REFERENCE

>*,, 0> '

NBS TECHNICAL NOTE 804

Review of

Computer

Networking Technology

National

Bureau

of

Standards
NATIONAL BUREAU OF STANDARDS

The National Bureau of Standards 1 was established by an act of Congress March 3, 1901. The Bureau's overall goal is to strengthen and advance the Nation's science and technology

and facilitate their effective application for public benefit. To this end, the Bureau conducts

research and provides: (1) a basis for the Nation's physical measurement system, (2) scientific and technological services for industry and government, ( 3 ) a technical basis for equity in trade, and (4) technical services to promote public safety. The Bureau consists of the Institute for Basic Standards, the Institute for Materials Research, the Institute for Applied Technology,

the Institute for Computer Sciences and Technology, and the Office for Information Programs.

THE INSTITUTE FOR BASIC STANDARDS provides the central basis within the United

States of a complete and consistent system of physical measurement; coordinates that system

with measurement systems of other nations; and furnishes essential services leading to accurate and uniform physical measurements throughout the Nation's scientific community, industry, and commerce. The Institute consists of a Center for Radiation Research, an Office of Measurement Services and the following divisions: Applied Mathematics — Electricity — Mechanics — Heat — Optical Physics — Nuclear

Sciences 2 — Applied Radiation 2 — Quantum Electronics 3

-— Electromagnetics 3 — Time

and Frequency " — Laboratory Astrophysics 3 — Cryogenics \

THE INSTITUTE FOR MATERIALS RESEARCH conducts materials research leading to improved methods of measurement, standards, and data on the properties of well-characterized materials needed by industry, commerce, educational institutions, and Government; provides

advisory and research services to other Government agencies; and develops, produces, and

distributes standard reference materials. The Institute consists of the Office of Standard

Reference Materials and the following divisions: . Analytical Chemistry — Polymers — Metallurgy — Inorganic Materials — Reactor

Radiation — Physical Chemistry.

THE INSTITUTE FOR APPLIED TECHNOLOGY provides technical services to promote

the use of available technology and to facilitate technological innovation in industry and Government; cooperates with public and private organizations leading to the development of

technological standards (including mandatory safety standards), codes and methods of test; and provides technical advice and services to Government agencies upon request. The Institute consists of a Center for Building Technology and the following divisions and offices: Engineering and Product Standards — Weights and Measures — Invention and Innovation — Product Evaluation Technology — Electronic Technology — Technical Analysis — Measurement Engineering — Structures, Materials, and Life Safety l — Building

Environment * — Technical Evaluation and Application * — Fire Technology.

THE INSTITUTE FOR COMPUTER SCIENCES AND TECHNOLOGY conducts research and provides technical services designed to aid Government agencies in improving cost effec- tiveness in the conduct of their programs through the selection, acquisition, and effective

utilization of automatic data processing equipment; and serves as the principal focus within

the executive branch for the development of Federal standards for automatic data processing

equipment, techniques, and computer languages. The Institute consists of the following

divisions: Computer Services — Systems and Software — Computer Systems Engineering — Information Technology.

THE OFFICE FOR INFORMATION PROGRAMS promotes optimum dissemination and

accessibility of scientific information generated within NBS and other agencies of the Federal Government; promotes the development of the National Standard Reference Data System and

a system of information analysis centers dealing with the broader aspects of the National Measurement System; provides appropriate services to ensure that the NBS staff has optimum

accessibility to the scientific information of the world. The Office consists of the following

organizational units: Office of Standard Reference Data — Office of Information Activities — Office of Technical

Publications — Library — Office of International Relations.

'Headquarters and Laboratories at Gaithersburg, Maryland, unless otherwise noted; mailing address Washington, D.C. 20234.

3 Part of the Center for Radiation Research.

3 Located at Boulder, Colorado 80302.

1 Part of the Center for Building Technology.
Review of Computer Networking Technology

Robert P. Blanc

National Bureau of Standards APR 2 9 1974

Computer Systems Engineering Division

Institute for Computer Science and Technology

National Bureau of Standards

Washington, D.C. 20234

\ ^

<F

Vau o? *

U.S. DEPARTMENT OF COMMERCE, Frederick B. Dent, Secretary

NATIONAL BUREAU OF STANDARDS, Richard W. Roberts, Director

Issued January 1974
National Bureau of Standards Technical Note 804

Nat. Bur. Stand. (U.S.), Tech. Note 804, 135 pages (Jan. 1974)

CODEN: NBTNAE

U.S. GOVERNMENT PRINTING OFFICE

WASHINGTON: 1974

For sale by the Superintendent of Documents, U.S. Government Printing Office, Washington, D.C. 20402 (Order by SD Catalog No. C13.46:804). Price $1.55
FOREWORD

This report is one of a series of publications produced

by the Institute for Computer Sciences and Technology, National

Bureau of Standards, under Grant AG-350 from the National

Science Foundation.

This grant supports a broad program of investigation into

the foundations of computer networking in support of scientific

and related educational efforts.

A listing of completed and planned publications produced

by the Institute under this grant follows:

1. Primary Issues in User Needs

D. W. Fife

Chapter 10 in Networks for Research and Education:

Sharing of Computer and Information Resources

Nationwide

MIT Press, Cambridge, Mass.

Expected Publication January 1973

2. Some Technical Considerations for Improved Service

to Computer Users

T. N. Pyke, Jr.

COMPCON, 1973

Seventh Annual IEEE Computer Society International

Conference

3. Computer Networking Technology - A State of the

Art Review

T. N. Pyke, Jr. and R. P. Blanc

COMPUTER Magazine

Computer Society of the IEEE

August, 1973

4. Review of Network Management Problems and Issues

A. J. Neumann

NBS Technical Note 795, October 1973

in
5. Annotated Bibliography of the Literature on Resource

Sharing Computer Networks

R. P. Blanc, I. W. Cotton, T. N. Pyke , Jr., and

S.W. Watkins

September, 1973

NBS Special Publication 384

6. Network Management Survey

I.W. Cotton

NBS Technical Note 805, expected publication December,

1973

7. User Procedures Standarization for Network Access

A. J. Neuman

NBS Technical Note 799, October, 1973

8. Review of Computer Networking Technology

R. P. Blanc

NBS Technical Note 804, December, 1973

9. Microeconomics and the Market for Computer Services

I.W. Cotton

Submitted for publication

10. Cost Analyses for Computer Communications

R. P. Blanc

NBS Report, expected publication December, 1973

11. Network User Information Support

A. J. Neumann

NBS Technical Note 802, expected publication December,

1973

12. Quality Service Assurance Experiments

R. Stillman

NBS Technical Note 800, expected publication December,

1973

13. A Guide to Networking Terminology

A. J. Neumann

NBS Technical Note 803, expected publication December,

1973

14. Research Considerations in Computer Networking

D. W. Fife

NBS Technical Note 801, expected publication December

1973

IV
CONTENTS

Page

1. INTRODUCTION 1

2. FUNCTIONAL CAPABILITIES OF THE COMMUNICATIONS

NETWORK 6

2.1 Interactive 6

2.2 Remote Job Entry 15

2.3 Minicomputer-based Systems 19

2.4 Computer-to-Computer 22

3. TECHNOLOGICAL FEATURES 26

3.1 Communications Circuits 26

3.2 Communications Hardware 31

3.3 Communications Software 36

3.4 Configuration 41

4. EXISTING NETWORKS . 45

4.1 TYMNET 46

4.2 CYBERNET 59

4.3 GE Information Services 70

4.4 MERIT 81

4.5 ARPANET , 91

4.6 TSS 104

5. CONCLUSIONS 112

REFERENCES 125

v

Review of Computer Networking Technology*

Robert P. Blanc

This report gives a descriptive summary of the technical

characteristics of existing computer networks, including data

communication technology and configuration related to support

of resource sharing services for a computer network. Included

are discussions of terminal support capabilities for the communications network and a development of relevant network

terminology. The report concludes with a comparative evaluation of existing technological approaches to networking.

Keywords: Computer networks; computer -to -computer transfers;

interactive terminals; minicomputer-based systems; network configuration; remote job entry; resource sharing.

1. INTRODUCTION

A review of the technical characteristics of existing

approaches to computer networking is presented for the purpose of comparative evaluation in relation to their potential

for supporting resource sharing for the Networking for

Science Program of the National Science Foundation [1] . For

the purpose of this review, a computer network is considered

to be any interconnection of an assembly of computer systems

and/or terminals together with communications facilities [2]

Resource sharing in a computer network refers to the sharing

among network participants of computer resources, including

hardware, software, and data files. The purpose is to make

available, nationally, special computer resources which are

not locally accessible, such as processors unique in speed

or storage capabilities, specialized programs of interest to

*This work has been supported by the National Science

Foundation under Grant AG-350.
a distributed clientele, and large data files too expensive

to reproduce. The coverage in this review is of sufficient

depth but general in scope so that it should be useful to

those planning, implementing, evaluating, and using other

resource sharing computer networks. It is impractical to

examine each technical detail of a network and relate it

directly to resource sharing, since such a one-to-one

correspondence is probably non-existent. But the features of

a network will be described and, in whole, related to a set

of network support requirements which are themselves prerequisites for sharing computer network resources. The categorization of the requirements will be by terminal or communications support, not by the end service provided, such as

program preparation or information retrieval. The purpose

in discussing these communications support functions is to

identify techniques for making the user interface to existing networks more flexible and effective and to provide

services that do not presently exist.

These support functions, which will be discussed in the

first part of the paper, are the capabilities of supporting

various classes of computer-based service, including interactive or conversational, remote job entry, where input and

output are performed with card readers and line printers

respectively, the support of minicomputer based systems such

as automated labs, and computer-to-computer communications.

Some of these capabilities are considered more essential

2
than others and this will be clarified in the text.

The next section is devoted to the description of the

technical characteristics which can be used to classify a network. These attributes will be described, and alternatives

in implementation, compared. Advantages and disadvantages

for application in the Networking for Science Program are

presented where possible. Of course, in most cases, it will

not be clear whether a particular feature by itself is advantageous. Rather, it is the combination of technical attributes

in the implementation of a network that can be evaluated. A

byproduct of these discussions is the development of a working terminology for the technological aspects in computer

networks.

The various technological approaches to networking in

several selected existing networks are then examined. Completeness has not been achieved, but a substantial number

of networks will be considered as representative of the

available approaches to network technology. These networks

include: TYMNET, the commercial timesharing network of Tymshare, Inc.; CYBERNET, the Control Data Corporation commercial network; GE Information Systems, the General Electric

Corporation commercial network; MERIT, the Michigan Universitys' network; ARPANET, the experimental network of the

Department of Defense Advanced Research Projects Agency; and

TSS, an experimental network of IBM 360/67 Time-Sharing

System computers. This report is confined to existing

3
approaches, and while suggested improvements in those

approaches may be indicated, there will be no attempt to

specify a new, but non-existent approach to network technology. A discussion of a network technology employing only

off-the-shelf concentrators and multiplexers may be important

for cost comparison, but is not addressed by itself in this

report. The technical aspects of such an approach are wellunderstood and in addition, its application to a network of

distributed heterogeneous computers is inconvenient at best.

Concentrators and multiplexers will, however, be considered

as specific network components, and at least one network

making extensive use of concentrators, along with more

sophisticated switching computers, will be discussed at

length.

Each technological approach will be discussed in terms

of technological characteristics and their ability to

support the functional requirements. In addition, each of

the networks possesses a number of salient features which

will also be identified. The information presented in this

section was obtained through personal contact with principals of the networks and/or through extensive investigation

of the literature including that referenced in the annotated

bibliography on resource sharing computer networks [3]

The last section evaluates the alternative approaches

indicating the desirable features and issues related to the

requirements of the Networking for Science Program. Although

4
it is difficult to separate technical and economic concerns,

this review concentrates on technical factors - desirable

technical characteristics and constraints of the technology

because of various mixes of performance requirements.

It is important to note that, although specific networks

are identified, there is not intent to favor or criticize

those particular networks. Rather these networks have been

chosen as well-known representations of important approaches

to networking technology and it is the review of those

technologies that is the purpose of this report.

NOTE: The identification of certain commercial equipment and

networks in this report is done in order to adequately identify

the network components and technologies discussed herein, and

in no sense does it imply recommendation or endorsement by

the National Bureau of Standards.
2. FUNCTIONAL CAPABILITIES OF THE COMMUNICATIONS NETWORK

Alternative technologies should be evaluated in terms of

their responsiveness to user needs. One approach is to

identify those needs and, subsequently select a communications

technology to satisfy them. This approach is iterative in

nature; that is, the best available data concerning user needs

is employed in evaluating different alternatives, and as the

data improves, so will the evaluation. At this point in time,

a number of basic requirements can be identified in terms of

particular types of usage that the communications network

should be capable of supporting. These types of usage to be

considered are interactive, remote job entry, communications

with minicomputer based systems, and computer- to-computer

transfers

2.1 Interactive

Interactive uses depend upon the use of teletypewriter

or equivalent terminals to communicate with a remote computer

system. Computer terminals are the mechanisms by which information stored in computer data banks becomes translated into

action by way of human beings. Of importance in evaluating

a communications network are the habits of the terminal users,

the characteristics of the terminals themselves, and the constraints placed on the communications network by these terminals . Code, speed, and mode of operation are important characteristics of terminals directly related to the communications

6
facility. There does exist a standard code for information

interchange, ASCII, as specified in Federal Information Processing Standards (FIPS) Nos. 1 and 15, and this code should

be supported by the network. For systems to be a part of a Federal installation, this support is in fact a requirement.

In general a network must impose a series of constraints to

operate effectively, and in the case of terminal to computer

communications, the standard code is a constraint which is

beneficial in that it diminishes the time and money spent on

interfacing terminals, a cost ultimately borne by the user.

Later when we examine the ARPANET we will see that its terminal interface is capable of supporting non-standard devices.

