NTIA-REPORT-79-13

Control Signaling in a

Military Switching Environment

R.' F. Linfield

Prepared for

U.S. Army Communications System Agency

Ft. Monmouth, New Jersey 07703

u.s. DEPARTMENT OF COMMERCE

Juanita M. Kreps, Secretary

Henry Geller, Assistant Secretary

for Communications and Information

January 1979
DISCLAIMER

The v~ews, opinions, and/or find~ngs contained

in this report are those of the author and should not

be construed as an official Department of the Army

position, policy or decision, unless designated by

other official documentation.

ii
PREFACE

The study reported here is one part of a continuing

program being conducted by the Institute for Telecommunication Sciences (ITS) for the u.s. Army's Communication Systems Agency (eSA) in support of that Agency's

Access Area Digital Switching System (MDSS) program on

project orders 501-RD~ 804-RD, and S09-RD.

Previous related reports have dealt with parametric

cost alternatives for localdigitaldistr.ibuti·on systems,

as well as with several topics in access area switching.

The latter has included preliminary switching hub

evaluations for the base environment, digital PABX

switching, and signa.ling technology in common use today •

Administration and technical monitoring of the

study contract was performed by Messrs. L.H. wagner and

T. Michelli, both with CSA. Technical and management

supervision of the program was provided by M. Nesenbergs

and Dr. P. McManam·on, both of ITS.

iii

TABLE OF CONTENTS

LIST OF FIGURES

LIST OF TABLES

LIST OF ACRONYMS

ABSTRACT

1. SYNOPSIS

1.1. Background

1.2. Network Control

1.3. Conclusions and Recommendations

2 ., TELECOMMUNICATIONS NETWORKS

2.1. Topology

2.1.1. Types of Networks

2.1.2. Examples of Access Networks

2.2. Terminals and Traffic

2.2.1. Types of Terminals

\ 2.2.2. Terminal Distributions and Densities

2.2.3. Traffic Generated

2.2.4. Traffic Classifications

2.3. Nodes

2.3.1. Circuit Switching

2.3.2. Store-and-Forward Switching

2.3.3. Switching Summary and Comparisons

2.3.4. Switching Control Systems

2.3.5. Switching Technology Developments

2.4. Links

2.4.1. Transmission Categories

2.4.2. Full Duplex and Half Duplex Channels

2.4.3. T-carrier Systems

2.4.4. Upgrading Local Loops

3. THE MILITARY SWITCHING ENVIRONMENT

3.1. Examples of Military Networks

3.1.1. Strategic

3.1.2. Tactical ",-

3.1.3. Non-tactical

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5.

3.2. Distinctive Characteristics

3.2.1. Survivability

3.2.2. Security and Privacy

3.2.3. Reliability and Availability

3.2.4. Adaptability

3.2.5. Compatibility

3.2.6. Special Serv~ces

3.2.7. Access Area

3.2.8. Performance

3.2.9. Cost

3.3. Future Trends

3.3.1. Digitization

3.3.2. Stored Program Control and Common-Channel

Signaling

3.3.3. Integration

3.3.4. Satellites and Packet Radio Systems

3.4. Network Forecasts

3.5. Transitional Interfacing

NEIJ."WORK CONTROL

4.1. 'Requirements and Issues to be Resolved

4.2. Advanced Signaling Techniques

4.2.1. Modes of Operation

4.2.2. Signaling on Separate Links

4.2.3. Control Procedures

4.2.4. Synchronization

4.2.5. Error Control

4.3. Performance Aspects

4.3.1. User Oriented Performance Parameters

4~3.2. Impact of Signaling System on

Performance

EXISrrNGAND PROPOSED SIGNALING SYSTEMS

5.1. Voice Network Protocols

5,.1.1. Cormnon Channel Interoffice Signaling (CCIS)

5.1.2. CCITT System No.6

5.1~3. CCITT System No.7

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102

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107

108

115

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132

144

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5.2. Data Network Protocols 154

5.2.1. Paralle~ Interfaces for ~evel 1 156

5.2.2. Serial Interfaces for Levell 156

5.2.3. Chara9ter Oriented Protocols for Level 2 158

5~2.4. Bit Oriented Protocols for Level 2 160

5.2.5. Packet Switched Protocols for Level 3 160

5.2.6. Network Control Architectures for ~ev~l 4 170

5.2.7. Performance Standards for Level 5 170

5.2.8. AUTODIN II Access Protocols 171

5.3. Satellite and Packet Radio Protocols 173

5.3.1. Pure Aloha 173

5.3.2. Slotted Aloha 174.

5.3.3. Reservation and Carrier Sense Protocols 174

5.3.4. Protocols for Mixed Traffic on Radio

Networks 175

6. APPLICATIONS TO THE ACCESS AREA DIGITAL SWITCHING SYSTEM 176

6.1. Signaling System Comparisons 178

6.2. Intranet and lnternet Gateways 178

6.3. Example of an AADSS Application La7

7 • RECOr1MENDED STUDY AREAS 193

7.1. Impact of Signaling Systems on Processor

Capabilities 193

7.2. Network M~nagement 196

7.3. Routing Strategy '196

7.4. Multiple-Access Protocols for Integrated

Satellite Networks 197

7.5. Network Performance Standardization 198

7~6. Traffic Capabi~ities 198

7.7. Computer Simulatio~ 199

8. REFERENCES 200

APPENDIX. SIGl'JAL {JNIT FORMATS AND CODES FOR CCITT

SIGNALING. SYSTEM NO~ 6 209

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LIST OF FIGURES

Figure 1-1.

Figure 1-2.

Figure 2-1.

Figure 2-2.

Figure 2-3.

A telecommunications network.

Access interfaces.

Network topologies.

Hierarchal network configurations for interregional access.

Hierarchal network configurations for local

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6

15

16

access. 18

46

83

87

51

70

73

76

83

81

42

43

34

24

28

31

Traffic rates generated by various types of

terminals.

Basic switching concepts.

The military switching environment.

Interrelation between circuit switching

functions.

Multiplexed PCM framing and signaling format

for T-carrier.

Figure 3-1.

Figure 3-2.

Figure 3-3.

Figure 3-4.

Figure 2-9. PCM multiplexing techniques.

Figure 2-10. Multiplexed PCM framing and signaling format

for CCITT recommended digital carrier.

Figure 2~11. Generalized cost curves for local distribution

networks.

Digital secure voice terminal block diagram.

Integrated network evolution.

Evolution of hybrid switch.

Exchangeable four-line modules to facilitate

analog-to-digital transition on PABX.

Figure 3-5. Analog and digital interfacing function

block diagram.

Figure 3-6. Digitized voice terminals.

Figure 3-7. Potential signaling interfaces to access area

digital switch for transitional network

environment.

Figure 2-8.

Figure 2-5.

Figure 2-6.

Figure 2-7.

Figure 2-4.

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97

162

155

148

146

135

137

140

143

133

124

104

106

110

113

116

118

100

4-5.

4-6.

4-7.

4-8.

4-9.

Figure 4-1. Network control procedures for voice and

data networks.

Figure 4-2. Interprocessor communication techniques.

Figure 4-3. Methods for distinguishing user and control

information.

Figure 4-4. Modes of operation for common channel signaling

system.

Interfacing protocol structures.

Error control schemes used on data links.

BRC, VRC and CRC error detecting schemes.

Cyclic redundancy check system.

Interface points for user-o'riented and

technically oriented performance parameters.

Figure 4-10. Mean access time on DDD network using push

button dialer and conventional signaling.

Figure 5-1. Network structure for disassociated common

channel interoffice signaling (CClS).

Figure 5-2. Control signaling- via common channel signaling

link.

Figure 5-3. Basic format of signal units used with common

channel interoffice signaling system.

Figure 5-4. Steps required for 10-digit call using CCIS.

Figure 5-5. ceIS signaling block structure.

Figure 5-6. Basic format of signal units used with

International Signaling System No.6.

Figure 5-7. Signaling block structure for International

Signaling System No.6.

Figure 5-8. Transmission schemes defined for Signaling

System No.6. 150

Figure 5-9. Functions and formats of proposed System No.7. 152

Figure 5-10. Interface standards promulgated by various

organizations.

Figure 5-11. Levell interface (X.21) for packet switched

network.

Figure

Figure

Figure

Figure

Figure

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Figure 5-12. Frame format for level 2 (HDLC) of X.25

protocol. 163

Figure 5-13. Call request packet used with the X.25

protocol. 165

Figure 5-14. Data packet format used with the X.25

protocol. 167

Figure 5-15. SENET/DAX packet format. 169

Figure 6-1. Access area digital switching system

interfaces. 177

Figure 6-2. Signal format comparisons. 179

Figure 6-3. Intranet gateway. 184

Figure 6-4. Internet gateway. 185

Figure 6-5. Transitional AADSS for regional internetting. 186

Figure 6-6. Local access at Ft. Monmouth, NJ, via coaxial

loop configuration. 188

Figure 6-7. Regional access to military reservations in

New Jersey area via T-carrier. 190

Figure 6-8. Strategic access to digital European backbone

via DSCS. 191

Figure 6-9. Terminal access via the AADSS. 192

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LIST OF TABLES

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59

Table 3-2.

Table 3-3.

Table 3-4.

Table 3-5.

Table 2-1. Projected Number of Military Terminals for

Generating Various Types of Traffic

Table 2-2. Number and Types of Terminals Proj~cted for

Two Military Base Environments

Table 2-3. Characteristics of Traffic Generated by Various

Types of Terminals

Table 2-4. Characteristics of Traffic Classes

Table 2-5. Comparison of Three Basic Switching Techniques

for Data Communication Networks

Table 2-6. Network Switching, Control and Signaling

Development

Table 2-7. Transmission Facilities in Common Use by U.S.

Communications Industry

Table 2-8. Digital Multiplexing Hierarchies Used in

North America

Table 2-9. Comparisons Between T-l Carrier and CCITT

Recommended Digital Carrier

Table 2-10. Cost Comparisons on a Per Drop Basis

Table 2-11. Estimated Cost for 6000 Terminal Local Access

Network at Ft. Huachuca

Table 3-1. Current and Projected Characteristics of DCS

Backbone Networks

Comparison of Commercial and Military Networks

Security Levels for AUTODIN I

Precedence Levels for AUTODIN I

Functions and Features Required in Military

Signaling Systems

Table 3-6. Number of Various Type Lines Served in Different

Size Areas

Table 3-7. Total Investment Costs to Replace Local Access

for Buried Outside Plant with Distributed Control 67

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Table 3-8.

Table 3-9.

Table 4-1.

Impact of New Technologies on Network Elements 68

Military Network Forecasts 78

Functions Performed by Various Types of Control

Table 4-2.

Table 4-3.

Table 4-4.

Table 4-5.

Signals

Signaling System Requirements for Military

Networks

Signaling Issues to be Resolved

Compilation of Parameter Values for Proposed

Federal Standard 1033

Some Performance Parameter Estimates for Various

Communication Networks

89

93

95

120

125

197

194

195

Between Terminals in Figure 6-9

AADSS Issues Summary

Number of Routing Control Steps for n-Node

Networks

Table 5-1.

Table 5-2.

Table 5-3.

Table 5-4.

Table 5-5.

Table 5-6.

Table 5-7.

Table 5-8.

Table 6-1.

Table 6-2.

Table 6-3.

Table 6-4.

Table 7-1.

Table 7-2.

Table 4-6. Impact of Signaling Systems on Performance of

Digital Networks 127

Summary of Telephone Signaling Systems

Recommended by the CCITT 131

Signal Information and Header Codes for LSD's 138

CClS Modem Characteristics 144

Heading Codes Allocated for System No. 6 147

Standard Interface for Serial Data Interchange 157

Character Oriented Codes 159

Recommended Standards for Data Networks 172

Traffic Characteristics for Typical User of

Data Communications Satellite 173

Common Channel Signaling Technique Comparison 180

Qualitative Signaling System Comparisons 181

Summary of AADSS Characteristics 183

Control Functions or Protocol Levels Involved

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AADSS

ADCCP

ACK

ACU

ANSI

AM

APC

ARPA

ARPANET

ARQ

ASCII

AT&T

AUTODI1'J

AUTOSEVOCOM

AUTOVON

BASCOP

BDLC

BER

BORSCHT

BSC

CATV

CCIS

CCITT

CCTV

CERCOM

CO

C,OMSEC

COl'JUS

CPODA

LIST OF ACRONYMS

Access Area Digital Switching System

Advanced Data Communications Control Procedure

acknowledgement of correct reception

acknowledgement signal unit

American National Standards Institute

amplitude modulation

adaptive predictive coding

Advanced Research Projects Agency

packet switched network developed by ARPA

automatic repeat request for error control

American Standard Code for Information

American Telephone and Telegraph Co.

automatic digital network

automatic secure voice communication network

automatic voice network

base communications program

Burrough's data link control

bit error rate

battery, overload, ringing, supervision, clock,

hybrid and testing

binary synchronous control, also called bi-sync

conununity antenna television, also called cable TV

common channel interoffice ~'-or interswitch) signaling

International Telegraph and Telephone Consultative

Committee

closed circuit television

Conununications and Electronics Readiness Command

(U . S. Army)

central office

communications security

continental United States (excluding Alaska, Hawaii,

and various territories)

contention-based priority oriented demand assignment

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CPU

CRC

CRT

CSA

CVSD

DCA

DCE

DCS

DDCMP

DEC

DECNET

DLC

DoC

DoD

DNA

DPClvi

DSCS

DSE

DrrE

EBCDIC

ECOM

EFTO

EIA

EO

EPABX

ESS

ETS

FAX

l;'DM

FDMA

FEe

PM

central processing unit

cyclic redundancy check

cathode ray tube

Communications Systems Agency (U.S. Army)

continuously variable slope delta, voice digitization

technique

Defense Communications Agency

data circuit-terminating equipment

Defense Communication System (terrestrial, strategic

backbone)

digital data communications message protocol

Digital Equipment Corporation

Digital Equipment Corporations Network Architecture

data link control

Department of Comrnerce

Department of Defense

Digital Network Architecture (NCR)

differential pulse code modulation

Defense Satellite Communications System

data switching equipment

data terminal equipment

extended binary coded decimal interchange code

Electronics Conunand· (U. S. Army)

encyrpted for transmission only

Electronic Industries Association

end office

electronic private automatic branch exchange

electronic switching system

European telephone service

facsimile

frequency division multiplex

frequency division multiple access

forward error correction

frequency modulation

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GPIB

HDLe

HP

Hz

lAM

IBM

IEEE

II'1P

I/O

ISO

ISU

ITS

lTD

LPC

HRC

LSI

LS'U

MBS

MF

MFDT

MTBF

MTTR

MUr-1

lViUX

NAK

NCA

NCR

NI4M

NPA

NTIA

O&M

PABX

proposed Federal Standard for user oriented performance parameters

General Purpose Interface Bus

high-level data link control

Hewlett-Packard Company

hertz, or cycles per second

initial address message

International Business Machines Corp.

Institute of Electrical and Electronics Engineers, Inc.

interface message processor

input and/or output

International Standards Organization

initial signaling unit

Institute for Telecommunication Sciences

International Teleconununications Union

linear predictive coding

horizontal redundancy check

large scale integration

lone signal unit

multiblock synchronization signaling unit

multi-frequency

multi-frequency dual tone

mean time between failures

mean time to repair

multi-unit message

multiplexer

negative acknowledgement

tJational Command Authority

National Cash Register Company

network management and maintenance signal

numbering plan area, also known as area code in telephony

National Telecommunications and Information

Administration

operation and maintenance

private automatic branch exchange

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PCM

P2SV

RS

RS-232C

RS-449

RS-422

RS 423

SAC

SAM

SATIN

SBC

SC

SCU

SDLC

SENET

SF

SNA

SSB

SSTV

SSU

STP

SU

SYU

T-CXR

TDM

1'Dlv1A

TRI-TAC

TTY

V.

VDP

pulse code modulation

phase two secure voice network

reco~nended standard by EIA

interface between DTE & DCE employing serial binary

data interchange

general purpose interface for DTE & DCE employing

serial binary data interchange

electrical characteristics of balanced voltage

digital interface circuits

electrical characteristics of unbalanced voltage

digital interface circuits

Strategic Air Command

subsequent addr(ess message

SAC Automatic Total Information Network

sideband coder

suppressed carrier

system control unit

synchronous data link control (IBM)

slotted envelope network

single frequency

Systems Network Architecture (IBM)

single sideband

slow scan television

subsequent signaling unit

signal transfer point in CCIS

signaling unit

synchronizing signaling unit

T-carrier

time-division multiplex

time-division ~ultiple access

Joint Tactical Communications Program

teletypewriter

CCITT designation for series of recommendations on

data transmission over telephone networks

voice digitization processor

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VRC

V.ID

V.ll

WATS

WE

WIN

WWMCCS

X.

X3.28

X3.44

X3.69

X.21

X.24

X.25

X.60

vertical redundancy check

electrical characteristics for unbalanced doublecurrent circuits, also known as X.26

electrical characteristics for balanced doublecurrent circuits, also -known as X.27

wide area telephone service

Western Electric Corporation

WWMCCS intercornputer network

World-wide Military Command and Control System

CCITT designation for series of recommendations on

data transmission over public data networks

American National Standard for use of communication

control characters of ASCII for Information Interchange (1976)

American National Standard for determination of the

performance of data communication systems (1974)

American National Standard for general purpose interface between DTE & DCE for synchronous operation on

public data network (Proposed)

general purpose interface between DTE & DCE for

synchronous operation on public data network

definitions for interchange circuits between DTE &

DCE on public data networks

interface between DTE & DCE for terminals operating

in packet mode on public data networks

common channel signaling for synchronous data

applications - data user part

xvii

CONTROL SIGNALING -IN A MILITARY SWITCHING ENVIRONMENT

R.F. Linfield*

A number of factors, including network topology,

traffic characteristics, switching technology, and

transmission media, affect the choice of control signaling techniques to be used in networks of the future.

In this report these factors are considered from a

military standpoint. It is projected that, over the

next two decades, all of the elements of military

networks will undergo a transition from essentially allanalog to all-digital systems. It is also possible

that a unified integrated network capable of handling

digital voice and data could evolve. During the forthcoming period of transition, as well as for the

ultimate network configuration, the control signaling

system will play a critical role in terms of both

performance and cost. This report examines that role

by evaluating several advanced signaling systems,

particularly those using common control links. Digital

systems that have potential application on both the

line and trunk sides of the digital switch are examined.

1. SYNOPSIS

One of the major issues which arises in the design, development and deployment of a telecommunications network is the

specification of network management and control. The design of

the control signaling system is a prime example. Because military

networks tend to evolve slowly, perhaps over a decade or two,

resolution now of the key control signaling issues could

influence, if not fully determine, the direct.ion future networks

will take in the 1980's and 1990 f s. This report provides an

introduction to these issues by reviewing the mOre important

advanced signaling concepts available today.

The report begins with a review of networks in general and

the military switching environment in particular (Sees. 2 and '3).

Next, the network control functions are defined, and ways and

*The author is with the Institute for Telecommunication Sciences,

National Telecommunications and Information Administration, u.s.

Department of Commerce, Boulder, Colorado 80303.
means for performing these functions are described (Sec. 4).

Specific signaling systems which are either in use or are

proposed for use are described in Section 5. Their applications

to the military switching environment are compared and evaluated.

Recommendations are made concerning choice of signaling schemes

for future military network controls (Sec. 6). The remaining

issues in the control area are summarized last. Ultimately,

these issues can only be resolved by conclusive field testing and

further study (Sec. 7). References are listed at the end

(Sec. 8).

The synopsis summarizes the background, key results, and

conclusions developed at in this report.

1.1. Background

Telecommunications means information transfer at a distance.

A switched telecommunications network, such as provided by telephone systems, provides economical telecommunications to many

users. This is because the many relatively low demand users can

share the network resources. Network resources consist of three

basic elements - terminals to provide transducer functions, nodes

to provide queueing, routing, and redressing functions, and links

to provide transmission functions (see Fig. 1-1).

Terminal functions permit coupling between users (human) and

between users' machines (computers). Node functions are performed by concentrators, processors, and switches. Link functions

are performed by a variety of facilities conveying information,

in electromagnetic form; e.g., wires, coaxial cables, radio or

optical fibers. Several users can share a terminal, terminals

can share the switches and switches can share the transmission

facilities to minimize information transfer costs.

Special signals (different from voice and data information

bearing signals) are conveyed over the links to control this

sharing process. The system devoted to these special control

signals is known as the signaling system. The signaling system,

like a nerve system in a human body, is pervasive throughout all

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the required network elements. Station signaling, over links

called lines, remotely controls the terminals; interswitch signaling, over links called trunks, remotely controls the switches.

Together, the station signaling systems and the interswitch

signaling systems control and manage the entire network.

Like all technologies, the evolutions of telecommunications

networks are shared by two driving forces: performance and cost.

A given network, more often than not, is a compromise between the

typically conflicting objectives of performance and cost. For

instance, a common carrier network user desires efficiency,

accuracy and reliability. He wants to minimize his costs by

using the network as little as is necessary to meet his needs.

At the same time, he may desire fast access with service always

available. The common carrier network supplier, in contrast to

the user, has economic reasons to sell as much service as possible;

to produce more income by sharing the network resource~ among

more revenue generating customers.

The military user of a network has many of the same needs as

the civilian user, but requires a higher degree of reliability

and availability. In addition, the military user has certain

unique requirements such as mobility, security and survivability.

The military user's service needs are different and the network

he uses must be adaptable to more dynamic conditions. The

military network operator may also be viewed as a paying consumer

rather than a producer (collector) of revenue. Such operators

must provide adequate service in a most economic fashion. This

economy can only be realized by careful consideration of all the

network elements, including cost to make, own, operate, and

maintain the terminals, .the switching nodes and the transmission

links. The importance of minimizing costs cannot be overemphasized. Almost one billion dollars per year is expended on

new installations, and on the operation and maintenance, of the

Defense Communication System (DCS)* alone. This is more than one
fifth of the yearly total expenditures for communications by the

Dept. of Defense.

Evolution of civilian and military telecommunication networks

must take into account compatibility with existing systems and

adaptability to future developments.. Adaptability and compatibility involve a number of complex network control and management

issues. The signaling strategy used for transferring control and

management information impacts the total network. The strategy

requires control signaling not only between all elements within

a given network (intranet control), but alsp between the elements

of independently operating networks (internet control). Intranet

control is concerned with internal network interfabes and the

remote control of the terminals, switches and transmission

facilities within the network. Internet control is conoerned

with the gateways for interconnecting networks which may be

separated functionally as well as geographically. This aspect is

discussed in Section 2.1.

The Access Area Digital Switching System (AADS~) provides

internet functional and geographic interfaces. The access inter~

faces provided by AADSS are indicated in Figure 1-2. The AADSS

provides internet control and traffic flow between tactica+, nontactical, and strategic systems', .plus commercial networks ..

In the past, internetting has had severe limitations bec~~$e

individual networks were designed and dedicated for specific

users. The engineering techniques and operating procedures

employed were seldom standardized. In the future, the technological and procedural limitations may be overcome by using b~oadly

applicable integrations of protocols, such as AID conversion,

hierarchal architectures, stored program controls, and common

channel interswitch signaling.

The forecasted evolution of military networks from allanalog, to analog-digital hybrids, and ultimately to all-digital

network configurations has already begun. Digitization, which

started at the strategic backbone level, is expected to progr~ss

to the local military base tactical and non-tactical level within

5
0'\

Figure 1-2. Access interfaces.
two decades. As this trend continues, it is entirely possible

that network services may also be combined. Some projections

describe a single network capable of handling all of the

military voice and data communications needs. The justification

for unification must be based on cost effectiveness of the total

system.

This study begins by examining the present status of

military networks, the projected traffic needs, potential switching and transmission technology and numerous cost factors. Since

signaling systems must interface with both new and old network

elements, this review is essential.

Future military telecommunications networks are expected to

handle an increasing number of widely dispersed terminals with

diverse traffic rates, transaction periods and acceptable

delivery times. Traffic projections are given in Section 2.2.

Terminal types will vary from massive data management systems

under distributed computer control, to traditional teletypewriters and telephones, to assorted sensors and monitoring systems.

Although data traffic is expected to increase, it is predicted

that over the next decade 90% of the military terminals will

serve voice subscribers. Approximately 1 million telephone

instruments will be connected to the Des global network from 1500

separate access areas ranging from small clusters «300) to

medium clusters (300 to 2000), to large clusters (>2000) of

terminals. Terminal densities will range from less than 10 per

square kilometer to over 10,000 per square kilometer. The

traffic generated by these terminals may vary in data rate from

less than 100 bits per second, for low speed teletypewriters, to

hundreds of kilobits per second for high resolution graphics and

imaging. Information exchange transactions will range from a few

hundred bits for interactive messages to ?everal million bits for

bulk data transfers. Delivery times will range from a few

hundred milliseconds for voice and video to several hours for

bulk data transfers will be either standard operating practice or

system requirements of the future.

7
Tying these terminals together, so that each can be connected to any other terminal, is a massive networking task.

Various sizes and types of switches are used at network nodes

depending on the amount and kind of traffic to be used. Switch

types generally fall into one of two categories: circuit

switches or store and forward switches. Hybrid switches may also

be used to handle both voice and data traffic. Advanced

switching technology utilizes a common control system which

consists of a computer processor with software programs stored in

a memory to perform the control functions.

The switching in stored program controlled systems may use

space division switching, time division switching, or combinations of these. In a space division switch, a matrix of

crosspoints is formed using metallic reed relays or electronic

semiconductors. In time division switches logic gates perform

the time slot interchange functions and no matrix is required.

Basic switchihg concepts including circuit, message and packet

switching are discussed in Section 2.3.

The switching nodes of the network are linked together by

various types of transmission facilities. The transmission

facility employed may be wire pairs, coaxial cable, fiber optical

waveguide, microwave radio or satellite, depending on such factors

as node separation, traffic load, and terrain traversed. The

links between terminals and nodes and between the nodes carry the

user information. They also can be used to transfer the control

information between the switch control systems (i.e., the processors) although a separate network may also be used for this

purpose. A description of some of the transmission facilities in

use today is given in Section 2.4.

The differences between the military switching environment

and the commercial switching environment are noted in Section 3.

Comparisons between the two environments are given in terms of

the following characteristics:

8
0 survivability 0 adaptability

0 availability 0 compatibility

0 security and privacy 0 services offered

0 reliability 0 performance

0 traffic 0 cost.

In addition, some military networks which are currently being

used are described. The effect of digitization, stored program

control, common channel signaling and service integration are

reviewed. Some projections are given regarding military networks

in the future. Emphasis is on the control and management of

these future networks.

This background material is essential to the understanding

of the report sections which follow because the switching environment has a considerable impact on the selection of signal systems

which control the network.

1.2. Network Control

The network control and management functions are considered

in Section 4. Specific systems used to accomplish these functions

for both telephony and digital data networks are described in

Section 5.

Various types of signals may flow on a network to control

the terminals, the nodes, the links, and the total network.

Advanced signaling techniques are considered for both circuitswitched networks and store-and-forward switched networks. Since

most of the network projections indicate a trend toward alldigital networks, the emphasis is on digital out-of-band signaling

systems for the line side of the switch and digital common channel

signal systems for the trunk side.

Signaling system requirements for military networks are

reviewed in Section 4.1 in terms of the distinctive characteristics of these networks. The signaling issues which must be

resolved in order to meet these requirements are also summarized.

Basic concepts used byrnost of the advanced signaling techniques,

including modes of operation, procedures, synchronization, and

9
error control, are explained in Section 4.2. The user-oriented

performance criteria used to evaluate and compare operating

systems are defined in Section 4.3.

The control procedures or protocols for controlling voice

networks using circuit switching are different from data networks

using message or packet switching. In Section 5, these differences are indicated by describipg typical examples of signaling

systems and the protocols they use.

Signaling formats, codes, and protocols specified by various

standards organizations for controlling several kinds of networks

are described in detail in Section 5.1 for voice networks and

in Section 5.2 for data networks. These include the cornman

channel interoffice signaling (CCIS) system and the CCITT

Signaling System No.6 for telephone networks, and the X.25

protocol for packet switched data networks.

A new signaling system is described in Section 5.1, denoted

CCITT No.7. This system is being developed for use on networks

which utilize both digital switching and digital transmission for

either voice and data. CCITT No. 7 is designed for 64 kb/s

operation and contains two parts, a message transfer part and a

user part. The message transfer part is common to both voice

and data circuits whereas the user part depends on the application. Other combination-type protocols are also discussed.

These involve "nesting" various protocol levels together in a

single format. The link, network, system, and user levels are

distinguishable by position of bit sequences in the master frame.

1.3. Conclusions and Recommendations

The report describes how the signaling system permeates and

impacts all the elements of a network, i.e., the terminal, the

switching and concentrating nodes, and the transmission links.

Thus, the ultimate choice of a signaling system for future

integrated digital networks requires the resolution of issues

which also involves all of the network elements.

10
Integrated digital networks for telephony (switching and

transmission) are already evolving in the commercial world.

However, this digitization process is being implemented in

different ways by different carriers. Some begin with an overlay

of digital facilities, others by replacement of obsolete transmission facilities or switching centers. Still others are

installing digital islands of networks which ultimately will be

connected by digital long-haul facilities. Just how this evolution will progress in the military world, and how fast is unknown.

In the interim transition period, all of the user's information,

whether it be voice or data is transferred to switching nodes

over dedicated subscriber lines. A switching node connects these

lines to other lines or to outgoing trunks. The interconnection

process may take many forms and use various media for control

purposes depending on local requirements for transmission signaling, numbering, etc. The need for compatibility between

existing equipments and evolving systems is a challenge to

designers of the military networks of the future. Some AADSS

applications are presented in Section 6. The AADSS is viewed as

an interconnecting gateway between military networks of various

types (e.g., strategic, tactical, and non-tactical) * and at

various levels (e.g., global, regional, and local).

Closely related to the integration of digital network

elements is the integration of the services that an all-digital

network could provide. New terminals and new switching systems

capable~of handling both voice and data are required to integrate

these services and thereby provide flexibility in the options

available to the military users. The extension of all-digital

formats to the subscriber's terminal and the use of an alldigital network to interconnect such terminals is a possibility

in the future~ When and if such a network should evolve, some

of the functions and features of the signaling system to be used

to control this unified system can be defined. The signaling

*Non~tactical networks included those which support base operations, e.g., fire, military police, medical, etc.

11
system should (1) be capable of using any link including those

normally used to handle data, (2) permit the addition of new

network elements and new service features using a multilevel

format, and (3) utilize a common discipline between data transmission and signaling so that a link of the control signaling

system is basically the same as any other data link.

Areas where further studies are recommended before a final

control system can be selected are outlined in Section 7. The

recommendations include work on:

1. The impact of signaling systems on the control

processor capabilities.

2. The optimal control system configuration for

network management functions.

3. Control processor requirements as a function of

routing strategy.

4. Multiple access protocols for application on

integrated satellite networks.

5. Network performance standardization parameters

and quantitativ,e measures of these parameters.

6. Traffic engineering studies to optimize the

total flow of intranetwork and internetwork

telecommunications traffic.

7. Computer simulation studies to aid in items 3,

5, and 6 above.

12
are reviewed. One should emphasize that it is the network

topology that ties all of the elements together. The network

control system utilizes its own, perhaps slightly different,

topology to distribute signaling messages.

The AADSS signaling system must interface with terminals

located in a military base environment. Interbase communications

proceeds from the intrabase level via the local access network to

a regional level via the regional access network and, at times,

to a global level via a backbone access network. The control

signaling system therefore is pervasive throughout all network

elements at all access levels.

In order to evaluate given signaling systems, it is

necessary to consider not only the network topology but the

terminals and the traffic they generate. One must consider the

basic switching concepts for both separate and integrated voice

and data switching, as well as the transmission media to serve

both analog and digital facilities.

, Although ultimately all of the network elements may utilize

digital technologies, it is useful to review analog implementations currently in use. One expects that the control signaling

systems developed in the future will be required, in many

instances, to interface with such 'existing network elements.

