 0.318 ms 0.55 0.505 ms

0.15 0.339 ms 0.65 0.609 ms

0.25 0.363 ms 0.75 0.779 ms

0.35 0.396 ms 0.85 1.20 ms

0.45 0.442 ms 0.95 3.18 ms

Figure 2b — Delay Table for Single Priority

258
Configuration:

10 Stations 256 Bit Packets including framing

1 Bit Time Station Latency

Exponential Arrivals 1Mbps Medium

Figure 3 — Service Delay

Priority Delay for 10 Stations

259
Configuration:

5 Stations 256 Bit Packets including framing

1 Bit Time Station Latency

Exponential Arrivals 1Mbps Medium

Figure 4 — Service Delay

Priority Delay for 5 Stations

260
Configuration:

3 Stations 256 Bit Packets including framing

1 Bit Time Station Latency

Exponential Arrivals 1Mbps Medium

Figure 5 — Service Delay

Priority Delay for 3 Stations

261
Configuration:

3 Stations 256 Bit Messages including framing

1 Bit Time Station Latency

Exponential Arrivals 1Mbps Medium

Figure 6 — Priority Class Service Shutoff

262
to one packet (consisting of only 19 data bytes) per token reception (note that the difference between the

delay of the highest and lowest priorities is only a factor of 2.5). Figure 7 shows the service delay for 40

stations when the token holding timer has been set such that one-packet-per-token transmission results.

Packets consist of 19 data bytes plus framing.

The increasing effect of the priority scheme can be seen in Figures 8, 9, and 10. Note that the

graphs do not show delays for low loads. This is because there is negligible difference at low loads

between the delays of the various priorities. One interesting effect is that as the number of stations

decreases, the delay of the lowest priority is greater than that of a network configuration with a higher

number of stations, and the delay of the highest priority is less. This is another indication that the priority

scheme is operating as intended, i.e., lower priorities are taking the brunt of the increasing delay.

The ratio between the delays of the high and low priorities can be seen in Figure 11. For three stations, this ratio is about 32, for 10 stations it is about 13, and for 20 stations approximately 5.5. The

actual values for the delays are given in Figure 12. Note that this chart only gives delay values for a network load of 0.95 — for loads less than this, the ratios between high and low priority delays decrease

rapidly. In comparison, the delay experienced by packets in an identical network configuration with the

exception that the entire network load was offered in a single priority is 5.8 ms. It should be remembered

that the greatest delay will be experienced at high loads with a low number of stations, short packets, and

single-packet-per-token service, so what we have been describing here is worst case behavior.

5. Conclusions

It is evident that unless very high network loads are expected in the ordinary course of events, the

network performs better without the added overhead and complexity of the priority operation. The

greatest effects of the priority scheme were obtained with very short packets, a low number of network

stations, and single-packet-per-token transmission. The only way to limit transmission to one packet per

token is to change the token holding timer such that there is only time for the transmission of one packet

when the token is received. However, this will tend to reduce the token holding timer to the point where

only very short packets are allowed (if only large packets are transmitted, this in itself reduces the effect

263
Configuration:

40 Stations 256 Bit Packets including framing

1 Bit Time Station Latency

Exponential Arrivals 1Mbps Medium

Figure 7 — Service Delay

Non-Exhaustive Service

264
1.0E-02

curves in order from top to bottom are

' for priorities 0 (lowest), 1, 2, 3,

Configuration:

20 Stations 256 Bit Packets including framing

1 Bit Time Station Latency

Exponential Arrivals 1Mbps Medium

Figure 8 — Service Delay

Non-Exhaustive Service

265
Configuration:

10 Stations 256 Bit Packets including framing

1 Bit Time Station Latency

Exponential Arrivals 1Mbps Medium

Figure 9 — Service Delay

Non-Exhaustive Service

266
Configuration:

3 Stations 256 Bit Packets including framing

1 Bit Time Station Latency

Exponential Arrivals 1Mbps Medium

Figure 10 — Service Delay

Non-Exhaustive Service

267
Configuration:

Varying Stations 256 Bit Packets including framing

1 Bit Time Station Latency

Exponential Arrivals 1Mbps Medium

Figure 1 1 — Priority Delay Ratio

268
Active

Stations

Delay by Priority 0 1 2 3 4 5 6 7

40 5.27 ms 4.27 ms 3.66 ms 3.41 ms 3.04 ms 2.71 ms 2.32 ms 2.14 ms

20 10.0 ms 7.06 ms 4.57 ms 3.90 ms 3.05 ms 2.38 ms 2.11 ms 1.79 ms

10 15.4 ms 7.18 ms 4.28 ms 2.93 ms 2.19 ms 1.70 ms 1.43 ms 1.22 ms

3 21.5 ms 6.69 ms 3.31 ms 1.90 ms 1.30 ms 1.00 ms 0.783 ms 0.661 ms

Figure 12 — Priority Delay Table

Offered Load of 0.95

of the priority scheme). It is also a difficult matter to generate a high network load with a small number

of stations.

We conclude that, in general, priorities are not a very useful feature of the 802.5 protocol. While

there are some circumstances in which priority operation is useful, they are rare. Given the added delay

and overhead generated under ordinary conditions by the use of the priority scheme, and the small set of

circumstances under which multiple priorities are shown to be useful, we believe that under general conditions, network traffic should be offered as a single priority, and that this will result in better service the

majority of the time.

269
References

[Ieee85]

[Pede87]

EEEE, Token Ring Access Method and Physical Layer Specifications, The

Institute of Electrical and Electronics Engineers, Inc., 1985.

J. H. Peden, Performance Analysis of the IEEE 802.5 Token Ring, Master’s Thesis, Department of Computer Science, University of Virginia,

January 1987

270
Performance Analysis of an

Integrated Traffic Slotted Ring

Muhammad Ali

Hermann J. Helgert

This paper describes the performance analysis of an integrated

traffic slotted ring by means of simulation, as well as determining

analytical models which provide best estimates for critical performance parameters. The network carries packet, voice and data

circuit traffics. The parameters considered are message delays for packet traffic and blocking probabilities for voice and data traffics.

The ring network under study is capable of carrying packet, voice and data

circuit traffic on a 8 Mbps slotted ring. The network, built by Laboratoire Central

de Telecommunications, is currently designed for carrying X.25 traffic on the

packet channel. The present study was done mainly to determine delays involved if the ring was to carry datagram traffic with lengths relatively larger than X.25

packets. Simulations were carried out to determine the performance

characteristics. Analytical models were determined which provided best estimates

for the message delays on the packet channel, and blocking probabilities for the

voice and data circuit channels. The service disciplines considered for the packet

channel were the Exhaustive, Gated and Limited service disciplines.

The ring latency, W, is a constant 125 (is, which is maintained at precisely

that value by a hardware unit, irrespective of the ring size, subject to a maximum limit for the ring. The latency per station is two bytes. The 125 as

time frame is slotted as shown in Figure 1. There are a total of 128 bytes in the

frame. The first byte has a pattern for synchronization. The first section

constitute the packet channel, the second section is for voice circuits, with each

byte slot being reserved for one voice channel. The third section is for data

circuits and similar to the second section. The format for the packet channel is given in Figure 1.

271
sync code

125 ms

0 1 n n+1 rn cm 1 127 - packet channel voice circuits 4- data circuits -

1 data n segment number — source station address —— destination station address — packet procedure token

Figure 1 Frame structure of the ring

r ack flag

crc

The first byte is a multiple use token. It indicates whether the packet channel

is free or in use. If it is in use, different patterns indicate whether the channel

content is a supervisory command, a complete message, the first segment of a message, an intermediate one, or the last one. The fourth byte indicates the

number of the segment within a message. The last byte is an acknowledgement flag

used by the destination to indicate acceptance of the segment or it's rejection by

the destination station.