This may at first appear desirable, but consider the scarce

resource, main memory, that must be devoted to the variety

of non-standard devices. In the ARPANET example, over onethousand (1000) words of memory of the twenty-eight thousand

(28,000) word minicomputer terminal interface are dedicated

to software and tables to interpret the code of the non-ASCII

devices [2]. It has been suggested that users of non-standard

terminals pay an additional charge for the added resources

that they consume, and this would appear to be one reasonable

alternative

Speed of terminals can range from 110 baud for a simple

teletype to 9600 baud and greater for a C.R.T. Terminal

with vector capability. Many communications networks are

restricted to speeds considerably lower than that maximum

7
on the high capacity outgoing channel (that connecting the

remote concentrator to the computer or computer front end)

and are even more restricted on the slow speed input channel

(that connecting the terminal to the concentrator or multiplexor) and are therefore incapable of supporting the higher

speed devices.

Speed recognition, the automatic identification of terminal speed by the network, although not a basic requirement*

is another capability worth considering in evaluating technologies to support interactive usage. When the terminal

interface servicing dialup ports is capable of supporting a mix of speeds without devoting particular ports to particular

speed devices, it will statistically provide better service

in terms of the probability of obtaining a free port for

attaching a device to the interface. That is, it can be

shown that the probability of obtaining a free port is greater

if all ports are accessible to any speed device than if subsets

of the ports are devoted to particular speed devices. The

physical realities of the two situations are: in the first

case a user can dial a single number regardless of the speed

of his device to obtain access to any port (ports are on a rotary) which will adapt to his device speed through automatic

speed recognition; or for the latter situation, the user will

dial, for example, three different numbers for three different

devices to obtain access to any of a subset of ports (the subset is also on a rotary)

8
The modes of operation of a terminal, which for many terminals is switchable, can be full-duplex or half-duplex transmission depending on the terminal itself, and the computer

system on the network with which the terminal is communicating

Half-duplex refers to the case where the user's keyboard is

connected to the printer at the terminal, and to the convention that when the computer is producing output it does not

respond to input from the user (except perhaps for a special

"break" signal) . When the computer is not typing it is in

a mode to accept input from the user [4] . Full-duplex refers

to the case where the user's keyboard is locally disconnected

from the printer mechanism. When characters are received at

the computer, they are returned to the user. The returned

character or characters may not be identical to the received

character; for example, some systems will return a control

character as an up-arrow followed by the character ("ETX"

is sometimes returned as "+c") . The process of sending the

received characters, exactly or modified, is often referred

to as the echo-plex mode of operation. Full-duplex, however,

also implies the separation of input and output to the point

where a user at a terminal can be typing input while the

computer is sending him output. In this case "echoing" will

be delayed until the output has been completed. It is not

unusual, nor particularly distasteful, to hear the terms full

duplex and echo-plex used interchangeably although it
is preferable to think of echo-plex as a subset of fullduplex (that subset related to the actual echoing of

characters)

The communications network must be capable of fielding

these alternatives. For example, if echoing is desired in

the full-duplex mode, then in the network it must be determined which communications component is to perform the echoing and when. Some networks allow the interactive user to

set and alter the mode as desired, while others are far less

flexible. As will be seen in later discussions, the proper

control of echoing is important to the efficient use of a network.

Line utilization generated by interactive usage is

another consideration relevant to communications technology.

Models together with actual data, for example, that derived by

Jackson, Stubbs, and Fuchs of Bell Telephone Laboratories, can

be used to predict communication facility utilization as a func

tion of the number of interactive users, their application, and

the speed of their terminals [5,6].

The "data stream" model derived is used to study the

characteristics of holding time, defined as the time from

connect to disconnect between a terminal and a computer

system. However, the model has general applicability in the

analysis of communication facility utilization and the tailoring of a communications system to the characteristics of interactive users and computer systems. Using the "data stream"

10
model it was found that system load can greatly effect holding times, that user delay or think time is a significant

component of holding time , that user send time is insignificant, and that computer transmit time is the smallest

component of holding time. So the model provides a tool for

determining the percentage of holding time during which useful work is performed and a means of studying ways to adapt

communications systems to terminal traffic characteristics.

Statistics are also available from several networks in

terms of the average number of characters generated per second

by interactive usages at specific data rates and the number

of interactive users that can be supported on a communications

link of a specified capacity. Specifically, it was learned

that on the TYMNET network using interactive users in the

speed range of 110 to 300 baud, the average traffic generated

by a user including both sending and receiving is 5.5 characters per second; and it is possible to support forty-six (46)

interactive users over a 2400 baud communications circuit.

Similar figures for the GE network indicate terminal duty

factors based primarily on terminal output statistics to be

7 characters per second for 110 baud terminals [7]

Using the model of Jackson and Stubbs, the figures from

GE and TYMNET and the preliminary data of a Network Measurement Machine [4] developed at the National Bureau of Standards,

the following tabulation has been generated:

11
Average Characters Per Second (Half Duplex)

Max User System Combined

Network Measurement Machine

(applied to scientific users)

TYMNET

GE

Jackson, Stubbs

(moderately loaded scientific)

10

10

10

30

2.3

0.5

7.5

5.0

7

21

4.5

10

10

15

0.48

0.27

0.8

8.8

2.1

8.3

3.6

1.5

4.0

(heavily load scientific)

(moderately loaded business)

In the above the maximum utilization "max", refers to

the channel capacity. The channel is composed of modems,

circuits, interfaces, and terminals, and its capacity is

determined by the most limiting component. In the preceding,

the terminal speed of 110 baud determines the channel capacity;

because the commonly used modems., 103 A's or equivalent, are

capable of 300 baud and voice grade circuits are capable of

operating at even higher speeds. The column labeled "user"

indicates the average number of characters generated per

second when the user has the channel. Typically this will

be during the period of user think time combined with user

transmit time. The column labeled "system" refers to the

average number of characters per second generated during the

time at which the system has the channel. This time will be

the sum of response time and transmit time. The "combined"

column is the average weighted by time.

12
This kind of data is essential for the analysis of the

relationship in terms of loading between the communications

network and the interactive user. In many networks, a concentrator is used to interface interactive terminals on

its low speed side to the network on its high speed side.

Assuming that the bandwidth of the minicomputer used as a concentrator in terms of speed and buffer space will not be

the limiting factor, then knowing the average traffic generated by a given speed terminal, and knowing the efficiency

of the communications discipline, the number of terminals that

can be serviced on a given high speed channel can be calculated

Average traffic statistics are somewhat useful since

concentrators employ asynchronous multiplexing; that is,

bandwidth under-utilized by some terminals will be utilized

by those requiring it rather than devoting fixed time slots

to terminals regardless of utilization as is the case with

synchronous multiplexing [8] . The averages though can be

somewhat misleading in that averaging holds only for large

numbers of terminals and the peaking effects are somewhat

unpredictable. So the averages are more meaningful for

concentrators that support large numbers of terminals. The

efficiency of the communications discipline indicates the

amount of actual data that can be transmitted in a message

block allowing the overhead. These factors all taken

together might give an accurate projection of compatibility

between a high speed communications channel and a number of

13
interactive terminals for a particular concentrator, but

the analysis in fact is somewhat more complex. Most

communication facilities are symmetric; that is, input bandwidth equals output bandwidth, while user interactions with

a system are typically asymmetric heavily favoring output or

traffic from the system to the interactive terminal. Therefore, total average traffic or average number of characters

transmitted is not as significant as the average number

received by a terminal from a system since traffic in this

direction will correspond to the greatest line utilization

and is the traffic that the line capacity must match. Further

more, these averages taken over a total network with a large

number of users and different kinds of terminals and systems

are not nearly as useful as burst characteristics relative

to particular kinds of uses, known terminal characteristics,

and given systems.

A Network Measurement Machine has been developed at the

National Bureau of Standards for measuring the activity of

interactive users, communications facility utilization, and

the performance of a network as seen by its users [4] . This

device interfaces at some point between users and systems

and, operating in transparency to both system and user,

records the entire dialogue, time tagging and identifying

as to source each character received. This makes it possible

to select networks, systems, mixes of terminals, and types

of users described by discipline or application and vary

14
whatever parameters are appropriate to arrive at the desired

results on traffic characteristics. For a system designer

this type of measurement makes it possible to tailor a communications facility to a specific class of users or to

vary the mix of users to accommodate an existing communications

facility. From the users standpoint, application of the

Network Measurement Machine allows a network subscriber to

specify performance requirements for a communications network,

and to then monitor the network performance to see that the

requirements are consistently met. For example, if response

time is the criterion it is possible to measure the communication network plus system performance (computer system with

which the terminal is interacting) or to measure only the effect

of the communications network. This can be done by comparing

data collected when the terminal is connected through the

network with that obtained when the terminal is connected

directly to the computer system bypassing the network. This

latter type of measurement should be particularly useful to

users of a communications network.

2 . 2 Remote Job Entry

Remote job entry is a computer usage mode employing card

readers, line printers, and possibly card punches located

remotely to submit jobs and receive output from a computer

system. Communication with a remote job entry device will

typically require a higher data rate than communication with

interactive terminals and synchronous instead of asynchronous

15
transmission as in the case of interactive terminals. With

asynchronous transmission, each character is initialized by

a start bit and terminated by a stop bit adding two bits of

overhead to each character transmitted. Synchronous transmission is continuous with regular timing, the bits of one

character immediately followed by those of another with no

start and stop bits and no pauses, thus giving the most

efficient line utilization. Besides being inherently faster

because of the elimination of start and stop bits, synchronous

transmission uses higher speed modulation techniques than

asynchronous transmission; specifically multilevel signaling

can be used. Also, synchronous transmission can tolerate a higher degree of distortion on high speed circuits than can

asynchronous transmission. In addition to the speed advantage,

there is also an advantage in the integrity of the data transmitted. The stream of characters is normally divided into

blocks for synchronous transmission. At the end of each

block an error-checking pattern is transmitted based on the

contents of the block. The receiving system must generate

the same pattern or an error is realized. Acknowledgement

of good data and timeouts and/or negative acknowledgements

for bad are used to control retransmission when necessary.

The specifications of the block size, the method of error

detection and correction, and the formatting and interpretation of the data transmitted determine a protocol.

16
The synchronous communication requires the development

of a synchronous protocol to establish the proper two-way

exchange of information between the remote job entry device

and the communications network. This protocol may vary

among devices and therefore requires flexibility in the

network to recognize and adjust to different devices, or

restrictions as to which devices are allowed on the network.

ANSI Standard X3. 28-1971 specifies procedures for the use of

communications control characters of American National Standard Code for Information Interchange (ASCII) in specified

data communication links.

Many existing networks provide adequate support for

interactive terminals, but make no provisions for remote

job entry devices in their communications network. This

raises an interesting question. Should remote job entry be

supported on the same communications facility as interactive

terminals? Remote job entry and interactive uses can entail

different traffic patterns and quantities, different communications protocols, different buffering requirements in the

communications network, different interfaces, and different

response requirements. It may be true that the network that

most effectively and economically supports interactive terminals

is not efficient for remote job entry. On the other hand,

the potential "economies of integration" of providing a single facility to handle both kinds of application cannot be

ignored, nor can the convenience to the user requiring both

17
of finding them on the same network. Therefore both courses

should be considered in the evaluation of alternatives.

As will be examined in more detail later, existing networks have taken different approaches to handle this mix of

needs. TYMNET is tailored for interactive traffic, and for

the moment, ignores remote job entry [9]. CYBERNET supports

remote job entry traffic on its wideband links [10] . GE

was developed for interactive use with background batch

systems accessible through the interactive network [11].

ARPANET will handle both with the same communications network,

but it is not clear what the effect on response time is of

mixing interactive and remote job entry traffic under conditions of heavy loading [12] . It should be realized that

the differences in the device characteristics are customarily

resolved at the interface and then transmission is synchronous

over the high speed circuits of the communication network. The

distinctions to the communications network are those of the

different kinds of traffic generated and the different response

times required. Interactive users will generate small messages

(block size may be constant but the actual data within the

block is small in amount) and require rapid, consistent response

times of less than two seconds. Remote job entry users may

transmit large messages or multi-block messages being interested

more in total throughput than in response time to any particular;

message. There has been a proposal to divide a communications

link into two logical channels, one being a demand channel

18
for short messages and quick responses, the other being a reservation channel for long messages [13] . The reservation

for that channel could be made over the demand channel. This

is meant to be particularly applicable to satellite links

and presents an interesting concept for continued research.

2.3 Minicomputer-based Systems

Minicomputers can communicate with large computer

systems for the purpose of providing support to the large

systems in the form of front end processing or device control, for example, or the minicomputer can be the receiver

of support from the large system where the system provides

a service more economically than can be performed in the

minicomputer, such as cross-compilation (advantages in speed

and editing capability), large computations, and large file

storage. In the former, minicomputers can be an integral

part of a network, acting as an interface between the user at

his terminal or remote job entry device and the network of

computers. This supporting role of the minicomputer is very

important when discussing network functions in relation to

components, but it is the latter interaction - the network

supporting the minicomputer - that is relevant to the

discussion of network capabilities and customer needs.

Minicomputers can relate to a network in a hierarchy of

three levels.. At the lowest level there is the minicomputer

or intelligent terminal which will be the user of the network analogous to a human user at a terminal. At the second

19
level is the interface to the network which quite probably

will also be a minicomputer. The interface is analogous to

the terminal interface (often a concentrator) in the case of

the human user. This interface for the minicomputer -based

system (hopefully a single interface can control many minicomputers) can do as little as provide access to the network

or can be quite sophisticated both in the services it provides

as an interface and the services it can provide locally which

will eliminate the need to access the network in some cases.

For example, a sophisticated interface could recognize and

resolve differences between the minicomputer based systems

and it could realize protocols for performing simple tasks with

remote systems on the network such as logging in. Furthermore, it could provide simple compilers and file storage to

the mini-based systems so that they could perform those

tasks without the necessity of even using the network. In

both cases the support can occur in varying degrees depending

upon the intentions of the designer. At the third level is

the network itself with its variety of host computers.

Such a hierarchy is presently being implemented at the Institute for Computer Research at the University of Chicago [14]

The minicomputer, which might be monitoring or controlling a laboratory experiment or controlling a number of

other minicomputers which are themselves controlling experiments, needs certain services from a larger computer or

computers on the network. For example, the minicomputer

20
could store its large files on the disk system at a larger

remote computer. A larger computer might have a cross-compiler

which has the capability of producing code which will run on

the minicomputer. Due to the relatively high cost of minicomputer peripherals [15] , a number of minicomputers could share

a set of peripherals located somewhere on the network. The

minicomputer could be sending data to a larger system for the

purpose of obtaining large scale computations.