This interface compatibility between the "old" and the "new" is

particularly important during the expected transition from allanalog to all-digital networks.

The networking concepts presented in this section are not

necessarily unique nor optimum. However, they do indicate

potential configurations and general trends.

2 . 1. Topo,logy

This section is concerned with the interconnecting links

and the structural form of the networks. Studies have been

conducted on network topology to show how the network can be

configured to maximize traffic flow and minimize cost. (See, for

example, Coviello and Rosner, 1974.) The topographic structure

13
of a network also affects survivability. Redundancy is incorporated in all network elements to insure that certain critical

users can maintain communication under a range of adverse

conditions.

2.1.1. Types of Networks

There are numerous network topologies which can be envisioned for connecting n nodes of a network together. In fact,

if n is large, it can be shown (Nesenbergs and Linfield, 1976)

th~t the total number of interconnecting possibilities approaches

2n /2. For n=lOOO (not an unreasonable number of nodes) the

number of possible network topologies is approximately 10150 ,000,

an astronomical number. Since each network must have at least

one route between any two nodes, the 2total number of possible

routes could be of the order of n

2 2n /2.

Figure 2-1 depicts several general network types with their

commonly used names. Real networks usually differ from these

ideal models. However, real networks can usually be viewed as

constructed from such simple subnetworks.

2.1.2. Examples for Access Networks

Figure 2-2 illustrates the topology for two network structures

which might be used to provide access from a local base environment to a global "backbone" network using a three level

hierarchal structure.

In Figure 2-2a the terminals in a local access area are

served by a local access switch. A group of such local access

switches are shown connected by a star network. This is typical

of the local loop plant for the telephone network on a military

base. Lines to telephone subscriber instruments radiate out from

a central office (CO) on the base and from private automatic

branch exchanges (PABX) in office complexes. The CO is connected

via trunks to an end office (EO) of the commercial common carrier

network, to tactical and non-tactical networks via cable or radio

links and to the AUTOVON, AUTODIN, and AUTOSEVOCOM backbone

,swi tching centers,. These Defense Communication System (DCS)

networks are described in Section 3.1.

14

/

\

\
(0) TREE

(c) RING or LOOP

(e)FULLY CONNECTED

(bl LINE

(d) STAR

(f) HIERARCH ICAl

Figure 2-1. Network topologies.

15
INTERREGIONAL

ACCESS SWITCH

REGIONAL

ACCESS SWITCH

LOCAL ACCESS SWITCH

(0) Interregional Access (Terrestrial Backbone)

(b) Interregional Access (Satellite Backbone)

Figure 2-2. Hierarchical network configurations for

interregional access.

16
The regional access network shown in Figure 2~2a uses a

partially connected network. This grid-like structure is typical

of that used by AUTOVON. The AUTOVON switches and transmission

facilities are leased from the common carrier industry for

private military use. Generally there are at least two indirect

paths connecting any switch pair in the network.

The interregional access nodes are connected by a tree network. This terrestrial backbone configuration 18 exemplified

by the digital European backbone (DEB) network for the Des.

An alternative hierarchy of networks is shown in Figure 2-2b.

Here a loop configuration interconnects terminals or PABX's in

the local access area, a fully connected grid connects the

regional access nodes, and the interregional access is Vla a star

network that has a satellite for the central node~ This satel~

lite backbone is typified by the Defense Satellite Communications

System (DSCS).

It is obvious that numerous other topologies can be envisioned. Combinations of several ideal topologies are also

used. For example, the terrestrial and satellite backbone

networks can operate in conjunction with each other to enhance

survivability and to carry the traffic load jointly.

The hierarchy may extend to even lower levels on a base as

indicated in Figure 2-3. The central office or the main post

switch serves not only individual terminals, but also remote

concentrators and private automatic branch exchanges (PABX·s).

The PABX may be located in an office complex or other cluster of

terminals. Substantial parts of the PABX such as concentration

and multiplexing may be remoted from the central hub of the PABX~

Note that in Figures 2-2 and 2-3 only one of several subhierarchical networks are indicated.

2.2. Terminals and Traffic

The.ultimate source and destination of nearly all information traversing the network is the user terminal~ One exception

to this is control information which may terminate in a node.

17
PABX

REGIONAL ACCESS

SWITCH

LOCAL ACCESS

AREA

OFFICE

TERMINAL

Figure 2~3. Hierarchical network configuration for

local access.

18
The user terminal is the input/output device which generates and

accepts the traffic handled by the network.

The types of terminals, their geographic distributions, and

the volume and kind of traffic generated by each terminal are

important factors in any network design. Terminals are discussed

in the following section.

2.2.1. Types of Terminals

There is no foreseeable limit to the types and variations

of terminals. Furthermore, new types are continually being

introduced. They range from the traditional voice terminals

consisting of a telephone handset with rotary or pushbutton dial;

to keyboard and printer terminals for teletypewriting and computer

access; to visual display terminals with cathode ray tube or

other optical readout~ Many terminals incorporate microprocessors

and memory. Software programs add useful intelligence and

processing power to such "smart" terminals. A program addition

can change the character of the terminal to meet changing communications requirements or to adapt to new applications.

The telephone h,andset is the most corrunon terminal in use

today. There are over 300,000 handsets in use by the military

now and the number is not expected to decline. Over 500,000

telephone terminals with about 1 million telephone instruments

(including extension telephones) are projected to be in worldwide use by all US military services in the mid 1980's (Wagner,

1977). Data terminals are experiencing their own relatively

rapid growth and could, over the same period, approach 50,000

terminals worldwide. Note that this still is only 10% of the

telephone terminal population.

The DCA has projected the number of terminals of different

types for the continental United States (CONUS) and overseas as

shown in Table 2-1. The average duration of the different

messages generated by these terminals ranges from several hours

for bulk data transfers, to less than a minute for voice calls,

and to a few seconds for interactive data transfers.

19
Table 2-1~ Projected Number of Military Terminals for

Various Types of Traffic

Traffic Type Number of Terminals

Voice

Interactive data

Computer data

Narrative

Facsimile

CONUS

400,000

13,500

2,250

6,000

3,300

Overseas

100,000

2,500

250

4,000

1,200

2.2.2. Terminal Distributions and Densities

The number of terminals in the military base environment

varies with the size and type of base. Projections made by

Nesenbergs and Linfield (1976) for the types and numbers of

terminals at Ft. Huachuca, Arizona, and Ft. Monmouth, New Jersey,

(and its environs) are given in Table 2-2. These projections are

for the 1980 to 1985 time frame.

Ft. Huachuca is a u.s. Army base located within a IS-square

kilometer service area. The total population of military and

civilian personnel is 10,000. The base is headquarters for the

U.S. Army's Communications Systems Agency. The population

density, which reflects terminal density is not uniform over the

15 km2 service area. In fact, 80% of the base residents occupy a

core area of about 4 km2 .

Ft. Monmouth and the surrounding access area (including

Camp Woods, Camp Evans, the CERCOM building and other nearby

installations) is the headquarters of the u.s. Army Communications and Electronics Readiness Command and the Communications

Research and Development Command (CORADCOM). Other tenants

include CSA, the Satellite Communications Agency and elements of

the Electronics Research and Development Command and the Aviation

Research and Development Command (AVRADCOM). The military,

civilian and dependent population of this access area is approximately 15,300 with about 10% being military personnel.

20
Table 2-2. Number and Types of Terminals Projected for

Two Military Base Environments

Estimated No. of Terminals

(1980 to 1985)

Terminal Type (nominal rate)

Telephones

-Analog (4 kHz)

-Digital (64 kb/s)

-Video (6 Mb/s)

computer Access

-Low speed (450 b/s)

-Med speed (3.6 kb/s)

-High speed (48 kb/s)

Teletype

-Low speed (300 b/s)

·High speed (4,8 kb/s)

Facsimile

- Low speed (9.6 kb/s)

-High speed (56 kb/s)

-Color (1.5 Mb/s)

Television

-CATV (1 way channels)

·CCTV (2 way channels)

-SSTV (2 way channels)

21

Ft. Hllachuca

5000

500

5

34

16

5

50

20

6

2

2

12

2

2

Ft. Monmouth

8000

1000

5

50

20

10

100

20

10

5

5

8

4

6
There are projected to be some 1500 access areas worldwide

in the early 1980's. These areas will range in size from small

(50 to 300 terminals) to medium (300 to 2000 terminals) to large

(over 2000 terminals) (see Wagner, 1977). The terminal density

in these areas may vary from less than 10 per square kilometer to

over 10,000 per square kilometer. These densities have been

classified by subregions in the local access area as low suburban, high suburban, low residential, high residential, low

office complex, high office complex, and so forth, using average

Subscriber density profiles for each subregion (Nesenbergs and

Linfield, 1976). This classification is useful for estimating

local access network costs. Some resulting estimates are given

in Section 2.4.4.

2.2.3. Traffic Generated

The volume, data Tate, duration and delivery delay for

traffic generated by various types of terminals is indicated in

Table 2-3. Only nominal values are shown in this table for most

of the parameters. Specific actual values may depart considerably from these nominal values. To be used, data rates must

conform to the DoD and FTSC standard rates for synchronous

transmission (FTSC, 1973). The standards basically establish

rates at increments of 75x2k bit per second for k less than 7, or

8000xk bits per second for k greater than 6. Thus the data rate

generated by any given terminal may vary over a fairly wide

range. Examples of this range for various types of terminals are

plotted on Figure 2-4. A wide rate variation is indicated for

digital voice terminals. The voice digitization rate depends on

the process employed. At the higher rates (e.g., PCM and DPCM)

the voice quality is generally higher for a given bit error rate

over the channel. Gold (1977) describes how a network could be

designed to adapt to these different digital rates and thereby

alleviate congestion. This is accomplished by slowly reducing

the voice digitization rate, with a corresponding slow degradation in quality, as the traffic load on the network increases. A

disadvantage is trre increased complexity of network control.

22
Table 2-3. Characteristics of Traffic Generated by Various Types of Terminals

S I sees to min

hrs

(Bursts in s)

N

W

Voice

PCM

LPC

Data

Data Base Update

Interactive

Query/Response

Bulk

Narrative

Alpha Coded Text

Text Editing

Facsimile

No Gray Scale

Half Tone Photo

Color

Video

Picture Phone

Color TV

Slow Scan TV

Volume

continuous bits

continuous bits

2 10 b/message

3 10 b/m,essage

10 4 bits/

transaction

10 5-10 8 bits/

transaction

3xl04 bits/page

10 3 bits/page

3xl05 bits/page

3xl06 bits/page

107 bits/page

continuous

continuous

continuous

Digital Rate

(one-way)

64 kb/s

2. 4 kbj's

2.4-16 kb/s

150 b/s-9.6 kb/s

150 b/s-9.6 kb/s

100 kbj's

75 b/s-9.6 kb/s

75 b/s-·9.6 kb/s

4.8 kb/s

9.6 kb/s

1.5 Mb/s

6.3 Mb/s

30 Mb/s

100 kb/s

Call

Duration

min

min

s to min

min to hrs

s to n1in

s to min

min

min

min

min

hrs

min

Delivery

Delay

<200 ms

<200 ms

s

<1 s

min to hrs

min to hrs

s

min to hrs

min to hrs

min to hrs

<200 ms

s

min
Vllllllf!!!!!!!!!1

11/1//1/11111111

RATE IN BITS PER SECOND

103 104

[ll]

lZZZZ]

r/711J

11/1/11)

IIIIIIIIIJ

rzzzJ

VIIIII

IIIIIIIIIIJ

DIGITIZED VOICE 10

2

,--r-I---r""1-r-1-r-\""'"'T""\-r-I-r-IT"""'T"II----,.\-~\ ---"'I~\ -r-I-r-\T"""'T\I-r-I-~\-r-\---r\---r-\~\""""'1,""""-,----I---""'1---'-1---0\---01---'1--'-1"""""11

LINEAR PCM

LOGARITHMIC PCM

D1FFERENTtAL PCM

CVSD

APC

SSC

LPC

CHANNEL VOCODER

FORMAT VOCODER

DATA TERMINALS

TTY

CARD I/O

MAG I/O

GRAPHIC CRT

OPTICAL SCAN

FACSIMILE

(-.J

;.>~

with FEC VlI///I///J

without FEC 177711!/!!J

IMAGERY

SSTV , ,!" ! ,JJJ I I

Figure 2,- 4 . Traffic rates generated by various types of terminals.
Similar approaches with similar advantages and disadvantages

could be used with other types of terminals and traffic.

2.2.4. Traffic Classifications

It is apparent from Table 2-3 that traffic can be classified

according to various characteristics. Three classifications are

generally used; Class I for continuous traffic, Class II for

burst traffic, and Class III for interruptable traffic.

Caviello and Vena (1975) characterized each class by examining a

variety of terminals including man/man, man/machine and

machine/machine operation. Their results are summarized in Table

2-4. They note that the major distinction between Class III and

Classes I and II is one of priority and service level, rather

than real technical considerations. Thus, although Class I and

Class II traffic are handled differently in a network using

circuit and packet switching, the Class III traffic can normally

be handled by either type of switch depending on specific

situations. Different switching networks and their traffic

handling capabilities are discussed in the next subsection.

2.3. Nodes

The nodes of a network may serve both switched and dedicated

subscribers. Nodal points may contain multiplexers, concentrators,

and switches in various combinations, as well as link interface

and signaling equipment.

A multiplexer combines the traffic from many channels onto

a few high-speed channels. Each of the few channels is required

to carry no more traffic than the individual input channels.

Fixed rules determine how channels are multiplexed.

A concentrator, in contrast, is called an adaptive, a

programmable, or an intelligent multiplexer. It observes the

intermittent nature of traffic on several input channels to

provide a more constant flow of traffic on a single output

channel. Unlike multiplexers, the single output channel cannot

carry the sum of traffic of the concentrator input channels if

all are busy simultaneously. Thus blocking can occur.

25
Class I

Table 2-4. Characteristics of Traffic Classes

Class II Class III

N

m

Continuous

Real Time Information

Connection Delay

Fixed Delivery Delay

No Error Control

Long Holding Time

Blockable

Compatible Users

Burst

Discrete Messages

Near Real Time

Short Delivery Delay

Potential Error Controlled

Short Total Lengths

Non-Blocking

Arbitrary Users

Interruptable

Not Real Time

Long Delivery Delay

Error Controlled

Indefinite Lengths

Non-Blocking

Arbitrary Users
Analog multiplexers and concentrators use frequency division

multiplexing (FDM) whereas digital multiplexers and concentrators

combine digital traffic in time slots by time division multiplexing (TDM). This TDM technique is the only practical method

to use in an all-digital network, since D/A and A/D conversions

can thus be avoided.

2.3.1. Switching

Two switching concepts may be employed at a node; a circuit

switch and a store-and-forward switch. The basic differences are

illustrated in Figure 2-5. In a circuit-switched network the

switch establishes a direct connection between terminals. This

connection is maintained for the duration of the information

tran~fer phase. When the phase is finished, the connection is

terminated. The information transfer takes place essentially in

real time and is completely transparent to the ~ser. Interconnecting cross points are either metallic or electronic for

the space division circuit switch. A "virtual" circuit switch

employs time division switching. Logic gates and a high speed

memory provide the time division switching in essentially real

time.

The control of a circuit-switched network for both time and

space division switching requires a signaling system to

establish, maintain and terminate the continuous communications

path between the terminals. Circuit switching is ideally suited

for the transfer of user information having a relatively long

duration and requiring real-time delivery, such as for voice,

video and facsimile (i.e., Class I traffic).

2.3.2. Store-and-Forward Switching

There are "'" two forms of store-and-forward switching:' message

switching and packet switching. Message switching involves

formatting the information to be transferred into long blocks of

data, called messages. Control information is added to each

block by the terminal before forwarding the block to the switch.

Ea9h block is stored at the switch until an end-to-end transmission path is available for use. In contrast to circuit

27
CIRCUIT SWITCH STORE and FORWARD SWITCH

I ._--_._--.-

I MASS I

I STORE

-j~

~, ----.. - I -- LINE/ -- ..... ... ... PROCESSOR ..... TRUNK ..... .... INTERFACE ....

~~

~,

STORED

PROGRAM

-II

•

•

STORED

PROGRAM

PROCESSOR

LINE/

SWITCH TRUNK

I NETWORK INTERFACE I+--

I I •

00 •

e METALIC SWITCH

e VIRTUAL CIRCUIT SWITCH

e MESSAGE SWITCH

e PACKET SWITCH

Figure 2-5. Basic switching concepts.
switching, network resources are not dedicated to specific users

but are shared by all on an "as available" basis. A message

switch must store and forward several relatively long blocks of

data from many users. The maximum block length and number of

blocks is limited by the storage capacity of the switch. Large

processors are required to keep track of all the blocks flowing

in the network. High capacity disks and tapes are used for

storage. Message blocks are often delayed for long times while

waiting to be forwarded. These random delays can be decreased by

restricting the block length, redistributing the storage and

controlling the storage access times. This leads to packet

switching.

Packet switches handle shorter blocks of data than message

switches. The maximum block length is usually limited to a few

hundred or thousand bits. These blocks are called packets. The

packet switch accepts and holds the packet only until a link is

available. Packets may be forwarded through the network via

varying routes. Smaller processors are required because each

switch does not have to keep track of all the packets en route.

The holding process is accomplished in core storage and buffers

rather than on disks and tapes. Thus packet switching holds the

information for a short time before forwarding, rather than

storing the information for a long time.

A packet switched network can provide two kinds of service

to the users - datagram service or virtual service. The basic

difference is the order in which packets are received. Datagram

service is more fundamental since in it individual packets are

delivered independently and may have only an approximate

relationship to the order of entry. Each packet must be fully

addressed when entered into the network. Virtual service, in

contrast, resembles a virtual circuit switch. At the destination,

data packets are delivered in the same order they are entered

into the network, even though they may be forwarded via different

routes. A finite delay is involved depending on the length of

29
the packets, the number of packets per message, and the number of

links traversed (Roberts, 1976).

Various procedures called protocols are used to control

terminals and data networks for store-and-forward switching.

Several levels of protocol are used to control the terminals,

the links and the switches. Control information called overhead

is contained in each message block or packet. The amount of

overhead required reduces the network efficiency. Some protocols

at various levels are described in Section 5.

2.3.3. Switching Summary and Comparisons

Figure 2-6 summarizes the basic switching concepts and

relates them to specific military applications. The AUTODIN II

network (see Sec. 3) employs pa~ket switching protocols which

support both datagram and virtual service to the users. The

AN/TCC-39 switch is being developed by the u.s. Army under the

TRI-TAC program. This switch will incorporate both virtual

circuit switching and message switching capabilities "for the

phase two secure voice (P2SV) network.

Table 2-5 compares the advantages and disadvantages of

the circuit, message, and packet type of switches when used for

data communications. The characteristics noted for each type of

switch in Table 2-5 can also be compared with the characteristics

of traffic classes in Table 2-4. It is apparent from this comparison that the circuit switch is ideally suited for some

information transfers whereas store-and-forward switching is

ideally suited for others. The circuit switch is inherently a

two-way transmission switch with little or no delay. It is most

suited for handling narrowband, lower speed transmissions which

occur continuously and for long periods of time, i.e., Class I

information.

The message and packet switches are inherently unidirectional.

Data encounter relatively long delays when passing through

message switches and relatively short delays through packet

switches. The message switch is most suited for interruptable

transmissions with long acceptable delivery times, i.e., Class

30
SWITCH CONCEPT

Figure 2-6. The military switching environment.

DATAGRAM

SERVICE

I

I

I

I

AUTODIN II

VIRTUAL

SERVICE

I

I

I

I

ARPANET

AUTODIN II

AN/TTC-39

(Tactical Switch)

AUTOSEVCOM II

I

STORE and FORWARD SWITCHING

I I

TIME DIVISION MESSAGE SWITCH PACKET SWITCH

( Virtual ) /

Circuit Switch· / /

"/

"" // AUTODIN I

ELECTRONIC

"

V /

CROSSPOINTS

AUTOVON

(ESS Switch)

AN/TTC-38

(Tactical Switch )

CIRCUIT SWITCHING

SPACE DIVISION

METALIC

CROSSPOINTS

I

I

I

I

AUTOVON

(Crossbar Switch)

AUTOSEVCOM I

W

r-'
Table 2·~5. Comparison of Three Basic Switching Techniques for

Data Communication Networks .

Circuit

AdYC)..I1t~<Je,~

voice compatible

Common Procedures

Fully Transparent

Disadvantages

Large Processing

tv'" w t Burden .

Requires Compatible

Terminals

Subject to Blocking

Message

Advantages

Code and Speed Conversion

Nonblocking

High Efficiency and

Channel utilization

Disadvantages

Large ProcessQrs

High Capacity Storage

using Disks on Tapes

Variable Delay

Poor Response to

Interactive Traffic

Packet

Advantages

Short Delay

Rapid Exchange of

Interactive Traffic

Code and Speed

Conversion

Disadvantages ...... '.... ,. . .....'. _.,-"--" ..,,.,

Many Small Processors

Core Storage/Buffers

Complex Routing and

Control

May be Blocked with

Short Delay
III data. The large storage capacity and processing capability

of the message switch are substitutes for more elaborate and

costly transmission facilities.

The packet switch is mOre suited to high data rate transmissions of information with burst characteristics, i.e., Class

II traffic. Less storage capacity and processing capability is

required in the packet switch. The holding delay is shared by

all users and therefore the buffer storage at each switch need

not be large.

Although different switching techniques provide better

performance for different classes of traffic, this does not

necessarily imply that different transmiss~on and switching networks are required.

The three basic switching concepts are not mutually exclusive.

A single network capable of handling voice and data could be

comprised of both circuit and store-and-forward switch~n9

elements. The evolution of an integrated (voice/data) switch for

potential military applications is described in Section 3.3.

2.3.4. Switching Control Systems

Since this report is concerned with network control, it is

useful to examine the circuit sw~tch and the store-and~forward

switch from the control standpoint. The signaling system

associated with the switch must perform three basic functions:

1. Interrogation which involves the transmiss:ion an,d the:

reception of control information.

2. Interpretation by a control unit to determine what, if

any, action is to be taken by the switch.

3. Interconnection to complete the data transfer (either

by operating the switch matrix or b'y fo,rwclrding da-ta

from storage, for example) .

The interrelationsh~p between these three functions is

shown in Figure 2-7 for a cireuit switch., In a moder'n electr'onic

switching system, the control unit is part of the central

processing unit (CPU). Associated with the CPU are several

memories containing, among other things, the stored programs.

33
INTERROGATION

INTERCONNECTION ----.

w INTERPRETATION ..

SIGNALING ..I

EQUIPMENT .....

~~

.,t ..

CONTROL LINE/TRUNK ;. - UNIT INTERFACE

)~ "

,If

SWITCH --

NETWORK -

TO OTHER

NETWORK

RESOURCES

Figure 2~7. Interrelation between circuit switching functions.
Control information received by the signaling equipment is

evaluated in the processor and, according to the program instructions, the switch matrix is activated. The switch matrix may use

metallic reed relays or electronic semiconductors for switching

cross points.

In a store-and-forward switch the processor itself makes

the connection and no switching matrix is required. Data blocks

with their associated control overhead are stored in memory or

queued in buffers until the processor can interpret their

destination, find an open route, and forward them.

The protocols for controlling data'terminal equipment and

link terminating equipment on a packet-switch network have

recently been standardized. The virtual service protocol denoted

X.25 by the CCITT is recommended as the international standard

for operating in the packet mode on public switched networks.

New standards are being proposed for datagram service. These

protocols and others used to control store-and-forward switches

are described by Folts and Cotton (1977) and reviewed in

Section 5.2.

2.3.5. Switching Technology Developments

The circuit-switching technology for telephony has undergone

considerable change since its inception. Table 2-6 indicates

some of the major developments which have occurred at approximate

20-year intervals beginning around 1880. The control unit has

progressed from completely manual to fully automatic, the

signaling from handcranked magnetos and aural addressing to push

button dialers and digital signals over cornmon signaling chan-

, nels; and the circuit switch control from operators with plugs

and jacks to microprocessors with stored programs.

Packet switched networks are a more recent introduction.

For example, ARPANET, conceived by the Advanced Research Projects

Agency, has undergone only a few changes since its operation

began in 1969. The network is designed to interconnect dissimilar

computers throughout the United States, allowing users and

programs at a computer center to access and interactively use

35
Table 2-6. Network Switching /Control and Signaling. Development

Interconnection Control Signaling

Circa

1880

1900

1920

concept

Analog

Analog

Analog

Switching

Technology

Plug & Jack

Step-by-step

Panel

Operation I Type tlSupervision I Addressing

Manual I Direct 11 Magneto I Aural

Mechanical IDirect It DC 1 Pulse DC &

Reverting

Motor Driven I Indirect 11 DC 1 PCl

W

0"\

1940

1960

1970

Analog

PAM

PCM

Cross Bar

Electronic

Microprocessor

Relay

Wired Logic

Software

Common

Common

Stor,ed

Program

DC+SF

SF

Digital

MF

MF

Digital
facilities at another center. Feinler (1976) and Feinler and

Postel (1976) provide a description of ARPANET resources and

protoc,ols , respectively. Most of the or~ginaldevelopm.entwork

on packet switched networks has resulted fr,om research activiti·es

on the ARPANET. The AUTODIN II netw~ork follow:s ulan'y of the same

conc,epts and is essentially a second generation packet switched

network (BBN, 1978).

2.4. Links

Military and cOInm'ercial networks ·currentlyuse va.rious

combinations of landlines, radio, and sat·ellites for the links

between nodes. These links in large trans·continentalnetworks

constitute nearly h,alf of the investment ·costof the network .

Major cost elements are the wires and cables Ln the landlines,

radio equipment as·sociated repeaters, and multiplexing equipment.

About one-third of this investment cost for transmission

facilities is for long-haul f·acilitiesand t'wo-thi'rds for local

access, primarily the lines to the terminals (or Local loops)

(Members of the Technical Staff, 1978)~

It is largely due to this high percentage cost of transmission facilities that networks are designed to share their

transmission resources efficiently in order to become economically

attractive.

2.4.1. Transmission Categories

Transmission facilities may be categorized into 1) local

loops for terminal connections, 2) short-haul facilities for

regional access, and 3) long-haul facilities for interregiona~

access.

A typical local loop for a telephone terminal consists of

a twisted pair of No. 26 gauge insulated copper wire. The

average length of a typical loop is abollt 2 km and the maximum

length is about 8 km. For analog voice transmissions over such a

loop, loading coils are installed approximately every 1.5 km

to improve the frequency response.

37
Short-haul facilities typically employ many pairs of twisted

wire in shielded cables. Frequency division multiplexing allows

several analog voice channels to be carried on each pair. For

example the type N carrier uses double sideband amplitude modulation and frequency multiplexing to yield a capacity of twelve

voice channels. The frequency of these 4 kHz multiplexed

channels spans the spectrum from 148 kHz to 196 kHz. Short-haul

facilities operate over distances of a few hundred kilometers.

A digital short-haul facility capable of handling 24 voice

channels was introduced by the industry in 1962. Called the

T-carrier, the facility uses pulse code modulation and time

division multiplexing to provide a true digital transmission

system. Since in this report we are concerned with digital

network elements and the control of these elements, the T-carrier

system will be described in more detail in a subsequent paragraph.

Long-haul facilities are designed for extremely efficient

transmission over intercontinental distances. Unlike the local

loop, or short-haul systems, multiplexing hierarchies permit

the transmission of thousands of voice channels via coaxial

cable, line-of-sight microwave channels and via satellites.

The satellite links employ multiple transponders each capable of

handling several thousand voice channels. Troposcatter radio

links are also used for global access.

Pertinent characteristics of several short-haul and longhaul transmission facilities commonly used by the telephone

industry for voice and data transmission are listed in Table 2-7.

2.4.2. Full-duplex and Half-duplex Transmission

It is common practice to denote a channel capable of unidirectional, either-way transmission as a half-duplex or 2-wire

channel. Two such one-way channels are required for full-duplex

or 4-wire operation. The actual channel separation may not be a

physical separation. For example, two channels, one for each

direction, may be carried in the same wire pair using frequency

division mul~iplex. A digital transmission facility operating at

38
Table 2-7. Transmission Facilities in Common Use by u.s. Communications Industry

W

\...0

Transmission

Carrier Channel ~otal ~ystem

Carrier Modulation caoacit Total System 6" Capaclty Repeater

Designator Scheme Media (one-way v· lce (two'-way VOiCE Spacing in km Length in km til ckts) ckts) ill

.r-!

+J a AM Open wire 16 16 13 300

.r-!

r--i

.r-! N AM Wire cable 12 12 13 300 U

m ON AM Wire cable 20 20 80 250

r--i T-1 PCM Wire cable 24 24 1.5 80

m

.~

I T-2 PCM vJire cable 96 96 1.5 800 +J

0 T-4 PCM Coaxial cable 4,032 4,032 1.5 800

..c

U)

K AM Wire cable 12 48 25 2,400

TD FDM/FM LOS microwave 1,200 2,000 50 6,000 til

OJ

.r-! TH FDM/FM LOS microwave 1,860 11,600 50 6,000 +J

.r-!

r--i L-l SSB/SC Coaxial cable 600 600 13 6,000

.r-!

U

m L-3 SSB/SC Coaxial cable 1,860 16,740 6 6,000

r--i L-4 SSB/SC Coaxial cable 3,600 32,400 3 6,000

::c

m L-5 SSB/SC Coaxial cable 10,800 97,200 1.5 6,000 I

tJ1

c EMT FDM/FM Troposcatter 120 120 300 5,000

0

t--l INTELSAT IV FDM/FM Satellite 300 per 6,000 N/A 13,000

transponder
baseband is normally half duplex. Two digital channels are

usually required to operate full-duplex. Note however that an

analog voice system operating at voice frequency is full duplex

and operates over two wires. Simultaneous tw.o-waydata transmission over this two-wire loop may require an additional pair of

wires for full-duplex operation. The voice network used for

AUTODIN is 4-wire.

Any given transmission facility may employ several full- or

half-duplex channels. Individual coaxial cables, for example,

are normally operated as one-way systems. The number of coaxial

cables in a bundle of cables may vary from 2 to 20, each cable

capable of handling many frequency multiplexed voice channels.

The L-5 system, for example, contains 20 coaxial cables in a

single large bundle. Since each coaxial cable can carry 10,800

voice chartnels, the total two-way system capacity is 108,000

channels. One pair of cables, however, is reserved for a spare

and the total system capacity shown in Table 2-7 is therefore

97,200.

The satellite system INTELSAT IV listed in Table 2-7 contains

multiple transponders in the satellite each capable of handling

300 one-way voice channels. Since there are 40 transponders, the

total number of two-way voice channels carried by the satellite

is 6000.

The channel capacity in terms of one-way voice channels and

the total system capacity in terms of two-way voice channels are

indicated in Table 2-7 for long-haul facilities.

2.4.3. T-carrier Systems

As mentioned prev~ously, digital T-carrier systems capable

of handling 24 voice channels were introduced in 1962 using

pulse code modulation (PCM) for voice channels (James and

Muench, 1972). The initial system, denoted as the T-l carrier*

used 0-1* channel banks at the facility terminations to digitize

*The T-carrier designations T-l, T-2, etc. and channel bank

designations 0-1, 0-2, etc. are Western Electric Co. designations

that have become generic terms for almost the entire industry.

40
- ---------------------------

and multiplex analog voice channels. Initially the digitization

process was accomplished in a D-l channel bank. Each voice

frequency was sampled 8000 times per second and quantized to 7

bit accuracy. An eighth bit was added for signaling purposes.

Later channel banks (e.g., D-2 and D-3) use 8 bit quantization

and signaling bits were borrowed periodically. This 8-bit

"encoding" is used today in D-l channel banks for T-l transmission where there are several links in tandem. A non-linear

quantization process is normally used to maintain a relatively

constant signal-to-quantizing distortion ratio over a wide range

of talker levels. Compression techniques quantize the low-level

samples with small steps and high-level samples with larger

steps.