The access method employed for the packet channel could be described as demand sharing using fixed size slots, following the categorization in [1], In the

present case there is one slot(channel) for the packet traffic. When a station has a message in queue, it checks the packet channel token to determine if the channel is empty. If it is, the message, or a segment of the message is placed in the channel,

and the token is set busy. The station maintains control of the channel till the

message is completely transmitted. It may further maintain control of the ring

depending on the service discipline. Once a station has finished using the channel,

it sets the token free to indicate that the channel is available. Thus it is the source

station which releases the channel and not the destination station. The channel may

be occupied by the next station 'down stream' which has message(s) to send. This

indicates that access control is based on a mechanism similar to hub polling.

When a station wants to reserve a free channel for a voice or data circuit, it broadcasts it's intention to all other stations, so that no other station tries to use

it. All stations have bitmaps indicating current utilization of the voice and data

channels. This avoids overhead bits on the ring. Since the traffics in different

sections do not interfere with each other, each traffic can be modelled

independently.

The message arrival distribution is assumed to be Poisson for the packet

channel, and identical for all stations. The message length is assumed to have a geometric distribution with a mean of 1000 bytes. Since the packet channel is generally smaller, a message may be divided into segments before transmission.

The last segment of a message may be partially empty. Now the probability

distribution function of message lengths is given by

272
k=1 .2

kc

P(y-k) - I p<i-p) m

r=(k-1)c+1

where p is the inverse of mean message length c is the segment length

k is the message length in units of segments

The walk time is considered to be constant and the sum of all walk times is W.

The transmission time for each segment can be considered to be W, irrespective of

it's length. Transmission error is considered negligible.

The mean and mean square message transmission times are given by

<m> = <y>.W

<m2> = <y

2>.W2

Smaller segments result in higher message transmission times. Using the

derivations by Takagi[2] for the case of exhaustive service discipline, the mean

message delay is given by

D m = N[A<m 2 >+ w(1 - A<m>) ]/ 2(1 - N A<m>) - W/2 + <m>

N A<m> < 1 where N is the number of stations

A is the mean message arrival rate at a station

w is the walktime.

For the case of gated service discpline it is

D m = N[A<m 2 >+ w(1 + A<m>) ]/ 2(1 - N A<m>) - W/2 + <m>

N A<m> < 1 For the case of limited service discipline with a limit of one message serviced

per visit, it is,

D m = N[A<m 2 >+ w(1 + A<m>) ]/ 2(1 - N A(w + <m>)) - W/2 + <m>

N A(w + <m>) < 1

Figures 2 shows the message delay characteristics as a function of segment

size. The graphs show the mean message delay results from both the analytical and

simulation models, using the gated service discipline. The difference in message

delays for the three service discipline is very small compared to the actual

message delays, both for the analytical and simulation models. Differences in message delays due to service disciplines increase with heavy loading of the

273
network (values of NX<m> or NX(w + <m>) close to 1 ) , but for loadings of

practical interest the differences are negligible.The delay due to the limited

service discipline is the highest, while exhaustive service discipline has the

lowest delay. The analytical and simulation results compare closely, for loadings

of practical interest. The difference may be due to the assumptions of exponential

distributions for length, as an approximation to a geometric distribution, in the

simulation model.

segment size in slots segment size in slots

Figure 2 message delays as function of segment size

For the voice circuits it was assumed that there are infinite number of

sources, which is a valid assumption if the number of sources are much greater

than the number of voice channels. If the sources are not much greater in number,

the approximation is still quite good [3]. The calls that are blocked are cleared,

and do not return again during the same study period such as the busy hour. The

call originations are characterized by a Poisson process, and the call lengths have

exponential distributions^]. The blocking probability is given by the Erlang B

formula [3] c P

b (voice) = (Ac

/c!) / (X A

x /x!)

x=0

A = Vn/

274
where is the mean call arrival rate during the busy hour,

is the mean length of calls c is the voice channels allocation (slots reserved for voice circuits)

The data circuit service can be modelled as a M/M/d/d case, with the arrival process as the total for the network. The buffer is of the same size as the number

of servers (channels), so the blocking probability is the probability of the buffer

occupancy being d.

d P b (data) = (R d

/d!) / (I R

x

/x!)

x=0

R = X

d

L

d/S

where is the mean arrival rate per second

L

d

is the mean message length in bytes

S is the service rate in bytes per second ( 8E+3/second)

d is the data channels allocation (slots reserved for data circuits)

Figure 3 shows the blocking probabilities as a function of channel allocation.