These services are obtained by the minicomputer through

interacting with the processes on a larger system much the

same way as a human user would. Difficulties arise, however,

in that the outputs from these processes are somewhat unpredictable and require responses from the observer. Since the

observer is a minicomputer, it must anticipate all system

responses, which as stated can be unpredictable. The introduction of a network of computer systems with which the mini

can interact to receive services compounds the problem as

the proliferation of heterogeneous systems makes responses even

less uniform. This indicates that a set of protocols should

exist on the network to standardize the interaction between

network computer systems and minicomputer based systems

that they serve.

A recent paper presents three (3) potential solutions to

the problem including : (a) special controlled protocol

channels in which a clean, well-behaved channel all the way

to the applications program is provided to the automation

21
user; (b) special filters at some point between the user and

the applications program to assure the automation user a uniform- looking network; (c) a very smart terminal node that

can take into account and react to a wide variety of messages

and initialization sequences [16]

The important consideration in evaluating an existing

network relative to minicomputer based systems is that it

be flexible enough to accommodate the communication needs

of the minicomputer and the development of the necessary

protocols. For example, sufficient bandwidth in the links

of the network connecting the minicomputer to the target

computer system must be available to facilitate the traffic,

anticipated to be more intense than that of interactive

human users. A file transfer capability would be useful,

for example in the application of cross-compilation where

source code and object modules are exchanged. Synchronous

communications between nodes is certainly important for

handling the volume of traffic generated at a satisfactory

speed. Therefore, although there may presently exist few

networks which satisfactorily support minicomputer based

systems, there do exist more networks commensurate with

the requirements for development of this support.

2.4 Computer- to-Computer

Computer-to-computer communication is comprised of interprocess transfers, where the processes are software processes

or programs running on different, separate host computers.

22
There are a number of kinds of applications for which

this kind of communications may be useful including: data

sharing, program sharing, load sharing, and multiprocess

solution to a problem. Data sharing is a mode of network

operation in which programs or data requests are sent from

various computers on the network to utilize a data base

resident at a particular node. This is particularly useful

when the data base is large and costly to transport or reproduce and the programs are relatively small. When the opposite

is true, large programs and relatively small amounts of data,

then program sharing may be appropriate. This is a mode of

operation in which data is sent to use a program resident

at a particular node. Load sharing is a mode of network

operation in which a workload is distributed among the

service computers on the network to achieve a balanced compute load at each site.

Multi-process computing is a mode of network operation

generating true process -to-process communications in which

an application requires the execution of programs concurrently

on several processors. One of the few examples of this type

of operation is the McRoss experiment which simulates an air

traffic control system through execution on distributed

PDP-10 TENEX systems on the ARPANET [17]. These kinds of

applications are certainly intriguing to the software

engineer but at this time are a limited utility to the

discipline-oriented customer interested in using a network

23
to support research, not as the object of research. To

allow for the possible incorporation of this form of communication at a future time, if it becomes stabilized and noncumbersome to use, requires a communications network consisting of links of considerable bandwidth to avoid propagation

delays and provide rapid interprocess communication between

computers, compatible to that internal to a computer. Sophisticated protocols will also have to be developed to facilitate

comfortable interaction between computers.

Special capabilities must be implemented to provide

for any kind of convenient computer-to-computer communication.

For a user to transfer a program from one computer to use a data base at another, for example, there should exist a standard, easy to understand and easy to use calling sequence

to trigger the interaction regardless of the systems in use.

Where such network control languages have been developed,

then computer- to-computer communications is an intriguing possibility for the applications user. Solving a problem with

programs running on distributed processes is certainly more

sophisticated than data or program sharing and at this point

in time does not constitute a mandatory capability for the

present considerations. In any case, for computer- to-computer

communications to be effective a substantial degree of reliability among participating systems is necessary when processes

are dependent upon all of those systems.

24
When evaluating existing networks, it becomes clear

that some are more capable of accommodating computer-to

computer communications than others. It must further be

determined if this capability has been developed at an

expense considerably greater than that required to satisfy

the other capabilities and if its usage results in the

deterioration of service for those other functions.

25
3. TECHNOLOGICAL FEATURES

In order to characterize the technology employed in an

existing computer network, it is necessary to consider the

features which are the components of the technology. There

are those features which can be used to describe the componential structure of the network and the interrelationships

of those components, and these will be described under the

category of communications. Other characterizations, take a more global view of the network, and these will be described

in the discussion on network configurations. Whenever practical it will be shown how a particular feature and the

manifestations of that feature relate to the functional

capabilities previously discussed. Particular features, when

considered in isolation, are not necessarily conducive to

analysis of their advantageous employment in a computer network, but rather it is the combined effect with other attributes that allows evaluation. Nevertheless, in some cases it

will be possible to make generalizations about the effectiveness of the usage of alternative components. The primary

purpose of this section, however, is to identify technological

features and clarify the terminology for use in the next

section, where existing networks are described and characterized.

3.1 Communications Circuits

In a computer network there are those circuits that interconnect the nodes of the network, circuits that connect ter26
minals with the network, and those circuits, logical or physical, which provide the end-to-end path between communicating

processes. Consider first the circuits that physically connect the nodes in a network. The transmission media here

can be voice grade lines or wideband and they can be owned,

leased from a common carrier, or dialup. Voice grade circuits

can be modulated to operate in the range of 110 baud to 9600

baud. Wideband lines operate from 19,000 baud to 250,000 baud

and some economies of scale exist if there is in fact a need

for that kind of bandwidth. For a more complete discussion,

see Reference 2. The functional requirements discussed earlier

could potentially be satisfied by voice grade lines, although

a lot depends on the topology and traffic patterns of the network, as it might, for example, be more economical to provide

a single very high speed link than to provide multiple lower

speed lines to accommodate known network traffic [18] . The

decision of whether to lease (or purchase) lines or to rely

on dialup facilities depends on utilization. First, dialup

lines are at present restricted to 4800 baud or less (2000

baud with common carrier provided modems) . This could cause

an unsatisfactory level of service to those functions performed

marginally at that speed, for example, remote job entry utilizing some of the faster and more sophisticated devices. In

addition, if holding times are significant then the dialup

facility will be uneconomical where toll calls are involved.

27
The second kind of circuit in a network is that connecting

a customer device - timesharing terminal, remote job entrystation, minicomputer based system, etc. - with the network

interface, possibly a multiplexer or concentrator. This circuit, part of the low speed channel side of the multiplexer

or concentrator, can operate at a maximum speed equal to that

of the circuit connecting nodes, but is usually sharing that

high speed circuit with a number of other low speed circuits

and thus operates at a considerably lesser baud rate. Voice

grade facilities are sufficient for most applications. The

speed achieved through wideband service can not normally

be utilized except, possibly, in a computer-to-computer

transfer where one computer is entering the network as a terminal port to interact with a remote computer doing likewise. Holding times with interactive terminals are usually

short enough to justify the use of dialup facilities, although

leased lines are sometimes employed in the more critical

applications to guarantee access. All of these physical circuits can be used as either full-duplex, allowing for twoway simultaneous transmission, or half-duplex, facilitating

two-way transmission but in only one direction at a time. This

is dependent upon the number of physical wires (2 or 4 wire)

,

or upon the modulation techniques used (2 wire can be used in

full-duplex mode)

Also the circuit can be synchronous or asynchronous,

the synchronous interface typically offering higher speeds and

28
better error checking at a higher cost. Ideally the network

should be capable of supporting both types of circuits for

terminal connections.

The third kind of circuit in a computer network can utilize

the other circuits in its establishment. This is the end-toend circuit which connects two processes, for example a program in a computer with a user at a terminal or two programs

in two different computers. In contrast, those circuits discussed previously connected two or more physical devices and

will be used in part to form this circuit, a physical or

logical connection between processes. If the circuit is

physical, then switching hardware will establish the circuit

making the proper connections at each switching point. If the

circuit is logical, than no direct physical connection exists

at any of the switching points, necessarily computers in this

type of communication between the input and output lines, but

instead, the connection is established in tables of the communications software of the computers establishing the connection, and is executed by other computers at each switching

point. In any case, whether the circuit if physical or logical, it remains in existence for the duration of the interprocess communication. When the circuit is physical, then

line or circuit switching is the terminology used to describe

the communications protocol. When the circuit is logical,

existing terminology does not adequately describe the con29
dition, but the term "message switching" has been employed.

Message switching, however, properly refers to communication

without the establishment of. a circuit, or perhaps it would

be more accurate to say that the circuit is established only

for the duration of a single message.

For discussions in this report, and to remain compatible

with similar discussions found in the literature, we will

make the following distinctions: line switching establishes

a physical circuit path, physically connecting the input path

to the appropriate output path at each switching point, and

exists for tht duration of interaction between processes;

virtual circuits are circuits that are established in the

tables of the switching computers logically connecting an

input port to an output port for the duration of interaction

between processes; message switching refers to the absence

of a circuit, logical or physical, relying instead on destination addresses found in the message itself so that the

switching computers can determine the appropriate path or

ports to use for the message to arrive at the known destination

For additional clarification line switching is commonly applied

to the public switched network accessed through dialing, the

virtual circuit concept is used in describing TYMNET [20],

and message switching occurs between switching computers in the

GE network [11] . In the ARPANET, a special variety of message

switching, called packet switching [19], occurs. In this case,

messages from a host computer are further divided, by the net30
work interface message processor, into packets for transmission

in the network.

It is not clear which kind of circuit technology is best

suited to a generalized network. Arguments exist favoring the

logical circuit and message switching concepts for making

better use of carrier facilities to increase effective throughput on a given speed circuit and for allowing greater flexibility in interprocess communication as to which process can

interact with what other processes over which circuit. This

is because many logical circuits can be established or many

messages switched with different sources and destinations

over a given physical circuit. Although this increased

capability employs a more sophisticated technology than line

switching and a proportionate degree of complexity in its

development and its subsequent use, it's importance is gaining

increasing acceptance [11, 19, 20].

3.2 Communications Hardware

Communications hardware encompasses devices for transmission,

switching, and termination of communications signals, such as

data terminals, modems, acoustic couplers, PBX equipment,

store-and-forward message switching computers, etc. Many of

these, such as modems and acoustic couplers, are not germane

to the discussion of existing networks. What is important to

the discussion of networks are the interfaces between the host

computers and the communications network, the interfaces between the users terminal devices and the communications net-

•31
work, and the message switching computer. Communications

network refers to the network exclusive of the user terminals

and host computers, or to paraphrase, those components of the

total network connecting the user terminals to the host computers. In ARPANET terminology, the host computer nodes and

the communication network are the "resource sharing computer

network", while the communications network is the "communications subnet" [19] . Selection of one type of network communications component over another is difficult to evaluate

without entertaining an economic argument, but where distinct

technical advantages exist relative to functional capabilities

they will be presented.

The interface between the host computer and the network

can occur in three different ways corresponding to three

different hardware considerations. First, the host computer

can be an integral part of the communications network, possibly performing a message switching function for the network

(where messages will be directed to itself or other nodes on

the network) , and thus requiring no special interface computer.

The IBM research network, Network 440 was configured this

way with a central computer acting as a host and a message

switcher [21] . Second, the host computer can be connected to

the network through the message switching computer, again

requiring no additional hardware other than the transmission

medium, as is the case with the ARPANET [19] . Last, the computer may be connected to a front-end computer which acts as

32
an interface to the message switching computer and therefore

the network, intercepting and controlling all input and output between the network and the host computer. The ARPA Network Terminal System (ANTS) could be applied as a front-end

computer in this manner [22]

The use of front-end computers has a lot of appeal in networking. It can perform the functions of network interprocess

communications control controlling the flow of messages between processes in the host computer and processes elsewhere

on the network, and it can be responsible for making and

breaking the connections between processes. These functions

can be performed in the host computer itself, but with three

distinct disadvantages:

(1) extensive revisions and additions to the host

system may be required;

(2) substantial overhead in the host computer may

be required to perform these functions;

(3) transportability is minimal or non-existant

except to computers with identical operating

systems.

Furthermore, a front-end computer in addition to acting as a host computer interface can also function as a terminal controller and possibly a peripheral controller. There are

however two disadvantages to using the front-end technique:

(1) two controllers may be required-- the standard

host controller plus the front-end computer;

33
(2) the host computer's network interaction is limited

to that supported by existing host communications

hardware and software.

The interface between the terminal devices and the communications network is usually a multiplexer or concentrator.

A multiplexer is a non-programmable device with predetermined

speed selections and a concentrator is a programmable computer.

Concentrators offer a number of advantages over multiplexers.

Multiplexers employ fixed time slots on the high speed link

to transmit data from the terminals and/or other devices being

multiplexed and cannot take advantage of under utilization

of some devices to accommodate increased utilization on others.

Concentrators, on the other hand, with their buffering capabil

ity and programmed logic can use the high speed link to the

optimal advantage of all devices [8] . Since concentrators

are programmable they can perform automatic speed recognition

to accommodate automatically different speed devices, whereas

multiplexers are limited to a single speed or predetermined

set of speeds. Concentration need not be performed on a computer devoted to that application but may be merely a function of a general purpose minicomputer which is also acting as one of the switching computers in a network such as is

done in the TYMNET [20] and ARPANET [23] . One seemingly

practical approach is to use concentrators as network nodes

and use multiplexers to access the concentrator ports when

terminal locations warrant it practical as is done on the GE

34
network [11]

The least attractive possibility for a network interface

is the use of the host computer itself. This situation occurs

when no special terminal interface has been developed for the

network so that terminals can access the network only by connecting to their host computers as is presently the case

with MERIT [24] and TSS [25]. This implies that to communicate with a remote system, processing must be done by both

the remote system and the users host system, driving up overhead and costs.

Message switching computers are also relevant to the

discussion of communications hardware. In order for a user

to establish a connection with a particular computer, he can

dial a unique number to make that connection, the number

being different for each computer to which he wishes to connect (the number can also be given to a computer which will

dial it by using an automatic calling unit). If a user, however, is to dial only one number, or be connected to one

leased line regardless of which computer o,r computers with

which he wishes to communicate, then a switching computer must

be employed in the network architecture. The switching computer performs the store-and-forward function, that is, a message is received enroute, stored only until the proper

outgoing line is available, and then retransmitted. In the

ARPANET, for instance, the design end-to-end delay through

several store-and-forward nodes is less than 0.5 seconds [26].