Quantizing 8000 samples per second with 8 bit accuracy

yields a 64 kb/s bit rate per voice channel. In the T-l carrier

system, 24 of these channels are multiplexed in 8-bit groups

into a frame. One bit is added for synchronizing purposes so

the total frame contains 193 bits. The T-l line format is shown

in Figure 2-8. A bipolar pulse train is used, where zero bits

are denoted by an absence of pulses and one bits are alternately

positive and negative pulses. Since there are 8000 frames per

second, the total bit rate is 1.544 Mb/s. The multiplexing

process can be accomplished either before or after the 8-bit

encoding process as illustrated in Figure 2-9. Most channel

banks multiplex before so that common encoder/decoder can serve

all channels. This process, however, precludes the possibility

of ~emoting the process to a single terminal, i.e., a digitized

telephone.

The T-carrier can be multiplexed to higher levels, to carry

more voice channels more effectively over greater distances. The

hierarchal levels common to the industry are shown in Table 2-8.

The T-3 carrier is only used internally at a switch center.

41
193 rd Bit 193 rd Bit

r 125,us -1

I I 1 v 1 - 1 • 'V J -. , "-I ._, - . I' - I'-I .. 1 . -I' -, - - ,_. '--I -, I

24 - 8 Bit Words -.... ........ ........ ......... .......... -.... -.... Sixth Frame -.... t Signaling Bit .......... ........ ......... .......... -.... .........

I I I I I I I I

[".J

I I I I r I I I I I I I

U I I I

Bipolar Format

1 . 0 I 1 I 0 I 1 I 1 . 0 1 ' Binary Code

r 5. 2 j1-s 1

Figure 2-8. Multiplexed PCB framing and signaling format for

T-carrier system.
FILTERING

&

SAMPLING

UNITS

2

F Analog COMMON

Lines 0 ENCODER PCM .. Digital Link & ... 0

0 DECODER

I

• N

a) Before Coding

UNITS

• PAM

C":NCODER / DECODER

UNITS

t

1

Analog

Lines

o

o

o

2

N

PAM 2

N

PCM .--.-----~ Digital Link

F ic.;'J.re 2- 9 . PCM multiplexing techniques.

43
Table 2-8. Digital Multiplexing Hierarchies Used in

North America

Carrier

Tl

TIC

T2

T3

T4

Tl Hierarchy

1

2

4

28

168

Voice Channels

24

48

96

672

4032

Rate (Mb/s)

1.544

3.152

6.312

44.736

274.176

The specifications for PCM multiplexors for the Digital

Radio and Multiplex Acquisition (DRAMA) system has a somewhat

different hierarchy. The first-level multiplexor performs the

analog-to-digital conversion and time-division multiplexing for

three, six, twelve and twenty-four voice channels. The digitization process for the 24 channel increment is essentially the same

as TI, i.e., 1.544 kb/s. The three, six and twelve channel modes

are used for Dses and the twenty-four channel mode for DeS. The

output parts from several first-level multip~exors can be combined

in a second-level multiplexor as follows:

Number of

1.544 Mb/s Ports

2

4

6

8

Number of

Voice Channels

48

96

144

192

Data Rate

(Ivib/ s)

3.168

6.336

9.504

12.672

Digital data can also be interleaved by replacing voice channel

cards in the first-level multiplexor. Data bit rates specified

are 0 to 20 kb/s and 50 kb/s asynchronous and 56 kb/s, 64 kb/s

and 128 kb/s synchronous.

The DeS system in central Europe is being upgraded to form a

Digital European Backbone (DEB) . ~errestrial microwave links

operating in the frequency band ~rom 7,125 MHz to 8,400 MHz are

phase-shift modulated to convey the 12.672 Mb/s rate corresponding

to two T2 lines or 192 digital voice channels.

44
In Europe and in many foreign countries, a different format

is used for commercial digit.al transmission facilities although

the basic PCM concept is the same. The European system uses a 32

channel system and format adopted by the Consultative Committee

for International Telephone and Telegraph (CCITT) & The thirtytwo 8-bit channels occupy a frame 256 bits long. No framing bit

is added for synchronization, so the total bit rate is 2.048

Mb/s. A 16-frame cycle makes up a multiframe. Multiframes occur

at a rate of 500 per second. Two of the 32 channels are reserved

for signaling and synchronization purposes. Therefore, there are

30 digital voice channels available. The signaling system uses

channel 16 for control information by allocating 4 bits per

multiframe for each of the 30 voice channels. Since multiframes

occur at a rate of 500 per second, each of the 30 voice channels

has a 2000 bls signaling rate. If common channel signaling is

used, channel 16 provides a 64-kbjs signaling rate just like any

other voice channel. The standard CCITT format is shown in

Figure 2-10.

Comparisons between the CCITT standard system and the T-l

carrier used in North America are given in Table 2-9.

A tutorial discussion of these basic digital transmission

concepts is given in CEEIA (1977). A detailed description of

T-carrier systems is given in Members of Technical Staff (1971).

The' international recommended standard is found in CCITT (1977e).

2.4.4. Upgrading Local Loops

The outside loop plant on military bases is comprised mostly

of thousands of pairs of pulp-insulated copper wires bundled

into pressurized lead sheath cable. Replacement of these outside

facilities is an expensive undertaking (CEEIA, 1977) e The

strategy for upgrading and digitizing this local access to

terminals is important in determining overall future costs of the

military networks.

Whenever possible, one can utilize the existing loop plant

rather than replace it. A twisted pair of No. 26 copper wire is

capable of handling unidirectional digital traffic. However, if

45
Multiframe Codeword

S F

151Z117 18 - - - - 31~ / , 22

/ ' /

"

/"

/"

/ /

"

,

F

JZI,I 2 3 4 5 6 78 9 10 II 12 13 14

I " ,

I "-

/

"

"

I "

m

Framing Codeword I 5~1 I 5-17 J Signaling Code

I I 4 Bits /Channe I

I ~: I

I ~!. I

I 5-15 ) 5-31 I

Figure 2-10. Multiplexed PCM framing and signaling format for

CCITT recommended digital carrier.
Table 2-9. Comparisons Between T-l Carrier and

CCITT Recommended Digital Carrier

Comparison Item

T-l Carrier

System

CCITT Recommended

Carrier System

8000/s

8 bit PCM

A-law

8

256

32

16

0.4882 l-lS

3.9056 WS

125 WS

2.048 Mb/s

30

64 kb/s

16th channel

dedicated

2000 b!s per channel

First character

dedicated

8000/s

8 bit PCM

l-l 255 law

8

193

24

12

0.6479 l-lS

5.181 us

125 l-lS

1.544 Mb/s

24

64 kb/s

1 bit borrowed in

6th frame

Signaling rate

Sampling rate

Modulation

Companding

Bits/time slot

Bits/frame

Time slots/frame

Frames/multiframe

Bit duration

Time slot duration

Frame duration

Bit rate

Voice channels/frame

Bit rate/channel

Signaling scheme

1.300 b/s/p~r

channel·

Synchronizing scheme 193rd bit

47
the loading coils are spaced at 1.5 km intervals, and if the

length of the loop exceeds 5 kilometers, then the data rate is

limited to a few hundred bits per second. The data rate can be

increased to 50 kb/s, and possibly more, depending on the

distance, by removing the loading coils. This could permit a

half-duplex digital voice channel to be transmitted over existing

pairs of wires. When the loading coils are replaced with digital

repeaters, it is possible for each wire pair to handle T-carrier

at 1.544 Mb/s and in some cases T-lC carrier at 3.152 Mb/s.

Since many digital systems require full-duplex transmissions, two pairs would normally be required for the T-carrier

type of link.

Another loop plant strategy involves the gradual replacement

of wire pairs with new technology. Suggested replacement schemes

include packet radio, packet satellite, lasers~ fiber optics, and

coaxial cables.-

A recent study by Nesenbergs and Linfield (1976) has

examined the cost~ of coaxial cable local distribution systems

in some detail. Cost estimates were made for a replacement of

the outside plant at Ft. Monmouth and Ft. Huachuca with either

buried oi overhead installations, using either star or loop

topologies. ~he cost was found to vary as a function of system

compon~nts, topology and method of control, as well as the

method used to install the network.

The results for one type of installation, using buried

coaxial cable with distributed control, at these two sites are

shoWh in Table 2-10. Inside plant costs include local switching

centers, concentration hubs, moderns, multiplexers, and the

link, but exclude installation and terminal costs. The outside

plant costs include the coaxial cable for trunks, feeders, drops,

amplifiers, and bridges as well as installation costs.

Table 2-11 compares the estimated total cost for a 6000

terminal, local-access network at Ft. Huachuca, assuming different types of installation (buried vs. overhead), different

48
Table 2-10. Cost Comparisons on a Per Drop Basis

(Buried Network with Distributed Control)

_.-.-.---------I-------...----..------...............-~----.....-.-{

Ft. Huachuca

Projected

Ft. 11orunouth

Projected

No. of Terminals

Area Served

Terminal Density

Bandwidth Required

Bandwidth/Terminal

Plant Cost Estimates

-Outside Plant

-Inside Plant

-Total Cost

Cost Per Drop

6,000

2 9 krn

667/km2

43 MHz

7 kHz

$1,480K

1,934K

$3,414K

5,300

10 km2

2 530/km

45 MHz

8. 5 kFlz

$1,304K

.2"O.33K

$3,337K

10,470

., 2 10 }\:m

, 2

l,047/km

156 Mliz

14.8 kHz

$2,247K

$7,752K

-Ou·tside Plant

-Inside Plant

-Total Cost/Drop

$ 2 4 5 $ 2 4 6 $ 2 1

..L. 5

32 2 3 8 ..,

..J 52 5 ........ -.-.. -'--..0.-'.

$ 5 6 7 $ 62 9 $ 7 4 0

49
Table 2-11. Estimated Cost for 6000 Terminal Local Access

Network at Ft. Huachuca (in 1976 dollars)

Underground Installation Overhead Installation

Star L<?op Star Loop

Centre Distr. Centre Distr. Centre Distr. Centre Distr.

I

Total Cost

($M) I 5.4 4.0 4.4 3.4 4.6 3.2 3.5 2.5

cost/Terminal f

($K) I

I 0.90 0.68 0.73 0.57 0.76 0.52 0.59 0.42

Cost/KBPS/Term. I

(¢ ) I 0.78 0.58 0.63 0.49 0.66 0.45 j 0.50 I 0.36

topologies (star to loop), and different control strategies

(centralized vs. distributed).

The results of the Nesenbergs and Linfield study indicated

that overhead cable installations are less expensive than~ underground cable installations. The star configuration appears more

expensive than the loop configuration and centralized control more

expensive than distributed control.

The cost figures given in Table 2-11 do not include any

terminal costs. Addition of terminal costs is expected to add

another 20% to 25% to the total cost (excluding any large and

expensive computer terminals) .

In general, it has been found that the local access network

cost per terminal drop varies also as a function of terminal

density and channel capacity or bandwidth per terminal. The

general cost relationship as a function of these parameters is

shown by the curves in Figure 2-11. The channel capacity

indicated is the number of Hertz required per terminal and assumes

32 kb/s and 2 b/Hz (i.e., 16 kHz) is required for a single voice

channel. These curves are used in Section 3.2 below to estimate

the replacement costs for local access networks.

50
1400

.,--..

(f)

\-

0 1000

0 Channel

0 Capacity

LO 20 kHz / Terminal l'-

Q)

D.

0

\0- 800 n

\-

<l>

a.

....

(/')

0

u

c ,600 (1)

t

..-

V>

(1)

>

C

1--'

0

..,..-

a. 400

0

U

200

Burled System with

Distributed Control

r - I " I I !

oL ._~_.__1_-:--LLLLL_

10 3

___------..L_'LLW

104

Averoye Terminal Density

in Area Served in Drops/km2

Figure 2-11. Generalized cost curves for local

distribution networks.

51
3. THE MILITARY SWITCHING ENVIRONMENT

Telecommunications networks used by the DoD have special

requirements and must meet specifications which are often more

stringent than those required of commercial networks.

In this section, the present differences are noted so that

the unique switching and signaling aspects of future military

networks can be predicted. This is necessary for understanding

the issues involved in network control. These issues and some

advanced signaling systems which may be used to resolve them

are discussed in Section 5. In this section we describe

pertinent military networks, their distinctive characteristics,

and projections of what future networks may be in the 1985 to

1990 time frame.

3.1. Examples of Military Networks

Military networks can be divided into three major categories,

global-strategic, tactical, and non-tactical. Each of' the three

has unique features to meet different requirements. Examples of

networks used by the military are given below. Unique characteristicsare defined in Section 3.2.

3~1.1. Strategic Networks

The global-strategic Defense Communications System (DCS)

is managed by the Defense Communications Agency (DCA) for the

combined military services, Army, Navy and Air Force. Major

components of the current network, DCS I, include the Automatic

Voice Network (AUTOVON I), the Automatic Digital Network (AUTODIN

I) and the Automatic Secure Voice Network (AUTOSEVOCOM I). These

networks sometimes share common transmission facilities which

include satellite links, high frequency and troposcatter radio

links, line-af-sight microwave links, and terrestrial wire and

coaxial cable links.

In addition, the DCS includes of several special purpose

networks such as the packet-switched computer network, ARPANET,

developed by the Advanced Research Projects Agency (ARPA). The

once experimental ARPANET was transferred to DCA for operational

52
use in 1975. Another DCA responsibility is the European Telephone System (ETS) which provides telephone service for U.~.

Forces in Central Europe. . A two-way data network called SATIN IV will provide secure

record communications between the National C'omrnand Authority

(NCA) and the commander-in-chief of SAC (CINCSAC)~ The dialed·

AUTOVON network is the prime candidate for SATIN IV backbone

because it has a greater survivability than AUTODIN, since the

number of nodes is limited in AUTODIN.

A network for connecting medium and large automatic data

processors is planned for the World-wide Military Command. and

Control System (WW1\1CCS). It is expected that this ~-vWMCCS Intercomputer Network (WIN) will utilize the AUTODIN II network to

connect 35 large- and medium-scale computer systems and remote

terminals at 26 locations around the world (Kuo, 1977).

Some characteristics of current and projected AUTOVON,

AUTODIN and AUTOSEVOCOM systems are given in Table 3-1. This

information was obtained from a number of sources including

Rosner (1973), Ochiogrosso et ale (1977) and Levine (1976).

3.1.2. Tactical

Tactical communications for ?ll military services are being

combined under a joint tactical conununications program called

TRI-TAC. Under TRI-TAC, the Army, Navy and Air Force are

developing a new series of mobile, interoperable communications

equipment for use by the tactical conunanders in the 1980's. Each

military service is assigned responsibility for certain system

components. For example, the Army is responsible for developing

the AN/TTC-39 switch along with other components. The switch

incorporates both circuit- and message-switching capabilities and

is designed to meet not only the tactical requirements but the

switching needs for portions of the Des strategic network.

3.1.3. Non-tactical

Non-tactical corrununica tions networks serve local mil.itary

bases and are the responsibility of each military departm~nt and

the base commander. Facilities provided' include telephone, record

53
Table 3-1. Current and Projected Characteristics

ofDCS Backbone Networks

Network Characteristic

No. of Subscribers

Call attempts/day

Switch type

No. of nodes

CONUS

Overseas

Access areas

Traffic (Erlangs/hr)

Backbone (CONUS)

Node

Computers

Remote Terminal

Messages/day

Switch type

No. of nodes

CONUS

Overseas

Backbone traffic

Average (b/h)

Peak (b/s)

Nodal traffic

Average (b/ s)

Peak (b/s)

Subscribers

Terminal rates

Wideband

Narrowband

Nodes

Traffic

54

Current

(1978)

5.0xl0 5

5 7.5:x10

Circuit

65

16

1,000

2,200

36

250

1,400

3.5xl05

Message

9

8

5 2.5xlO

6 1.OxlO

1,400

50 kb/s

2.4 to 9.6 kb/s

AUTOVON

Unknown

Projected

(1985/1990)

b I.OxIO

Unknown

Hybrid

43

20

1,500

2,500

42

2,500

25,000

7 1.2xlO

Packet

24

7

1.4xl010

1.Cxl08

5 5.0xlO

6 1.25x10

10,000

9.6 kb/s

20

Unknown
and computer data, video imagery such as facsimile and television

(MITRE, 1976). Many of these services are presently provided

with separate networks. Ultimately, a fully integrated multiservice system might be used, but only if found to be cost

effective.

The U.s. Army Communications Command (ACC) is responsible

for the communications on some 400 Army bases throughout the

world. A Base Communications plan (BASCOP) is being developed

which provides the system architecture for future upgrading of

the base plant (see CEEIA, 1977). Networks for integrating

multimode communications and electronic facilities to satisfy the

Army's base-level information processing and transfer needs is

also being evaluated under the Army Base Information Transfer

System (ARBITS) program (see MITRE, 1976).

3.2. Distinctive Characteristics

The special requirements of military networks and certain

characteristics which distinguish them from commercial networks

can have a considerable impact on the network management and

control. These distinctive characteristics are defined and

described in this section. Table 3-2 compares the behavior of

commercial and military networks in terms of pertinent characteristic parameters, e.g., survivability, availability, security,

etc. Major distinctions arise also between strategic and

tactical networks. Non-tactical systems often use commercial

equipment, although the traffic loads may differ.

3.2.1. Survivability

Military networks are subject to unusual hazards like

,jaIT@ing and enemy action which cooonercial networks are not

designed to contend with. The ability of a network to combat

jamming and to survive destructive forces determines survivability.

In the military environment it is essential for the network to

provide communications to critical users at all times. A

responsive control system, capable of real-time fault isolation

and dynamic network reconfiguration and anti-jamming capability,

55
Table 3-2. Comparison of Commercial and f1ili tary Networks

Ul

en

Characteristics

Survivability

Availability

Security and Privacy

Reliability

Dynamics

Adaptability

Compatibility

Special Services

Access Area

Cost

Performance

Commercial Networks

Limited by economics

Priorities usually not assigned

Low level privacy for certain

transactions only

Moderate MTTR

Relatively stable traffic

statistics

Limited spare capacity

High inertia system, slow to

adapt to changing environment

and technology

Interoperability dictated by

economics

Limited to specific users

Depends on loop and switch costs

and terminal density

Networks implemented to produce

revenue

Depends on markets

Military Networks

Extremely high

Redeployment

Adaptive routing

Preemption priorities

Require precedence

Input and output alerts

Low blocking probabilit~

I

Multi-level security

required on all transactions

Low MTTR

Large uncertain traffic

statistics

Spare capacity required

for peaks

High flexibility to suit

military operations

Interfacing required

between tactical, nontactical and strategic

networks

Unique military

Required by many users

Depends on geographical

layout

Network cost to be

minimized

Depends on special

military requirements
enhances survivability. Reconfiguration of the network topology

is accomplished at the switch nodes and is ultimately limited by

the availability of hardware, such as links between nodes. A

critical factor in network design is the network topology and the

control strategy for that topology. A mesh or grid network for

example provides more alternate routes. Its control program must

be more versatile to take advantage of these alternatives.

Distributed control, rather than centralized control, aids

survivability. Thus, star networks and hierarchal structures

with centralized control may not be desirable.

3.2.2. Security and Privacy

Network security protects the addresses and information

being transferred from disclosure either accidentally or intentionally. Privacy refers to the prevention of unauthorized use

of the network, either for sending or receiving information.

Both security and privacy require 1) limiting access to authorized user terminals, 2) providing physical security to network

elements, 3) shielding elements to prevent acoustic, visual or

electromagnetic interception of information, and 4) address and

text encryption for protection during transmission. The encryption process has considerable impact on network control.

Military networks utilize multilevel security. The security

classifications used in AUTODIN I are listed in Table 3-3.

Table 3-3. Security Levels for AUTODIN I

Level Designator

Top Secret T

Secret S

Confidential C

Restricted R

Encrypt for Transmission Only (EFTO) E

Classified (clear transmission) M

Unclassified U

57
The letter designations shown in the table are used in message

headers, they are part of overhead control signals to identify

the security level. When the designator is received at a node it

must be checked against the security classification of the

destination terminal. A secure route must be found to that

terminal. And finally, if the above two conditions are met, the

appropriate action can be taken by the switch.

In some instances the control signals themselves may require

encryption. For example, signaling encryption may be used to

prevent disclosure of traffic statistics, volumes and addresses.

However, when signaling encryption is used, these signals must be

decrypted at each switch to be interpreted. Control signals are

then "clear" internally at a node only and electromagnetic

security measures must be taken. If non-secure signaling is

acceptable, secure information can be circuit switched in

encrypted form. This may not be feasible in a store-and-forward

switch where the control information is added as overhead to each

information block.

3.2.3. Reliability and Availability

Reliability in the context used here is concerned with the

mean time between failure (MTBF) and mean time to repair (MTTR)

of the network elements and the subsystems which comprise these

elements. Thus the structural properties including hardware

and software components of the network determine reliability.

The network management and control system monitors each element

and subsystem. It measures performance degradations and aids in

preventive maintenance. When likely failures are indicated, the

system isolates the fault and takes corrective action in order to

reduce the MTBF.

Availability refers to the probability that a user will have

access to the network. The system must meet the demands of the

user when and if he wants to use it. Users of military networks

often must contend with the limited resources available. When

contention occurs, priorities are established using multiple

precedence levels. In AUTODIN I, for example, the precedence

58
levels established for CONUS are given in Table 3-4. The

handling time or speed of service from receipt at the origination

node until delivery at the destination node is also given in the

table. It is estimated that 50% of the messages handled by

AUTODIN I are processed at the routine level and approximately

1% are processed at the flash level. A 16-level precedence

scheme is planned for AUTODIN II (Shah, 1977).

AUTOVON I employs a flash override precedence level, in

addition to the four levels shown in Table 3-4 for AUTODIN I.

When lines or trunks are busy the AUTOVON circuit switch automatically preempts lower precedence calls in progress.

These precedence procedures add considerable complexity to

the control of military networks and significant overhead must

be added to the control signaling system.

The preemption requirements can have considerable impact on

the signaling strategy employed. In-band signaling systems

currently in use on many telephone networks require the existence

of an idle channel before signaling can even begin. If all

trunks are busy one must be preempted for a higher precedence

call. If the higher precedence call cannot be completed, the

channel is tied up during the attempted access, previous call

disconnect and signaling disconnect times. The preempted caller

may reaccess. With out-of-band signaling on a per-trunk or

common-channel basis, preemptable calls need not be interrupted

until the completion of a precedence call is verified.

Table 3-4. Precedence Levels for AUTODIN I

Level Precedence Designator IIandling Time

I Flash Z 10 minutes

II Immediate 0 30 minutes

III Priority P 3 hours

IV Routine R 6 hours

59
3.2.4. Adaptability

The nodes, and sometimes the links of tactical networks are

often highly transportable and must be able to adapt to a mobile

environment. Switches require rugged construction to withstand

shock, vibration and severe temperature changes. The weight,

size, and number of such switch nodes may be quite limited.

Strategic networks usually are not transportable but they

are subject to changing military or worldwide conditions. These

conditions could necessitate large changes in traffic loads and

geographic coverage.

Non-tactical networks must also adapt to a changing environment. Sudden growth and declines in the base population are

common. Operation and maintenance personnel have a large turnover. Computers, processors and terminals are being moved as

required.

These changing environments affect network control in a

number of ways. The terminal registration or numbering plans

must adapt to contingencies, including the possibility of·

assigning identification numbers independently of geographical

location. The changing traffic loads require network reconfiguration, which in turn requires revisions of routing plans and

route control.

3.2.5. Compatibility

A military network, be it strategic, tactical, or nontactical, usually must interface at one or more points with other

networks of the commercial type. Because the interconnecting

networks are often dissimilar, the network control requires

conversion at the interface point. This conversion process

includes transmission media and mode, code, speed, and format for

not only control signals, but user information as well.

As the digitization and possible integration of military

networks progresses, the signaling compatibility during the

transition period is an important factor to consider. The signaling system interface compatibility is discussed in detail in

Section 3.3.

60
3.2.6. Special Services

There are numerous functions and features available with

the newer switching systems which incorporate stored program

control. Advanced signaling techniques are required to bring

these functions and features to the user. Commerical equipments

have many desirable functions and features available in a variety

of combinations. Specific military needs are not yet well

established, but will undoubtedly evolve into special combinations of services and additional function and feature packages.

This unique aspect will entail modification to equipment hardware and software, and particularly to the network control

system.

Table 3-5 lists some of the more important functions and

features the military user may require. Certain of these functions and features are unique to the military; others are already

available in the commercial market. They are included here

because they either expedite communication services or increase

survivability, reliability, and availability.

In general, special services have an impact on signaling

systems because they increase the number of signal types used

for control. For store-and-forward switches, the signaling

overhead and the switch complexity are both increased to handle

the new functions.

3.2.7. Access Area

For commercial networks, such as the public switched telephone network, the size of a local access area handled by a

central office (i.e.,the exchange area) is limited by the cost

of the loop facilities, especially in sparsely populated areas,

and the cost of the switching facilities in densely populated

areas.

The size of the local access area for a military network may

involve other considerations. The access area is described as a

geographic area comprising one or more post, camp, station or

base entities. The actual area may be determined by factors

other than loop and switching costs. The location of the

61
Table 3-5.

Switching

Functions

Functions and Features Required in

Military Signaling Systems

Service

Features

Network Managemerlt

- traffic measurements

- performance trends

- equipment monitoring

- failure detection

- reconfiguration

Routing Plan

- adaptable

- saturable

Numbering Plan

- flexible

- code restriction

- translation

Internetting

- access limiting

- signaling conversion

Response Phase

- waiting

- transfer

- pickup

- forwarding

- conferencing

- camp-on

Direct Access Phase

- preset dialing

- automatic dialing

- abbreviated dialing

- night anSvl(~r

- pickup

Information Transfer Phase

- security

- privacy

- accuracy

- class of service

Priority Access Phase

- preemption

- precedence

- intercept

62
recognized access nodes is one such factor. Potential service

boundaries constitute another factor. In the CONUS, the local

access network may service an area whos~ diameter ranges from 1

km to possibly 50 km. The entities served within this area would

include all military traffic from Army, Navy and Air Force

installations.

Different military users may have different communication

requirements and therefore different 'signaling requirements.

Thus, the military access adds to the signaling system complexity.

Table 3-6 illustrates, as an example, the number of lines of

different service types as functions of area size. Area size is

presented in terms of total number of lines. The table also

indicates the number of digital trunks required to serve such an

area (Linfield and Nesenbergs, 1978).

3.2.8. Performance

Network users are interested in performance from their

standpoint~ This standpoint is depicted in such perform~nce

parameters as user-tQ-user delays, information throughput

(including overhead), grade of service (blockingprob~bility),

and accuracy (error probability). The proposed Federal Standard

1033 (FTSC, 1977) lists some 26 primary and secondary useroriented performance standarqs. Although these standards are

being developed for digital data communications, many of them are

also applicable to digital voice systems. The performance para~

meters in FS 1033 are defined for three phqses of the telecom~

munications process: the access phase, the information transfer

phase and the disengagement phase. Performance during each phase

is judged in terms of efficiency, accuracy, and reliability.

Numerical values are not specified, only the criteria which

describes the performance during each phase.

The performance of a network is largely determined by the

network control, strategy and signaling system used, particularly

performance. during access and termination phases. Although the

signaling system itself affects neitherinforrnation transfer nor

63
Table' 3-6. Number of Various Type Lines Served in Different Size Areas

.Area Size in Number of Lines Served

Services II 50 100 200 300 600 1,000 2,000 4,000 5,000 110,000

Analog - Phones 44 88 175 262 525 880 1,760 3-,520 4,400 1 8,800

Analog - Slow Data 1 I 2 I 4 I 6 I 12 J 18 I 361 72 I 90 I 180

Digital - Phones II 4

Digital - Slow Datall 1

()) 1Digital - Fast Data

9 J 18 I 271 5-4 I 88 176 352 440 880

11 2 J

:I

6 9 18 36 45 90

11 3 5 10 20 25 50

--------------------~----~----~----~----~----~-----~-------~------~------~-------

Digital - Trunks 15 30 60 90 J 180 300 60011,.20011,500 3,000
control signal transfer, accuracy and reliability are important.

These signaling aspects are discussed in Section 4.

3.2.9. Cost

The DoD views the DCS as a revenue consumer as distinct from

common carrier networks which are revenue producers (Rosner,

1973). Thus, whereas a common carrier desires to sell as much

service as possible to users of the network, the Des is designed

to provide only the service required to meet military needs and

at a minimum cost. The importance of cost cannot be overemphasized. The DeS costs about $500 million per year to operate.

Another $400 million per year i~ invested in new systems. This

$900 million expended per year represents about 22% of the DoD

expenditures for communications, command and control (Babcock,

1977) .

The current DCS contains three differently switched systems,

the circuit-switched AUTOVON network, a message switched AUTODIN

and the packet-switched ARPANET. As new generations of networks

emerge, the direction they take will depend largely on cost.

Digital networks, when fully implemented, are expected greatly to

reduce the operation and maintenance (O&M) costs. However,

during the transitional phase from all-analog to all-digital

elements, the O&M costs may temporarily increase.

As noted previously, the local loop plant costs can dominate

the network cost. Unfortunately, upgrading existing loop

facilities can be an expensive proposition. Using the curves in

Figure 2-11 and the projected total of 1 million telephone instruments and 500,000 terminals, it is possible to estimate the total

cost required to replace the wire pairs in the loop plants with

coaxial cable. An average terminal density of 1000 terminals per

square kilometer has been projected for 'Ft. Monmouth in Table

2-10. Using this value and assuming a 4 kHz bandwidth for analog

phones andl6 kHz bandwidth for digital phones*, the costs per

terminal are $450 and $780, respectively. The total cost of the

*The 16 kHzdigita1 bandwidth assumes a DPCM digitized voice

system operating at 32 kb/s and 2 b/Hz signals.

65
local access network replacement is $225 million for analog

telephones and $390 million for digital telephones. Telephone

terminals amount to 95% of all terminals. The other 5% consists

of data terminals and computers which require additional digital

capacity. These results are summarized in Table 3-7.

Another major cost item for digital switches in a military

environment is the software development costs. These costs have

been growing rapidly, due largely to the specialized labor

required. The unique military features could be expected to

foster this trend unless special steps are taken to standardize

software packages and requirements. The use of higher level

languages for programming switching software could be used to

increase programmer productivity. This, however, adds to the

overhead of assembly-language instructions produced.

3.3. Future Trends

Future changes in military communications networks will

result from innovations in technology. These changes are driven

by the market demand for new services and better performance at

lower cost. Several recent technologies have found applications

in network elements. These include digitization, stored program

control, common channel signaling, traffic integration, and

satellite systems.

It is important to review these innovations, because the

sections that follow are concerned with the network control of

future telecommunication systems. The selection of advanced

signaling systems must be based on this emerging technology,

to insure that these systems best meet the user's needs.

Table 3-8 summarizes the impact new technologies have on

major network elements. One technology that is not listed is

large-scale integrated circuitry (LSI) and very large-scale

integrated circuitry (VLSI). This manufacturing process is, of

course, a major factor in reducing cost, size, weight and power

consumption in all network elements. At the time when these

devices are incorporated into large networks of the future, the

66
Table 3-7. Total Investment Costs to Replace Local Access

for Buried Outside Plant and Distributed Control

Total No. of Terminals

No. of voice terminals

95%

~nalog Voice

500,000

95%

Digital Voice

500,000

No. of voice subscribers

(2 ext per terminal)

/

Bandwidth/terminal

Bit Rate (2 bits/Hz)

Average density

4/terminal drop

Total cost (excluding

terminal costs)

67

1,000,000 1,000,000

4 kHz 16 kHz

N.A. 32 kb/s

1000/km2 1000/km2

$450 $780

$225 million $390 million
Table 3-8. The Impact of New Te·chnologies on tletwork Elements

Technology

Digitization

User Terrninals

Provides easier endto-end security.