The analytical and simulation results match very closely and follow the indicated curves.

Figure 3 blocking probabilities for voice & data circuits

275
Conclusion

The message delays due to different service disciplines shows very little difference, specially for larger segment sizes. The mean message delay for the

limited service discipline is the highest, followed by those for the gated and

exhaustive service disciplines, as expected. Fairness between stations can be

increased by using the limited service discipline, with price of a very small

increase in the mean message delay. The comparison between the analytical and

simulation results for the packet channel suggests that the analytical models can be used, under the given assumptions, to determine mean message delays in place

of the more expensive simulation methods. Since the analytical results for the

voice and data circuits match very closely with the simulation results, the former can be used effectively to determine the blocking probabilities under various load

conditions.

REFERENCES

[1] B.K. Penny & A.A. Baghdadi, "Survey of computer communications

loop networks:Parts 1 and 2", Computer Communications, vol. 2,

pp 165-180,244-261,1979.

[2] H. Takagi, "Analysis of polling systems", The MIT Press, 1 986.

[3] R.L. Freeman, "Reference manual for Telecommunication

Engineering", Wiley-lnterscience, 1985.

[4] J.L. Hammond & J.P. O'Reilly, "Performance Analysis of Local

Computer Networks, Addison Wesley, 1986.

[5] J.F. Hayes, "Modeling & Analysis of Computer Communications

Networks", Plenum Press, 1984.

[6] W. Bux, "Local Area Subnetworks: A Performance Comparison",

IEEE Trans, on Commun., Vol. COM-29, No. 10, Oct. 1981

[7] G.S. Blair, "A Performance Study of the Cambridge Ring",

Computer Networks, Vol 6 , 1982.

276
NBS-114A (rev. 2-8C) U.S. DEPT. OF COMV. BIBLIOGRAPHIC DATA

SHEET (See instructions)

1. PUBLICATION OR

REPORT NO. NBSIR 87-3516

2. Performing Organ. Report No, 3. Publ ication Date FEBRUARY 1987

4. TITLE AND SUBTITLE

Workshop on Factory Communications

March 17-18, 1587

5. AUTHOR(S)

Robert Rosenthal, Editor

6. PERFORMING ORGANIZATION (If joint or other than NBS, see instructions) 7. Contract/Grant No. national bureau of standards ___

DEPARTMENT OF COMMERCE 8. Type of Report & Period Covered

WASHINGTON, D.C. 20234

9. SPONSORING ORGANIZATION NAME AND COMPLETE ADDRESS (Street, City, State. ZIP)

10. SUPPLEMENTARY NOTES

| | Document describes a computer program; SF-185, FIPS Software Summary, is attached.

11. ABSTRACT (A 200-word or less factual summary of most significant information. If document includes a significant

bibliography or literature survey, mention it here) Factory and laboratory automation requires computer communication networks capable

of providing rich connectivity among diverse computer systems, sensors, actuators,

robot controllers and other devices. Protocol specifications for networks- capable

of meeting manufacturing and technical office requirements are evolving nationally

and internationally. These networks enable distributed computer applications

within the factory; and, with these distributed computer applications, automated

design and manufacturing is possible.

These workshop proceedings report recent efforts of government, industrial and

academic researchers in factory communication networks. Four major research

areas are addressed: the application of manufacturing automation protocols (MAP)

in factory networks, the application of non-MAP protocols, design and simulation

tools and analytic and simulation modeling.

12. KEY WORDS (Six to twelve entries; alphabetical order; capitalize only proper names; and separate key words by semicolon s) computer networks; factory automation; local area networks; MAP; modeling;

simulation

13. AVAILABILITY 14. NO. OF

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