35
Each message contains an identification of the sending device

and an address for the receiving process allowing the switching

computer to route messages from any source to any destination.

The switching function can also be performed by mapping the

input ports of the switching computer to the appropriate

output ports [20] . Besides the inherent flexibility in software switching, the store-and-forward technology allows for

optimal line utilization, since interprocess messages can be

mixed with interprocess messages involving different host

computers and terminal devices on the network. Also, the

switching computer can perform other functions such as concentrating terminals and interfacing to service computers.

3.3 Communications Software

Communications software can best be discussed in relation

to the control it exhibits over a network. The ARPANET uses

a well-structured layered approach [27] and the terminology

describing this structure is useful in discussing network communications software. At the lowest level is the synchronous

communications protocol governing the exchange of information

between switching computers [19]. Next, the so-called first

level protocols control the exchange of information between

a host computer and its interface. Using these two protocols

in its implementation is the level two protocol controlling

the exchange of information between host computers [28]

Finally, there are the level three protocols which control the

information exchange between actual processes where a process

36
may be a computer program or a user terminal [27] . At this

level are the protocols which are very relevant to the user

in relation to the utility of the network. In the case of

ARPANET third level protocols exist to allow for initial connection to a remote process, to control communication between

a terminal process and a computer process, to transfer data

and files between processes, and to define remote job entry

communications

For a network to truly be a utility, there should exist

in the hierarchy, a network control language which is concerned with human considerations related to the use of the

diversity of systems that can exist on a network. In the

network control language there should be a set of standard

commands to perform common simple and intermediate system

control functions regardless of the system being used on the

network. For example there is a set of simple functions such

as logging in, listing files and directories, etc., which

should certainly be standardized in addition to some of the

intermediate functions such as running common compilers and

executing jobs.

The control functions from the lowest level on up can be

distributed to various network components. It is useful to

consider different possibilities to aid in understanding some

of the concepts applied and the relationship of alternatives

to networking considerations. If switching computers are involved some network component must be responsible for establish37
ing an initial connection between two processes desiring to

communicate (a customer terminal and a host computer program

for example) . This type of control can be centralized in the

network (even though topologically the network is distributed)

,

such that every process desiring to communicate with another

process on the network will do so by initially connecting

to the same computer which will establish the desired connection

[20] . Since a single computer is establishing all initial

connections it may be more convenient to approach standardization in the connection mechanism relieving the user of the

burden of learning a variety of protocols for the different

systems on a distributed network. There is, however, enormous

dependency on that single computer, and it is essential that

backup systems be incorporated into the architecture. Another

possiblity is to allow the interfaces to the network of the

involved processes to be responsible for establishing the

initial connection, thus distributing that function to all

nodes on the network [23] . This removes the dependencies of

those connections from any one computer implying that failures

will cause only local disturbances.

The control of traffic flow through the network is another

software function. The traffic flow between the various

communications components will be typically controlled by

ordinary line control procedures [29] . The flow control

between processes can involve special software in the computers on which those processes are running. This is the

38
situation with the ARPANET where interprocess flow control is

host resident [28] . An alternative is to use the regular

software controller of a particular computer to control interprocess communication. This can be done if all of the computers

and systems are similar [25] or if a front-end device performs

a mapping function between the network and the software controller [24] . Another alternative is to have a central computer be responsible for flow control accepting inputs from

all processes in remote computers desiring to communicate [21]

The separation of communicating processes and the computer or

computers which regulates the traffic flow between them can

lead to the capability of sending information to a process on

a computer which is not at the time available.

When a message can transverse more than one path in a network from a source to a destination, then the communication

software must perform a routing function. If the network is

circuit switched, then a physical connection is established

at each switching point, and remains so established for the

duration of the connection [25] . The software involvement

is minimal, in this case, and can, for example control an

automatic calling unit. If a logical or virtual circuit is

established, then communications software residing in some

network component or components must select the optimal path

for the establishment of that circuit [20] . This can be done

by comparing the conditions of the possible circuits to select

that with some combination of least traffic and least errors.

39
In a message switching network, each individual message, is

routed dynamically, or as' in the case of ARPANET, messages

are divided into packets, each of which is dynamically routed.

Considerable attention has been given to the development of

routing strategies for a packet switched network such that

circuits are efficiently utilized and high throughput can be

obtained [19, 30]

Another facet of network control is the measurement of

network performance. It is possible to generate statistics

to monitor the internal performance of the communications network, for example, queue lengths, message sizes, routing,

etc., to aid in tuning the system and providing fault detection and correction [31, 32]. Another set can be recorded

to provide more of an external measurement on network usage

[4] . Recordings of traffic flow between nodes can be analyzed

to determine proper distribution of facilities including

placement and number of service computers, switching computers,

and device interfaces, and bandwidth and topology of communication circuits. In addition usage statistics can be kept of

the different processes available at the service sites and of

the generic types of services available such as timeshared,

batch, or graphics. Both the internal and external measurements will aid in determining the effectiveness of the computer

network and the appropriate distribution of services.

40
3.4 Configuration

The preceding discussions have described network parameters

which identify the network technology at a local level in terms

of its components. To get a more global representation of a network technology, it is helpful to examine networks in terms

of configuration and configuration alternatives. Alternatives

will be considered relating to network topology, composition,

size, and distribution.

Network topology can be centralized or distributed. Centralized networks, also called star networks, are those in

which all nodes connect to a single node. This is a description of topology, only, and implies nothing about location of computing power, network control, or user distribution.

The alternative topology is distributed where, in the limit,

each node is connected to every other node, although the

terminology is commonly applied to topologies approaching this

full connectivity. There exists a third alternative which

more accurately describes a certain condition of a distributed

network. This is the ring configuration commonly used in describing the Distributed Computer System [33] . Observing the

topology a ring can be seen connecting the nodes of the network. In the case of TYMNET a number of such rings are then

themselves interconnected forming a multiring structure [20]

This is a special case of a distributed network.

For a network of distributed computers, a distributed topology would seem more practical for connecting existing computer

.41
centers, in addition to the reliability factors that favor a distributed network. If it were the intention in building a network of constructing one large and reliable computing

facility to which all customers could connect, then a centralized (or some modification of a centralized) topology

might be in order [34] . If the intent is to connect to existing distributed centers, and not radically disturb the balance

of computing power, a distributing topology is implied. There

exists a potential arrangement using advanced communications

technology where a network can be a centralized network while

the computing power remains distributed. Consider using

satellite communications to replace existing earth-bound longline circuits. Transmission from any user (human or computer)

to any process can travel to a satellite and, using the satellite's broadcasting ability, to the target [13]. This topolog

ically, is a centralized network that allows for distributed

computing power, and worth mentioning because a potentially

powerful transmission medium is considered. To avoid ambiguities

it is probably helpful to qualify statements of centralization

or distribution with references to topology, computing power,

switching, and network control. In the satellite example

distributed computing power with central switching is the

desired description.

As the terms composition, size, and distribution are fairly descriptive, little development is necessary other than the

definition of alternatives. Network composition can be homo42
geneous or heterogeneous referring to the host computers

on the network. If the network is homogeneous then all host

computers are the same and if heterogeneous, all are not the

same. The term heterogeneous is sometimes applied to networks having computers which are different models from the

same manufacturer [21]. For this report, the most meritorious

are those that are capable of connecting to the majority of

existing large scale computing systems and not just different

models of a computer. One of the details included in the

survey in chapter 4 of existing networks will be a list of the

actual computers connected.

The size of the network in terms of the number of nodes

and distribution or actual location of those nodes is also

important. For purposes of this report, it should be evident

that a network can support on the order of thirty (30) to

fifty (50) nodes with a national, and possibly, international

distribution.

A number of technical factors have been identified to

describe a network communications technology and characteristics

of configuration have been discussed, and at the same time,

a working terminology has evolved. Whenever possible, technological factors were considered independent of the other factors comprising a network technology, and evaluated using generalities as to their effective application in the Networking

for Science Program. It is now possible to investigate existing networks and discuss the technology applied using the

43
aforementioned terminology subsequent to the comparison of

those varied approaches and the technology or technologies

that they represent.

;

44
4. EXISTING NETWORKS

It was not considered practical to attempt to review

every existing network. The networks chosen are considered

a fair representation of the various current approaches to

network technology for networks of computers having potentially national scope. It is important to distinguish

between a technology, which is the primary concern of this

paper, and implementations based on that technology. This

distinction is necessary because networks not included in

this report may be reasonable alternative implementations

employing the technologies represented by those chosen.

In this section selected networks are classified in

terms of the technology or technologies that they represent.

For each network, following a brief overview accompanied by

graphical representations of the network, the communications

enployed will be identified in relation to communications

circuits, communications hardware and communications software using terminology developed in the previous section.

Following this is a description of the network configuration,

again relying on the aforementioned terminology. Then, if

a network possesses any salient features related to the

technology employed and in the author's opinion, particularly

noteworthy they will be itemized. Finally, the network

technology will be related to the previously discussed

desirable support capabilities for a computer network.

45
4.1 TYMNET

TYMNET is a distributed network of heterogeneous

computers (see Figure 1) . It exists primarily to connect

the users of the time-sharing service of Tymshare, Inc., to

the computers of Tymshare. However, because of the wide

distribution of communications facilities and the flexibility

in host and terminal interfaces the network lends itself

nicely to the connections of non-Tymshare host systems and

their users nationally distributed. This is different

from the kind of service that can presently be obtained

from common carriers in that value is added to the carrier

circuits because of the front-end, store-and-forward, error

detection and retransmission, and concentration processing

capabilities of the mini-computers which are an integral

part of the network.

Overview

The network consists of approximately ninety (90)

minicomputers, Varian 620' s, interconnected by common

carrier voice grade facilities. The Varian 620, when

employed in TYMNET, is called a Tymsat and serves in two

separate capacities, as a base Tymsat and as a remote

Tymsat. The base Tymsat is responsible for acting both as

a message switching computer in the communications network

and as an interface to the network for host or service

computers. When the host computer is one of those

generally supported by Tymshare, such as an XDS 940,

46
DEC PDP-10, or IBM 370, the Tymsat has been programmed as a communications controller replacing the standard components

for that function. Computers of a kind not employed by

Tymshare have thus far been interfaced to the net through

the Tymsat where the Tymsat sends and receives input and

output appearing as a rotary of asynchronous modems to the

standard communications controller for that machine, e.g.,

an IBM 270X or 3705 for an IBM 360 or 370. Presently under

development is a Tymsat to replace the IBM communications

controller. This will eliminate the need for the IBM 270X

or 370X and allow for greater flexibility in terminal

support. The base Tymsats are each connected to a host

computer in either a one-to-one or one-to-two fashion

(connecting approximately forty host computers throughout

the country) and to one another either directly or through

an intermediate base Tymsat in a multiple ring or distributed

manner. The circuits used are from 2400 to 9600 baud

synchronous, full-duplex, private leased lines.

Terminal Interface

The remote Tymsats act as store-and-f orward computers

and as concentrators for user terminals. Each is capable

of supporting up to thirty-one (31) asynchronous full-duplex

modems allowing for terminal speeds in the 110 to 300 baud

range. In addition, some remote Tymsats with added hardware

and software can support hardwired terminals in the 600 to

1200 baud range. The remote Tymsat can identify a terminal

47
by the first character typed as to its baud rate and carriage

return delay time. It is possible to allow a terminal to

connect with two different baud rates, one for input and one

output. In addition, conversion to and from ASCII with the

accompanying overhead is provided for EBCDIC or Correspondence coded terminals. The remote Tymsat also performs echoing when a user terminal is operating in echo-plex mode. The

remote Tymsats are connected to the base Tymsats, and thus

the host computers, in what is best described as a multiple

ring configuration. A circuit passes through a number of

remote Tymsats and one base Tymsat using store-and-forward

techniques to exchange information between any remote Tymsat

and any base Tymsat. The high speed links to the remote

Tymsats (versus the low speed links which connect the terminals) are again 2400 to 9600 baud synchronous, full-duplex,

private leased lines. Data is blocked and buffered and the

usual error checking and retransmission, as necessary, is

incorporated allowing for virtually error-free transmission

between Tymsats. Error detection between terminal and Tymsat

is accomplished through character echoing.

Character Transmission

When transmitting interactive traffic in a network there

are two related problems for which TYMNET has implemented an

interesting solution. The first is concerned with transmitting characters in echo-plex mode, which is almost always

the situation when interacting with full duplex systems. The

48
problem is to echo a character without any uncomfortable

delays to the user. An uncomfortable delay would result,

for example, if a user typed two characters in succession

before the first was echoed. To meet this "instantaneous"

echo requirement it is necessary to transfer a character

through the network to the service computer, assigning

the character and its echo a high priority. Of course,

transmitting a single character is quite costly in terms

of overhead since each unit of transmission must contain the

appropriate header and error checking information. This is

to identify the data as to source and destination and to

insure its integrity. The TYMNET implementation provides

a solution by having the remote Tymsat (that to which the

terminal is connected) perform all echoing requiring

immediate response (i.e., type ahead does not require

immediate response) . The second problem is that of reducing

the "overhead to data" ratio in message transmission.

Remote echoing helps by making it possible to buffer, prior

to transmission through the network, an entire line of input from a user; because it is the line and not each

character in the line that requires a response. The TYMNET

approach to the blocking of data goes further to ameliorate

the overhead problem by collecting, at each Tymsat, the input from multiple sources destined for the same output port,

to form a single block. The maximum block size is 66

characters and the attempt is to approach that maximum prior

49
to transmission thus' increasing efficiency in terms of the

message overhead to data ratio. Of course there is computational overhead at each node (Tymsat) since sending nodes

must assemble the characters of a block and receiving nodes

must disassemble them.