Simplifies interface

with transmission

facilities

Concentrators and

Switches

More effectively

managed

Reduced size, weight,

and power consumption

Modularity

Transmission

Facilities

Greater n.oise

immunity

Eliminates marginal

operations

Improves bandwidth

utilization

Eliminates modems

Performs more

functions

0\

co

Switch~ng

with

Stored Program

Control

More features to user lAdds flexibility

Permits cornmon-channellSimplifies hardware

signaling on trunk

side

Permits commonchannel interswitch signaling

.Common Channel

Signaling

Voice/Data

Traffic

Integration

More signals for more

services

Reduces access time

Permits interconnecting broader

community of

terminals

Greater reliability

Reduced cross-switch

delays

Increases traffic

handling capacity

Adds complexity

Adapts to best

traffic handling

procedure

Faster operations

Requires special

link testing

Resistent to

interference

Single facility

handles all

classes of

traffic

Pooling excess capacity increases

efficiency
economies realized could provide order of magnitude reductions in

costs for manufacture, operation, and maintenance.

3.3.1. Digitization

Although it appears that future digital technology wi~l

permit reductions in space, power and eventually the cQst of

telecommunications equipment ,there. are both advantages and

disadvantages to the digitization of future networks. These are

summarized in the following paragraphs for each network element.

As noted previously, the dominant terminals in the network

will probably continue to be the voice terminals; i~e., the t~lephone instruments. Digitizing voice terminals has certain·

advantages, not the least of which is security. A block diagram

of such a terminal is shown in Figure 3-1. Digital encryption

devices generate signals which are less susceptible to deciphering

then encrypted analog signals by unauthorized persons. At the

same time, such devices are becoming less expensive when bas~d on

LSI circuitry. In the future, digital voice terminals may red~ce

the bandwidth required for voice transmission. For example, a

di~itized voice channel using linear predictive coding (LPC)

requires approximately 2. 4 kb/sfort~ransmission. Four such

voice channels can be carried over an analog voice-grade line at

9.6 kb/s which normally handles one full-duplex voice link.

Digital voice terminals may uLtimately be capable, of interfacing

with other dig~tal computers, e.g., a voice-operated computer.

The digital transmission of signals over bot~analog voice

band and data links offers several advantages. Digital signals

are amenable to regeneration and are therefore more resistant to

noise over long distances. The reliability of a digital link is

greater. No modems are required. Accuracy is improved using

simply implemented error control schemes. LSI interfacing equipment is less expensive to build. The nodes, digital switch~.s"

concentrators, and multiplexors are simpler and easier to design.

Mixers, filters, and converters are no longer required. The LSI

microprocessor can be used to control not only the switches,

but to provide intelligence in the terminals as well.

69
MICROPHONE PROCESSOR

... j

o KEYPAD I I l I I

'SIGNAL

SIGNALING

and

CO·NTROL

--.

ENCRYPTION

DECRYPTION ...-1----II

.--

LINE

INTERFACE

(DIGITAL)

TRANSMI:T

RECEIVE

L J---L-----J CONTROL

POWER ... '

EARPHONE I I ~---l PROCESSOR

Figure 3-1. Digital secure voice terminal block diagram.
Digital signaling systems provide larger signaling alphabets

for control of more functions and providing more services to the

user. Robust signaling systems (which are less susceptible to

environmental degradations) use digital techniques for the same

reasons 'given above.

There are also some disadvantages including the cost of

digital telephones and the problems associated with full duplex

operation over 2-wire loops using digital transmission. Software development costs for the switch is another disadvantage.

Some digital transmission is being implemented with the DCS

Digital European Backbone (DEB) in central Europe. This trend

may continue and could encompass the local base networks in the

future according to the Base Communication Plan, BASCOP (CEEIA,

1977) if proven cost effective.

3.3.2. Stored Program Control and Common-Channel Signaling

The use of software, stored in the memory of a processor, is

expedient to control space- and time division switches. The

stored program control (SPC) adds flexibility and operating speed

to the switching process. The practical advantages of centralized SPC -were demonstrated in 1965 with the introduction of the

No.1 ESS by Bell Laboratories. More recently, decentralized

control has emerged. Decentralization uses microprocessors and

LSI technology. Decentralization distributes the processing

functions of network control, but to do so, it places a greater

burden on the signaling systems. More signaling is needed to

allow the network processors to communicate with each other. The

interprocessor communication links may take many forms and these

are described in Section 4.2. One form of particular interest is

common channel signaling. It provides the SPC with new capa~

bilities by virtue of its increased signaling capacity relative

to conventional signaling. The SPC switching, in conjunction

with common-channel signaling, may provide the adaptability

required by military networks in the future. Simple software

additions permit accommodation to new services as needed. However, the cost effectiveness must" still be demonstrated.

71
3.3.3. Integration

As military networks for strategic, tactical and non--

tactical communications evolve to meet the needs of the future,

it is possible that a unified common user network could result

with some sacrifice in performance. The advantages and disadvantages of a single integrated network capable of handling

voice and data have been discussed by many workers including

Rosner (1973), Coviello and Vena (1975), Esterling and Baku

(1975), Occhiogrosso et ale (1977), Gitman etal. (1977), Ross et

ale (1977), and Schutzer and Ricci (1976). The cost effectiveness of such a unified network has been evaluated under the

ARBITS andAFBITS programs (MITRE, 1976) for base communications.

Integrated networks are designed to serve communities of

users with different traffic types by sharing switching equipment

as well as transmission capacity. Figure 3-2 illustrates how

integrated networks could evolve from completely separate voice

and data networks, to networks which share only the transmission

facilities, and finally to a completely integrated network.

Many current networks' share transmission facilities as shown in

Figure 3-2b. Digital data from data terminals are converted to

analog form using modulators and demodulators (modems) for

transmission on analog facilities. In an alternate form, the

analog (voice) information may be digitized using various coding

and decoding (cod~c) schemes for transmission on digital

facilities. In either case the switches at the nodes are

separate circuit and store-and-forward (S&F) type switches.

It is also feasible to combine these nodal functions into a

single entity, so that a completely integrated network is

obtained (see Fig. 3-2c). The principal advantage of this is that

the integrated network can serve a- greater number of terminal

types. The total cost of transmission facilities can be reduced

by optimizing the sharing possibilities. There may also be some

disadvantages. The integrated network may only be economically

achieved by sacrificing performance to certain classes of users.

72
CIRCUIT

TERMINALS CONCENTRATORS TRANSMISSION SWITCHES

ZS

VOICE ZS-------

ZS

DATA

Sand F

(a) Separated Facilities

CIRCUIT

I

I

Sand

®F

(b) Shared Transmission Foci lities

HYBRID

(c) Integrated Facilities

Figure 3-2. Integra~ed network evolution.

73
Increased complexity may increase vulnerability to catastrophic

failures.

An integrated network may take different forms. One form

can use digitized voice terminals and digital transmission

facilities, including digital (or virtual) circuit switches.

However, the advantages of store-and-forward switching are lost

for this network.

Another alternative is to packet switch all of the traffic.

Packet switching of digitized voice signals, however, is still in

the experimental stages. Gold (1977) describes ARPANET experiments conducted with packetized voice. Two problems arise for

packet voice. One is related to overhead and the other to

quality degradation due to variable delays through the network.

A packetized virtual circuit proposed by Forgie and Nemeth (1977)

whereby network links are preassigned to the user during dial-up

could reduce these problems. Packet overhead is reduced by

distributing destination data to the nodes during dial-up and the

packets merely carry an identifier. The reduced overhead

increases efficiency and the fixed routing eliminates the

variable delay.

Circuit and packet switching networks can be combined using

a "hybrid" switch, where the network adapts to the traffic type

by selecting the appropriate switching technique. An example,

described by Coviello and Vena (1975), and GTE (1975) is the

slotted envelope network (SENET). A constant, self-synchronizing

master frame is divided into time slots, which can be dynamically

allocated to different traffic classes, and switched accordingly.

One packet-switched channel is assigned to control virtual

connections for class I traffic. Other classes of traffic carry

control signals along with the messages.

The evolution of a hybrid switch, from an all-analog system

to an all-digital system, is depicted in Figure 3-3. In Figure

3-3a, the control signaling for the switch is furnished on a per

channel basis using an in-band signaling system on the lines and

trunks. In Figure 3-3b, the common-channel signaling is used.

74
---------------------",--_.'_..."._~>"._--

The signaling information for several lines and trunks is

transferred over a separate data link using modems to interface

with the link. A digital time-division switch establishes virtual

connections for both voice and data. Since analog facilities

are used for transmission, the digital conversion occurs in the

interface to the lines and trunks. In Figure 3-3c the processor

and memory are expanded and additional lines and trunks are added,

so that data can be packet switched and voice can be circuit

switched. The control signaling for voice circuits is handled

by one of the packet switched data channels. Figure 3-3d is the

hybrid switch. Digital data and voice are both packet switched

and circuit switched using the slotted envelope transmission

facilities. The control signals are handled as if they were

digital data.

The evolution of military networks from all-analog to

combined analog and digital and ultimately to all-digital configurations is already progressing. Voice and data integration,

on the other hand, is still in exploratory R&D stages, as noted

previously.

3.3.4. Satellites and Packet Radio Systems

It is expected that satellite communications will continue

to playa role in future military network operations. Satellite

intermediate message processors (IMP's) have been developed for

use on the ARPANET. These satellite IMP's use random access

packet broadcast techniques pioneered by the ALOHA system. A

large number of geographically distributed users are connected to

a central computer via satellite radio links (Abramson and Kuo,

1973; Schwartz, 1977). Satellites could, in the future, be

exploited to provide access on the regional and local levels

as well as in backbone structures if costs are more attractive

than terrestrial alternatives.

Packet radio system concepts are also being developed for

use in terrestrial tactical networks and local distribution of

data to offset the high cost of local distributions (Kahn, 1977).

'75
COMMON

(b) Virtual Circuit Switch a Analog Transmission

SIGNALING (

EQUIPMENT "7' "'" ...

C

CENTRAL

PROCESSOR

UNIT

t

STORED ,~

PROGRAM TIME

CONTROL DIVISION H INTERFACE 1 - and MEMORY SWITCH

ANALOG

LINES

and TRUNKS

CENTRAL

PROCESSOR

UNIT

STORED

PROGRAM

CONTROL

and MEMORY

(a) Analog Switching a Transmission

Figure 3-3. Evolution of hybrid switch.

DIGITAL

LINES

nd TRUNKS

{d)lntegrated Switch a Transmission Facilities

PACKET

SWITCH

CENTRAL 4- TIME

PROCESSOR DIVISION ... .....

UNIT MULTIPLEX

t j~

STORED

PROGRAM CIRCUlT CONTROL L...+ SWITCH and MEMORY

PACKET

SWITCHED

LINES

and TRUNKS

MEMORY

SIGNALING

and

DATA Sand F

PROCE·SSOR

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STORED TIME CIRCUIT

PROGRAM DIVISION SWITCHED

CONTROL . LINES

and MEMORY SWITCH and TRUNKS

(c) Digita I Circuit a Packet Switch

Separate Transmission Facilities

......,]

0....,
Control of the packet switched systems, which statistically

share a service entity (e.g., a satellite or terrestrial subsystem), is beyond the scope of this study. However, it is

important not to Qverlook such controls when selecting the

advanced signaling systems for future military communication

networks.

3.4. Network Forecasts

Predicting the future is always risky, particularly in the

telecommunications area which is continually undergoing both

innovative technological and service-requirement changes. Near

term predictions over the next 5 years appear fairly reliable

because changes can be expected to occur slowly due to the large

investments in existing facilities. The network configurations

projected 20 years in the future are, of course, far less reliable.

Some recent forecasts have been made for the strategic DCS

network. See, for example, Krevsky et ale (1972), Rosner (1973)

Paschall, et ale (1976), and Levine (1976). The BASCOP program

includes plans to digitize local facilities (CEEIA, 1977). The

ARBITS program could lead to integrated services in the local

non-tactical environment (MITRE, 1976). Tactical networks are

being developed for use by all military services in the 1980's

under th~ TRI-TAC program (Hoover, 1977).

Us~ng these plans and programs, plus information from other

sources, the general characteristics of current, transitional and

long range military network configurations are estimated in

Table 3-9. These projections indicate only the broadest expected

network characteristics. They are useful in defining network

control and management procedures that are, (a) compatible with

the imminent transitional networks, and (b) adaptable to networks

expected in the more distant future. Several advanced signaling

system alternatives are discussed in Sections 4 and 5. Some

aspects which pertain to the transitional period are covered in

the following subsection.

77
Table 3-9. Military Network Forecasts t-----' _0..._---

Networks

·Strategic

Current

(1970's)

Separated by technology

type and by service

type

Mostly analog, some

digital

Transitional

(1980 to 1990)

Combined technology

types, some integrated

by service

Many digital

Long Range

(1990's)

Combined strategic, tacticaJ

and non-tactical with

integrated services

All digital

I I I I -

·Tactical Mostly analog with

separate subnets

Analog/digital hybrids,

some internetting with

strategic networks

All digital

-J

00

·Non-tactical Separated classes,

mostly analog

Integrated data, Integrated voice, data and

separate voice and video, slow speed imagery, separate

analog/digital hybrids video (TV)

Terminals and Traffic

Voice

Data

Over 95% analog

Some intelligent

90% analog,

10% digital PCM

Mostly intelligent

10% analog,

90% digital, with

lower digitization rates

Added intelligence with

microprocessors

! I I I

Nodes

Switches Mostly analog, step-bystep, crossbar, some

electronic ckt. sw. and

digital message switch

Analog and digital, electronic. ckt. sw. with tim1 Mostly digital, hybrid

division multiplexing, packet and circuit,

separate packet switch time division

Concentrators

Multiplexors

FDM

FDM

FDM and TDM

FDM, Statistical TDM

StatisticalTDM

Mostly STDM
Links

Current

{1970's)

Table 3.9 (cont.)

Transitional

(1980 to 1990)

Long Range

(1990's)

I I I _ -- ......,]

\.0

Long Haul

Short Haul

Local Loops

Network Control and

Management

Signaling System

Lines

Trunks

Mostly analog with

linear repeaters, LOS

microwave, some satellite, coaxial cable,

troposcatter

Analog and T-carrier

Mostly analog, quasianalog, 2- and 4-wire

pairs

Deterministic routing,

manual and semi-automatic control

dc and ac, in-band and

out-band

MFDT addressing, SF

supervision

Analog/digital, linear

and regenerative

repeaters

TI, T2, T4 carrier

Digital and quasianalog, 2- and 4-wire

pairs, Tl carrier and

coaxial cable

Deterministic routing,

semi-automatic control

dc and ac (in-band)

ac (in-band) and

common channel

Digital with regenerative

repeaters, satellite, LOS,

coaxial cables

Satellite, packet radio,

fiber optics, millimeter

waveguides

Quasi-analog and digital

coaxial cab~e, fiber optic

cables, T-carriers

Adaptive routing, automatic

control

Digital, in-slot,

channel

associated

common channel

some non-associated
3.5. Transitional Interfacing

Interfacing new digital networks with the existing analog

networks raises several issues of signaling compatibility.

Several of these issues were considered by Linfield and

Nesenbergs (1978J and will only be summarized here.

The details of future network configurations are unknown.

Therefore, the selection of advanced signaling systems for future

networks cannot be divorced from issues of adaptability. Integration of voice and data on a single network is one such issue.

Should such integration occur, the network controls would have to

differ from those existing now on separated networks.

Adaptability and compatibility durin9 the transition period

and beyond is a key factor in the development of access area

digital switching systems (AADSS). The AADSS program must be

concerned with control signaling between elements of a given

network (intranet control). During the transitional period, the

intranet signaling system must interface with both analog and

digital facilities. The interfaces for intranet and internet

control are described in the following paragraphs.

Intr~netControl Interfacing

An example of the transitional interfacing problems is th~

irttranet control of the telephone network. As the digitization

process progresses from the highest hierarchy backbone switches

to regional, local, and PABX switch levels, and ultimately to the

terminals themselves, more interface gateways arise. Although

their unit costs may be somewhat reduced, the total expense

increases. Overall, the interfacing issues become more and more

important. Figure 3-4 indicates one approach to the problem at

the PABX level (Linfield and Nesenbergs, 1978). Exchangeable

modules are used to facilitate analog-to-digital transition. An

electronic switch with SPC and time division switching is used to

perform the switching functions. The interface modules shown

provide analog to digital conversion for up to four analog lines

or digital multiplexing for digital lines. Digital T-carrier

80
CPU MEMORY MAG TAPE

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SWITCHING BUS

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CONFERENCING

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TONES

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MODULE

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f f f if ••,. f f f 1 TI, 18 TI

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18 11

REMOT~

CON. CHAN.

TERM.

T2 I

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DISTRrSUTION

SWITCHING

MODULE I

nfT. ~i8 •ITn

REMOTE BP t ...-------......-...... COM. CHAN. COM. CHAN.

02/D3, TERM.

IU

Tt 256 kbls -. _. - I

I ~4 I

TRUNK I EXCHANGEABLE t

I MODULE t

~ANAl~ D~I~l~~

00'

Figure 3-4. Exchangeable four-li~e modules to facilitate

analo~-to-digital transition on a PABX.
trunk transmission facilities are assumed, although a combination

of digital and analog trunks could be used.

A major consideration in any terminal loop or line interface

is the BORSCHT. This is an acronym used by the telephone

industry to cover all the functions required in a line interface

circuit. Most of these functions are also required on trunk

side, but for somewhat different purposes. The basic functions

of BORSCHT are

B Battery feed

o Overload protection

R Ringing

S Supervision and signaling

C Codec and clock

H Hybrid (2-wire to 4-wire)

T Test.

A block diagram shown in Figure 3-5 indicates the hardware

relationships between elements of BORSCHT. It assumes digital

switching for both analog lines and trunks, and digital lines and

trunks. Other examples of BORSCHT for the digital line interface

to a telephone are given by Melvin (1978).

It is apparent that nearly all the elements of BORSCHT

appear in the interfaces of Figure 3-5. The battery supplies the

dc current for dialing and for the microphone on the line side.

The battery voltage is typically 48 volts. The dc circuit sees

a low line impedance of about 400 ohms. The ac terminating

impedance matches the telephone cable of either 600 or 900 ohms.

A conventional carbon microphone requires 20 to 80 rnA of line

current.

Overload protection is required on all incoming lines and

trunks. This protects the more sensitive circuitry from damage

due to high voltage transients induced by lightning or power

lines. Typical protection levels are for 1000 volts and 1 ampere

surges lasting about 1 millisecond. Greater protection is

required for sensitive solid state switching devices.

82
en

CD

W

>

Wu

W

0::

a..

en

CD

Ien

z

0::

Ia..

TO OTHER ANALOG

LINES and TRUNKS

-.J

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21- 22

~~

u ~---..

SUPERVISION

OVERLOAD

PROTECTION t--------4 .---~ 0 ---J Ww

I- w

t------""""'"i CD~l.L

OVERLOAD

PROTECT ION t----e-----t----1

OVERLOAD

PROTECTION t------4

4-WIRE

ANALOG

TRUNK

2-WIRE

ANALOG

LINES

and TRUNKS

TRANSMIT

COMMON

UNiT

RECEIVE

COMMON

UNIT

en

CD

w

2

w

u

W

0::

U

a..

en

CD

Ien

2

a::

I-

, ,

TO OTHER DIGITAL

LINES and TRUNKS

STORED

PROGRAM

CONTROL

and MEMORY

OVERLOAD

PROTECTION

OVERLOAD

PROTECTION DIGITAL

LINES

and TRUNKS

Figure 3-5. Analog and digital interfacing function

block diagram.
Ringing power is required on the line side. A conventional

telephone instrument requires a nominal 20 Hz tone at 86 volts

rms to ring the bell.

Supervision and address signaling from a conventional telephone requires 15 rnA for dc hook status control and rotary

dialing of de pulses.

The clocking, codec, and channel filters are required to

match the analog loop to the digital switch. The channel

filter is nominally 200 Hz to 3400 Hz bandpass.

The hybrid converts 2-wire analog line to a 4-wire system.

The hybrid is on the terminal side of the codec since digital

carriers are inherently unidirectional. The line hybrid must be

designed to prevent voice degradation due to coupling between the

transmit and receive paths. Echo cancelling may also be used in

the hybrid. The hybrid is not required on 4-wire lines and

trunks.

The test circuitry provides a means to switch the line to a

test facility. On the trunk side, test signals are injected to

insure proper operation of the trunk. This is particularly

important when common channel signaling is used, since the

. control signals are not conveyed by the trunk. On 4-wire trunks,

a typical 2010 Hz test signal is looped back to the source to

indicate a continuity check. On 2-wire trunks, a 1780 Hz signal

is used in the outbound direction and 2010 Hz signal is returned.

During the transition from an analog voice network to a

digital voice "network, conventional telephones may be attached to

some form of voice digitization processor (VDP). If a secure

link is required, the processor may be followed by a communications encryption device (COMSEC unit) as shown in Figure 3-6a.

All the elements of BORSCHT must get by the VDP and the COMSEC

units, if a conventional telephone instrument is used.

Ultimately, a low-cost digital voice terminal may be

developed using LSI circuitry. A simplified block diagram of

such a unit is indicated in Figure 3-6b. Certain time slots are

allocated for inbound and outbound digital control signals. No

84
, ~I: DIG~?ZI.~fION ., I: .COMSEC F==8 TR.ANSMIT . PROCESSOR'r--------..;;...--~·, t====8 RECEIVE

4-WIRE

LOOP

(a) Secure Voice Terminal (Transitional)

OVERLOAD

PROTECTION L---~ TRANSMIT

BIPOLAR

CONVERTER

FILTER

and TEST MICROPHONE

00

U1

KEYPAD

LOUDSPEAKER TONE

GENERATOR

LOGIC

CIRCUITS

POWER

UNIT

RECEIVE OVERLOAD

PROTECTION

CLOCK c

UNIPOLAR

CONVERTER DECODER

SIGNAL

GENERATOR

and DETECTOR

(b) Digifized Voice T-erminal (wifhLSICircuifry)

F!LTER and

A;MPLIFJER

Figure 3-6~ Digitized voice terminals~

EARPHONE

VISUAL DISPLAY
hybrid is required. The control signals operate a signal

generator ~nd speaker for both alerting and call progress tone

generation. P6wer is furnished from the switching node" via a

phantom circuit on the 4-wire line. Clocking is derived from the

incoming line. All incoming and outgoing control operations are

performed by the logic circuitry. Except for the power, the

remalnlng elements of BORSCHT are passed digitally.

Irtternet ~ontrol Interfacing

The future AADSSmay be required to interface with and to

provide internet controls to strategic, tactical ~nd non-tactical

networks, as well as to commercial networks. During the transitional phase of digitizing and integrating, the required control

interfaces may be ektremely complex. Figure 3-7 indicates most

'of the potential interfaces which must be required in AADSS

for switching voice circuits, whenever both analog and digital

transmission facilities are interconnected.

In addition, data circuits may have to be passed through

the same switching node. Existing data networks use a variety of

signaling schemes. The AADSS must act as a gateway between these

networks by performing mode, code, speed, and format conversions

on the control signals. Possible gateway concepts are described

in Section 6.

Figure 3~7 does not imply that every AADSS should be

implemented with every known interface capability. Many of the

interfaces indicated are used selectively by the commercial

telephone networks in their end offices around the country. The

AADSS for a given r~gion needs only to incorporate the interfaces

required to match the nearest end office.

Th~ following sections 4 and 5 specify those advanced

signaling systems which have potential network control applications at the AADSS. The emphasis is on digital networks which

may be either integrated or not. Digital signaling techniques

for both the line side and trunk side are considered. The

signaling systems used for circuit-switched voice differ from

86
NOTES:

SPC - STORED PROGRAM CONTROL

PCM - PULSE CODE MODULATION

MUX - MULTIPLEX

MOD-MODEM

87

IT' . Analog Transmission --.,.-- Digital ransmlSSlon

Facilities I Facilities

Potential signaling interfaces to access area

digital switch for transitional network environnent.

Figure 3-7.
those used for packet-switched data, but the technique can be

combined for controlling hybrid (circuit and packet) switching.

4. NETWORK CONTROL

In Section 2 the basic elements of a telecommunications

network were described. In Section ~, the unique characteristics

of the military switching environment were presented and future

forecasts were made for the military network. In this section

the network control and management aspects are considered.

Two types of signals flow on a network: user information

and control information. The user information or traffic

(e.g., voice, data, video), which flows between terminals,

represents a useful service to the customers. It justifies the

existence of the network. Control information is necessary to

maintain this flow.

Control information can be classified into three basic

signal categories.

1. Signals that operate switches and terminals to

establish, maintain, and terminate the transfer of

user information. These control operations are

usually on a real-time or short-time basis.

2. Signals that control the links to ensure efficient,

accurate and reliable network transmissions.

3. Signals that manage the overall network, its elements,

and its operations on a longer term basis.

Table 4-1 lists the major network elements and gives

examples of their basic functions that require the transfer

of control information. This transfer or exchange of information

for network control requires a special signaling system. Since

all network elements are involved, the signaling system permeates

the entire network. Communication processors at the nodes must

exchange information with the terminals and with each other.

This exchange may be accomplished over the same links used

for transferring user information or over separate links. The

88
Table 4-1. Functions Performed by Various Types

of Control Signals

Terminal Control Signals

-Attending

-Addressing

-Alerting

-Supervision

Node Control Signals

-Registration

-Translation

-Path Searching and Selecting

-Routing

Link Control Signals

-Synchronizing

-Error Protection

-Testing

Network Management Signals

-Flow Control

-Status Monitoring

-Performance Assessment

-Reconfiguring

-Recording

89
control facilities which perform all these control functions are

therefore equivalent to the nerve system of the network. They

sense user needs and act accordingly. This nerve system which

remotely controls the terminals, the nodes, the links, and the

entire network is the subject of this section. The field of

network control is called "signaling", or more appropriately

"control signaling."

The Consultative Committee of International Telegraphy and

Telephone defines signaling as (CCITT, 1973a):

"Signaling: The exchange of electrical information (other than speech) specifically concerned with

the establishment and control of connections, and

management in a cornmunicationsnetwork."

This definition applies primarily to circuit switched telephone

circuits. However, if the parenthetical expression were

amplified to be "other than user information, e.g., speech, data,

and video" the definition would also apply to all signaling

networks. Data, of course, can be sent over voice networks using

unmodified telephony signaling to establish, maintain and

terminate the connection. However, the transfer of data may be

expedited by adding special control signals. Error control

signals are a good example. The parity check, repeat request,

as well as other additional signals are called "overhead" since

no user information is involved. Overhead signals are required

to provide the orderly exchange of user information over the

network. The procedures or arrangements established by overhead

are called protocols. Certain standard protocols are commonly

used on voice circuits - e.g. saying "hello" to start a conversation and goodbye to terminate one. On data links various levels

of protocols are used to exchange user data and to ensure the

proper flow of information over the network. These protocol

levels are described in a subsequent section.

Figure 4-1 shows separate network control procedures for

voice and data networks. Examples of specific signaling systems

used under each breakdown are indicated on this figure.

90
NETWORK CONTROL PROCEDURES

DATA NETWORK PROTOCOLS VOICE NETWORK PROTOCOLS

r0 0 0 0

0 0 0 0

0 0 0 0

RI R2 cellT NO.6 CCIS

CCITT NO.3 CCITT NO.7 (USUALLY)

CCITT NO.4

CCITT NO.5

CHARACTER BIT

ORIENTED ORIENTED

0 0

\,,0 0 0

0 0

DLC ADCCP

ANSI HDLC

BSC (IBM) SOLS

DDCMP

PER

CHANNEL

I

I I

IN-BAND OU1--OF-BAND

COMMON

CHANNEL

I

I I

ASSOCIATED NON-ASSOCIATED

Figure 4-1. Network control procedures for voice and data networks.
Traditional signaling systems incorporating in-band and

out-of-band signaling techniques were reviewed by Linfield

and Nesenbergs (1978) for telephone networks. In this section,

11 advanced signaling" techniques are considered for both circuit

switched networks and stare-and-forward switched networks.

Since most network projections indicate a trend toward alldigital networks, digital control signaling systems are emphasized.

Both terminals and switches are involved with signaling.

Signaling systems are considered separately for the line side and

the trunk side of the switch. The emphasis here is on out-ofband digital signaling for digital terminals and common channel

interswitch signaling for the trunks. These two signaling techniques are essential for accommodating both voice and data on an

integrated network.

In the sections which follow it is assumed that the nodes of

the future military telecommunication networks will be automatically run by processors with stored program control. The

interprocessor control information will be transferred digitally

using some form of signaling system as described in Section 4.2.

Specific systems are described in Section Sfor voice and data

networks.

4.1. Requirements and Issues to be Resolved

Initially, the major requirements of the signaling system

used to control a military telecommunications network can be

listed in a qualitative way. This is done in Table 4-2 where the

unique characteristics of the military environment given in

Section 3.2 are listed and the corresponding signaling system

requirements are summarized. The justification for these requirements can be found in Section 3.2. In practice, such a list of

requirements is seldom complete. Special, tailor-made requirements do arise often. Nevertheless, the list indicates the kinds

of factors imposed on a network by the signaling system that must

be considered.

92
Table 4-2.

Characteristic

Survivability

Security/Privacy

Availability

Adaptability

compatibility

Special Services

Accuracy

Efficiency

Reliability

Performance

Signaling System Requirements for

Military Networks

Signaling Requirement

Furnish redressing functions (e.g., adaptive routing, network reconfigurations) to

restore critical links during catastrophic

failures.

Control encryption/decryption devices and

identify source, destination and routing.

Administer priority controls and reroute/

accept overflows.

Handle wide range of data rates and

traffic classes with signaling blocks

independent of message lengths. System

should be open-ended to add network

elements and have flexibility to introduce

new functions.

Applicable to half- and full-duplex operations with mode/code/format and speed

conversion.

Spare signaling alphabet or extendable

formats and spare link capacity to provide

new fea tures.

Variable redundancy for error detection

with automatic correction (see Sec. 4.2).

Monitor and vary traffic flow for optimum

throughput, locally or overall.

Able to reconfigure network elements to

avoid faults and minimize downtime.

Signaling must be error free and transparent. User information and control

information must be readily distinguishable (also see Sec. 4.3).

Cost Simplicity for numerous terminals.

Complexity permitted for more

intelligent terminals. Cornmon discipline

for integrated networks. Accounting of

services supplied. I-------------=--------........::::...=-----------------------J

93
In order to meet these requirements, there are a number of

signaling issues and tradeoffs that must be resolved. These

issues are summarized and listed in Table 4-3. Two categories of

issues, technical and operational, are shown in the table.

4.2. Advanced Signaling Techniques

Past and present day signaling systems use direct current

(dc), multifrequency (MF) or single frequency (SF) techniques.

They are known to suffer from slow speed limitations, insufficient

number of unique signals, false operations, fraudulent manipulations, and mass trunk seizures (Dahlbom, 1977).

All of these limitations can be eliminated with advanced

signaling techniques. These are possible for modern electronic

switches that are controlled by processors and stored programs.

The related common channel sigonaling systems have been introduced

and are becoming more widespread. Common channel signaling is

defined by the CCITT as follows (CCITT, 1977d):

itA signaling method using a signaling link cornman to

a number of channels for the transmission of all signals

necessary for traffic via these channels."

Since future military networks are expected to evolve into

configurations that employ digital terminals, digital switches

and digital transmission facilities, network control must also

undergo a transition into the digital world. This means that the

processor-controlled switches must communicate with each other

using digital signaling systems. For economic reasons, several

information channels can share a common signaling channel. When

the voice digitization process extends to the subscribers instrument, digital signaling on a per-terminal basis must realistically

follow.

The advanced signaling systems considered here are digital

systems using cornman channel signaling between circuit switches

(on the trunk side) and some form of digital signaling to and

from the terminal (on the line side), on a per-terminal basis.