Virtual Circuits

Blocks are transmitted through the network over fullduplex virtual circuits. These circuits are established at

log- in time and exist for the duration of the interaction

between terminal and processor. The circuits are not

physical but exist as table definitions in the software of

involved Tymsats associating an input channel with the

appropriate output channel at each switching point (Tymsat)

A circuit is established by the network supervisor (computer

program) in active mode (Sam) . TYMNET provides multiple

paths for circuits in the event of line failure. When a user connects to TYMNET he is originally communicating with

the Sam which after the appropriate exchange of information

will establish a circuit from that user's terminal to the

proper service computer by selecting the appropriate Tymsats

to complete the virtual circuit. The Sam is so named because

it is a function which can be overtaken by one of three

backup supervisors in the event of failure. A backup

supervisor will become active by detecting a failure, polling

the Tymsats to get network status information, and assuming

the active role. This is performed transparent to users

50
other than those who may have been connected to the processor running the Sam that failed. It should be noted

that in the event that a processor fails, its base Tymsat

can still function in the role as a network store-and-forward

computer. Since virtual circuits are established and fixed

for the duration of processor interactions, recovery from

a physical line failure is not as clean. When a line fails,

users communicating through virtual circuits using that line

in their definition must reconnect.

At this time the TYMNET computer network can have 900

individual users simultaneously connected to the network.

The communications network exhibits a high degree of

reliability with a stated average 1.4 failures per node

per year. Presently, most usages are through interactive

terminals because the TYMNET does not support remote job

entry. However, high-speed printers exist in many field

offices to which large volume printing may be routed.

51
H

52
Communications

Communications Circuits

Internode - circuits are voice grade, leased, synchronous

operating at 2400, 4800, 7200, or 9600 baud.

Terminal to network - circuits are full duplex dial-up

or hardwired operating at 110 to 300 baud

(dial-up) , 600-1200 baud (dial-up in certain

cities) , and 600-1200 baud (hardwired)

Interprocess - virtual circuit established at log-in

time by the network supervisor in tables in

the store-and-forward nodes.

Communications Hardware

Single host interface - a Varian 620 (8K of 16 bit

words, 1.8 ysec cycle time) called a base

Tymsat, also acting as a store-and-forward

node.

Dual host interface - a Varian 620 L-100 (12K of 16 bit

words, 1.0 y.sec cycle time).

Terminal interface - a Varian 620 called a remote Tymsat

also acting as a store-and-forward node.

Each remote Tymsat supports up to 31

asynchronous, full-duplex terminals and

performs speed recognition, local editing,

and character conversion.

53
Communications Software

Initial connection - all users initially connect to the

same processor, the supervisor in active

mode, (Sam), which will establish the

optimum communications circuit between the

user and the appropriate processor on the

network. Alternate routing occurs during

leased line outages. The Sam also acts as

a service computer on the network.

Protocols - computer processes communicate by looking

like terminals to one another.

54
Configuration

Topology

Distributed - service computers, store-and-f orward nodes,

and user terminals are well distributed and

interconnected in a multiple ring configuration . Actual distribution - international connecting 53 cities

throughout the nation in addition to a link

to Paris , France . Composition

Heterogeneous - XDS 940

XDS Sigma 7

DEC PDP 10

DEC PDP 9

DEC PDP 15

IBM 370/155

IBM 370/158

IBM 360/40

IBM 360/50

IBM 360/65

IBM 360/67

Burroughs B6700

Honeywell G275

Univac 1108

CDC 6400

55
Size - approximately 40 host computers, over 90 communications nodes serving 900 individual users

simultaneously.

56
Salient Features

TYMNET is reliable with an average number of failures

of 1.4 per year for a Tymsat and an error rate of less than

9

one bit in 4 x 10 bits transmitted.

The communications facility is efficiently utilized

supporting approximately 46 users operating at speeds of

110 to 300 baud on a 2400 baud circuit.

Terminal recognition is automatic with baud rate,

character rate, and carriage return delay time detected and

input and output speeds may be set independently.

The network supervisor in active mode which controls

all connections can be transferred in the event of failure

to another processor without effecting users. It maintains

the current status of all network activities.

TYMSHARE will lease the communications service (TYMNET)

exclusive of its host systems, allowing a customer to connect his own computer to the network to serve his own users.

This service is charged for on the basis of value added to

common carrier facilities and operates under FCC Tariff

Number 260, Shared Facilities. The service is presently

used by many organizations including the National Library

of Medicine in Washington, D. C.

57
Terminal Support Capabilities

Interactive

Code - network code is ASCII although terminals mayoperate in ASCII, EBCDIC, or Correspondence code.

Speed - 110 to 300 baud dial-up and 600 to 1200 baud

hardwired (requiring on-site Tymsat)

Mode - full-duplex.

Remote Job Entry

RJE support does not exist but is planned for future

implementation. A synchronous protocol may be developed

in a Tymsat-like device to support RJE devices of the

IBM 2780 variety.

Minicomputer-based Systems

No minicomputers are on the network other than as an

integral part of the communications net (Tymsat) . There

is no support for synchronous communications at the user

level and the bandwidth is presently limited to 9600

band on the high speed circuits.

Computer- to -Computer

One computer can connect as a terminal to another and

vice versa allowing for interprocess communication.

Files can be transferred from one processor to another

in this manner.

58
4.2 CYBERNET

CYBERNET is a commercial network of large to super scale

Control Data Corporation (CDC) computers and it operates as

a division of CDC. These computers have been connected to

a communications network (see Figure 2) to form a national

computer network satisfying a number of important objectives

Reliability is increased by allowing a user to access an

alternative computer in the event of a failure of his local

computer. Workloads can be leveled among the participating

systems by routing jobs from loaded systems to unloaded

ones. Computers can be better utilized by specializing in

applications to which they are best suited and the network

allows for the routing of jobs requiring those applications

to the proper computers. Large data bases can be kept at

selected sites and need not be copied at each site to be

accessible since the network will provide access as required

Also access to the super computer CDC 7600, when connected,

will be more generally available with the national CYBERNET

network. This presentaion is limited to the U.S. but CDC

is evolving similar networks using the same technology in

Europe and Australia, and on a smaller scale, in other

i countries

Overview

The technology employed by CYBERNET is difficult to

characterize. It is heterogeneous in that it interconnects

different models of Control Data computers including the

59
.

CDC 6600, CDC 3300, and CDC 6400, and plans exist to connect

a CDC 7600. The communications links interconnecting com

puters are a combination of voice grade and wideband, and

even include a satellite link. The switching mechanism

employed is mostly line switching but a special use of message switching exists. The topology, control, switching,

and computing facilities are all distributed. CYBERNET

appears to be an add-on network where new technologies are

incorporated as needed, void of commitments to particular

hardware, software, or communications facilities.

Centroids

The bulk of the computing power for the network is

performed by the centroids. The centroids are all currently

CDC 6600' s operating under SCOPE operating system, although

because of the SBC acquisition, plans call for connecting

IBM 370' s. Access to the centroids is primarily via wideband batch terminals (see "Terminal Interface") . The

centroids are interconnected with full duplex 40,800 baud

wideband communication links. The switches employed are

multiway wideband switches. These switches connect to

each other in a distributed fashion, to the centroids, one

per switch, to the nodes, one or more per switch, and to

the wideband batch terminals. The CYBERNET concept of nodes

will be discussed later. The switches allow the terminals

to access the facilities of the network providing flexibility

in that the terminal can connect to any centroid, and provid60
ing down time back up and load leveling.

It should be pointed out that load leveling is not

automatic, but involves a human decision to use, by dialing,

one facility rather than another. Also down time back up

for any job is entirely dependent upon that job not using

permanent disk files, since the file systems are host

dependent.

Nodes

The term node as used in CYBERNET refers to computers

which act as processors and store-and-forward computers.

The nodes are actually CDC 6400' s using the KRONOS timesharing operating system. Their main purpose is to provide

time-sharing service to low-speed interactive terminal users.

It is, however, possible to access the batch computers -

CDC 6600' s - from a low-speed terminal indirectly through

the CDC 6400' s. Jobs can be submitted via terminal to the

CDC 6400 KRONOS time-sharing system destined for the CDC

6600 SCOPE batch system and KRONOS will forward the job to

SCOPE using a wideband switch. One paper presents plans

for the nodes to have a number of important capabilities for

load balancing and resource sharing [10]

Terminal Interface

Terminal support on CYBERNET includes support for interactive or conversational terminals, remote peripheral pro-

• cessors, satellite computers, and computers with terminal

facilities. The interactive terminals are those of the 110

61
to 300 baud range and using ASCII. The remote peripheral

processors are conventional computer peripherals driven at

a remote site. The typical configuration is a card reader,

a line printer, and a modem controller. Supported are low

speed (2000-2400 baud), medium speed (2400-4800 baud), and

high speed (40,800 baud) devices using 300 card per minute

(CPM) and line per minute (LPM) readers and printers, 600

CPM and LPM readers and printers (planned) , and 1000 CPM

and LPM readers and printers respectively. The CDC 200

User Terminal is the standard CYBERNET device for low and

medium speed remote peripheral processors. It is a hardwired rather than programmable peripheral device. The

satellite computers are programmable peripheral processors

and appear to the network as the hardwired remote peripheral

processor. They are distinguished by their ability to do

off -net processing and, when on-net, can support the higher

speed readers and printers. The satellite computers presently used number over 25 including: Honeywell 200 series,

Univac 9200' s, and IBM 1130's. The last class of terminals

are computers with terminal facilities which are computers

which have been programmed to behave as terminals to the

network. This is the manner in which the larger scale IBM

360' s access the network. At this time nearly 800 remote

peripheral processors, satellite computers and computers

access CYBERNET. In addition the CDC 6400 KRONOS system

and the CDC 6600 SCOPE are programmed to appear as terminals

62
to the net while simultaneously serving terminals connected

to them.

The look alike technique is important for computer-tocomputer communications on CYBERNET. The standard remote

peripheral processor for CYBERNET is the CDC 200 User

Terminal, but other devices have been programmed to look to

the network like the CDC 200 including: CDC 8090, CDC 160A,

IBM 1130, UNIVAC 9200, UCC's COPE, Honeywell 200, and IBM

360/30 and larger IBM 360 's. The nodal computers can look

like wide band terminals to the centroids and to each other

This is an uncomplicated method of performing computer-tocomputer commmunication and renders it less difficult for

incorporating new systems into the network.

63
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Communications

Communications Circuits

Internode - full-duplex 40,800 baud wideband facilities

between wideband switches. A plan calls for

switching nodes being developed which will

initially be connected by 2400 and 4800 baud

lines [10]

Terminal to network - voice grade operating a 110 to

4800 bits per second and wideband facilities

at 40800 bits per second to connect terminals

which include interactive terminals, remote

peripheral processors, and satellite computers.

Interprocess - all interprocess communication, node to

centroid for example, is via a look alike

technique where one process appears as a standard network terminal to another. These

communications are established by direct

connection or connection through a wideband

switch.

Communications Hardware

Host interface - multiway wideband switch can interface

host or hosts to the network.

Terminal interface - terminals connect (multiplexed or

individually) through the host computers,

the nodes, or the wideband switches. Ter65
minals can be computers.

Communications Software

Initial connection - user selected through appropriate

settings on wideband switch. Also done

automatically by nodal computers.

Protocols - computers communicate through look-alike

techniques appearing as network standard

terminals or peripherals to the network.

66
Configuration

Topology

Distributed - nodes, centroids, and user terminals are

distributed.

Actual distribution - computer centers exist in at least

12 cities around the country and links exist

to over 45 cities including Honolulu and

Anchorage, Alaska. KRONOS systems are in

Washington (3) , and Los Angeles and over 30

metropolitan areas are covered with local

phone service.

Composition

Heterogeneous - CDC 6600

CDC 6400

CDC 3300

Peripherals - IBM 360

CDC 3150

CDC 8090

IBM 1130

UNIVAC 9200

Honeywell 200

Size - at least 9 CDC 6600 SCOPE batch computers and 4

CDC 6400 KRONOS time-sharing computers with

a large number of users (approximately 600

simultaneous users on KRONOS and 250 RJE

stations on SCOPE)

67
Salient Features

Many companies add value and resell at a higher price

via terminal the facilities of CYBERNET and do therefore

not have to have their own computers to be in that business.

Bulk discounts apply.

Several packages are offered through CYBERNET by independent companies on a royalty basis including a dynamic

structure package, a time series analysis package, a 1970

Census Tape processing package, a data management system,

and a crystallography package.

Super computer processing power is made available on a nationally accessible basis to the paying customers of

CYBERNET.

68
Terminal Support Capabilities

Interactive

The code is ASCII, speeds are in the 110-300 baud range

although nothing in the design precludes higher speeds,

and the mode is half-duplex.

Remote Job Entry

Support is extensive centered around the CDC 200 User

Terminal which is a non-programmable device supporting

a card reader, a line printer, and a modem controller.

Other devices can be programmed to look like this device

Card readers and line printers operate in the speed

range of 300-1000 plus cards per minute and lines per

minute

Minicomputer-based Systems

Minicomputers have been programmed to look like either

remote peripheral processors or as so many terminals to

the network.

Comput er - to - Computer

Computer communicate by look alike techniques to the

network.

69
4.3 GE Information Services

The General Electric network is a commercial network

implemented to make available the computing services of

General Electric Information Services. The network encompasses over 300 cities in North America, Japan, and Europe,

making it international in scope. The communications network allows a user to make a local phone call to connect to

the main computing facilities in Cleveland, Ohio, providing

services ranging from light computation to remote batch

processing. The network has the interesting capability of

connecting customer in-house computers to GE network service

computers for the purpose of exchanging files. This will

be discussed later.

Overview

The GE network is a centralized, hierarchical network.

Topologically , all communications paths lead to the central

facility. The central facility itself is composed of a number of processors with varying capabilities, switching

computers which allow flexibility in the choice of userprocessor connections, and central concentrators which

establish initial connections and act as interfaces between

the processors and the network. There are presently two

switching computers and each central concentrator is connected to both of them. They are able to switch messages

between central concentrators. Each central concentrator

acts as a network interface for from one(l) to six (6)

70
processors. Moving out from the central facility, each

central concentrator can connect to a maximum of eight (8)

remote concentrators. At the present, more than fifty (50)

remote concentrators are in service. The remote concentrators typically have forty-eight (48) ports connected to

the public switched network using local loops, foreign

exchange, and frequency division multiplexers. The FDM

multiplexers can multiplex up to nine ports on a voice

grade circuit. Finally, there is the user at his terminal

who can dial a local number and through the hierarchy of

communication, establish a connection with the desired

processor.

Remote Concentrators

The remote concentrators are Honeywell 416 or GE Diginet

1600 minicomputers. They act as network interfaces for terminals of speeds up to 300 baud. They are responsible for

recognizing terminal speed, code, and other characteristics.

All non-standard codes are converted to ASCII before transmission onto the network. There is no echoing (characters

are printed by the terminal when a key is depressed) and

transmission is line at a time, that is, characters are

buffered until a line is completed before transmission on

the network. The remote concentrator can also perform line

and character deletion editing. In addition, it is able to

recognize whether a terminal is entering a command or data

(as in file building) and only interrupt the central system

71
when a command has been entered. Each remote concentrator

is connected to a central concentrator by a pair of 4800

baud voice grade lines. There also are being developed

remote high speed concentrators using Diginet 1600's [11].

These will support synchronous terminals up to 4800 baud.

Included in the set of synchronous terminals are computers,

and in particular, an OS teleprocessing package has been

developed to permit IBM 360 or IBM 370 computers to interface through the high speed concentrator. The high speed

concentrators will initially connect to the central concentrators over dual lines at 9600 baud. At this time remote

job entry devices and IBM 360 or 370 computers connect to

the central facility via 2000 or 4800 baud dialup circuits.

Central Facilities

The central concentrators are Gepac-4020 ' s . They storeand-forward data to and from terminals in their role as

interfaces between the host and the network and also between

remote concentrators and the network. Host or central

computers are connected in a variety of ways depending on

the host involved including direct memory to memory interfacing and remote communications channels using synchronous

communications protocols. The central concentrator determines to which system the user is to be connected at logon time. If it is a system that is not connected directly

to the concentrator, then the concentrator will send all

72
messages from the user to a switching computer which will

examine the destination address for each message and route

it accordingly. The circuits are two-line links in the speed

range of 4800 to 14400 baud. The switching centers which

provide the connections between concentrators are Gepac4020's or Diginet 1600's. There presently exist two switches

which operate independently. The host or central processors

themselves can be either G635's, G605's, G265's, or HISI

6000's.

Computing Services

The network and its central system offer different kinds

of computer services depending on the host computer being

used. The Mark I time-shared system offers BASIC, ALGOL,

and a simplified version of FORTRAN. The Mark II timeshared service is an outgrowth of Mark I and offers extended

capabilities including geographically distributed transaction

oriented processing, and it runs on the Honeywell G635. The

most interesting service is the Mark III which combines the

Mark II time-sharing system and the GECOS III remote-batch

system running on the larger Honeywell 6080. All on-line

activities are handled by the Mark II (called Mark III

foreground) which is optimized for that kind of work. Large

batch tasks are done by GECOS (called Mark III background)

likewise optimized for batch. A high-bandwidth interface

allows background work to be submitted by Mark II across the

interface to GECOS. Therefore, batch jobs originating from

73
low or high speed terminals connected to Mark II, can be run

on the background GECOS and the output sent back to a file

on Mark II where a user can print it on a high speed printer

or scan it with his terminal. The high bandwidth communication channel between Mark II and GECOS is actually a shared disk device. This is not to imply that the two have

a common file system, on the contrary, the file systems are

independent and distinct.

Interprocessing

GE ' s facility known as Interprocessing is an exciting

concept. A customer is permitted to use his in-house

computer for production- type processing, then transfer his

files through the GE network to the central computers, to

make them available on-line throughout the international

network for customer interactive query and update. Periodically, the updated files are transferred back to the customer's in-house computer. Pontiac Division of General

Motors uses the Interprocessing capability of the network

maintaining order and delivery information for its thousands

of automobile dealers [-35] .

For the future, there are indications that GE will expand

the Mark III foreground - background concept to include other

manufacturer's equipment for running the background jobs,

including IBM systems and possibly the Control Data 7600 [34].

Also considered is the possiblity of supporting remotely

74
located background systems such that a customer owned and

operated computer could be connected to the network as a background system. This would allow a customer to couple

his machine and applications to the massive on-line capabil

ity of the GE network.

75
Communications

Communications Circuits

Remote to central concentrator - dual 4800 baud voice

grade.

High speed remote to central concentrator - dual 9600

voice grade.

Central concentrator to switching center - dual links in

the speed range of 4800-14400 baud.

Terminal to network - maximum for asynchronous terminals

is the 300 baud and 4800 baud for synchronous.

Low speed terminals connect to frequency

division multiplexors which connect to the

remote concentrators.

Interprocess - user terminals are connected through their

remote and central concentrators to the processor or processors connected to that

central concentrator or, using message

switching, are connected to processors on

other central concentrators ( concentrators

to which their remote concentrators are not

directly connected)

Communications Hardware

Host interface - hosts interface to the network through

the central concentrators which are GEPAC4020' s. More than one processor (six maximum) can be connected to a central concen76
Itrator

Terminal interface - asynchronous terminals, after

multiplexing, interface to the network

through the remote concentrators which are

Honeywell 416's or GE Diginet 1600's. Synchronous terminals will in the future interface through high-speed concentrators using

Diginet 1600's. At this time synchronous

terminals must dial the central concentrator

facilities

Communications Software

Connection - central concentrators connect user terminals

to the appropriate processor using information in the user number.

Traffic flow regulation - distributed between remote

and central concentrators.

Protocols - computers can connect to the network using

BTAM (Basic Terminal Access Method) operating

under OS. This facility is limited to IBM

360's and IBM 370's.

77
Configuration

Topology

Centralized - all communications feed into a central

system in Cleveland. Within that central

system the compute power is distributed among

a number of processors.

Actual distribution - the network extends via local

telephone to over 250 cities in the U.S.,

nine cities in Canada, Mexico City, San

Juan, London, Birminghan, Manchester,

Brussels, and Amsterdam.-

Composition

Heterogeneous - GE 635

GE 605

GE 265

HISI 6000

(plans call for background systems employing IBM and

CDC machines [34]) . Size - Approximately 50 remote concentrators in 13 U.S.

and European cities. Each concentrator can

have 48 user ports. Maximum capacity is on

the order of 2000 simultaneous users. The

total customer base is 3000 firms with over

10,000 individual users [7].

78
Salient Features

A service called Interprocessing is offered in which

files from a customers own computer may be transferred

through the network to the GE system computers for on-line

access by the customers terminals, possibly, distributed

t

throughout the country. Updated files can be periodically

sent back to the customers computer.

In addition to the time sharing services, background

systems, interfaced to the network through the time sharing

systems, run batch jobs submitted via terminal or remote

job entry stations.

Plans call for the introduction of IBM and possibly CDC

equipment into the network for running background jobs

lessening incompatibility problems with customers facilities.

79
Terminal Support Capabilities

Interactive

Code - ASCII, EBCD, Correspondence.

Speed - 10, 14.8, 15, and 30 characters per second.

Mode - half-duplex.

Remote Job Entry

Synchronous support is provided for the IBM 2780 binary

synchronous line discipline and compatible devices. The

high speed code is EBCDIC. The connection is via 2000

bit per second dialup to the central facility although

high speed remote concentrators are being developed.

Minicomputer-based Systems

Synchronous interfaces and sufficient bandwidth exists

to support these systems although no such applications

presently exist.

Computer -to -Computer

With the appropriate communications adapter (IBM 2701,

2703, or a 3705 emulating a 2701/2703) and a GE-provided

software package, certain computers can connect to the

high speed service as does a remote job entry device.

These are all IBM 370' s and IBM 360's models 22 and up

using IBM OS-BTAM. The interconnection is used mainly

for large file transfers.

80
4.4 MERIT

MERIT is a state network developed with funds provided

by the National Science Foundation, the State of Michigan,

and the three participating universities: Michigan State

University, University of Michigan, and Wayne State University. It interconnects the computing facilities at these

three universities, IBM 360/67 's at two nodes and a CDC 6500

at the third (see Figure 3) , and provides a working model to

study the problems associated with the use of remote computing facilities, that is, the use of computing facilities

at a university different from the university of the user.

The several problems being analyzed, in addition to the

technical development of the network, include the following:

the provision of a mechanism for exhanging computer dollars

among the universities; the inclusion of the smaller state

universities into the network and finding a suitable

mechanism for payment of the communication facilities; and

the problem of informing potential users of services that

are available on the network and the detailed procedures

of using the network resources [36]

Communications Computer

The technology used in this network goes beyond the

much simpler directly connected host-to-host network by

employing communications switching computers which act as

interfaces between the host computers and the network, and

also as store-and-forward nodes. The Communications Computer

81
is a Digital Equipment Corporation PDP-11/20 with 16K of 16-

bit memory. The primary purposes of the Communications

Computers are the following: to multiplex logical connections over physical communications links communicating

with other nodes using a binary synchronous communications

protocol, to receive messages from their respective host

computers for transmission on the network, and to receive

messages from other nodes for delivery to their hosts or

for forwarding to another host.

The interface between the Communications Computer and

the host computer is a special hardware device. It allows

for the transmission of software determined length data

records to and from the memory of the Communications

Computer (CC) performing whatever memory alignment operations

are required for word lengths which differ from that of the

CC. It also provides a multi-address capability which allows

the host to treat the CC as many identical devices. This

simplifies the task of programming the host computers since

the CC does all the multiplexing functions.

Network Control

To interface a host to the network requires no changes

in the operating system of that host computer. Instead the

network features are extensions to the operating systems,

requiring, only that device support for connections, which

* appear similar to other peripheral devices, be added to the

system. Therefore, when a connection is established be32
tween processes in independent host computers, the connection behaves as an input/output device in each host. An

initial connection is established to prepare the "connection

device" for record traffic by a single user request. The CC

propagates this connection request through the network to the

remote process and then the connection is completed with the

allocation of a "connection device" to each process. Traffic

is exchanged through the read/write mechanisms of the host

computers under the control of a network master/slave

mechanism to regulate traffic flow. The master, the remote

end of the connection, can always initiate the sending of a record to the slave. The slave is restricted to send one

record on each go-ahead from the master. The record traffic

regulation mechanism, as can be noted from the preceding, is

resident in the host computers.

Circuits

The original communications circuits interconnecting the

CC's were 2000 baud dial-up facilities. Each CC was interfaced to a maximum of eight (8) direct dial telephone lines.

To establish full duplex channels, two (2) dial up telephone

lines were used to form a single communications channel, thus

giving each CC a maximum of four (4) full duplex channels.

These channels could be evenly distributed among the three

(3) computers in the network, giving two links between each

pair of computers, or they could be distributed unevenly to

match the traffic requirements. A single automatic calling

83
unit was employed by each CC to automatically dial lines as

required. This single unit by a special multiplex arrangement was shared among the direct dial telephone lines.

In the fall of 1973 the dial-up communication channels

will be replaced with point-to-point channels operating at

4800 baud. These full duplex channels require only one

line rather than the two lines previously used.

The number of logical connections between computers is

a function of facilities available to service those connections themselves. These logical connections are multiplexed by the CC over the physical connections.

Command Language

A network command language exists for communicating with

remote hosts. Commands exist for logging into remote hosts,

copying files to and from remote hosts, transmitting and

receiving records with remote hosts and a number of other

useful activities. Since network access is only through a

local host computer, an overhead in terms of local processor time and money is associated with communicating with

remote hosts.

84
MSU

CDC 6500 SCOPE

CC

CC

WSU

IBM 360/67 MTS

UM

IBM 360/67 MTS

Figure 3

85
Communications

Communications Circuits

Internode - circuits are voice grade, dial-up using

Bell Telephone 201A's to operate at 2000

baud. Full duplex is achieved by associating

two (2) dial-up lines with each channel.

Eight lines stem from each node for sending

and receiving, thus forming 4 full duplex

2000 baud channels. Upgrading to point-topoint lines using Bell Telephone 208'

which operate at 4800 baud. Each full duplex

circuit will require one line.

Terminal to network - access to network is only through

host computers. No special terminal interface exists.

Interprocess - connection between a pair of processes

appears as an I/O device in each host. Once

allocated to a process, a connection is

dedicated to that process.

Communications Hardware

Host interface - interfaced through Communications

Computer (CC) (DEC PDP-11) which can also

act as a store-and-forward node. The CC

appears as a number of identical but separate

I/O devices to its host.

86
Terminal interface - only through host computer.

Communications Software

Initial connection - function is distributed and

accomplished by allocating a connection

device ( host thinks its an I/O device)

Protocols - network interactive service allows users to

access resources on a remote host and to

control file transfers and process-toprocess computing. Network batch allows users

to submit batch jobs on their local host

for subsequent execution and/or printing

elsewhere.

87
Configuration

Topology

Distributed - nodes exist in three universities, one

node per university.

Actual distribution - Michigan State University, Uni

versity of Michigan, and Wayne State Uni

versity, all in Michigan.

Composition

Heterogeneous - IBM 360/67

CDC 6500

Size - 3 nodes with one host computer at each.

88
Salient Features

A special multiplexing system was developed to allow

an automatic calling unit to be shared by eight (8) dialup lines.

A network command language exists for communicating

with remote computers and provides some file and record

transfer capability.

Communication interface units have been designed to

permit the use of wideband facilities if desired at some

future time.

Host software has been greatly simplified by allowing

the host to treat the communications computer as several

peripheral devices.

89
Terminal Support Capabilities

Interactive

Code - no terminal interface but Communications Computers

translate local host character strings to ASCII.

Speed - as determined by host through which terminals

interface

Mode - as determined by host.

Remote Job Entry

Protocols - none.

Device support - as supported by host. No separate RJE

network interface exists.

Minicomputer-based Systems

Interface - no special interfaces or software support

other than what may be supported locally by a host computer.

Computer -to -Computer

Protocol - exists for host to host communications.

Implementation - network connections appear as I/O

devices to a process.