94
Table 4-3. Signaling Issues to be Resolved

Operational Issues

·clocked vs. non-clocked links

·circuit switching vs. store-and-forward switching vs.

hybrid switching

·cornmon channel vs. per channel signaling for circuit

switching

·end-to-end vs. link-by-link signaling plan

·compelled vs. non-compelled signal sequences

·one-way vs. two-way trunks

·centralized vs. decentralized control strategy

·location dependent vs. independent numbering plan

Technical Issues

·parallel vs. serial transmission

·synchronous vs. non-synchronous formats

-bit-oriented vs. character-oriented codes

·forward error correction vs. correction by retransmission

95
-------------------------

Digital data transfers via store-and-forward type (e.g., packet)

switches will undoubtedly continue to use overhead control

information added to blocks of data. Both cases require interprocessor communications on the trunk side of the switch.

Internally, computer processing systems are logically

organized for parallel interaction. The internal codes are

usually some form of binary coded decimal, where four-bit

characters are sufficient to represent one decimal digit. Two 4-

bit Characters may form a one 8-bit byte which is the typical

internal code, although l6-bit, 26-bit and 32-bit bytes are also

used. Each 8-bit byte represents one of 256 possible commands,

instructions, symbols or numbers needed within a computer. One

such set of 256 codes is the extended binary-coded-decimal

inforrnatioh code (EBCDIC) used in IBM System 360 computers.

Although the internal transfer of information within a

computer is often accomplished in parallel with the above 8-bit

bytes, the external transfers either to other computers or to

external peripheral devices are usually accomplished serially.

The requirement for parallel to serial conversion depends on the

separation distance of the communicating entities. Serial transmission becomes cost effective at larger separations. When

processors are collocated, parallel transfer may be used to

increase the speed and to avoid the need for parallel to serial

converters.

Figure 4-2 depicts three methods for interprocessor

communications. Since the internal codes are in parallel, a

parallel bus can be used when processors are collocated or are a

short distance apart. When they are separated as in Figure 4-2b,

the interconnection over serial transmission facilities requires

parallel-to-serial conversion at the transmit end and serial-toparallel conversion at the receive end. These parallel-to-serial

and serial-to-parallel conversions are accomplished with a

communication interface. The control functions to be converted

may be an integral part of the processor and use a dedicated data

link for this purpose. Often a separate front-end processor

96
\.0

-...j

PARALLEL. _ BUS

PROCESSOR PIS ) ( SIP PROCESSOR

A CONVERTER CONVERTER B

""

(b) Colocated Processors with Serio I Bus

PROCESSOR FRONT-END ( FRONT-END PROCESSOR

A PROCESSOR PROCESSOR B

(c) Separated Processors with Separate Front End Communication Processor

Figure 4-2. Interprocessor communications techniques.
is used as shown in Figure 4-2c. Other functibns required for

serial transmission are added in the front end, such as synchronization characters or bits, and error control information.

Both processors exchange information serially over the link

whenever serial is more efficient and less costly. The serial

link, however, can be viewed as a means for transferring parallel

data but usually at a reduced speed.

Serial transmissions may be synchronous or non-synchronous.

With non-synchronous systems, each code word, or byte, or

character may occur independently of the others. Start and stop

bits are added to distinguish them. This addeo overhead reduces

the link efficiency. Synchronous code formats utilize the line

more efficiently but require bit, word, and frame clocking between

the communication interfaces.

In the following subsections it is assumed that the

signaling communications between switch processors is basically

the same as between any separated general purpose processors.

The signaling system includes a front-end processor for the

communications interface and the control information is transferred over some channel using serial transmission of synchronous

data. Digital signals convey the control information. The

transmission facilities operate digitally at baseband or in a

quasi-analog form using modulation and demodulation (modem)

techniques.

4 . 2 . 1. Modes of Opera ti'on

User information and control information proceed through

the nodes of a network as electrical signals. It is necessary

to distinguish between these two types of electrical signals.

When all the control information is transferred on a separate

channel dedicated for this purpose, the signaling is known as

common channel signaling. When the control information is

transferred on the sante channel as the user information, the

separation process becomes more difficult. The three most common

current methods for separating the two types of information are:

98
a) To use a unique sequence of bits (characters) which are

recognizable as control signals.

b) To allocate certain time slots in a time division

multiplexed channel for control characters only.

c) To specify fixed positions in a sequence of bits

strictly for control information, perhaps of

distinguishable length.

These three methods and the separate channel method are illustrated in Figure 4-3. Each method is described in more detail in

the following paragraphs.

Unique Code Words

Word or byte discrimination can be achieved by adding a

qualifier bit to each word or byte. The qualifier bit could

always be a I for user information and a 0 for control information, for example.

Alternative forms are the EBCDIC 8-bit code and the American

Standard Code for Information Exchange (ASCII). Both of these

codes allocate certain sequences of bits for control characters

and other sequences for alpha-numeric characters. The ASCII

code, for example, is a 7~bit code with a possibility of 128

distinguishable characters. Wherever bits 6 and 7 are both zero,

the remaining five bits indicate one of 32 possible control

characters.

One or more redundant parity bits are often added to a

character-oriented code for error control purposes. The American National Standard Institute's data link control (DLC) code

is defined as a 7-bit per character code with an odd parity bit

added for synchronous transmission and an even parity bit added

for non-synchronous operation (Gray, 1972). When these control

characters are transmitted, the corruption of even one bit upon

reception can convert control information to user information

and vice versa. These false indications cannot be entirely

avoided, when all possible code combinations are available to some

users on the same link, or when transmission errors occur. A

99
• Control Information o User Informa tion

COMMUNICATION __ COMMUNICATI.ON lNTER~CE ~-~~~~~~~~~~-~~ INTER~CE

(0) Unique Code Words

'-----.II .....1 _~I .....1_--,I •

COMMUNICATION .... COMMUNICATION

INTERFACE ""'" INTERFACE

(b) Fixed Position in Frome

I I I I I I

COMMUNICATION COMMUNICATION

INTERFACE INTERFACE

(c) Allocated Time Slots

......

- COMMUNICATION COMMUNICATION

INTERFACE INTERFACE .... .....

(d) Separate Lines

Figure 4-3. Methods for distinguishing user and control

information.

100
similar character confusion problem occurs for traditional inband signaling on a telephone link.

Fixed Position in a Frame

The AUTODIN I message switch assigns control characters to

fixed position format within frames. Idle line characters are

transmitted continuously to establish and to maintain bit

synchronization and character framing between the terminals and

the switch. Whenever a message is to be sent, the idle characters

are interrupted and a header is transmitted to indicate the start

of a block. The switch recognizes this start character, as well

as subsequent control characters by their prescribed order in the

block. The control character codes are the same as other message

characters. Because of their unique positions in the block the

two character types can only be confused when timing err~rs

occur. Note that a 7-bit character code permits 128 control

characters in this format.

This technique has several disadvantages. Due to buffer

storage a delay is required to detect control characters and to

correct errors. Throughput is reduced by overhead and coordination procedures, and there is considerable terminal complexity.

(Hamsher, 1967).

Allocated Time Slots

At least three variations of digital signaling systems are

used to allocate time slots in the transmission channel. Time

division multiplexing interleaves both the user and control

information on the same channel, and generates a continuous bit

stream. The two types of information are distinguishable by

their time occurrence and synchronization is essential.

The slot allocation method defines the signaling technique.

Three such methods are defined by the CCITT (1973a):

a) Speech digit signaling - Certain digit time slots of

digitized speech are periodically used for signaling.

b) In-slot signaling - Signaling associated with a channel

and transmitted in a digit time slot permanently (or

periodically) allocated in the channel time slot.

101
c) Out-slot signaling Signaling associated with a channel

but transmitted in one or more separate digit time

slots not within the channel time slot.

Speech digit signaling is similar to traditional signaling

systems, except that digital transmission facilities are used.

For example, a typical telephone terminal currently uses dc

signaling on the subscriber loop for line status control, and

either dc pulses generated by a rotary dial or dual frequency

tones generated by push buttons for addressing. Traditionally,

the interswitch signaling over analog facilities uses an in-band

frequency tone (2600 Hz) for trunk status information and multifrequency signaling for addressing. The status tone is applied

during idle conditions and removed during a busy condition.

Associated with a switch there may be a common pool of senders.

The function of a sender is to pulse 2 out of 5 tones for each

address digit over a trunk. A detector~requestor combination

(also from a common pool) at the receive end detects these tones

and makes the appropriate next circuit connection. Over digital

facilities (allocation method (a) above), the idle and busy

conditions are distinguished by transmitting a 0 or 1 state

during a designated time slot in the digitized speech channel.

The in-band, multifrequency address tones are coded the same way

as voice signals.

In-slot signaling (method (b) above) is used with T-carrier

systems as described in Section 2.4.

Out-slot signaling is used with the international digital

carrier systems recommended by CCITT. It was also described in

Section 2.4. Other examples of in~slot and out-slot signaling

systems are discussed in Section 5.

4.2.2. Signaling on Separate Links

When a separate channel is used to transfer the signaling

information for several user channels, the technique is known

as common channel signaling or common channel interswitch

(interoffice) signaling (CClS).

102
Cornman channel signaling systems eliminate most of the

limitations of speech digit and in-slot signaling systems. The

control information is transmitted in digital form using baseband

transmission facilities or quasi-analog facilities. A high-speed

bidirectional link is used to transfer many forward and backward

signals between processors. In a network with many nodes, the

signals may be transferred on a link-by-link basis and processed

at each node. This mode of operation is called the associated

mode. The associated mode is cost effective when there are a

large number of trunks between nodes which also terminate the

common channel signaling system. When the number of trunks is

reduced in parts of the network, the non-associated mode may be

used.

There are two forms of non-associated signaling, fully

disassociated and quasi-associated. The disassociated mode

employs a completely separate network for signaling purposes.

The nodes of this network, called signaling transfer points

(STP), relay data from one signaling link to another. The quasiassociated mode combines the associated and disassociated modes.

It is used in locations where direct trunk groups between

switches are not large enough to support economically an

associated extra CCIS link.

The three modes of operation: associated, disassociated,

and quasi-associated, are illustrated in Figure 4-4. In the

associated mode of Figure 4-4a the common channel signaling link

along its entire length, including end points; closely tracks the

interswitch trunk groups served. In the quasi-associated mode

the cornmon channel signals are not necessa.rily conterminal, with

all trunk groups served as shown in Figure 4=4b. In the disassociated mode there is no close or simple association between

the cornmon channel signaling links and the trunk groups served.

The disassociated mode permits nodes to communicate via signaling

links even though there are no functioning connecting trunks.

This capability is useful for performing network management

103
TRUNK GROUP

SWITCH

'---------'

COMMON SIGNALING CHANNEL

(a) Associated Signaling

/

/

./

/

/

"

"

"

"

"

(b) Quasi-associated Signaling

(c) Disassociated Signaling

Figure 4-4. Modes of operation for common channel

signaling systems.

104
functions as well as control functions. Examples of eelS systems

are described in Section 5.1.

4.2.3. Control Procedures

The procedures for controlling the flow of data between two

terminals, as well as between two switching nodes, can be divided

into discernible groupings or control levels called protocols.

Five levels have been distinguished as follows (see Folts (1977)

and des Jardines and Brosi (1977)):

Levell. The physical level which provides mechanical and

electrical specifications for communicating across a

physical medium.

Level 2. The link logical control level which enables

message sequences to be transferred across the physical link.

Level 3. The cOTI@unication logical control level for

establishing end-to-end connections through the network.

Level 4. The system logic control level to manipulate

terminal resources.

LevelS. The user level for exchanging user resources and

networkwide resources.

It is possible to define interface protocols for all five

levels at a single point in the network. The interface point is

traditionally selected between approximately defined data

terminal equipment (DTE) and the data circuit terminating equipment (DeE). Since each level depends only on lower levels and is

independent of the higher levels, it is possible to standardize

control procedures on a level-by-Ievel basis.

The interface structure and protocol levels are indicated in

Figure 4-5a. Messages containing user information or control

information may be transferred across the interface layers by

segmenting the message as shown in Figure 4~5b. The control

overhead required for each protocol level is added starting at

the highest level and then progressing outward to the lowest

level. This message structure maintains the transparency required

by the layered control functions.

105
USER PROTOCOLS

SYSTEM PROTOCOLS

NETWORK PROTOCOLS

1-- '------------' '--------'

p~~t~'8&cs

(0) Interface Structure and Levels of Protocol

USER INFORMATION I~_~

I

SYSTEM INFORMATION p __=q

I~-N-E--L.~-W-O-R-K-IN-F-O-R-M-AT--,-II-ON--I

I I

P q '

LINK INFORMATION

INTERCHANGE CIRCUIT

(b) Segmented Message Structure for

Transfering Information Between Leve Is

Figure 4-5. In~er£acing protocol structures.

106
A number of standard protocols have been defined by national

and international standards organizations for the lower levels.

These are discussed in Section 5.2.

The protocols may be implemented in hardware or software, or

both. The protocols transfer user information and control

information between node processors and between host computers,

as well as among various terminals.

4.2.4. Synchronization

Synchronous transmission is usually employed in signaling

systems because it permits more speed and reliability in control

signal transfers. No transmission time is wasted, for instance,

on the extra start and stop bits required in nonsynchronous

systems. Various levels of synchronization must be recognized.

They include bit, byte, word, character, block, entire message,

or frame and multiframe synchronization.

Bit synchronization is required to aid the detection process,

character sync to define the code, and block or frame sync to

separate multiplexed codewords.

Various schemes are employed to maintain bit integrity over

a network. The schemes fall into two categories: clocked and

nonclocked. The clocked approach provides synchronization

throughout the network by controlling the timing at each node

from the same master clock. In a unclocked network the timing at

certain nodes may be independent of other nodes. Buffers and

bit stuffers are used to retime incoming bits to match a local

clock.

The advantages and disadvantages of various network synchronization schemes are not considered here. Some synchronization techniques used in control signaling systems are included in

specific system descriptions. A recent paper by Harrington

(1978) examines the network time and synchronization requirements

for various network topologies. The emphasis of Harrington's

review is on circuit switched networks, however, he does consider

message, packet and integrated voice/data networks synchronization.

107
Network timing can be derived by either synchronous or

asynchronous methods. The synchronous approaches employ clocks

slaved to a common master clock, with frequency averaging and

external time references. Asynchronous approaches use atomic

clocks with buffer compensation or less precise crystal controlled

clocks and bit stuffing.

The synchronization technique to be employed on a given

network depends not only on the~topology, but on other factors

such as reliability, survivability, and maintainability. The

complexity of the synchronization technique can also have a

considerable impact on cost (GTE, 1975).

4.2.5. Error Control

Transmission errors are known to occur due to noise and

signal distortion on the link. Noise results from natural causes

(e.g., lightning) and man-made causes (e.g., switching noise,

interference from crosstalk and jamming). Signal amplitude and

delay distortions are caused by link imperfections. Binary

errors occur when noise distortion causes the information

receiver to interpret a 1 as a 0 or vice versa. The resulting

inaccuracies impact the network and its use in different ways.

The accuracy required by a user depends on the type of traffic

generated and its subsequent use. For example, accuracy is

extremely important when transferring software programs, critical

figures, such as targeting information and coordinates, and for

signaling information such as destination addresses and priority

information. Accuracy is less important with other types of

information such as narrative, record and sensor data. When the

latter information is transmitted in the clear there is typically

sufficient inherent redundancy for a user to interpret the

message correctly.

Digital voice transmissions pose different accuracy requirements. Acceptable error rates depend on the voice digitizatiion

process used. For instance, a typical pulse code modulation

108
(PCM) system operating at 64 kb/s is at the intelligible threshold

with end-to-end error rates of one in 100 bits. The same

intelligibility can be achieved with 1 in 10 4 bit error rates

using an LPC system operating at 2.4 kb/s.

The error rate on data links and signaling links can be

reduced in a number of ways. Transmission error rates of 1 in

10 6 bits can be reduced to 1 in 10 9 bits with fairly simple

error detecting and correcting schemes. More complicated error

control schemes permit almost any accuracy level. All of these

schemes involve redundant coding which increases overhead

and reduces throughput.

Figure 4-6 shows a simplified breakdown of error control

schemes. There are two basic schemes which have been used

separately or together. Error correcting codes may be used in

forward error correcting schemes (FEC) to recognize and to

correct individual bit errors. Either block or convolutional

fEe codes are applicable to blocks of data. Error detecting

codes only indicate the existence of some unknown number of

errors in a given block. Corrections can then be made by

requesting a repeat (ARQ) transmission of that block. There is

a small probability that some errors may pass undetected.

The simplest error detection scheme is a duplicate copy.

A single parity check bit on a group of bits only provides

protection against a single, or any odd number of bit errors in

that group. More parity bits, as in cyclic redundancy checking,

can protect against more extensive bursts of errors.

The FEC schemes require no reverse channel but are more

complex to implement, especially at the decoder. The ARQ schemes

require either a half-duplex or full-duplex channel.

In detection and retransmission schemes, the network retransmission procedures must be coordinated for all communicating

stations. Once errors are detected in a block, the entire block

is retransmitted until the error detecting code indicates no

errors. Blocks must be numbered so that the transmitter and the

receiver can tell which block is being acknowledged (ACK) via the

109
ERROR CONTROL

SCHEMES

ERROR CORRECTION CODES --l ERROR DETECTION CODES

I HYBRID (FEe 8 ARQ)

FORWARD ERROR CORRECTION (FEC) DETECTION 8 RETRANSMISSION (ARQ)

I J o DUPLICATE COpy PARITY CHECKS

VRC

LRC

CRC

Figure 4-6. Error control schemes used on data links.
reverse channel. A negative acknowledgement (NAK) is returned

for blocks received in error. When several blocks are outstanding on different links of the network the ACKs and NAKs are

numbered corresponding to the appropriate block and pair of

nodes.

Acknowledgement blocks may be tagged with the transmission

block numbers. The ACKs and NAKs are returned using 9 or 1 bits

in the appropriate position. The relatively short acknbwledgement block may be part of a longer reverse information block. It

may contain a joint or separate error check field.

The transmitting station must store outstanding blocks until

a positive ACK is received.

Currently used control schemes are described in the following paragraphs.

Forward Error Correction

When a reverse channel is not available or its cost is

impractical, forward er~or correction can be used to reduce the

error rate over the channel. Even with the sophisticated codes

required for FEC, however, the protection against errors is not

great, especially during extended periods of channel disturbances.

Thus, FEC has not found significant application for telephone

network signaling systems over the terrestrial data networks

(Burton and Sullivan, 1972).

with the FEC scheme, error presence is detected and the

likely bits are corrected at the receive terminal or node.

There is no need for retransmission. Several FEC codes have been

developed to correct single and multiple errors in a block or in

a convolutional stream.

One of the simplest FEC block codes is the Hamming code

(Hamming, 1950). It can detect all double errors, or it can

correct any single bit error. This code requires m check bits

for every 2m-m-l data bits. Typical values for m are 3,4,5,.~ ..

Parity Checks

Processors often use internal parity checks. This checking

can extend to the links as well. A single parity check on a

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112
/

~R.pos.

BIT POS:------

2

3

4

5

·6

7

8

N

,

LRC

PARITY

P4

R5

R6

P7

p

o

(a) Vertical and Longitudinal Parity Check Scheme

ONE BLOCK .'--~ __- ...JA'--,__- _

\

b2 b3 C1 C2 C3

II

II

em

\ I \ --JI ------~V---~ ---..I "-------V

DATA FIELD CHE.CK F1ELD

(b) CycHc Redundancy Check Scheme

Figure 4-7. ARC, VRC· a.nd. CRe error detJ3cting sche!nes.

113
where b. represents binary numbers and n is the total number of

1

data bits in the block. The length of the check field c. is m.

1

Encoding is accomplished by adding to P(x) a remainder R(x)

such that the combination P(x) and R(x) is divisible by a selected

generating polynomial G(x).

At the receiver, the entire received block is tested whether

it is or is not a multiple of the generating polynomial G(x).

If the remainder is 0, the received polynomial is divisible by

G(x), and the message is assumed correct. If the remainder is

not 0, some kind of error has occurred (Peterson and Brown, 1961).

Although the polynomial coding circuitry is considered to be

more complex then the VRC or HRC scheme, the transmission

efficiency can be higher. For a 3% redundancy (24 bit CRC

remainder added to 800 bit block) the undetected error probability is 'always below 10- 8 For a 10% redundancy, on the same

800 bit block, the probability of undetected errors is at least

10- 27 (Martin, 1973).

The length of the check field can vary with the number of

data bits in a block. Common channel signaling systems CCITT No.

6 and CCIS use signaling blocks containing a 20-bit signaling

field with an 8-bit CRC field. The proposed CCITT No. 7 system

is expected to employ a 64-bit signaling field with a 16-bit CRC

field.

Error control procedures for data transmission itself also

can include some form of CRC. The Advanced Data Communications

Control Procedure (ADCCP) for example uses a 16-bit CRC for a

variable length data field. The AUTODIN II binary-oriented

procedure Mode IV is similar to ADCCP, except a 32-bit CRC is

used.

Error detection and correction schemes can be implemented

using existing processor hardware or software when a computer is

part of the system. A separate hardware implementation is

relatively complex and expensive, but it reduces the load on the

processor. A combined implementation simplifies the process by

114
using software for check arithmetic. Whiting (1975) describes

two software implementations, one emulating the hardware method

and the other using a table of partial remainders for CRC schemes.

A hardware implementation of CRC using shift registers and

an 8-bit check code of the form

G(x) = x8 + x2 + X + 1

is shown in the block diagram of Figure 4-8. This is the

method recommended by CCITT (1977).

The shift registers of the encoder (Fig. 4-8a) are initially

set to zero, gates A and B are enabled, and gate C is inhibited.

All of the information bits and control bits in the block to be

checked are clocked into the input. These bits generate the

remainder within the shift register, while simultaneously

appearing at the output. After the desired number of bits per

block has been clocked in, the A and B gates are inhibited (the

source is stopped!) and the gate C is enabled. The register is

then shifted for eight additional counts. This appends the

remainder, i.e., the required check bits, to the output sequence.

The corresponding decoder is shown in Figure 4-8b. The

storage registers are initially set to zero, block synchronizing,

and gates A, Band E are enabled while gate D is inhibited. The

information bits of the word to be checked are clocked in. Gates

A and B are inhibited, gate D is enabled. The eight check bits

are now clocked in. If the block contains no errors all inputs

to the error detector will be zero. Otherwise, some-lis appear

at the error detector and retransmission is requested.

The generating polynomial recommended by the CCITT No. 6

system detects alII, 2, and 3 bit errors in a 28-bit signaling

word, because the minimum distance between code words is 4.

4.3. Performance Aspects

Section 3.2 noted several distinctive characteristics of the

military switching environment and the importance of the network

control system for that environment. Here, the relevant performance aspects are explored. There are several ways in which

115
I

I

L_

GATE A

j--J

/--l

0'\

INPUT • -..

(0) Encoder

GATE B .' ~OUTPUT

GATE A

GATE E

INPUT" • ./ GATE BI .. OUTPUT

(b) Decoder

Figure 4~8. Cyclic redundancy check system.
t h econt ro Isi gna 1 itl'g S ystern l.tnpactsbtl U5:e r rYer £:0rm'Cl-nce •

T-he seway:s·aret"e \T iew-e:cihere ,)together wit-h ;some 'quailtit.·a t i \Ie

numb-ersfor the state of t'heperfortnahce :ofa.iff~,l>eht t.y~re-s·Gf

signalin9 sy~tems~

Th'(~u:ser :p'erformance paraIlletersar;e :stJe-cifiet1 at t:i1.€ user.!

syst-ern int.:erface .~This ihizerracemaydiff:et from t.·ft·e p'Q:a.nt-s :Ett:

whi-ch the sy;st-emp:ertf-orlTtane,errre·a'surements :ac'emade '.. The'bl'(jt~k

di~9ram bf ~i~Ute 4~9 ~larifi~s the di5tift~tibfi betW~~h th~~@

interfacepolht;s,. Thenetwo;r,k informatibn t~t·ahsfer ihtieff'ace J>S

betwe'en t.he ·datat.'erminal equipItre,h.t (b'TE) and the "d·alta :e.i:rdti~t

termi nat ing e;qaip,rtteh t {DeE 'i II? hysica1 .~ e~ectr i~ea1) anti pifb'hOBoi

standards areu,suallydefih-ed fGrt~his poiht,.

Ih cbhtr~$tt th~ tiS~r perform~fid~ tl£itQtia ~~~ d~tih~d a~

the user/'system functiQnalinterface" Asah e}tampietJ,ft.-hiltS

distinction it i's u·seru-l toco!C1sider a simpl:(e t:elet:ypewriter~t6~

teletypewritercQ.mmuniic.atlt)h s'er\Ticebetweetltwoopet~tor,sover

a dedicated tr·ansmit~si()n .llne.The tlsetiifif:erfaceisl3,et:w$en the

operator and t.he telet:ypewriterahd t.h,einfe>rmatibntta.nsfef

interface isb'etwe;eh the ti~let.ypeW'rit:er ·a.nd t.'h:elihe ih't7Jaffa:ce

unit. This line interface m·ay b:ea fuod-efufor'2thaleg tra.nsm;Ls'§~9h

faciliti'es, or a. titrtediVisi6h rnultipleKer f'or digital facilitie-s.

In the tel~typ~wrlter ~~ample, whef~ d~dl@at~d fa~~li~~ i~

used ,coht:.r·o1si gna 1 i n3J is rL:eqUired. Tn@ph.ysi ;~a<lafi-tl e l;eetr i €,a i