90
4 . 5 ARPANET

ARPANET is a distributed network of heterogeneous computers employing store-and-f orward packet-switching communications techniques. There has been interest shown in making

ARPANET technology publicly available, possibly on a regulated basis. At this time there is a proposal by Packet

Communications Inc., to offer a packet- switching service

providing the host interfaces, network control programs,

communications links, and terminal processors to compete for

the data communications market [37] . This proposal stands

as a formal application to the FCC along with an application by MCI Data Transmission Corporation (MDT) to offer a similar kind of service. Two other corporations, Telenet

Communications Corporation and Telecommunications Network

Inc., have also expressed an interest in this type of service . Overview

The communications network or communications subnet is

composed of interface message processors (IMP) and terminal

IMPS (TIP) connected in a distributed fashion by 50,000

baud synchronous wide band circuits (see Figure 4) . The

IMPS are Honeywell DDP 516's with 16,000 words of memory

which act as interfaces for host computer systems and as

store-and-forward nodes in the network. A host is connected

to an IMP and thus to the communications subnet by a channel,

and if the host is very distant from the IMP, it can con91
nect via a synchronous communications line using block

control procedures [38] . The latter method is appealing in

that it eliminates the requirement of locating the IMP, a component of the communications subnet beside the host

computer that wishes to connect to it. It is conceivable

that the communications subnet could become physically independent and hosts would connect to the nearest IMP using

the synchronous link. Presently each IMP can interface

from one to four host computers to the network. The software interface between host and IMP is called the network

control program (NCP) . It is responsible for initiating

connections, regulating traffic flow, and realizing network

protocols between all communicating processes. The NCP

resides wholly in the host computer thus making the host

an integral part of the communications network for communications to and from the host. The NCP is closely related to the supervisor of the host system [39] and is difficult and time consuming to implement for even first rate

systems personnel. Proposals have been presented to transfer some of the NCP functions to the interface processor

and possibly diminish the developmental efforts of interfacing a new host [40] . In addition, some organizations

are investigating the use of minicomputers as front end

processors performing the NCP functions in addition to such

functions as peripheral control. It is important to realize that tradeoffs do exist in the different approaches.

92
Moving the NCP into a front end processor eliminates the

need to make extensive alterations in the host system

software and can reduce the overhead associated with running the NCP in the host computer. However, all communications to and from the host computer would be limited to the

capabilities of the host's standard software and hardware

controller.

Packet Switching

Host-to-host messages are passed from the sending host

to its IMP, where they are broken into packets (thus the

term "packet switching" used in describing the ARPANET) and

relayed to their destination by the network of IMP's using

store-and-forward techniques. Packet routing is adaptive to

the conditions of the network considering failures and loading characteristics and, in general, the several packets of

a single message may follow different routes. The destination IMP will reassemble the message and deliver it to

proper host. The overhead of dynamic routing should be

compared to the advantages gained over techniques where a route is chosen for communicating between two processes on

the basis of communication facilities loading and then not

dynamically altered for the duration of the connection.

Protocol Levels

Protocols in the network are constructed according to a layered approach. The lowest level protocol is a synchronous

communications protocol governing traffic exchange between

93
the IMPs. Error checking is extensive and retransmission

takes place as necessary. The so-called "first level"

protocol governs the exchange of information and data between host and IMP. The "second level" protocol governs

the logical exchange of information between Network Control

Programs in communicating hosts. The "third level" of

protocol refers to any communications occurring between processes in host machines. Such protocols include the initial

connection protocol, which can be used by all processes to

initially communicate, the file transfer protocol, the remote job entry protocol, and the telnet protocol. Some

third level protocols make use of other third level protocols.

For example, the remote job entry protocol as recently defined uses the telnet protocol which in turn uses the initial

connection protocol, and it also uses the file transfer protocol. The hierarchy of protocols is such that all higher

level protocols use the lower level protocols to function.

Terminal Interface

TELNET is a special third level protocol which defines

a network virtual terminal and permits all terminals on the

network to provide a similar interface to processes in any

host. This allows for the use of any terminal supported by

the network, and support is extensive, with any host system

on the network even if a host could not support a particular

terminal through its standard terminal controller. A special

IMP which is augmented by the addition of 12K of memory and

94
a multi-line controller can provide direct network access to

terminals (dial-ups, leased, and hardwired) without going

through any host by providing the TELNET function and NCP

itself [41]. This device is called the TIP. It has portal

access for 63 terminals but due to buffering restrictions

may be constrained to support a smaller total number when

some of the terminals are operating at higher speeds. Synchronous interfaces are presently not provided. Input speed

can be as high as 2400 baud asynchronous and output speed

can be set up to 19200 baud under special conditions where

the terminal supplies clocking. A great deal of flexibility

exists to accommodate the peculiarities of different terminals.

A user can specify code, speed, when to transmit (on-line

feed for example) , and how to echo and by which network

component echoing is to be performed. Some of this flexibility is, however, accompanied by considerable cost in

overhead, particularly the ability to support the variety of

devices using non-standard codes. This overhead cost can

be measured by the large amount of main memory reserved to

facilitate these devices [2]

Character Transmission

In the servicing of interactive terminals on the ARPANET

there exists a potential problem area. This occurs when terminals are interacting with a system in echo-plex mode. The

echoing is often not one-to-one, that is, a particular

character inputted from a terminal may require a different

95
character or character string to be echoed. Also certain

characters require an immediate host system reaction. There

fore the user will choose to have echoing performed by the

host system rather than the TIP, which can only echo characters exactly as inputted at the terminal. This requires

that packets be sent through the network containing only

one character if the echoing process is to maintain pace

with the input process. Since there are approximately 150

bits of overhead per transmission, then approximately 158

bits must be transmitted of which only 8 bits are data.

Added to this are 158 bits of additional overhead for the

echoed character transmitted from the host system. Even

if characters are sent in larger blocks, six (6) at one time

for example, the result will be very choppy echoing, in

addition to an overwhelming overhead of approximately 700

percent. With moderately or heavily loaded network, this

kind of overhead would certainly be intolerable. The solution involves putting additional sophistication in the TIP

so it can echo characters as the host system would. Since

the host systems are heterogeneous, there would have to be

a mechanism for exchanging control information between a host and TIP to notify the TIP of idiosyncracies in echoing.

Then with characters being echoed at the TIP to which a terminal is connected, it would be possible to buffer an

96
entire line to form a packet for transmission on the network

This problem and potential solutions are presently receiving

attention in the ARPANET community.

97
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98
Communications

Communications Circuits

Internode - circuits are leased, full duplex, synchronous,

50,000 baud wideband. It is possible to use

other speeds and 230,000. and 9600 baud have

already been employed in two applications.

Terminal to network - leased or dialup, full or half

duplex, asynchronous. Speeds can be from

75 to 2400 baud on input and up to 19200

baud on output.

Interprocess - packet switched at communications network

level (IMP to IMP)

Communications Hardware

Host interface - host is connected through the interface

message processor (IMP) which also acts as a message switcher. Four hosts can be interfaced through one IMP. The IMP is a Honeywell DDP-516 or 316 with 16000 words of

memory.

Terminal interface - terminals are interfaced through

the terminal IMP (TIP) concentrator which

also acts as a message switching node. Up

to 63 terminals can be interfaced by one

TIP (this limit is theoretical as buffering

problems prevail before it is realized depending on terminals speeds) . The TIP is a 99
Honeywell H-316 with 28,000 words of memory.

Communications Software

Initial connection - this function is distributed and

carried out by network control programs which

are host resident.

Protocols - four levels of protocol exist with the highest level controlling interprocess communication. At this level exist the initial connection protocol, data and file transfer protocols, remote job entry protocol, and telnet protocol (see text of this section)

100
Configuration

Topology

Distributed - hosts, IMP's, and TIP's connected in a distributed fashion.

Actual distribution - national coverage with 38 locations

throughout the country, with connections to

Hawaii, Norway, and England.

Composition

Heterogeneous - IBM 360/44/65/67/75/91

IBM 370/145

DEC PDP 10

DEC PDP 11

Honeywell H645

XDS Sigma 7 Burroughs B6700

CDC 6600

Size - approximately 40 host computers connected through

21 IMPs and 18 user nodes or TIPs (terminal

users can also access network through host

computers).

101
Salient Features

A minihost system (ANTS) has been developed at the University of Illinois to run on a PDP-11 [22] . That minihost

can be used to provide device support and user support not

available through the TIP.

A telnet protocol exists which allows for the use of any

terminal on any system by mapping the real terminal into the

network virtual terminal.

A remote job entry protocol has recently been developed.

Dynamic routing techniques are used to route messages

through the network.

New versions of IMP software are loaded from a control

center passing from IMP to IMP without the necessity of manual intervention at each site.

102
Terminal Support Capabilities

Interactive

Code - network code is ASCII.

Non-standard terminals are mapped into ASCII at

the interfaces for transmission on the network.

Speed - 75 - 2400 baud.

Mode - full-duplex with half-duplex support provided.

Remote Job Entry

A Protocol exists requiring an operator using TELNET and

a terminal to initiate the various phases. There exists

no special RJE device support or synchronous interface.

Minicomputer-based Systems

No synchronous interface to allow for useful employment,

but an NCP has been written for a minicomputer (PDP-11)

to allow it to act as a minihost.

Computer- to -Computer

Protocols exist for host-to-host communication (see text

of this section)

103
4.6 TSS

TSS is an experimental network of homogeneous computers,

all presently IBM/360 model 67' s, designed and implemented

as a joint effort by Carnegie-Mellon University, Princeton

University and IBM Research Division. The project was conceived to investigate the uses and advantages of computer

networks while keeping modifications to existing hardware and

software to implement that network at a minimum. The network

is point-to-point, dialup, and involves no front end computers,

message switching, or special interfaces for terminals (see

Figure 5)

Operating System

All of the systems are IBM 360/67 's operating under the

time-sharing system TSS/360. Systems modifications to TSS

were intentionally kept at a minimum. Only the following

were necessary: (1) the basic system initialization routine

was modified to permit the processor to accept incoming calls

from other installations; (2) the task initialization routine

was modified so a task can determine whether it is operating

on a primary (local) or secondary (remote) processor; (3) the

central terminal communication mechanism was modified to detect communication with another location and therefore make

calls to the Computer Access Method (developed for networks)

,

rather than the customary Terminal Access Method, which in

essence allows a primary process to appear as a terminal to

a secondary process, and thus facilities normally available

104
to terminals are at the disposal of the primary process;

(4) the task termination routines and the task abnormal termination routines were modified to transfer control to the

communications logoff routine if processing is occurring on

a secondary processor.

Communications Controller

The hardware used is the standard communications contoller for IBM computers, the hardwired IBM 2701 Transmission

Control Unit. The unit must have a Synchronous Data Adapter

and communicates using the Binary Synchronous Communications

protocol. This allows for the half duplex mode of operating,

that is, two-way nonsimultaneous transmission. Data flow is

in one direction at a time, then the IBM 2701 control unit

must reverse the line direction at the end of each exchange.

The IBM 2701 controls a 2000 baud dialup modem for pointto-point switched operation. The IBM 2703 Transmission

Control Unit can be substituted for the IBM 2701 in this

application because transmission speeds greater than 48000

baud are not used.

Computer Access Method

The communications software, residing in the IBM 360/67,

which was created for the network to access the hardwired

IBM 2701 or IBM 2703 Transmission Control Unit is called the

Computer Access Method (CAM). CAM has the TSS standard

access capabilities of open, read and write, error recovery,

check (to check the completion status of the previous I/O

105
operation), and close. When communication with another computer is taking place, I/O calls are made to CAM which effectively makes a processor appear as a terminal to the other

processor so that processors communicate in their normal I/O

stream having all the facilities normally available to a terminal at their disposal.

Network Commands

A number of network commands were developed to control

communications with remote processes. One command establishes

the initial communication with a remote processor. This is

accomplished by dialing automatically the correct number to

establish the communications link between the two independent processors. Another command can interrupt the communication with the network and temporarily return control to the

TSS command interpreter. This command has a complementary

command which can terminate the suspension and return control

the network command interpreter to resume the previously

interrupted interaction. The send data set and receive set

commands are able to transfer entire files between remote

processors. Finally a command exists to terminate communications with a remote processor.

106
l J

NASA LEWIS *, BELL TELEPHONE LABS •

IBM RESEARCH

RUTGERS • PRINCETON

CARNEGIE MELLON

Fig\sre 5

107
Communications

Communications Circuits

Internode - 2000 baud dialup lines operating in halfduplex.

Terminal to network - no separate interface. Connection

is through host computer only.

Interprocess - line switched. A switched physical circuit is devoted to each pair of communicating

processes

Communications Hardware

Host interface - host is an integral part of the communications network and connects through its

standard controller (IBM 2701) . Terminal interface - none other than through host computer.

Communications Software

Connection - Computer Access Method (CAM) residing wholly

in host system establishes physical connection

by dialing for 2000 baud switched circuit,

and uses it for the logical connection between processes.

Protocols - simple protocols to allow for initial connection to a specific node, initiation of

computation process, disconnection from a node,

test outstanding responses, and send and

receive data sets.

108
Configuration

Topology

Distributed - hosts are distributed and communicate

through dialing.

Actual distribution - Carnegie Mellon University, Prince

ton University, Rutgers University, IBM

Research, Nasa Lewis, Nasa Ames, and Bell

Telephone Labs.

Composition

Homogeneous - IBM 360/67.

Size - 7 host computers.

109
Salient Features

The design philosophy of using existing communications

components and the terminal look alike techniques (a processor

looks like a terminal to the processor with which it communicates) enable the processors to be easily interconnected for

the purpose of experimenting with networks.

A simple network control language exists enabling a user

to perform some unsophisticated but useful functions.

110
Terminal Support Capabilities

Interactive

Code - no terminal interface other than host computer

exists

Speed - as determined by terminal/host interface, usually

10 or 15 characters per second.

Mode - half-duplex when using standard terminal/host

interface

Remote Job Entry - no special support.

Minicomputer-based Systems - no special support.

Computer- to -Computer

Using the Computer Access Method (CAM) one computer connects as a terminal to another and vice versa allowing

for interprocess communication.

Ill
5.0 CONCLUSIONS

Various approaches to computer networking technology

have been realized in the six networks discussed. The relative merits of the different types of components and configurations were discussed in previous chapters. It is useful

now to consider briefly these network technologies relative

to their potential application in the Networking for Science

Program. Only the communications network, or in ARPANET

terminology the "communications sub-net," will be considered.

The communication network will, however, include considerations

relevant to the host interface and of course the terminal

interface. Furthermore, when discussing the alternatives, it

is important to distinguish between a network technology and

the implementation of that technology. For example, to implement a particular technology as it appears in an existing

network, it might be possible to build an identical or customized version, connect to the network, or lease service from

the network. These implementation alternatives are not

addressed in this particular report.