interface anctsomefor·m 'of link co-nerbl .{ levels land20f $'e@·i,

4.1.3) are bothneedt:2d" alt.n.·bu'gh there are noswitc'hh6des

preSQht. The high~r e6httfil Pfbt6~61§ (l~V@l~ j. 4 ihd ~) ~f~

characteri§ti6 bf th~ §Wi~~h~d hetwo~k ~hVi~bfifu~ht,

~~~mpl~~ fif lev~l p~btdd61§j Whidh ~ith~t ~Ki§t ~t hav~ b~~h

proposed a~ ~t~haatd~. ~~@ d~g~fib~d ift B~~ti~fi §,

T,he followlng sUbsections sUmm.arize thepefftJrm€1.hee

parameterS fGr th~ u§@£/~y~tem fUh~tidhaliht~f~ae~, ~h@

irnpact of the signaling system on these paramei.:§:fs should be

determined at l~a~t bh a qti~litatiVe b§§1§4

4 • 3.1. _tJ$.:~_t.__ Ot)~~Dt~~. ~e~ff9ttn.a_rlc'e1i)~~c3.Jn~,,~~,~J~

the s~ledt~bh bf dbfttrOl St~at~gi~§j ~§ w@il a§ th~ §igftilifiij

syst~m it~~lf~ has an important beaiihg ~fi th~ p@~f6t~artc~ of a

117
USER A

DTE

--------

USER / SYSTEM

FUNCTIONAL INTERFACE

USER B

INFORMATION

,.....---- TRANSFER CHANNEL-~

INTERFACE

COMMUNICAT ION

NETWORK

Imaginary communi/cation links for logic level

protocols

Physical communication links for user information

and control information

Figure 4-9. Interface points for user-oriented and

technically oriented gerformance parameters.

118
telecommunication network from a user standpoint. Just how

important the effects are depends on the criteria used to judge

the performance.

User-oriented performance standards have been proposed by

the Federal Telecommunications Standards Committee (FTSC, 1977)

and by the American National Standards Institute (ANSI, 1974).

These standards provide methods for describing the performance

of digital communication systems in terms of the service they

provide to the user. When specific numerical values are obtained

by measurement or by calculation, the standards permit meaningful

comparisons to be made between systems offering equivalent

services, while using similar or different facilities and

procedures.

It is important to examine at least one of these proposed

standards in detail, so that the effect of the signaling system

on specific performance parameters can be evaluated.

The development of proposed Federal Standard 1033 is

described in detail by Seitz and McManamon (1978).

Table 4-4 lists twenty-six proposed digital communications

performance parameters. Kirnrnett and Seitz (1978) have calculated

values for these performance parameters for three different types

of communication services. Their estimates are also shown in the

table. The three services used in these examples are: 1) a nonswitched private line, teletypewriter-to-teletypewriter communication service that connects only two operators, 2) a circuit

switched teletypewriter-to-teletypewriter service connecting

selected pairs of operators through the public switched network,

and 3) a message-switched teletypewriter-to-teletypewriter

service providing communication between pairs of operators

through an automated store-and-forward switching center.

The performance values shown in Table 4-4 are calculated

parameters based on assumed transaction profiles and performance

outcomes. They are not intended to be used in characterizing

or comparing actual offered services. They do, however,

demonstrate how the three basic systems differ in performance.

119
Table 4-4~ Compilation of P2rameter Values for Proposed Federal

Standard 103J

PERFORMANCE

PARAMETER

NONSWITCHED

PRIVATE LINE

Example

CIRCUIT-SWITCHED

EXAMPLE

MESSAGE-SWITCHED

EXl'"MPLE

Access Time 5.2 Seconds 37.5 Seconds 15.4 Minutes

I

Incorrect Access PrObabilitYJ o 1.1 x 10- 4 o

Access Denial Probability 1.7 x 10... 3 1.1 x 10... 2 6.8 x 10-4

Block Transfer Time 68 milliseconds 68 milliseconds 16.2 minutes

Bit Transfer Time 68 milliseconds 68 milliseconds 16.2 mi nutes

Block Error Probability 1 .5 x 10- 4 1.5 x 10- 4 2.5 x 10-4

6 x 10- 7 1.1 x 10-4

I . . - 5 .'. ; . - 4 I l '. - 5 '. ..' . - 4 II .. ..5 .... .'.' " - 4 (Bit Error Probabiltty i1.5XlO~ P(ble)~ l.5exlO l~·5eXIO "- P(ble}~1.5exlO J~.5eXI0 P(bl e )2.5exl0

[BlOCk MisdeliveTy prObabilit~ 0 ,. I

\-I

1'0

o

Bi t Mi sde 1i' very Probabi 1i ty () 1. T x 10- 4 6 x 10- 7

Bit Loss Probability 1~.7 x 10- 5

I

:Extra Block Probability i 0

; I

Block Loss Probability 4.7 x 10- 5 6.8 x 10- 4

6.8 x 10- 4

o

T.7 x 10- 3

1.7 x 10- 3

7 x 10,-7

-----

I! Extra Bit Probability 0' I

o 7 x 10- 7
Table 4-4 (continued)

o

a

a

o

3 x 10- 3

1.5 seconds

1. 1 x 10-4

42 minutes

93 UIT hours

a

a

0.4

0.02

5.i7 x 10- 5

1.4 x 10- 3

38 minutes

2. 2~) seconds

8.2 UIT hours

I

I

a

o

0.19

0.8

4 x 10-4

1 hour

8 x 10-4

0.67

43 UIT hours

Extra Bit Probability

Outage Probability

Outage Duration

Service Time

Between Outages

Disengagement

Denial Probability

User Access Tiue Fracti·on

Disengagement Time

User Message Transfer

Time Fraction

User Block Transfer

Time Fraction

User Disengagement

Time Fraction

I

_Bit Rate Efficiency 25 percent I~ 68 percent L o8 percent

I 0.5 seconds

f---I

N

\--1
This enables each to be designed to meet different end-to-end

service requirements.

Payne (1978) has measured some of the user performance

parameters on the ARPANET. These measurements include access

time, incorrect access probability and access denial probability

during the access phase, as well as disengagement time and disengagement failure probability during the disengagement phase.

This measurement program is continuing, with emphasis on the more

pertinent performance parameters during the information transfer

phase.

Network control plays a major role in determining many of

the performance parameters, but not necessarily all of them. The

network topology, switch design and transmission state also

affect performance.

Coviello and Vena (1975), in their justification for network

integration, indicate that the performance, in terms of blocking

probability for circuit-switched networks and in terms of

queueing delay for store-and-forward switching networks, depends

on the number of trunk facilities between nodes. In a military

environment, the spare capacity required to handle the peak loads

could be considerable. For example, although five voice channels

may be sufficient to handle the average number of calls between

two nodes, ten may be needed to achieve a 0.02 blocking probability for the actual random statistics of call arrivals. Thus,

on the average, such trunks are utilized only 50% of the time.

Due to the uncertainty of call statistics; many of the performance parameters can only be specified as design goals, not

requirements. Some of these design goals are given in Section 5.

4.3.2. Impact of Signaling Systems on Performance

It is difficult to determine quantitatively the impact of

the signaling system on all of the 26 performance parameters

listed in Table 4-4. Yet, signaling must have some impact on

nearly all of them, because without controls the user information cannot be transferred.

lL2
The signaling system has a major impact on the access phase

and disengagement phase of a circuit switched network. However,

these impacts need not be similar. For example, once (with the

aid of CCIS) a path has been established through the circuitswitched network, the error control of user data no longer involves the common signaling channel. Error control is added to

the data as overhead. In a store-and-forward switched network

the path-establishment and error-control overhead do not follow

separate paths but are appended to each block of information

bits.

The affect of different dialing and signaling systems on the

access and disengagement times of the public, circuit-switched,

telephone network can be derived from the results of a recent

survey by Duffy and Mercer (1978). The mean access time for

direct-distance dialed (DOD) attempts is indicated in Figure 4-10

for a user with a dual tone multifrequency (DTMF) dialer. This

dialing time is approximately doubled when a rotary dialer

generates dc pulses instead. The connect time from end of

dialing to ring return is almost 11 seconds for conventional

signaling on a per channel basis (e.g., System Rl) as shown in

Figure 4-10. When all the switch nodes are controlled with SPC

processors and common channel interoffice signaling (eCIS) is

used, this connect time is expected to be reduced to 2-3 seconds.

(Note: Signaling systems Rl and CCIS are described in Sec. 5.1.)

Table 4-5 summarizes the total mean access times and disengagement times for plain old-fashioned telephone service (POTS).

The message transfer time for these telephone networks

depends on the distance. Most long-haul carrier systems have a

propagation time of 0.004 ills/km. The average propagation delay as

a function of connection length is given below (AT&T, 1970).

2.5

4.0

15.0

connection Length Distance in km

Short 0 to 290

Medium 290 to 1170

Long 1170 to 4700

123

Average

Propagation Delay

in Milliseconds
LDISCONNECT

BUSY

f--i

f\)

If...:..

7.05

1.65

iRING.

I 8.55

4.65

,ANSWER

Figure 4--10. Mean access time on DDD network using pushbutton dialer and

conven~ional signaling.
Table 4~5. Some PerforrnCince PCirarneter Estirna.tes for Various Communication Networ'ks

(]J

()

.~

o

>

Network

POTS

POTS

P,OTS

Priocedure

BotCl,ry dial

Conv.Sig

MF dial

Conv. 5ig

MF dial

CClS

Access

Time

35s

28 s

20 s

Transfer

Time

2,,5 .... 15ms

2.5.... 15 ID:S

2.5-15 IDS

Disengagem,ent

Tim,e

4 s

4 s

3 s

Ref.

(1) (6)

.( 1 )(6)

(1) (6)

(1) Duffy & Mercer (1978) (3) Estimated

(2) Kimmett & Seitz (1978) (4) Payne (1978)

Packet Sw. I 50 k baud

(ARPANET) . Virtual Ckt. Serve

Nonswitched

Private Line

Circuit Sw.

TTY

I -I Circuit Sw.

u-" Cd - CQmp oTerm

~I Message Sw.

'Off-line

Message Sw.

On-line

No dial

150 baud.... BSC

Rotary dial

150 baud-BSC

MF dial

2400 baud-BSC

150 baud

Poll,ed

150 baud

Datagram

5 53 I 68ms I 0.5 s I (2 )

-

40 s I 68 ms I 4 s I ( 2 )

-

33 s 5ms 3 s (2 )

15 min. 16 min. 1,.5 s (2 )

0 I 16 min. 1.5 s ( 3 )

-

9 s I 93 ms I 5 s I ( 4 ), (5 )

(5) Kleinrock & Naylor (1974)

(6) AT&T (1970)
These average ranges of propagation delays are indicated in

Table 4-5 for POTS.

Values of access, transfer, and disengagement times for

several data networks are also shown in Table 4-5 for comparison

purposes.

The transmission rates in b/s (bits per second) are related

to baud speeds for each data network link. The baud is a measure

of the carrier modulation rate or signaling speed for analog

facilities. With a true binary system, 1 baud=l bit/sec. The

conversion from words/min to bits/sec is obtained as follows

bls = wlm x ch/w x b/ch x m/s.

Assuming 6 characters per word and 10 bits per character

(including overhead bits for error detection), a 150 word per

minute teletypewriter operates at 150 w/mx6 ch/wxlO b/chxl/60 mls

or 150 b/s.

Three of the data networks use binary synchronous control

(BSC) protocol for signaling the control information. BSC is

a character-oriented link control procedure. The other data

networks utilize bit-oriented link control procedures. These

character and bit-oriented procedures are described in Section 5.2.

Table 4-6 summarizes the signaling system impacts on

certain users performance parameters in a qualitative manner.

It is assumed that entirely digital networks are used, and that

the digital parameters apply to voice as well as to data communications. Although this table seems fairly subjective, it is

useful in evaluating specific signaling systems for potential

application to the AADSS. Those items that are denoted "major

impact" should carry more weight in selection of suitable systems.

Note, for example that the signaling system has little effect on

the information transfer rate on a circuit switched network because

signaling impacts only the access and disengagement phase. On a

packet switched network providing datagram service the destination information is contained in each packet and thus added

control overhead reduces the user's information rate.

126
Table 4-6. Signaling System Impacts on

Performance of Digital ~etworks

Signaling System Impacts

For Clrcult For J?acket'

Switched Network Switched Network

Performance Parameter Major Moderate Minor Major Moderate Minor

Access Time X X

Incorrect Access

Probability X X

Access Denial

Probability X X

Block and Bit Transfer

Time X X

Error Probabilities X X

Information Transfer Rate X X

Disengagement Time X X

Disengagement Denial X X

Outage Probability X X

Service Time Between

Outages X X

Outage Duration X X

127
5. EXISTING AND PROPOSED SIGNALING SYSTEMS

It was indicated previously in Figure 4-1 that there are

different control procedures for different digital networks.

In particular, the controls for store~and-forward switching

differ from circuit switching networks. In this section these

differences are examined in detail. Typical examples are given

of signaling systems and their protocols. Numerous protocols

have been standardized either commercially, nationally or internationally. Because many protocols are very similar to each

other, we will select and emphasize one or two control procedures

from each category.

5.1. Voice Network Protocols

The CCITT (l977a) has standardized a number of signaling

systems for general use in international automatic and semiautomatic circuit switching networks. These are designated by

serial numbers 3, 4,S, and 6. Systems No.3, 4, and 5 are

designed to operate over analog trunks ona per-channel basis.

All require in-band signal_ing. System No. 6 is a common channel

~ignaling syst~m which operates digitally over quasi-analog

or digital facilities. The CCITT is also developing a common

channel signaling system for synchronous data applications on

public data networks. This new signaling system is expected to

be officially designated as System No.7 (CCITT, 1977b).

In additioh, two regional signaling systems have been

standardized by the CCITT (1977c). System R-l is commonly used

in North America and System R-2 is used in Europe.

All seven of these systems are briefly described below.

System No. 3 - It is an international in-band system adapted

for one-way operation. A single frequency 2280 Hz is used

for both on-hook supervision and pulse type addressing.

system No. 4 - This system is similar to No.3, except that

two in-band frequencies, 2040 Hz and 2400 Hz, are used for

end-to-end operation over one-way links. System No. 4 is

used exclusively in Europe.

128
System No.5 - System No. 5 is the international in-band

system for two-way operation. It uses two frequencies, 2400

Hz and 2600 Hz, for link-by-link supervision; and two-outof-six frequencies (700, 900, 1100, 1300, 1500, and 1700 Hz)

for addressing.

System No. 5 bis - The uiJo. 5 bis" system uses the same

supervisory signals as No.5, i.e., 2400 and 2600 Hz. The

six in-band address frequencies however have now an expanded

function. They provide forward and backward signaling of

control information during the call establishment phase.

System No.6 - This is the international system designed

for digital two~way signaling over a common channel. The

digital signals may be transmitted over a quasi-analog

channel at 2400 bls or over digital channels derived from

the PCM multiplexer at 4 kb/s. The North American eelS

system is similar to System No.6. The differences between

the two will be noted in subsequent detailed descriptions.

System No.7 - This system is being developed for general

application in public synchronous digital networks for

voice, data, integrated voice/data services. Proposed No. 7

characteristics are given in Recommendation X.60 by the

CCITT (1977b), but many specifics are still under consideration. The functional structure of System No. 7 is divided

into a user part and a message transfer part. This makes

No. 7 useful for other communication needs besides control

signaling. It is designed to operate over 64 kb/s digital

channels but it can be used over lower rate channels.

System Rl - This is the standard system currently used on

large portions of the u.s. telephone network. In many areas

the system is being replaced with the common channel interswitch signaling (CCIS) system, that is similar to System

No.6. System RI operates over analog or digital transmission facilities. The supervisory or line status signals

use a continuous idle tone of 2600 Hz over analog links and

129
associated in-slot signaling over T-l carriers. The latter

is done by borrowing the eighth bit of each channel, once

per every 6 frames (see Sec. 2.4.3). The address digits are

transmitted in-band, link-by-link, using two out of six

frequencies. The six frequencies are 700, 900, 1100, 1300,

1500 and 1700 Hz. The system is also suitable for signaling

on satellite links. No digital version of address signaling

is specified in Rl. However, the tones can be digitized

like voice signals and handled as the so called speech

digit signaling. Certain periodic time slots, normally used

for digital speech, are then used for signaling.

System R2 - System R2 is another analog/digital signaling

system. Its analog version uses out-of-band supervisory

signals at a frequency of 3825 Hz. The digital version uses

one of the 32 channels which make up the 2.048 Mb/s European

digital carrier system. The address digits are in-band and

continuously compelled. Forward signaling frequencies are

1380, 1500, 1620, 1740, 1860,and 1980 Hz. Backward

signaling frequencies are 1140, 1020, 900, 780, 660, and 540

Hz. The out-of-band supervisory signals cannot be transmitted via an analog input to a PCM channel bank. However,

a digital version of R2 has been developed. In this version,

in-band address signals are applied to the analog inputs

of the digital speech circuits.

Table 5-1 summarizes the characteristics and capabilities

of the signaling systems reconunended by the CCITrr'. It is

apparent that Systems 6, 7, Rl and R2 are all suitable for

digital networks, however, only 6 and 7 are designed for cornman

channel signaling between digital switch nodes with SPC.

System No.6, CCIS, and System No.7 are described in the

following subsections. The basic protocols for CCIS and System

No. 6 are the same. The differences noted are largely due to the

130
Table 5-1. Summary of Telephone Signaling Systems Recommended by the CCITT

f-J

W

f-J

AddressSystem Supervision ina Suitable For Potential Uses Reference

ITlme

Analog Digital toivisiotl

Out of [trans- Trans- Common Swi tell Oper- Satellite DirecIn Band Band In Band mission mission Channel lNud~~-) ation Link tion

No. 3 2280 Hz None SF X End to 1 way CCI'I'T, 1960

Pulsing End

No. 4 2040 Hz None SF X End to 1 way Euro!>c) CCITT, 1973a

and Pulsing End

2400 Hz

No. S 2400 Hz None MFDT X Link X 2 way US and CCITrr, 1973b

and by Europe

2600 Hz Link

No. 6 Binary Binary Binary X X X X Link X 2'way Europe CCITT, 1977a

(CCIS) Code Code Code (Quasi) by (US)

Link

No. 7 Binary Binary Binary X X X Link X :: way r ( )I)() ~; (' ~; . CCITT, 1977b

Code Code Code by US

Link j':uruI)( .

Rl 2600 Hz None MFDT X X One Link X 2 way US CCITT, 1977c

Only by

Link

R2 3825 Hz MFDT X X One Link 2 way Europe CCITT, 1977c

Only by

Link
fact that System No. 6 was developed for use internationally,

whereas eelS is being implemented for domestic use in the u.s. on

toll networks. System No. 7 is a newer system still under

development. Its fundamental concepts are radically different

from System No. 6 and ceIS.

Only the basic concepts, code formats, and operating

procedures of these three common-channel signaling systems are

defined in the following paragraphs. More details can be found

in CCITT (1977a) for System No.6, and in Ritchie et ale (1978)

for eelS. Some of the basic concepts of System No. 7 can be

found in CCITT (1976), CCITT (1977b) and Appenzeller (1977).

5.1.1. Common Channel Interoffice Signaling (CCIS)

The eelS system was introduced in the North American toll

network in 1976. By 1980, CClS is expected to control over

300,000 intertol1 trunks between electronic switching centers

(Cann, 1977).

The eelS network is usually operated in a disassociated mode

over quasi-analog links. The network is arranged as shown in

~'igure 5-1 which shows how celS controls interconnecting links

between two separate regions. There are 10 such regions in

CONUS, and two signal transfer points (STP's) within each region.

The two STP's within a region are each connected to two switching

centers, as well as to three other STP's, to provide redundancy

in case of signal link or STP failures.

A typical signaling link is designed to operate at 2.4 kb/s.

It can carry the signaling information for up to 3000 trunks.

Normally,however, each eelS link is assigned 1500 trunks.. This

means that in a case of failure, the load of the inoperative link

can be added to another operating link. Signaling link operation

at 4.8 kb/s has been contemplated to minimize' delays and to

increase capacity over busy links (Dahlbom, 1972). Usually,

only two STP's are involved in establishing, maintaining and

disengaging a call. This is necessary so that answer signal

delays are acceptable during the transition to an all celS

network.

132
RIEGION 1I

Signaling

Trunk Groups

Signal Transfer

Point

I i 1'{ ......

REGION I Switch

Center

w

w

Figl.lre 5-1. Network structure for disassc)ciated common channel

interoffice signaling (eelS).
A block diagram of the eCIS system is shown in Figure 5-2.

Separate links are identified as information transfer links, data

links, and signaling links. Control information, generated in

the processor, is in accordance with the CCIS signaling unit

format. It is delivered in parallel, using either 8 or 16 bit

bytes, to the block denoted signaling terminal. This terminal

stores the outgoing information in an output buffer. Redundant

error control coding is added to the outgoing signaling units.

This error control is based on the previously described CRC,

with error detection and retransmission. Each signaling unit

(SU) contains 20 control information bits and 8 error check bits.

A sequence of eleven signaling units and one acknowledgement unit

(ACU) define one block~

The analog link interface equipment shown in Figure 5-2

modulates outgoing signals and demodulates incoming signals.

The modem operates in the synchronous mode. Special synchronizing

signaling units (SSU) are transmitted continuously, whenever.

other types of signaling units are not being sent. The SSD's

maintain bit synchronization and SU synchronization, whereas the

ACU is used to maintain block synchronization.

Upon recep~ibn and demodulation ata distant processor, the

output data are delivered to a CRC decoder and the error check is

performed. Error free signaling units are transferred to an

input buffer of the signaling terminal where they are stored

until they can be processed. Acknowledgement signals are

returned to the originating terminal either to confirm reception

of error~free SU's or to request retransmission of SU's received

in error. Thus, all unacknOWledged SU's must be kept stored at

the originatinq terminal until a correct acknowledgement has been

received.

The CCIS systern,like any common channel system, requires a

special continuity check feature over the trunks being connected.

This feature is indicated in Figure 5-2 by the continuity check

134
TO

OTHER

SIGNALING

LINKS

..-_._£.~ TR~~KS

SWITCH OTHER

NETWORK SWITCHES

SIGNALING

TERMINAL

INTERFACE

EQUIPMENT

•

•

•

TRUNKS

DATA LINK

SIGNALING LINK:

\ I

TRANSFER LINK

INTERFACE ,- .1

EQUIPMENT ...----,

\ /

SIGNALING

TERMINAL

\ /

CONTINUITY CHECK

TRANSCEIVERS

SWITCH

NETWORK

Figure 5- 2 . Can trol signaling via common Cl1c-l11nel s icyn_al ing 1 ink. ,

LINES

TO

TERMINALS

W

In
transceivers. During the call set-up phase for a 4-wire trunk,

one transceiver is selected from a common pool. It is briefly

connected to the chosen trunk at the originating switch. A loopback connection is made via the destination switch and a 2010 Hz

signal is transmitted over the loop. A proper level return signal

verifies that the trunk is operable. Over 2-wire trunks the

continuity check signal is 1780 Hz outbound and 2010 Hz on the

return. Transceivers are needed at both ends of the trunk for

the 2-wire continuity check.

Signaling Codes and Formats for CCIS

The format used for the various types of signaling units in

the eCIS system are indicq.ted in Figure ,5- 3. As noted previously,

each signaling unit consists of 28 bits, eight of which are

parity-check bits. The other 20 always contain a three-bit

header, which defines the type of unit being sent. This leaves

17 bits for various types of control information.

The three-bit headers occupy positions 1 through 3 of the SUe

They are transmitted first. The heading code indicates eight

different types of signaling units as follows.

Type

Telephone Signals

System Control, Management

or Maintenance Signals

Acknowledgement Signal

Initial Signaling Units

Subsequent Signal Units

Acronym

LSU

LSU

ACU

ISU

SSU

Heading Code

000, 001

010, 100

III

011

101

110

The four LSD's used for telephone signals are further distin~

guished by signal information bits 4 to 7, that immediately

follow the heading code. This is shown in Table 5-2. The

remaining bits 8 to 20 contain the signaling information.

136
BIT NUMBERING SCHEME

IHEADING ISIGNAL INFO. I BAND NUMBER ] CKT. NO, I CHECK I

LONE SIGNAL UNIT (LSU) or INI"TIAL SIGNAL UNIT (ISU)

III 0 I SIGNAL INFORMATION I CHECK I

SUBSEQUENT SIGNAL_ UNIT (SSU)

o ACKNOWLEDGEMENT INDICATOR ACK. NO. I BLK. NO. CHECK

W

........i

ACKNOWLEDGEMENT SIGNAL UNIT (ACU)

II I I I0 I I I I0 I I I 0 0 0 I §\ SEQ. NO, I CHECK I

SYNCHRONIZATION SIGNAL UNIT (SYU)

II I I I 0 I I I I CONTROL INFORMATION I CHECK I

SYSTEM CONTROL UNIT (SCU)

I ISIGNAL INFO. BAND NO. or MISC. INFO. MGMT. INFO. CHECK

ONE-UNIT MANAGEMENT SIGNAL

F igur:3 5- 3. Basic format of signal units used withcommon channel interoffice

signaling system.
Table 5-2. Signal Information Codes for LSD's

I--J

w

00

Header Codes

Bits 1,2&3

Sig.

Info. Codes

Bits 4,5,6&7 000 001 010 100 111

0000 Xv Failure Blocking Ack X

0001 Continuity Refused Unblocking X

Ack

0010 X Confusion X X

0011 X X X

0100 X Trunk Congestion X Busy Network Management

(International)

0101 X Switch Congestion X X Signaling Management

(International)

0110 X X Release X

Guard

0111 X X Answer X Signaling Management

(no charge) (Unbanded)

1000 Clear for Reset Answer X Network Management

(charge) (National)

1001 Block X X X Signaling Management

(Banded)

1010 Unblock X Clear Back Address Complete

#1 No charge

1011 X X Reanswer Address Complete

coin

1100 X X Clear Back Address Complete

#2 charge

1101 X X Clear Back X

#3

1111 X X Reanswer X

X indicates spare
If the header identifies an ISU (code 101), then bit 4 indicates the type of ISU, while bits 5 to 7 indicate the number of

SSU's which follow. This multi-unit message (MUM) may contain up

to eight subsequent signal units in tandem. It provides an

efficient means for transmitting large groups of related information. Each SSU requires only a 3-bit identification header. The

17 remaining bits can all contain control information, such as

address, line coordination, etc.

Signaling a Call

The CeIS steps involved in a successful completion of a 10-

digit call are shown in Figure 5-4. Only the first 20 bits of

each signaling unit are indicated in the figure. The other 8

bits are parity check bits. In North America, a 10-digit long

distance call normally requires three decimal digits for the

numbering plan area (NPA) , three decimal digits for the central

office code, and four decimal digits for the terminal as follows.

NPA

N B X

OFFICE CODE

NNX

TERMINAL

XXXX

where N is any digit between 2 and 9, B is 0 or 1, and X is any

digit between 0 and 9.

At least 30 bits are required for the 10-digit address.

However, it is simpler to assign 4 bits to each of the N, B,

and X. Then, the above address consumes 40 bits. When a caller

goes off hook and dials the destination address, the address is

stored in a register by the processor. An initial address

message is formed consisting of an ISU and an appropriate number

of SSU's. The signaling information bits in the ISU indicate

that four SSU's are to follow. This ISU contains 13 other bits

(8 through 20), which are to designate the trunk. Thus, there

are 8192 different trunk labels available.

The first SSU following the ISU contains information concerning the service desired, such as the satellite links·

139
ORIGINATING PROCESSOR

SENDS INITIAL ADDRESS MESSAGE

ISU I 0 I 0 0 I I I TRUNK DESIGNATION

SSU I I 0 I ROUTING INFORMATION

SSU I I 0 0 0 0 0 0 N 0 0 0 Cfl X

SSU I I 0 0 N N X X

SSU I I 0 0 X X X FILLER

DESTINATION PROCESSOR

I I COMPLETES DESIGNATED TRUNK LOOP

CHECKS CONTINUITY .- - - - - - - --+-- -

-----1

____ --JX

LSU \0 0 0100 0 I l TRUNK DESIGNATION I I CONFIRMS CONTINUITY

CONFIRMS ADDRESS COMPLETED ---------+-1--"'II () O}I I 0 0 1 TRUNK DESIGNATION J LSU

r--:

..

AUDIBLE RING RETURNED -- -- -- -- -- -- -- --+-. -- -- -- -- -- -- -- -- -- -- -- -- -- -- --

CUSTOMER ANSWERS, STOP RING1NG

c

CALLED CUSTOMER HAS ANSWERED,

BEGIN CHARGING

-------+-1-----11 0 I 0 II 0 0 0 I TRUNK DESIGNATOR I LSU

CALLED CUSTOMER HANGS UP

STOP CHARGING, RELEASE BACK

CONNECTION

--------+-1-----.0110 .1 011· 0 I 01 TRUNK DESIGNATOR) LSU

LSU \0 0 011 0 -() -0-1 TRUNK DESIGNATOR RELEASE FORWARD CONNECTION

READY FOR NEXT CALL -~~~~~~~~~1~~1010101 1 oj TRUNK DESIGN~OR ) LSU

Figure 5-4. Steps required for IO-digit call using CClS.
included, echo suppressors involved, or special line features

required.

The next three SSU's contain the dialed IO-digit address.

Upon receipt of this address, the destination processor closes

the loop on the designated trunk to check continuity. An LSU

from the originating office verifies the continuity check.

Another LSU at the destination office confirms that the address

is complete and rings the called station. An audible ring is

returned to the caller. When the called customer goes off hook,

an LSD is returned so that charging can begin. Either the

calling or the called customer may hang up first. In either

case, the nearest processor sends an LSD to indicate release,

stop charging, and clear all the involved line and trunk units.

Release forward and release backward signals indicate that

both customers are on-hook. A final LSD is transmitted to

confirm that the trunk is ready for another call.

There occur, of course, many other situations which are not

shown in Figure 5-4, such as customer busy, trunk congestion,

inoperative number, etc. When these situations occur, an

appropriate LSD is returned to the originating processor. It

takes appropriate action and, if necessary, alerts the calling

customer on the progress of the call.

The priorities for various types of signals are as follows.

Priority

I

2

3

4

5

6

7

8

9

Signal Type

ACU (Error ACK or NAK)

Fault Information

Retransmit Answer Signals

Answer Signals

Retransmit Telephone Signals

Telephone Signals

Retransmit Management Signal

Management Signals

SYU (Synchronization signaling

unit)

141
These priorities establish the signaling sequence when conflicts

arise at the transmitting terminal.

Signaling Block Structure

As noted previously the SU's are combined in groups to form

a 12-unit block. The block structure containing the initial

address message from Figure 5-4 is shown in Figure 5-5. Direction of transmission is to the right. Therefore, the blocks are

numbered in reverse. Synchronizing signal units, SYU's, are

sent periodically and whenever there is a need for fillers.

Every twelfth SU is an acknowledgement unit (ACU). The ACU

format contains the identifying header (011) and eleven acknowledgement bits, one for each SU in a block. A zero bit indicates

positive acknowledgement, and a one bit indicates negative

acknowledgement. These eleven bits are followed by three bits,

which identify the number of the previous block being acknowledged,

and three other bits indicating the sequential number of the

block just completed by this ACU. Since the three bits can be

coded for up to eight blocks, there may be this number of blocks

in doubt, and outstanding on the link.

A typical CClS link is operated at 2.4 kb/s data rate. Since

there are 28 bits per SU and 12 SU's per block the acknowledgement

total time required to receive 8 blocks is

28x12x83 = 1.12 seconds.

2.4xlO

Of this time (96 SU's), the time for 64 SU's is available between

emission of an SU and subsequent reception of the ACU containing

its acknowledgement. The number, 64 SU's is based on a CClTT

recommendation (CClTT, 1977d). The remaining time for the other

32 SU's is required for processing. Thus the maximum loop

propagation delay is about 750 ms. When negative acknowledgements

occur, multiple loop delays must be allowed for.

Transmission

The CClS signals are normally transmitted full duplex over a

4-wire link. An 1800 Hz carrier is modulated by the binary

142
/

BLK I IBLK 8 IBLK 71 BLK61BLK 5 IBLK 41BLK 31 BLK 21 BLK ItBLK8

t---J

w

Figure 5-5. eelS block structure.
signaling bits using a quaternary or four-phase shift keying

(QPSK) at 1200 baud. Each differential phase increment contains

two information bits as follows:

Bits

00

01

11

10

Phase Differential

45°

1350

2250

3150

The actual bit rate is 2400 b/s.

The modern characteristics are summarized in Table 5-3.

Table 5-3. eelS Modem Characteristics

Modulation:

Modulation Rate:

Bit Rate:

Demodulation:

Operation:

Link:

Carrier Frequency:

Carrier Envelope Frequency:

Timing Frequency:

differential four-phase

1200 baud

2400 b/s

differential coherent four-phase

full duplex

4-wire

1800 Hz

600 Hz

2400 Hz (one cycle/bit)

5.1.2. CCITT System No.6,

System No. 6 is the international common channel signaling

system recommended by the eCITT. It was initially standardized

in 1968. After extensive field testing, it was recommended for

in-service application in 1972 (Bernard, 1974).

The basic No. 6 protocol is very similar to eelS, although

the present specifications for System No. 6 provide for operation

over digital data channels at bit rates of 4 kb/s and 56 kbjs,

and at 2.4 kb/s over quasi-analog channels.

System No. 6 is oriented toward the associated mode of

operation rather than the non-associated mode used by eelS.

144
This is so, perhaps, because Syst'emNo. 6 was designed to handle

fewer and relatively shorter trunks than eelS. These differences

will become apparent in the following paragraphs. The operating

procedures for signaling are the same as given for eelS.

Signaling Codes and Format for System No.6

The signal unit formats for System No. 6 are shown in Figure

5-6. A five-bit header is used. This reduces the trun_k desig·-

nator field to 11 bits. The five-bit header provides 32 distinct

headings. Currently, only about 25 of these are used. Theothers

are being retained as spares for regional and national needs.

When the first two bits of the header are zero, subsequent

SSU's are indicated. The next two bits then give the number of

S5U's in the multi-unit message.

A three-bit header 011 is used f·orArCU'S. This leaves sufficient bits for unique identification of all blocks in the

sequence, for the identity of a block being acknowledged, and for

the eleven signal units per block.

The heading code 11101 is used for synchronizing signal

units (SYU's), system control units (SeD's) and the multiblock

synchroni zing unit (MBS) . The distinction bet'weenthese units

is obtained from the immedia-tely following four-hit field (i -" e. I

bits 6 to 7).

Table 5-4 summarizes the heading codes used in System -No" 6

'rhe various codes used in System No. 6 are given in theAppendix~

Signaling Block Structure

The block structure for System No. 6 is similar to the eelS

block structure, except that in No.6, eight consecutive blocks

can be synchronized using the MBS signal unit. Such eight

consecutive blocks are called a multi-block. The MBS format

provides a five-bit sequence number in bit positions 13 through

1 7 to index 32 mul tiblocks. Thus ,the maximum number ofbloc_ks

in the error control loop is 256 rather than the 8 blocks used by

CClS. See Figure 5-7. This multiblock structure permits speeds

of operation up to 4 kb/s over both 1.544 Mb/s and 2_048 Mb/s

primary multiplex channels, as well as at 56 kb/s~ The maximum

145
BIT POSITION

I 2 3 4 5 6 7 8 9 10 II 12 13 1415 16 17 18 19 202122232425262728

LABEL

l. HEADING ISIG. INFO I BAND NO I CKT. NO I CHECK

LONE SIGNAL UNIT (LSU) OR INITIAL SIGNAL UNIT (lSU)

~_LG_T_H....... I --.S_IG_N_A_L_IN_F_O_R_M_A_T_IO_N -L-__C_H_

EC_K__---J

SUBSEQUENT SIGNAL UNIT (SSU)

10 I I I ACK INDICATORS ISEO. NolsEO. Nol CHECK

ACKNOWLEDGEMENT SIGNAL UNIT (ACU)

II I I 0 I II I 0 I U=I 0 0 0 I I l_S_E_Q_oN_O~I C_HE_C_K_-----,

SYNCHRONIZATION SIGNAL UNIT (SYU)

II I I 0 I O_I_o_o-.lI~__CO_N_T_R_O_L_IN_F_O__..L...-__C_H_E_C_K__

SYSTEM CONTROL SIGNAL UNIT (SCU)

II I I 0 I II 0 I II CONTROL INFO CHECK

MULTIBLOCK SYNCHRONIZATION SIGNAL UNIT (MBS)

II I I 0 JJO I 0 I I BAND NO l!'v1GMT -I~I---- __C_H_EC_K__----,

Figure 5-6.

MANAGEMENT SIGNAL UNIT (MSU)

Basic format of signal units used with

International Signaling System No.6.

146
Table 5-4. Heading Codes Allocated for System No.6

Code Signal Unit

000 SSU

all ACU

10000 ISU

10001

10010

10011 Subsequent address message

10100 (SAM)

10101

10110 -

10111

11000

11001 International Telephone Signals

11010

11011

11101 SCU, MBS, and NMM

147
1---1

0.'"

SIGNAL

UNIT

BLOCK

MULTIBLOCK

INFORMATI,ON

20 BITS

CHECK

8 BITS

------~

Figure 5-7. Block structure for International Signaling System No.6.
permissable loop propagation time with and' without the multiblock'

counter is given below. The MBS unit is required at high data

rates over long paths.

Link Data Rate

2.4 kb/s

4 kb/s

56 kb/s

56 kb/s

Transmission

Loop Delay in IDS

740.

448

32

1520

MBS

no

no

no

yes

Transmission of System No~ 6 signal data over quasi~analog

channels is essentially identical to that fOr the eelS system.

In addition, System No. 6 can be used on digital signaling links

with 2 kb/s, 4 kb/s and 56 kb/s data rates. The basic transmission schemes are shown in Figure 5-8. Over these digital

facilities, the 2 or 4 kb/s signaling data are transferred

serially to the interface equipment.

At the interface the bit stream is modulated onto the 64 kb/s

voice channel, with 16 bits of the voice channel assigne.dto each

bit of the 4 kb/s signaling channel. The entire 16 bits are

inserted into the time slots allocated for signaling in each

frame of the multiplex equipment.

Data transmission at 56 kb/~ is also feasible over a

2.048 Mb/s primary multiplex channel. The 28 bits of the SU

are placed in bit positions 1 through 7 of four 8-bit bytes and

transferred serially at 64 kb/s. The four bytes are in~erted

into designated time slots of four successive f:;r:ames.

5.1.3. CCITT System No.7

Study Group XI of the eCITT is studying a new common channel

signaling system for use on networks which utilize digital

switching and digital transmission for integrated voice and data.

The design emphasis is on simplicity and flexibility. The basic

concept differs considerably from eelS ~nd System No.6, although

signaling units of fixed length are still envisioned. The System

will be optimized for 4-wire operation over digital links at

149
2·4 kb/s ......I.,~~'·MODEM·I, 2.4.. kb/s. ,.... .. I~ FDML., I ,

-----

I

SYSTEM

NUMBER I 2.4 kb/s 6 INTERFACE 56 kb/s __ 2.048 Mb/s - SIGNALING -- ADAPTER MULTIPLEX

TERMINAL I

,

U1

o

I' 20r 4 kb/s - INTERFACE

ADAPTER

1.544 Mb/s I 64 kb/s .1 or 2,048 Mb/s .....-

M-ULTIPLEX

Figure 5-8. Transmission schemes defined for system No.6.
64 kb/s. It will also be capable of working efficiently at lower

bit rates and over analog links. Recommendations for a complete

and detailed specification of this new con®on channel system,

denoted Signaling System No.7, are expected by 1980.

Only certain general aspects can be given at this time. The

known features concern the message transfer part of the system.

Study Group XI has indicated that the basic structure of

Signaling System No. 7 will be divided into two clearly defined

functional parts: a user part and a common message transfer

part, as shown in Figure 5-9. The user part resides at both ends

of a message link. It may utilize several different message

processors, each making use of the message transfer part, e.g.,

call control, management, action, etc. The message transfer part

includes the signaling data link and performs the link control

functions, such as error control.

One signal unit format being considered is exemplified in

Figure 5-9b. The signal unit length will be a multiple of 8

bits. In the example shown in the figure, the total unit length

is 80 bits, half of which are user data. The other 40 bits are

divided into link control fields, as indicated on the figure.

The forward sequence number identifies the signal unit. A

backward sequence number identifies some previously received

signal unit which is being acknowledged either positively or

negatively by the acknowledgement indicator. The signal unit

indicator field identifies the type of signal unit being sent.

Four types of signal units are provided to permit expansion of

the user data field. They are 1) lone signal unit, 2) initial

signal unit or multi-unit message, 3) subsequent signal unit, and

4) final signal unit. Multiple unit messages cannot be interrupted, because interruption may cause excessive queueing delays.

The maximum number of signal units in a multi-unit message may

have to be limited to approximately six.

The service indicator field provides 16 code combinations to

direct the user data to an appropriate user part, e.g., telephone

call control, data call control, network management, maintenance

management, signaling system control, etc.

151
A

rFORWARD'

SEQUENCE

NUMBER

f-.'

l.r~

['0

I I

USER MESSAGE SIGNALING MESSAGE V USER

MESSAGE DATA MESSAGE

PROCESSING CONTROL LINK CONTROL PROCESSING

\ I \ I \ / V v v

USER PARTS MESSAGE TRANSFER PART USER PARTS

(a) Functional Blocks

BACKWARD CHECK

SEQUENCE ARE USER DATA CODE lNUMyBER J l y ..• J

I 881TS I 881TS 1±14BITSI 40 BITS IT! 1481TS I

r I L---.,

(SIGNAL 'SERVICE' .).. A~.-

UNIT INDICATOR RETRANSM ISSION ACKNOWLEDGEMENT

INDICATOR INDICATOR INDICATOR

(b) Signa I Unit Formats

Figure 5-9~ Function and format of proposed system No. 7~
The service indicator field identifies the user data for the

different user parts for message processing. If the signal unit

is being retransmitted because of a previous negative acknowledgement, this is indicated by the bit state in the retransmission indicator field.

Error detection is provided by including check bits in each

signal unit. A 14-bit check sequence is shown in Figure 5-9b.

An alternative format uses 16 check bits and the retransmission

and acknowledgement indicator bits are borrowed from the forward

and backward sequence numbers. Error co~rection is normally

performed by retransmission. However, forward error correction

may be provided as an option. Alternative methods for reduction

of retransmission requirements are being considered. This

involves repetative retransmissions during the idle periods of

either the last signal unit sent, or of all unacknowledged signal

units stored in the transmitter buffer.

The user data field fdr a lone signal unit will probably

contain a four-bit header, to identify groups of signals, and a

four-bit information field to identify the type of signal

within each group. The remaining 32 bits are then available for

labeling, addressing, and other information, depending on the

signal unit type.

There are, as mentioned above, many detailed specifications

and aspects of the system which require further study. Unresolved issues include the exact form of the polynomial to be

used in generation of check bits, synchronization procedures,

arrangements for insuring signal service availability and

continuity, encoding of telephony signals in the user data field,

performance characteristics of the signaling data link, and

service requirements under normal and fault conditions.

The ultimate objective of System No. 7 is to provide a highspeed signaling system which is more suited for the digital environment. It should be capable of serving both dedicated and

common user networks. The System No. 7 capabilities would

153
include signaling over point-to-point terrestrial and satellite

links including two satellite links in tandem. The latter

requirements may be included to maintain essential voice communications. Point-to-multipoint operation may also be feasible.

5.2. Data Network Protocols

In Section 5.1 three common-channel signaling systems were

described, which are now either used or proposed for telephone

networks. The eelS and eCITT No. 6 systems have been implemented

for use nationally and internationally on circuit-switched voice

networks. System No. 7 is being developed for international and

national use to control circuit-switched, synchronous data networks. One of its main goals is to shorten the access time for

data calls. The system may also be used to control voice networks and for maintenance and administration purposes. In this

section /the protocols for data networks are considered, including

protocols from Levell through Level 5 as defined in Section 4.2.3.

Level 1 standards define the interface between data terminal

equipment (DTE) and data circuit-terminating equipment (DeE).

These standards or combination of level 1 standards provide the

physical, electrical and functional interchange to establish,

maintain, and disconnect the physical link between the DTE and

DeE or between two DTE's. Thus they provide a common means of

connecting a computer directly to peripheral equipm~nt or

remotely via communication equipment. This connection may be made

using a serial or parallel interface. The serial interface transmits the data one bit at a time either synchronously or nonsynchronously. It is commonly used when long communication links

are required between terminals. The parallel interface transmits

data in groups of bits (usually 8-bit bytes) and is used over

short links because it is more efficient.

Figure 5-10 indicates the various standards which have been

recommended by standards organizations for use in serial or

parallel interfaces. The breakdown includes parallel and serial

interfaces. Some serial interface standards define functional

154
LEVEL 1 STANDARDS

I I

PARALLEL INTERFAC-ES

I

I

I

GPI B (HP)

488-1975 (IEEE)

MC -1-1975 (ANSI)

:.--l

(J'~

Ul

SERIAL INTERFACES

I I I

FUNCTIONAL 8 MECHANICAL ELECTRICAL

I I

I · -------.

: VO~AGE OPERATED CURRENT LOOP

I I

I I I

I I I

RS-232C (EIA) RS-232C (EIA) WE- 303

V.24(CCITT) V.28 (CCITT) (WESTERN

V.21 (CCITT) RS-422(EIA) ELECTRIC)

RS- 449(EIA) RS-423 (EIA)

V.II(X.27)(CCITT) V.35(CCITT)

V.10(X.26)(CCITT)

MS -IB8C (DOD)

Figure 5-10. Interface standards promulgated by various organizations.
and physical characteristics, others define only the electrical

characteristics while still others combine all characteristics.

5.2.1. Parallel Interfaces for Levell

The General Purpose Interface Bus (GPIB) was originally

developed by a commercial industry for interfacing programmable

electronic test equipment. It has been adopted by both the IEEE

and ANSI and is widely used for transmitting data bits in

parallel between test equipment and computer peripherals. The

parallel bus utilizes a 24-wire cable to carry eight parallel

data lines and 8 parallel control lines. The data are transferred in 8-bit bytes at rates up to 1 Mb/s for short (1 meter)

cable lengths. Up to 15 devices separated by 15 meters can share

the bus at lower rates.

5.2.2. Serial Interfaces for Level 1

The serial interfaces indicated in Figure 5-10 are divided

into voltage operated and current loop categories. The current

loop interface WE 303 is used by AT&T to interconnect high speed

terminals to wideband modems operating over private lines.

Various versions of the interface provide data rates from 19.2

kb/s to 230 kb/s over short cable lengths. A binary "1" is

signified bya current less than 5 ma and a binary "0" by currents

between 5 rnA and 23 rnA for a 100-ohms circuit.

The CCITT V.35 standard is a high-speed interface similar

to WE 303.

Voltage-operated standards typically operate at lower speeds.

The international standards recommended by CCITT are in many

instances similar to the reconunended standards (RS series)

promulgated by the EIA. Table 5-5 compares the pin numbers and

functions of the RS-232C and CCITT V.24 standards.

The RS-232C interface provides an unbalanced connection

between the DTE and the DCE. Two voltages of opposite polarity

in the 5 to 25 volt range are required. Cable distances are

denoted to be about 15 meters and data rates to 20 kb/s. This

standard is gradually being replaced with RS-422 for balanced

connections and RS-423 for unbalanced connections. These new

standards alter the electrical characteristics of the interface

156
Table 5-5. Standard Interface for Serial Binary Data Interchange

.,----~-------___ir__-------_r__------------------.

RS-232C (ErA)

Designation

AA

BA

BD

CA

CB

CC

AB

CF

CCITT V.24

Equivalent

101

103

104

105

106

107

102

109

Description

Protective ground

Transmit data

Receive data

Request to send

Clear to send

Data set ready

Signal ground common

return

Received line signal

detector

SCF 122

SGB 121

SBA 118

DB 114

SBB 119

DD 115

Secondary receive line

detector

Secondary clear to send

~econdary transmitted

data

Transmit signal element

timing (DCE)

Secondary receive data

Receive signal element

timing (DeE)

SCA

CD

CG

CE

CH/CI

DA

120

108

110

125

111/112

113

157

Secondary request to sene

Data terminal ready

Signal quality detector

Ring indicator

Data signal rate

selector

Transmit signal element

timing (DTE)
signals and specify voltage levels which are more compatible with

digital logic levels currently used in TTL circuitry. The span

of RS-422 is 1200 meters at 100 kb/s and up to 12 meters at 10

Mb/s. Using RS-423 for unbalanced connections the span is either

60 meters at 10 kb/s or 12 meters at 100 kb/s.

RS-449 specifies the mechanical pin assignments and connector

types used with both RS-422 and RS-423.

5.2.3. Character Oriented Protocols for Level 2

Level 2 protocols can have an important impact on overall

network costs. This is because the transmission facilities and

the software to control these facilities are a major cost factor.

Therefore, the link control procedure must be simple to implement

and should provide efficient and reliable communications.

There are two basic types of level 2 protocols. The cornmon

type is a character-oriented protocol such as the Binary

Synchronous Control (BSC) protocol used by IBM or the data link

control (DLC) recommended by ANSI. These character-oriented

protocols use a code set whereby certain codewords are allocated

for control purposes and other code words for user information.

A header field specifies the control parameters and an information

field contains the control characters. The character codes used

in ASCII and EBCDIC are indicated in Table 5-6. The 7-bit ASCII

code is the American National Standard Institute version of the

ISO code R646. The same code is also recommended by the CCITT as

International Telegraph Alphabet No. 5 (ITA-5). The 8-bit EBCDIC

code is used in IBM 360 computers.

A 7-bit code (e.g., the R646 ISO code) gives 2 7=128 code

word combinations. These provide 95 printing symbols, a space,

and 32 control characters. Bits 6 and 7 are both zero when

indicating a control character as shown in Table 5-6. The

8-bit EBCDIC code gives 256 combinations. About one quarter of

these are control characters.

Other codes use a shift character to allow more symbols to

be represented than is possible with a one-to-one correspondence

between symbols and code patterns.

158
Table 5-6. Character Oriented Codes

Binary Code

R646 (ISO)

ITA-5 (CCITT) EBCDIC

Symbol Meaning ASCII (ANSI) (IBM)

NUL Null 000 0000 0000 0000

SOH Start of heading 000 0001 0000 0001

STX Start of text 000 0010 0000 0010

ETX End of text 000 0011 0000 0011

EOT End of transmission 000 0100 0011 0111

ENQ Enquiry 000 0101· 0010 1101

ACK Acknowledgement 000 0110 0010 1110

BEL Bell alarm 000 0111 0010 1111

BS Backspace 000 1000 0001 0110

HT Horizontal tabulation 000 1001 0000 0101

LF Line feed 000 1010 0010 0101

VT Vertical tabulation 000 1011 0000 1011

FF Form feed 000 1100 0000 1100

CR Carriage return 000 1101 0000 1101

SO Shift out 000 1110 0000 1110

SI Shift in 000 1111 0000 1111

DLE Data link escape 001 0000 0001 0000

NAK Negative acknowledgement 001 0101 0011 1101

SYN Synchronization 001 0110 0011 0010

ETB End of transmission mode 001 0111 0010 0110

ESC Escape 001 1011 0010 0111

159
In general, if M combinations of an n-unit code are used

as shift commands, the total number of commands will be M(2 n -M)

Wh "h' . h" M' 2n - 1 h' . 2(n-l) " 1C' 15 maXlmum wen ,,':::: . T1S glves 2 posslble

symbols compared with 2n symbols without shifting (Davies and

Barber, 1976).

5.2.4. Bit-Oriented Protocols for Level 2

Bit-oriented protocols are procedures used for arranging

messages in frames. A frame is a sequence of control and information bits which is delimited by a flag at the beginning and end

of a sequence. The flag is a unique code word. Transparency is

~chieved by insuring that none of the bit sequences between the

flags Gorre~p6nds to a flag during transmission.

The AmeriCan National Standards 'Institute (ANSI) has adopted

a bit-oriented level 2 protocol called the Advanced Data Communications Control Procedure (ADCCP).

The International Standards Organization (ISO) protocol is

callQd high-level data link control (HDLe) and the International

Business Machines (IBM) protocol is denoted synchronous data

link control (~DLC). All are bit oriented, data link control

protocols (i.e., level 2) and all are similar procedures. The

HDLC prbtocol is described in more detail in the next section

since it is ~sed to support the level 3 protocol, X.25, used on

packet-switch networks.

The AU~ODIN II network will use a protocol similar to HDLC

and ADCCP in one of its operating modes (Sevcik, 1977).

5.2.5.!}Packet-Switched j jProtocols \ \for Level 3

The CCITT, supported by ISO and ANSI, recently adopted the

X.25 protocol for interfacing with public packet-switched networks

(CCITT, 1977b). Level I of X.25 specifies that the physical,

electrical, and functional characteristics shall be in accordance

with level 1 of X.21. The CCITT X.21 and the U.S. version ANSI

X.2l is a simple interface implementing a minimum number of

interchange circuits toconnectDTE's to any telecorrununication

network. The standard provides full transparency (bit sequence

independence) for data transfer.

160
Level 2 of X.25 employs the HDLC protocol specified by ISO

for control between two nodes on the packet-switched network.

Level 3 of X.25 defines the formatting of packets and

control procedures for end-to-end communications over several

links in tandem. Each of these X.25 levels is described in more

detail below.

Levell

Figure S-ll illustrates the X.21 functional interface and

circuit designations. Call establishments and terminations are

provided by the control circuit C. This control circuit also

provides transparency by separating the data and control functions.

The indicator circuit I is used to signify when the establishment

phase is complete and data transfer can begin. Since the interface is transparent, the user can employ any higher level

protocol. The transmit circuit T and receive circuit R are used

for transferring information, containing both user data and logic

level control information.

The X.21 interface uses a IS-pin connector between the DTE

and the DCE. The electrical interface characteristics correspond to Recommendation X.27 or RS-422 for the DCE. Either X.26 or

X.27 may be used for the DTE.

The X.2l protocol uses the character oriented International

Alphabet No. 5 (lAS). It is similar to ASCII for the higher

levels, such as the establishment phase of a call (Volts and

Cotton, 1977). Basically, X.21 is a bit-oriented protocol for

the data transfer phase. The user is free to use his own format

protocol and his own frame synchronization for data transfer

(see Folts, 1977). Only the physical-electrical level 1

characteristics of X.2l are applied to packet switching. Packet

switched networks use X.25 at levels 2 and 3.

Level 2

The HDLC protocol is specified for X.25 to provide error

and flow controls over the access link between the DTE and the

network. The HDLC format is shown in Figure 5-12. The number

across the top of the figure indicates the order of bit transmission. 161
INTERFACE POINT

.. -I TRANSMIT. T L...-

.- -. - r

RECEIVE R

LINK

-. , -

-

DTE ----- CONTROL,C DCE

INDICATION I

NODE

, -

SIGNAL TIMING, S

I~

- .J ,~

........

-

I J I

I f--J

C'\

N

Figure 5-11. Levell interface (X.21) for packet switched networks.
2 3 4 5 6 7 8

1 I 1 8- BIT FLAG I 1 I

0 I I I I I I 0

8- BIT ADDRESS FIELD

a-BIT CONTROL FIELD

INFORMATION

PACKET

16-BIT FRAME CHECK

-

SEQUENCE

8-BIT FLAG

Figure 5-12. Frame format for level 2 (HDLC) of

X.25 protocol.

163
The check sequence contains 16 bits, derived from the

1 '·1 16 125 1 . h . d generator po ynom1a X+X +X +. Transparency 1S ac leve

by using O-bit insertion when six or more lis appear in sequence,

except for the flag itself.

All HDLe frames start and end with an eight-bit (octet) flag

consisting of tpe bit sequence 01111110. A single flag may be

used to signify both the start and the end of a frame.

The eight-bit address field indicates whether the direction

of commands and responses is from DCE to DTE, or from DTE to DCE.

The control field contains a command or a response, and

sequence numbers where applicable. Three types of formats are

used to distinguish between information transfer, supervisory

functions and control functions.

The information field contains a packet of information whose

octet Length and makeup is described under level 3.

Level 3

The X.25 interface recommends a maximum data field length of

128 octets, where an octet is one 8-bit byte. The network may

disassemble these ,128 octets into smaller packets for internal

switching. The recommendation also allows for other data fields,

b~t they must be of fixed length. Normally, if more user data

are required, a full data packet is marked by the DTE with a "tobe-continued tag". This tag indicates that more data follow in

the subsequent packet. Packets generated by a source DTE are

transmitted in an ordered sequence to the destination.

Different types of packets are used from DeE to DTE and

from DTE to DCE. Both types include packets for call setup,

clearing, data, interrupts, flow control, reset, and restart.

Figure 5-13 illustrates the call request packet format. This

format is used during the call establishment phase to set up a

virtual circuit between origination and destination terminals.

The 4-bit format identifier indicates the general format of

the rest of the header and the sequence numbering scheme employed.

164
8 7 6 5 4 3 2

I f J

FORMAT IDENTIFIER

I r I

CHANNEL GROUP

NUMBER

LOGICA·L CHANNEL NUMBER

PACKET TYPE IDENTIFIER

CALLING DTE ADDRESS

LENGTH

CALLED DTE ADDRESS

LENGTH

DTE ADDRESS

- - - - - ,-- - - - - - - - - ...----.-,------~----I

° ° ° o

o o FACILITY LENGTH

FACILITIES

~: CALL USER DATA ....

11Iooo-. --_-<_0---1-6_O_C_T_ET_S_.) --~J

Figure 5-13. Call request packet used with theX.25

protocol.

165
The logical channel group number and the actual channel

number identify the virtual circuit. These two numbers appear in

subsequent packets, except for restart packets.

The packet identifier in octet 3 distinguishes the particular packet type being transmitted, i.e., request, data, interrupt, clear, etc.

There are address fields that occupy one half of an octet.

These semi-octets indicate the length of the address in semioctets, for the called DTE and the calling DTE. The actual

addresses are contained in octet 5 and subsequent octets.

The fields provide a means for communicating with optional

user facilities or to realize other special features via the DTE.

The field is not required in applications where peripheral

equipment is not used.

The user data are sent in the octets following the field.

In the call-request packet the ~aximum length of this data field

is 16 octets.

If more user data are to be transmitted, then additional

packets are formed using the data packet format of Figure 5-14.

Since a virtual circuit has already been established with the

call-request packet, these additional data are identified only by

their logical or packet number. The third octet contains transmit and receive sequence nurnb~is and one indicator bit, to show

when more data follow.

The X.25 protocol provides the interface to. a packetswitched data network to provide virtual service to the user.

Additional studies are being conducted by CCITT and ANSI to

extend the interfate to provide datagram service, where all the

appropriate control and address information is contained in each

packet. Thus, in contrast to virtual service, there are no call

establishment or disengagement packets involved in datagram

service.

The recommended X.25 protocol for packet-switched networks

is a good example of how the various protocol levels interact.

When communicating'digital information, levels may appear

166
8 7 6 5 4 3 2

I I I

FORMAT IDENTIFIER

I I

CHANNEL GROUP

NUMBER

LOGICAL CHANNEL NUMBER

RECEIVE

SEQUENCE

NUMBER

MORE

DATA

IND.

TRANSMIT

SEQUENCE

NUMBER

o

USER DATA

(125 OCTET MAXIMUM)

Figure 5-14. Data packet format used with the X.25

protocol.

167
independent of each other. At the same time, levels can be

formatted together by nesting one protocol within another.

Failure of a higher level protocol does not affect the lower

level. Also, changing the format or some other characteristics

of a high-level protocol does not change the lower level

protocols.

This nesting concept leads to combination protocols, whereby

different higher level protocols are used with common lower level

protocols. Such nesting extends the functions and services

realizable by a variety of terminals.

The concept of a general-purpose protocol to integrate different networks has been used before. One example is reported

by Nichols (1976). The information field portion of an HDLC type

frame, as shown in Figure 5-12, is used in the example to

transfer control information for terminal classes ranging from

low-speed, five-level-code, teletypewriter terminals, to mediumspeed, eight-level-code, intelligent terminals, and to high-speed

computer terminals.

The SENET concept described by Coviello and Vena (1975) and

GTE (1975) uses a high-level control protocol imbedded in the

information field of the ADCCP protocol. It dynamically controls

the virtual circuit-switched and packet-switched channels. The

protocol includes a CClS type signaling scheme for the circuitswitched and packet-switched channels. The SENET/DAX format,

which is compatible with ADCCP protocols for link control is

illustrated in Figure 5-15.

System No. 7, itse~f, appears to be evolving as a combined

protocol due to the separation of functions between the user

part and the message transfer part. The user part does not have

to apply just to subsystems of the processor at a given node.

The user part could contain data for any other remote user

terminal.

Several u.s. manufacturers are offering inexpensive data~

link control chips which support more than one protocol.

Protocol conversions are handled by software and can be changed

168
o

...J

W

lJ..

Z

o

a::

o

lJ..

Z

8 765 432

1 I I I ITT

FLAG

ADDRESS

CONTROL

MESSAGE TYPE

TRUNK IDENTIFICATION

ORIGINATiNG SWITCH

DESTINATION SWITCH

TRANSIT COUNT

DATA 8 CONTROL(CCIS)

INFORMATION

(0 - 240 OCTETS)

END OF MESSAGE

FRAME CHECK SEQUENCE

FLAG

Figure 5-15. SENET/DAX packet format.

169
using the same hardware. One multi-protocol LSI chip can support

character-oriented protocols from a terminal and bit-oriented

protocols for node-to-node communications (Weissberger, 1978).

As network complexity and user diversity increases, protocol

conversion may be handled with distributed microprocessors. Such

microprocessors can be placed at the nodes, as part of the

signaling terminal or in conjunction with the switching processor. A microprocessor implementation of the X.25 protocol is

described by Shearin and King (1977) for levels 1 and 2, to

provide full duplex operation of data links at rates up to 3

Mb/s.

5.2.6. Network Control Architectures for Level 4

Level four establishes the system control procedures. This

could be processor subsystem control or peripheral-device control.

Level four protocols coordinate the transfer of data between a

users application program and the operating system.

Currently no standards have been recommended although several

computer manufacturers are developing protocols for this level.

Since these system control architectures are still under extensive development and test, a description is beyond the scope of

this report. Information on some of these protocols is given by

McFayden (1976), Wecker (1976), and Conant and Wecker (1976).

5.2.7. Performance Standards for Level 5

ANSI (1974) describes the American National Standard X3.44-

1974 for the determination of the performance of data communication systems. This standard is applicable to synchronous and

non-synchronous data communication links employing the ASCII code.

Performance criteria are established from a user viewpoint and

include system capacity, delay factors~, accuracy and availability.

A similar standard has been proposed by the Federal Telecommunications Standards Committee (FTSC). Denoted FS-1033 thi~

Federal Standard defines user-oriented performance parameters in

terms of efficiency, accuracy and reliability for three communication functions, namely access, information transfer, and

170
disengagement. These FS-1033 parameters with sample parameter

values for different types of networks were discussed in Section

4 .3.

The X3.44-1974 and FS-1033 standards are considered level 5

since they involve performance evaluation at interface between

the terminal and the user.

Table 5-7 summarizes the known standards for interface

levels 1 through 5. Definitions for the standard designations

used and the promulgating agencies' acronyms are given in the

list of acronyms at the beginning of this report.

5.2.8. AUTODIN II Access Protocols

AUTODIN II is a packet switched network being developed to

meet the data transmission requirements of DoD users primarily in

CONUS. Network applications include interactive, query/response,

bulk data transfer and narrative. The four basic protocols under

network control (i.e., level 1 through level 4) are being

implemented for AUTODIN II. Different modes of access at certain

levels provide support to a variety of terminals and services.

For example, at level 3 the network supports either datagram or

virtual service. The user access protocols specified for

AUTODIN II are summarized below*.

Levell

Defined by MIL STD 188C or EIA RS 232C.

Convertible to MIL STD 188-114 and EIA RS 449.

Level 2

Mode I: Full duplex, character oriented, binary synchronous

(currently used by message terminals to access

AUTODIN I).

Mode IB: Full duplex, character oriented, binary

synchronous (ANSI X3.28 and BSC (IBM)).

Mode IIA: Full duplex, character oriented, asynchronous.

Mode lIAR: Half duplex, character oriented, asynchronous.

*Because the system is under design and development some changes

are expected. The information given was obtained from DCA

"System Performance Specification (Type A) for AUTDON II Phase-I

dated February, 1977.

171
Table 5-7. Recommended Standards for Data Networks

International United Stat.es

Level Current

1 V.24/V.28

(CCITT)

New

X.21 (CCITT)

V.10 (X.26)

(CCITT)

V.ll (X.27)

(CCITT)

Current

RS-232C (EIA)

New

RS-449 (EIA)

RS-423 (EIA)

RS-422 (EIA)

BSR-X3.69

(ANSI)

2

3

4

5

1745 (ISO) HDLC (ISO)

X.25 (CCITT)

DLC (ANSI)

BSC (IBM)

X3.28 (ANSI)

X3.281 (ANSI)

ADCCP (ANSI)

BDLC (Burrough)

SDLC (IBM)

DDCMP (DEC)

X.25 (CCITT)

SNA (IBM)*

DNA (NCR)*'

DECNET (DEC)*

DCA (UNIVAC)*

FS 1033 (FTSC)

(Proposed)

X3.44-1974

(ANSI)

Blanks indicate no applicable known standard.

*Examples under development by industry (not necessarily

complete).

172
Mode VI: Full duplex, bit oriented, binary synchronous

(compliant with ANSI advanced Data Communication

Control Procedure, ADCCP).

Level 3

3A: Datagram Service

3B: Virtual Connection Service (analogous to CCITT X.25)

Level 4

Terminal handler protocol provides interprocess control via

operator key commands to network virtual terminal.

5.3. Satellite and Packet Radio Protocols

A variety of protocols have been developed for use on radio

and satellite channels. These are particularly useful when a

multiplicities of data sources transmits randomly to a central

destination (e.g., a number of access terminals using the same

computer). Usually such traffic comes in bursts and has a very

-low duty cycle. Table 5-8 gives some examples of the peak and

average data rates for different types of user terminals.

When the peak to average ratio is high, as shown in the

table, then it is possible to connect a large number of geographically distributed users to a central computer.

5.3.1. Pure Aloha

A number of protocols have been developed to provide random

access using the same radio frequency channel for packet radio,

Table 5-8. Traffic Characteristics of Typical User

of Data Communications Satellite

Average Peak

Data Rate Data Rate

User (b/s) (b/s)

Input Terminal (e.g., TTY) 1 10 2

Output Terminal (e.g., CRT) 10 10 4

Remote Job Entry (RJE) 10 2 10 4

Computer to Computer 10 4 10 6

173
and satellite communications. Most of these random access

techniques have evolved from the pure Aloha system (see Abramson

and Kuo, 1973). This pure Aloha system uses a protocol whereby

all terminals transmit randomly. Because of the bursts of

traffic and because each user transmits at will, there is a nonzero probability that two or more sources may transmit simultaneously and the message (or packet) could not be delivered.

Actually this probability is quite low and even with a large

number of users the channel utilization (i.e~, the maximum

throughput) is 1/2e* or 18% of the channel rate. Thus, for a

typical channel rate of 50 kb/s, a large number of users (~250)

can transmit peak bursts of data at 50 kb/s with an average rate

of 0.18x50xl0 3=9 kb/s.

5.3.2. Slotted Aloha

The average rate can be increased to lie or 37% of the

channel rate using a slotted Aloha protocol. With slotted Aloha,

all users are synchronized to a common clock and transmit only

in specified time slots. The probability of simultaneous transmissions occuring is reduced by a factor of two. This improvement however is obtained with the added complexity of synchronizing all user terminals (Kleinrock and Lam, 1973).

An additional increase in throughput can be obtained if the

slotted Aloha is augmented with the additional feature of

"capture". When two packets are transmitted simultaneously the

packet with the higher signal level "captures" the receiver and

is not lost. Therefore, the throughput is increased. The amount

of this increase can be shown to be 57%. Again this increased

throughput is obtained at the cost of the added complexity.

5.3.3. Reservation and Carrier Sense Protocols

There are still other schemes for increasing the throughput of an Aloha type of network. The reservation Aloha protocol

*e=2.718.

174
is based on the user's requesting a transmission slot in advance.

The carrier-sense protocol permits the user to transmit a packet

only when no carrier exists. These carrier sensing schemes

should theoretically be capable of an average throughput rate

which exceeds 80% of the channel rate. However, such benefits

are achievable only when the users are not widely distributed

geographically. The delay in sensing the carrier is only a small

fraction of the slot interval.

5.3.4. Protocols for ~ixed Traffic on Radio Networks

Typical channel traffic may not be comprised entirely of

bursts, as it often includes data from continuous sources, such

as voice and facsimile. When such a mixture occurs, one talks of

integrated voice and data traffic. The slotted envelope network

(SENET) concept described by Coviello and Vena (1975) appears

useful for integrated radio networks. A special protocol which

is related to reservation Aloha is required in this application.