TSS

The last three networks outlined were the research networksTSS, MERIT, ARPANET. TSS technology can be distinguished by

a number of important features. Looking at the configuration

it is distributed and homogeneous. The homogeneity certainly

impedes broadening the use of TSS, and is not easily overcome

as the network is composed of computers of the same model and

112
same operating systems. The communications circuit and hardware are also limiting factors. The circuits are circuit

switched rather than message switched which is not necessarily

detrimental but is somewhat less flexible. The host interface

is a non-programmable IBM 270X which eliminates the front end

processing possiblity and thereby restricts the kinds of computers that can be interfaced. No separate terminal interface

exists. The fact that there is no multiplexing of communications

circuits further hinders usage. The general principle of the

TSS approach, that is, having computers dial one another and

communicate through their terminal handling facilities has

some merit as it allows for rapid, relatively inexpensive

startup. The concept would have to be modified in order to

include heterogeneous systems, line multiplexing, and separate

terminal interfaces.

MERIT

The MERIT network technology with its additional sophistication overcomes some of the drawbacks of the TSS approach.

It does in fact include heterogeneous computers. Two different kinds of computers and three different operating systems

are connected. The host interface is a Digital Equipment

Corporation PDP-11 minicomputer that provides some front-end

processing capability and an interface flexible enough to handle more than one kind of computer. Better circuit utilization

is achieved through line multiplexing. However, there exists no

direct terminal interface which implies that the minimum connection

113
mechanism is through a host computer system. Furthermore, since the

MERIT technology has been applied to interconnect only three

computers, it has not been demonstrated that the technology

zould be applied unaltered to a large network of distributed

computers and users.

ARPANET

The ARPANET technology appears to be the most sophisticated

network technology presently in operation on a national basis.

It connects heterogeneous computers of many different manufacturers in locations throughout the country. It is distributed in topology, switching, control, and computing power,

making it extremely flexible for alternatives in each category.

The communication circuits are message switched utilizing 50

kilobit per second wideband links. The host interface is a programmable minicomputer which as an interface is able to

accommodate many different kinds of host computers including

more than one simultaneously, and as a node in the network,

has message-switching store-and-forward capabilities. In

addition it has the ability to perform dynamic routing through

the network. The terminal interface is also a minicomputer

capable of supporting a large number of terminals, running

in a wide range of speeds, operating in different modes, and

using standard as well as non-standard character codes.

It is perhaps because of these extensive capabilities

that there exists unanswered questions concerning the ARPANET

technology. For instance the host interface through its

114
sophistication and complexity is quite difficult to implement.

It requires a great deal of time (actual amount of time and

money is presently undocumented) for a systems programmer

familiar with both the interface and the host operating

system. This is because the network control program resides

in the host computer itself. There are presently efforts

underway on a limited basis, to move the network control

program out of the host into a front end computer [22] , and

there are proposals which favor protocols for interprocess

communication with less host dependency [40] . It is important to point out that a large number of NCP's have already

been implemented for a variety of computer systems which could

be used in newly attached identical systems. However, in the

present state of the ARPANET, the network control program and

its associated overhead of CPU time and core used must be

considered as directly related to the communications network.

The terminal interface processor (TIP), as noted, has

extensive capabilities to handle selected speeds, codes, and

modes of operation. Nevertheless, there do exist shortcomings

with the TIP. It can only support asynchronous terminals and

in order to connect with most remote job entry devices

synchronous support is needed. One way to implement synchronous support is to contain it in the already overburdened

TIP. Another approach, one which has already come into

existence in one instance, is the use of a minicomputer as

a mini-host system on the net, providing the desired device

115
support. The Center for Advance Computation at the University of Illinois has developed such a system using a DEC PDP11. The system is called the ARPANET Terminal System, or

ANTS for short [22]. At any rate there needs to be a standard network-wide remote job entry device controller.

Network traffic in terms of quantities and characteristics

is another potentially troublesome area. With peak loading

of roughly ten percent capacity, it has never really been

adequately demonstrated that the network will not deteriorate

under conditions of maximum loading. Besides quantity, the

characteristics of the traffic may result in poor network

performance. To date most of the traffic has been that

associated with terminal-to-computer interaction. It is an

open question what will happen to response times when mixes

of interactive (single packet) and file transfer or remote

job entry (multiple packet) traffic occur. Analysis, however,

has shown that success is highly probable with significantly

different degrees of overhead for different traffic mixes [12].

Finally, there exists a very human problem that occurs

when trying to use the network, and is likely to occur with

any very heterogeneous network. With the proliferations of

systems, it is cumbersome to learn the system command

sequences necessary to perform even simple transactions, such

as logging-in on each system. One possible solution would

be to implement a higher level protocol (fourth level?) to

standardize some of the basic interactions between a user and

116
a system. This protocol could be realized in the hosts, TIPs

or quite conceivably on mini-hosts systems such as that under

development for remote job entry devices.

CYBERNET

The commercial networks- -GE, CYBERNET, TYMNET-- have

developed technologies which appear to have successfully

satisfied their own needs and in some cases could be useful

for applicability in the Networking for Science Program.

CYBERNET is a distributed network in all respects. It is

heterogeneous in that it connects different models of Control Data computers. Other manufacturers' computers can

connect as peripherals to the network by looking like a CDC

network standard device. The look-alike technique is used

quite extensively in the CYBERNET approach even for communications between host systems. The communications circuits

are varied using wideband, voice grade, and public switched,

but for the most part wideband circuits are used for interconnecting computers. Instead of using message-switching

and store-and-forward computers, wideband switches are

employed so that various interconnections are established by

dialing, or having dialed, the appropriate number. The host

interface to the network is the wideband switch. Apparently

batch terminals can also interface to the network through the

switches but normally interface directly through the host

systems. Interactive terminals interface through different

host systems--the CDC 6400' s running the KRONOS time-sharing

117
system- -which can then themselves access the batch systems.

The technology does not at this time seem particularly suitable as the technology for the Networking for Science Program.

The use of wideband switches rather than message-switching

computers limits the interconnections that can occur. No

host interface other than the switch exists and no terminal

interface exclusive of a host system exists.

GE Information Services

The GE approach represents an entirely different kind of

network technology. It is a centralized network with all

host computers interconnected but located in a central location. It is, however, possible to connect a customer computer at the periphery of the network for the purpose of

exchanging files with one of the central systems. This innovation has been put into practice to make customer's batch

created files available on-line to a national distribution

of the customer's own users [35]. The network is heterogeneous

with different models of Honeywell computers running different

operating systems. Plans call for connecting to the network

computers from other manufacturers [34] . Also the concept

of allowing customers to connect their own computers to

exchange files with the central system and utilize the vast

communications network is planned to be expanded.

The communications circuits are message-switched with

the apparent restriction that communications are with one of

the central systems. The host interface is a computer, but

118
only systems at the central site are integral to the network.

As mentioned, however, certain customer computers can interface as remote job entry devices. Communications are directed

to the central systems so that if many customer computers are

connected to the network it is not clear that they could

establish direct communications with one another using the

GE communications network. Also terminals connected to the

network through the terminal interfaces communicate with the

central system and may not be able to communicate with customer computers. This centralization is the drawback to use

of the technology per se in a distributed network although

the extensive coverage and capabilities of the net may warrant

further consideration.

It is useful to point out that the technology itself may

be applicable to networks of distributed computers in two

different ways. First, the central complex may be viewed as

a large message switching center. All communications between

computers connected to the remote concentrators at the periphery

and customer's terminals could be routed through the central

switch. Second, the technology used to interconnect the computers at the central complex could be applied to a network

of distributed computers. Both of these are conjecture.

TYMNET

TYMNET has already entered the business of offering only the

services of the communications network exclusive of the Tymshare

host computers. In such cases customers interface their own

119
host computers and their users, their own terminals, and thereby utilize the TYMNET national data communications network.

This reportedly is permitted under Federal Communications

Commission Tariff 260 covering joint use agreements. The

customers make a contractual arrangement with Tymshare for

use of a certain percentage of the capacity of TYMNET for

handling the communications traffic between the customer's

terminals and customer's host system or systems. The Telephone Company then bills the customer for the specified

percentage of the communications charges. In addition, Tymshare bills the customer for value added services. These

include the utility of the host and terminal interfaces along

with the store-and-forward capability of the communications

computers. Also included is the involvement of the Tymshare

host computer running the supervisor which establishes all

initial connections.

The TYMNET technology has a number of characteristics

that could make its usage a powerful convenience. It has

been demonstrated that it can support a network, well distributed, with nodes, computers, and users all over the

nation and in Europe with a clear possiblity of connecting

new host systems and users wherever they may presently exist.

The host computers themselves are heterogeneous being different

models and from different manufacturers and it appears conceivable to connect computers of any of the primary manufacturers

using their standard interface equipment. The communications

120
circuitry is voice grade connecting the switching nodes which

are store-and-forward minicomputers. The voice grade circuits

appear to be quite sufficient for the interactive traffic that

exists on the network and some rather impressive figures exist

to indicate a high usage of the facilities through efficiencies

in line concentration. For example forty-six (46) interactive

users are supported on a 2400 baud facility.

Switching is performed by establishing a virtual circuit

through the nodes of the network. The systems supervisor,

the system initially contacted by every user, sends information to the nodes to set up a virtual circuit from user process (terminal) to server process (computer) . This virtual

circuit concept would seem to decrease communications overhead once the circuit is established, although there is the

disadvantage that dynamic re-routing cannot occur to accommodate circuit failures. Nevertheless, the virtual circuit

reportedly results in nearly an 80 percent efficiency in line

utilization rather than the 60 percent efficiency associated

with true message switching [7] . This is due primarily to a reduction in overhead due to the use of only a single character to address a virtual channel. Furthermore, the central

supervisor performing all connections presents an advantage:

only the supervisor has global knowledge of the network so

that the individual nodes can be handled independently relative to software changes and debugging. In the event of a failure at the supervisor node, a backup system will take

121
over, first polling the network to determine its status.

The host interface to the network is a minicomputer, which

for the sake of convenience, can make the network appear as

terminals to the host's regular terminal interface, although

the interface is more sophisticated in the case of Tymshare

host computers. Interfacing new computers in the past has

not been particularly difficult. The terminal interface is

also a minicomputer acting as a concentrator and a storeand-forward node. It can handle terminals of a variety of

speeds and when communicating with some of the host sytems

will perform local echoing.

The network technology appears quite capable of satisfying

the communications needs of a network of computers distributed

throughout the country, and large numbers of interactive

users (presently 900 simultaneously) . With a present average

number of failures of 1.4 per year per node, reliability is

another asset. One big drawback to the network is its lack

of support of synchronous devices and remote job entry. This

situation may not be permanent as plans call for the implementation of synchronous device support to control certain

types of remote job entry devices. Also batch processing is

anticipated on the Digital Equipment Corporation PDP-10 computers which have become a part of the network. Also the incor'

poration of large IBM computers into the system is underway

for more remote job entry and batch processing.

122
Concluding Statement

In keeping with the purposes of this report, the discussion

has been exclusive of economic considerations, concentrating

on functional capabilities. All of the networks discussed

have particularly worthy features. It is, therefore, worthwhile to suggest at this time that a single network technology

need not be selected. It is entirely possible to use the

best features of each network technology matching the task to

the proper network.

Although the discussion of the usage of services provided

by the host computers of the networks did not fit into the

context of this report, it is important to the concept of

multiple networks. Suffice it to say that if the services

are readily available and compatible with the needs of user

groups, than the most convenient access may be through the

networks to which the hosts are already attached.

Two of the networks discussed- -TYMNET and ARPANET- -are

presently offering a computer communications service to

which a customer can connect computers acting as host systems,

as well as terminals. Each has considerable experience in

host computer and terminal interfacing and is representative

of a computer communications network to which customers can

connect their servers and users, nationally distributed, with

a minimal effort in communications development. Both networks,

resultant of both their present state of development and the inherent characteristics of their technologies, represent important

123
advantages and also possess serious limitations, the details

of which are described in the text of this report.

124
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,

New Jersey 1972, p. 111-146.

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of Hawaii Technical Report B73-2, March 1973. Also

125
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,

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126
27. Crocker, S., J. Heafner, R. Metcalfe, and J. Pastel,

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, 'A System for Interprocess Communication in a Resource Snaring Computer Network, "CACM, Vol. 15, Issue

4, April 1972, p. 221-230.

127
41. Kahn R E., "Terminal Access to the ARPA Computer Network, Computer Networks , ed. R. Rustin, Prentice-Hall

N.J. , 1972, p. 49-63.

ft U. S. GOVERNMENT PRINTING OFFICE : 1 974— 542-653/1 27

128
U.S. DEPT. OF COMM. BIBLIOGRAPHIC DATA

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1. PUBLICATION OR REPORT NO. NBS TN-804

2. Gov't Accession No.

3. Recipient's Accession No.

4. TITLE AND SUBTITLE

Review of Computer Networking Technolo gy

5. Publication Date January 1974

6. Performing Organization Code

7.

T.

AUTHOR(S)

Robert P. Blanc

8. Performing Organization

PERFORMING ORGANIZATION NAME AND ADDRESS

NATIONAL BUREAU OF STANDARDS

DEPARTMENT OF COMMERCE

WASHINGTON, D.C. 20234

10. Project/Task/Work Unit No. 6502372

11. Contract/Grant No. AG-350

12 . Sponsoring Organization Name and Address National Science Foundation

Washington, D.C. 20550

13. Type of Report & Period Covered Interim

Fourth Qtr. 1973

14. Sponsoring Agency Code

15. SUPPLEMENTARY NOTES

16. ABSTRACT (A 200-word or less factual summary of most significant information. If document includes a significant

bibliography or literature survey, mention it here.) This report gives a descriptive summary of the technical characteristics of

existing computer networks, including data communication technology and

configuration related to support of resource sharing services for a computer

network. Included are discussions of terminal support capabilities for the

communications network and a development of relevant network terminology.

The report concludes with a comparative evaluation of existing technological

approaches to networking.

W. KEY WORDS (Alphabetical order, separated by semicolons) Computer networks; COKipute r- to - Compute r transfers; interactive terminals; minicomputer -based systems; network configuration; remote job entry; resource sharing.

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