Such a protocol is being developed by ARPA and DCA.

This broadcast satellite channel transmission protocol

currently is being implemented for experimental evaluation on

INTELSAT-IV and is described by Jacobs et ale (1977). Called

CPODA, for Contention-based Priority Oriented Demand Assignment,

the protocol is designed to handle packetized voice and data

traffic in a multiple access satellite broadcast channel. The

channel can be shared by hundreds of earth stations while

generating an arbitrary mix of bursty and non-bursty traffic.

Jacobs and his colleagues define these traffic types as block

traffic and stream traffic. For both types it is assumed that

the messages have differing precedence levels and delivery delay

requirements, differing load distributions among the earth

stations, and either multiple or single destination addresses.

The CPODA protocol is based on the reservation system which

has been shown to be more cost effective than TDMA, FDMA and

simple ALOHA techniques (Roberts, 1973). The reservation system

175
employed ,with 'CPODA utilizes a centralized controller to inform

all oth~r, preSently idle, stations of their scheduled transmission times. Several CPODA protocol concepts are currently

being evaluated using four earth stations in the Atlantic region.

rhe data rate from these stations is either 64 kb/s or 16 kb/s

for both transmitting and receiving. Three of the stations use

satellite !MP's whi~h are part of the ARPANET. Signals are

broadcast from the INTELSAT-IV-A. The fourth station uses an

unattended COMSAT earth terminal.

6. A'PPLICATIONS TO THE ACCESS AREA DIGITAL SWITCHING SYSTEM

The access area digital switching system (AADSS) nlay be

viewed as an interconnecting gateway for local networks. The

gateway provides intracornmunications at the base and tactical

level. It interconnects with global-strategic networks, which

may be terrestrial (DCS) , satellite (DSeS), or combinations of

these networks. Finally, the AADSS must provide the interface

between the military networks of various types and the public

switched networks on a world-wide basis.

The interfacing structure is depicted by the diagram in

Figure 6-1. The AADSS performs duplex protocol transformations

by matching the transmission facilities o,f the various networks

indicated in the figure. This AADSS role must be accomplished

during the military transition frofu analog to part digital, and

all-digital network configurations. Ultimately, it may be used

toirtterface with - and even to be a part of - a fully integrated

voice and data network.

A major dissimilarity between networks is reflected in the

means by which they are controlled, i.e~, in their protocols.

Mbstprotocols in use, or under present development, have not

been standardized. T~is includes protocols for random access

satellite links and packet ra~io links.

176
}'igure 6-1. .Access ar~a digital .s'vi-tching ...sy.stem

interfaces.

177
In the following subsections, we compare the various circuit

switch and packet switch protocols that have been described

for use on digital facilities. We also review the role of the

AADSS for protocol transformations at various inter- and intranetwork interface levels.

6.1. Signaling System Comparisons

The signal formats for common channel signaling using

digital techniques are compared in Figure 6-2. The CCIS, as

well as CCITT System Nos. 6 and No.7, provide formats for

common channel signaling over telephone networks. The ADCCP,

SDLC, HDLC formats are used primarily for controlling data links.

Table 6-1 compares the various signaling techniques in terms

of transmission facilit~es, signaling rate and operating mode.

Table 6-2 provides a qualitative comparison between

conventional analog signaling systems and three types of digital

systems. The digital signaling mixtures are those on a per

channel basis, and the common channel signaling system operating

in either the associated or non-associated mode. The comparison

parameters used represent the unique military environment

characteristics described in Section 3. Although Table 6-2 is

largely subjective, it appears that both common channel interswitch systems corne close to meeting most of the AADSS objectives.

6.2. Intranet and Internet Gateways

Individual AADSS installations may face different requirements. There may be a variety of regional applications and

services. At the local level, the AADSS may be viewed as a

protocol transformation system that interfaces dissimilar

terminals (see Fig. 2-3 in Sec. 2.1.2). In this intranet application, the AADSS performs protocol, mode, format and speed conversions for various interacting terminals.

At the regional level, the AADSS need not interface with any

terminals directly, but it does provide gateways between switch

178
.....,

-....J

"-0

HEADER INFORMATION FIELD ERROR CHECK

3 BITS 17 BITS 8 BITS

CCIS

HEADER INFORMATION FIELD ERROR CHECK

5 BITS 15 BITS 8 BITS

CCITT #6

HEADER INFORMATION FIELD ERROR CHECK

24 BITS 40 BITS 16 BITS

CCITT #7

"

FLAG HEADER INFORMATION FIELD ERROR CHECK FLAG

8 BITS 16 BITS N BITS ........ 16 BITS 8 BITS ..

ADCCP /S;DLC

Figure 6-2. Signal format cOffi?arisons.
Table· 6-1. Common Channel Signaling 'rechn.iqueComparisons

,.....,

00

o

Type

eelS

CCITTNo. 6

celTT No. 6

CCITT No. 7

ADCCP

Transmission

Quasi-Analog

Quasi-Ana.log

Digital

Digital

Digital or

Quasi-Analog

Rate

2.4 kb/s

2.4kb/s

4 kb/s to 64 kb/s

64 kb/s (max)

Unspecified

Operating Mode

Nonassociated

Associated and

Quasi-Associated

Associated and

Quasi-Associated

Associated and

Nonassociated

Associated
Table 6-2. Qualitative Signaling System Comparisons

I---J

OJ

Compatibility

Survivability

Reliability

Adaptability

Mobility

Service Choices

Security/Privacy

Complexity

Relative Cost

.11 conventiO..". nal

(Analog)

High

High

High

Low

Med

Low

Low

Low

Low

Per Channel

(Digital)

High

High

Med

rv1ed

High

Med

Med

Med

Med

Common Ch.

(Associated)

Med

Med

Med

High

High

High

High

Med

Med

Common Ch.

(Non-associated)

~1ed

Low

(with single

route)

~1ed

High

High

High

High

High

High
nodes of different types for local and backbone networks. This

AADSS aspect was depicted previously in Figure 2-2.

The AADSS characteristics must vary as the applications

change from local, to regional, and possibly to inter-regional

levels.

A summary of the basic characteristics of each level is

given in Table 6-3.

Figure 6-3 illustrates a common channel signaling system

operating between two terminals with different protocols, but

connected to the same network. Here the protocol transformation

occurs on the line side of the switch. The common channel

signaling system between the switches may be operated in the

associated or non-associated mode, using a cornmon protocol for

communicating between the control processors. The cornmon

protocols may differ from those used by either terminal.

Figure 6-4 depicts the signaling interface transformations

between networks. Here the AADSS serves as an internet gateway.

Network A may be a local digital distribution system on a

military base. From network A, data are transferred to the

backbone network via the AADSS where a protocol transformation

takes place. Both data and converted signaling information are

forwarded to another network via digital or analog trunks.

Network B may be the Des backbone and network C may be a common

carrier, for example.

A time division multiplexer is shown as the trunk interface

for network A. The digital trunk may be a T-carrier with Tl, T2,

or even T4 trunks, depending on the traffic loads.

A block diagram of the AADSS for regional internetting is

shown in Figure 6-5. Both analog and digital trunk interfaces

can be added or deleted in modular form as the interconnecting

networks undergo future transitions. The basic switch is

~ssumed to be a hybrid type, capable of handling both circuit

switched voice and packet switched data. The control signals

are transferred via the packet switched networks, just like any

other data.

182
Table 6-3. Summary of AADSS Characteristics

Application

Local

Network Topology IStar

Loop

Tree

Terminals IVoice

Data

Traffic IAnalog and Digital

Regional

Loop

Grid

Sparsely Connected

None

(Intern1odal)

Primarily Digital

(Analog to DCa)

Interregional

Tree

Grid

Nearly Fully Connected

None

(In ternodal)

All Digital

Links IWire Pairs

T-Carrier

-Packet Radio

Fiber Optic Cable

Coaxial Cable

I-J

(X)

w Switching

(all SPC) Separated

Space & Time Div_

T-Carr i~9r

FDM

Hybrid

Space &

Time Di~lision

Microwave Radio

Coaxial Cable

Satellite Radio

Hybrid

Space &

Time Division

Signaling System

·Protocols Ispecial Purpose

-Mode IPer Channel &

Associated

-Transmission IAnalog, Quasianalog & Digital

·synchronizationlcentral Clock

TransfoJ::-mable

Associa1:ed &

Quasi-associated

Quasi-ar1alog &

Digital

Undeternn.ined

General Purpose

Transparent Protocol

Associated & Quasiassociated

Quasi-analog & Digital

Undetermined

ARQ on Terrestrial

-Error Control CRC with ARQ CRC wi tIl ARQ FEC on Satellite
I/O TRANSPO,RT ,FORMATION

SIGNALING

EQUIPMENT

SIGNA'LING

'EQUIPME'NT

I/OTR,ANSPORT FORMATION

Figure 6-3. Intranet gateway.

184

SUBSCRIBER

PREMISES

LOCAL lOOP

SWITCHING

NOD'E

TRU'NK

SW1TCH:ING

NODE

LOCAL LOOP

SUBSCRIBER

PREMISES
DIGITAL TRUNK

SIGNALING

EQUIPMENT

NETWORK

A

NE'TWORK

B

PROTOCOL A

NETWORK

C

F~gure 6-4. Interne~ gateway.

185
TO

REDUNDANT

PROCESSOR

ANALOG .... ., PCM T-CXR CHANNEL

/ INTERFACE BANK

• • TIME • • SLOT

INTERCHANGE .- SERVICE "'"

and GENERATORS

DIGITAL T-CXR DIGROUP _ "' T-CXR • • ,/ TOM / INTERFACE TERM SWITCH · •

• •

- CCIS SIGNALING ROUTING PROGRESS - TERM CONTROL CONTROL CONTROL

t

MEMORY CONTROL PROGRAM

STORE PROCESSOR STORE

"-

I .-

PRINTER CONSOLE DISPLAY MAGNETIC

TAPE

J--l

OJ

0"1

if)

z

:::::::i I ANALOG

o TRUNKS

z

.-J

<!

Z

<..9

if)

0

w

I DIGITAL

g TRUNKS

(f)

if)

<!

Figure 6-5. Transitional AADSS for regional internetting.
The signaling for circuit switched networks may be multiplexed onto the T-carrier. Therefore, the interswitch signaling

is normally operated in the associated mode, although in some

cases a quasi-associated mode could be used. A general-purpose

protocol provides the means for interprocessor communications and

for transferring user data.

For signaling, the information field conveys the control

data. For data transfer the same field conveys user data. The

transmission channels are therefore interchangeable.

As an example, it is possible to envision a packet-switched

network protocol which utilizes data packets for transferring

user data, as X.25 does. At the same time it could utilize

control packets containing the eClTT System No. 6 format or

eelS format to handle circuit switched calls.

6.3. Example of AADSS Application

A local digital distribution system for Ft. Monmouth, New

Jersey, using coaxial cable has been evaluated ,in terms of cost

parameters by Nesenbergs and Linfield (1976). This access area,

with a population of approximately 15,000 has one terminal for

every three persons. Over 95% of these terminals are mainline

telephones. Current traffic estimates, assuming all terminals

were operated simultaneously, could probably be accommodated

with about 45 MHz of channel bandwidth. One network configuration envisioned to handle this traffic is shown in Figure 6-6.

A coaxial cable, essentially identical to that used for cable

television, is used to connect a series of switching and concentration hubs around the base. Primary control to these access

hubs is located at a centralized hub, in this case, Vail Hall.

The network configuration may be either a star or a loop, the

latter being used in Figure 6-6. Connection between subscriber

terminals and the hubs (not shown on the figure) may be twowire pairs, or additional coaxial cables using a tree, loop or

star subnet topology.

187
O.5km

~. i

Ap.proximate Scale

r -------.J

\

\

)

/ ,A //\ Q9 /"

""-v/ \ HosPita~// \ ./

\ //

v

Reservation

BOundary~ .. '. ~",......- .------------.... --- "'\

/~vr--.." // SATeOM \

/r- -- -_.. _.-. / ~-- \

( -- \ Training - --- --.../ Squire Hall \

I School \ \ \

\ Vail Hall Video Headend Engr /) /

L------· (

To Camp Woods

a ECOM Building

O'J

0',)

To Earle NAO

a Camp Woods

Figure 6-6. Local access at Ft. Monmouth, NJ via coaxial loop

configuration.
The central hub, shown as a square in Figure 6-6, is also

connected to other military facilities in the area using Tc~rrier lines. Ultimately, these lines connect to the strategic

backbone network. This is depicted in Figure 6-7. All of the

military bases, including Army, NavYr and Air Force facilities

in the New Jersey area, are interconnected using a line or tree

network. Access to terrestrial backbone switches is accomplished

at both ends of the line to enhance the survivability. In

addition, access to a Defense Communications Satellite System

(DSCS) terminal provides connectivity to the Digital European

backbone (DEB) and hence to the military bases in Western Europe

(see Fig. 6-8). The DEB consists primarily of line-of-sight

microwave facilities linking the backbone switches. Troposcatter

facilities link across non-accessible regions to switching centers

in Spain, Turkey, Africa, etc. This is exemplified in Figure 6-8

by the link from the southern end of the DEB in Italy to a backbone switch in Spain.

Figure 6-9 is a block diagram of the nodes and links required

to connect voice or computer terminals in Ft. Monmouth to voice

or computer terminals somewhere in Europe, e.g., Spain in this

example.

Access to public networks in CONUS may be made either from

the dial central office (DCO) on the base as shown in Figure 6-9

or from the AADSS which serves as the Dca for several dispersed

clusters of terminals.

Access to tactical networks and terrestrial or satellite

backbone networks is also via the AADSS. Figure 6-9 depicts

access to the DEB via DSCS terminals. In Europe the digital

backbone may include LOS microwave links, troposcatter links

and coaxial cable links. Ultimately, connections are made to

local terminals through another AADSS which also provides the

interface to the European telephone system.

A computer terminal operator in Western Europe who desires

the use of a system resource (e.g., files and data, processors,

virtual terminals) in Ft. Monmouth is concerned with exchanging

189
To Terrestial Backbone

(Netcong NJ) Bayonne

Military Port

Facilities

CERCOM

Bldg

ci?

Approximate Scale

I

IOkm

Earle

Naval Ammunition

Depot

Earle

Pier

Fort

Monmouth

Camp Evans

McGuire

Air Force Base

Fort Dix

Military Reservation

Lakehurst

Naval Air

Station

To Terrestial Backbone Switch

(Cederbrook NJ)

Figure 6-7. Regional access to military reservations in

IJew Jersey area via T-carrier .

190
fOOOkm

II

Approximate Scale

\.0

Fiqure 6-B. Strategic access to digital European backbone via DSCS.
G

N

Voice

Terminal

Figure 6-9. Terminal access via the AADSS.

LOS

Microwave

Links
information between these resources and with the content of the

processor languages emitted and recognized by these resources.

This user-oriented control function is the level 5 protocol

discussed previously. It depends on all the previous levels 1

through 4 for proper operation. Thus the terminal operators at

each end of the link view the network from level 5 and the network elements perform the other functions leading to this level.

In Figure 6-9 the level 4 functions may consist of several

separate protocols for particular applications such as file

transfers and access to virtual terminals, e.g., CRT terminals,

printers, and other displays. These protocols may reside in the

DTE itself. Level 3 protocols for communicating over an arbitrary

network reside at the DCa or more likely in the AADSS level 2

protocols reside at every node so that control is maintained

between each node. Level 1 is the lowest level required to

establish, maintain and disengage communications between the

network resources. The purpose of the furiction and application

for these five levels are summarized in Table 6-4.

7. RECOMMENDED STUDY AREAS

This report has shown that the signaling system affects the

operation and the performance of the network. Thus, the ulti,mate

choice of a signaling system requires the resolution of issues

which involve the entire network and all of its elements. Basic

issues concerning the network characteristics and the switching

node capabilities must be resolved before the final signaling

system is selected. These basic issues are listed in Table 7~1.

A regional AADSS is concerned with all of these issues, but

rather selectively in different areas.

7.1. The Impact of the Signaling System on Processor

Capabilities

This report has reviewed how user and cant,rol information

may proceed through several nodes of a network, and how it may

193
Table 6-4. Control Functions or Protocol Levels Involved Between Terminals

in Figure 6-9

r-:

\..CJ

...

Level

1. Interface

Control

2. Link Control

3. Communications Control

4. System

Control

Function of

Protocol

Establish, maintain

and disconnect

physical link

Ensure reliable data

transfer over single

link

Ensure reliable

transfer via

multiple links and

node s , i. e . ,

Network Transport

Defines system

resources to be

virtually interconnected

Purpose

Mechanical, electrical

and functional interchange between DTE and

DCE

Provides control

between two physical

nodes

Formatting, routing,

network management

Formatting data information fields and

characterizing information transfer

Application Area

Examples

Between terminal to

line circuit on base

Between DCa and

AADSS

Between AADSS's

Remote terminal

control to

peripheral devices

5. User Control I Actual inforTIia tion

exchange betv.leen

resources

User level coordination of operations

tTsers of remote

terminal and interaction between network resources
f--J

\.0

lJ1

Network

o Topology

o Routing Strategy

o Numbering Plan

o Centralized vs.

Distributed Control

o Interworking

o Link Capacities

o Error Control

o Security

Table 7-1. AADSS Issue Summary

Switching

o Voice/Data Integration

o Hub/Concentration Point

Locations

o Hardware and Software Roles

o Features and Services

o A/D Conversion Points

o Traffic Requirements

o Blocking Requirements

o Delays Requirements

Signaling

o Type of System

o Operating Mode

o Transmission

o Security

o Interfacing
be transferred between networks using different control procedures. It is assumed that the nodes of future military networks

will be automatically operated by processors with stored program

control. The interprocessor communications may take different

forms, such as eClS, CClTT System No.6, CCITT System No.7,

HDLC, ADCCP, etc. Each form affects the traffic management

capability of the processor in a different way. A study is

recommended to evaluate these processor capabilities. This study

should establish the speed and memory requirements of the control

processor as a function of traffic statistics, signaling system,

scheduling and routing strategies and, to the extent possible,

the service features furnished to the users. These factors are

clearly interrelated. They also depend on other system issues,

such as the degree of centralization or decentralization permitted

for the network control. Service definitions, service requirements, and performance objectives also playa role. The degree

of integration of various traffic types and nodal capacities for

such traffic substreams, are likely to be critical parameters.

7.2 Network Management

Long-term status monitoring, failure detection, redressing,

and maintenance functions, as well as other aspects of network

management, require further study. Different control system

configurations appear suited for performing different network

management functions. For example, Rosner (1978) has indicated

that the maximum traffic throughput is achieved using decentralized control, whereas the assurance of network integrity tends

toward a more centralized control implementation. This and other

aspects, such as active-versus passive management control schemes,

must be resolved before an optimum network control system can be

defined, designed, and finally implemented.

7.3. Routing Strategy

The number of network control steps required for routing

random traffic through networks with generally unknown topologies

196
is another pertinent study area. Some estimates developed

earlier by Nesenbergs and Linfield (1976) showed that the number

of control steps is proportional to the IO'garithm of the number

of possible routes through a network. This number of possible

routes is reduced as the network structure is progressively

betterdefin~ed. Table 7-2 shows the number of routing contro.l

st'eps, Or binary ·decisions, required for several network

topolo'gies as a functi'onof the number of nodes, n. These

relative task proportions provide a basis for estimating

processor requirements,such as amount of logic cireuit·ry, the

magnitude of software, and the storage capacity required to

store, forward, and to route calls.

Table 7-2. Number of Routing Control Steps for n-node Networks

Configuration

Arbitrary Network

~rbitrary Tree

Arbitrary Ring

Simplest Star or Ring

constrained Tree

constrained Rihg

Practical Tree

Practical Ring

Proportionality Factor

2

n

nlog 2 h

n

log2n

n to nlog2h

lo'g 2n to 11

.....,n

7.4. MUltiple Access Protocols for Integrated Satellite Networks

The multiple-access protocols for packet radib and satellite

broadcast systems are still in the experimental evaluation stage~

One advanced protocol (CPODA) is being tested on ARPANET satellite IMP's. The system utilizes the slotted envelope network

(SENET) concept in conjunction with a reservation ALOHA type of

protocol. The CPODA protocol was discussed briefly in Section 5~2.

Although the general approach appears to be sound and cost

effective, the detail specifications for this protocol have hot

197
been finalized. Further studies are expected to result in a

number of modifications to the protocol.

'7.5. Network P~rformanceStandardizatioh

In AADSS, just as in other tactical, non-tacticalj and DeS

backbone networks, the traffic handling capabiLity has the

ultimate effect on usefulness and cost-effectiveness of military

systems. Straightforward traffic increase and forced overloading

of facilities results in det~rioration of networkperforrnance.

While qualitativ~ly this appears simple enough, the causes and

effects are by no means clear quantitatively.

It appears important t6 agree first what parameters should

be used for AADSS rtetworkperformance. The previously mentioned

delays, blocking probabilities, throughput efficienciesj and

perhaps other characteristics, may be good initial candidates for

network performance description.

At the second level, network performance standardization can

only be completed by assigning numerical values to the above

parameters. Thus, for instance, the standard blocking probability for circuit switched telephone lines could be specified

as 1% at the end offices. For trunks, depending on various

facto~s, one could specify a lower standard blocking probability,

such as perhaps O~2%. For message and packet switching, standard

delay numbers may be quite important. End-to-end delays impact

on the quality of digitized packet voice~ Lihk and node delays

relate to buffer size, processor speed, and channel speed

requirements.

7.6. Traffic Capabilities

The basic requirement for AADSS is to carry telecommunications traffic. Three technical descriptors are needed to specify

that traffic:

(a) The total traffic volUme (in bls or Hz) through given

gateways.

198
(b) Individual traffic substreams, generated by different

user scenarios, their service modes, speeds and access

statistics.

(c) The service requirements of the constituent users.

The network performance standards, outlined earlier under

Section 7.5, must be sufficient to provide for the demands of

(b) and (c). And in the interests of economic viability, it must

do that at the highest overall traffic volume (a).

It appears, therefore, that AADSS intranetwork and internetwork telecommunications should be traffic engineered to carry

the total flow in the most efficient, cost effective, mannerwhile satisfying the DoD network performance requirements.

7.7. Computer Simulation

Computer simulation is an ideal tool for resolving some of

the problems associated with the studies recommended above.

Tests have already been conducted at ITS using a PDP-ll/40

computer and the ARPANET to measure user performance parameters

(see Payne, 1978). This facility could also be used to simulate

certain AADSS functions on the digital computer and to evaluate

signaling protocols on a packet-switched network including, for

example, routing strategies, performance standardization and

traffic capabilities.

199
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pp. 22c-l to 22c-4.

Rosner, R.D. (1978), Control strategies for computer networks,

National Radio Science Meeting, Sponsored by u.s. National

Committee of URSI, Boulder, CO, January.

Ross, M.J., and E.A. Harrington (1977), Unique characteristics

of military switching networks, Proc. of National Electronics

Conference, Chicago, IL, ~, pp. 212-217.

Ross, M.J., A.C. Tabbot, and J.A. Waite (1977), Design approaches

and performance criteria for integrated voice/data

switching, Proc. IEEE, ~, pp. 1283-1295.

Schutzer, D.M., and F.J. Ricci (1976), Design and implementation

of a PAC-switched network, trends and applications 1976:

Computer networks, Gaithersburg, MD, pp. 182-186.

Schwartz, M. (1977), Computer Communications Network Design and

Analysis (Prent i0e-Hall, Inc., Englewood Cliffs, NJ),

ch. 14.

Seitz, N.B., and P.M. McManamon (1978), Digital communication

performance parameters for proposed federal standard 1033,

Vol. I: Standard parameters, NTIA Report 78-4, NTIS,

5285 Port Royal Road, Springfield, VA 22151, Accession

No. PB 283-580/AS.

Sevcik, P.J. (1977), Autodin II subscriber access protocols and

interfaces, NTC Record, Los Angeles, CA, ~, December,

pp. 37:6-1 to 37:6-6.

Shah, A.N. (1977), Impact of military requirements on packet

switching network design, Proceedings of National Electronics

Conference, Chicago, IL, ~, pp. 218-221.

Shearin, E.A., and L.L. King (1977), Microprocessor implementation

of CCITT Recommendation X.25 (levels 1 and 2) protocol,

Proc. of Computer Network Symposium, NBS, Gaithersburg, MD,

pp. 125-130.

Wagner, L.H. (1977), Access area digital switching system,

Proceedings of Second Annual Seminar, Armed Forces Communications and Electronic Association, Ft. Monmouth, NJ, pp. 39-57.

20G
Waxman, R., J. Wold, and W. Gluckoski (1975), Integrated

switching/multiplexing/technical control, Final Technical

Report No. RADC-TR-75-R, RADC Contract F30602-73-C-0335,

Computer Science Corp., Paramus, NJ.

Wecker, S. (1976), The design of DECNET - A general purpose network base, IEEE Electro '76, Boston, MA.

Weissberger, A.J. (1978), Data-link control chips: bringing order

to data protocols, Electronics, ~, No. 12, June, pp. 104-

112.

Whiting, J.S. (1975), An efficient software method for implementing

polynomial error detection codes, Computer Design, March,

pp. 73-77.

20,

APPENDIX. SIGNAL UNIT FORMATS AND CODES FOR CCITT

SIGNALING SYSTEM NO. 6

This appendix is reproduced from Recommendation Q.257 of

the CCITT specification of Signaling System No.6, Volume VI.2,

CCITT, Sixth Plenary Assembly (Orange Book), ITU, Geneva,

Switzerland, 1977.

209
SECTION 3

SIGNAL UNIT FORMATS AND CODES

Recommendation Q.257

3.1 GENERAL

3. I. I Types ofmessage and signal unit (SU)

Signalling and other information carried by the common signalling link is transferred by means of

messages consisting of one or more signal units.

A signal unit (SU) is the smallest defined group of bits on the signalling channel and contains 28 bits.

Dependent upon the number of signal units necessary to transmit one message, the message is called a -

one-unit message or a multi-unit message.

3.1.1.1 One-unit message, lone signal unit (LSU)

A one-unit message is a message which is transmitted entirely within one signal unit. Such a signal unit is

called a lone signal unit (LSU). It is designed to transmit either:

a) a single telephone signal,

b) a signalling-system-control signal, or

c) a management signal.

3.1.1.2 Multi-unit message (MUM)

A multi-unit message (MUM) consists of 2, 3, 4, 5 or 6 signal units in tandem. It is designed to transmit a

number of related signals (e.g. address signals) in an efficient way. A special case of the mulit-unit messages is

the initial address message, which is the only one which can have six signal units in tandem and has a minimum

ofthree signal units.

3. I. 1.3 Initialsignal unit (ISU)

The flTst signal unit of a multi-unit message is called the initial signal unit (ISU).

3.1.1.4 Subsequentsignal unit (SSU)

The second and any following signal unit of a multi-unit message are called subsequent signal units

(88U).

210 VOLUME VI.2 - Rec. Q.257
32 GENERAL

3.1.2 Basicformats

3.1.2.1 Basicformat ofa lone signal unit

The basic format of a lone signal unit is shown in Figure 5/Q.257.

XXXXX/XXXX/XXXXXXX/X~XX/XXXXXXXX

1-5 6-9 10-16 17-20 21-28

Heading Signal

information

Band

number

Label

Circuit

number

Chc~:k

FIGlJRI·: 5/0.257 -- Basic format of: - a lone signal unit

- an initial signal unit of a multi-unit message

The basic format of a lone signal unit is not used in all cases. Where a different format is used it is shown

in the sections relating to individual signal units.

3.1.2.2 Basicformat ofa multi-unit message

The format of the initial signal unit of a multi-unit message is shown in Figure 5/Q.257. The use of a

special code in the signal information field (bits 6-9) distinguishes an initial signal unit from a lone signal unit. See

3.1.2.1 above.

The format of a subsequent signal unit of a multi-unit message is shown in Figure 6/Q.257.

OO/XX/XXXXXXXXXXXXXXXX/XXXXXXXX

1-2 34 5-20 21-28

** Signal information Check

* Subsequent signal unit heading code

** Length indicator

FIGlTRF 6/Q.257 - Format of a subsequent signal unit of a multi-unit message

For some messages.. the signal information field of a subsequent signal unit (bits 5-20) can be

sub-divided.. notably in address messages where the field is divided into four 4-bit parts.

VOLUME VI.2 - Rec. Q.257 211
GENERAL

3.1.3 Codesfor the general parts ofsignal units

The interpretation of a message depends upon a system of codes in various parts of the message.

33

3.1.3.1 Heading

The heading is used to identify the type of:

a) group ofsignals"

b) message., or

c) signal.

The heading generally consists of the first five bits of the signal units (bits 1-5). There are two exceptions

to this rule" viz.:

all subsequent signal units are identified by the same 2-bit heading code 0 0 (bits 1- 2)~

the acknowledgement signal unit is identified by a 3-bit heading code 0 I I (bits 1-3).

The heading codes are allocated as follows:

00

01000

01001

01010

010 I I

oI I

10000

10001

10010

100 I I

10100

10101

101 1 0

101 11

11000

11001

11010

11 0 1 1

11100

I 1 1 0 1

11 1 10

1 1 1 1 1

Subsequent signal unit

Spare (reserved for regional and/or national use)

Acknowledgement signal unit

Initial signal unit of an initial address message

(or of a multi-unit message)

Subsequent address message (one-unit message

or multi-unit message)

International telephone signals

Spare (reserved for regional and/or national use)

Signalling-system-control signals (except

acknowledgement signal unit) and· management signals

Spare (reserved for regional and/or national use)

The heading code allocation is also shown in Table I at the end of this Section.

VOLUME VI.2 - Rec. Q.257

212
34 GENERAL

3.1.3.2 Signal iriformation

Signal units with a 5-bit heading code have a signal information field of four bits (bits 6-9). The signal

information field is used:

a) to define a particular signal within a group ofsignals being defined by the heading code.

b) to define a sub-group within a group of signals~ or

c) to indicate that the signal unit is an initial signal unit and that the subsequent signal unit(s) contain(s)

a number ofsignals belonging to the group of signals defined by the heading code.

For case c).. the signal information code 000 0 is used except \\'ith heading code I 000 0 \vhich alpnc is

sufficient to identify the signal unit as an initial signal unit.

The allocation of signal information codes is shown in Table I at the end of this Section.

3.1.3.3 Label

Messages which relate to a speech circuit (ot a group or sub-group of speech circuits) nlust carry a lahd

to identify that circuit (or group of circuits). Only one label per message is used.

To identify a group of up to sixteen speech circuits.. a 7-bit band number is used (bits 10-16).

To identify a circuit within a group of up to sixteen speech circuits. an additional 4-bit code (circuit

number) is used (bits 17-20). See Figure 5IQ.257.

This provides a total of II bits which can be used to identify 2048 speech circuits.

Label codes will be assigned by the Administration concerned.

The label field position is in bits 10-20 of either a lone signal unit or an initial signal unit of a multi-unit

message. Subsequent signal units of multi-unit messages do not require a label. Where a 7-bit band number alone

is sufficient to identify the destination of a signal (e.g. some management signals)~ bits] 7-20 can contain some

further signalling information.

3.1.3.4 Length indicator

Subsequent signal units have a length indicator field of two bits (bits 3-4) to indicate the number of

subsequent signal units contained in a: multi-unit message. Each subsequent signal unit of a multi-unit message

carries the same length indicator. The codes used are sho\\'n below.

Length indkator

Number of

subsequent signal units

Initial address Other

message multi-unit messages

I - 00

2 01 OJ

3 10 10

4 II II

5 00 -

VOLUME VI.2 - Rec. Q.257

213
GENERAL 35

The length indicator 0 0 has a different, but unambiguous meaning in the initial address message because

the initial address message has a minimum requirement of two subsequent signal units.

3.1.3.5 Check

Every signal unit has a check field of eight bits (bits 21-28) for error detection purposes (see

Recommendation Q.277).

Recommendation Q.258

3.2 TELEPHONE SIGNALS

3.2.1 Initial address message (IAM)

The initial address message (lAM} is the first message of a call. It is a special case of the multi-unit

message as it consists of a minimum of three signal units and a maximum of six signal units. It can contain

different types of information - address signals (including ST), other routing information~ and the filler

code .- under the same heading code.

3.2.1.1 Format ofthe initial address message

,The format of the initial signal unit is shown in Figure 5/Q.25 7.

The format of the subsequent signal units is shown in Figure 6/Q.257 except for the subsequent signal

units numbers 2-5 in which the signal information field (bits 5-20) is sub-divided into four 4-bit parts so that four

address signals can be carried in each of these subsequent signal units.

The subsequent signal units of an initial address message do not require the 5-bit heading or II-bit lahel

as this in(ormation is already contained in the initial signal unit.

The number of address signals available for transmission determines the length of the initial address

message.

3.2.1.2 Codes used in the initial address message

a) Initialsignal unit

The 5-bit heading code 1 0 000 is used.

The signal information code 0 0 0 0 is used.

The assigned label code is used.

b) Subsequent signal unit (number 1)

The heading code 0 0 is used.

The length indicator is coded as appropriate (see Recommendation Q.25 7 3.1.3.4).

Bit 5: country code indicator:

o country code not included

I country code included

Bit 6: natureof circuit indicator:

o no satellite ~ircuit in the connection

I one satellite circuit in the connection

Bit 7: echo-suppressor indicator:

o outgoing half-echo suppressor not included

I outgoing half-echo suppressor included

Bit 8: spare (reserved for international use) I)

.t These bits are coded as 0 at present.

VOLUME VI.2 - Rec. Q.258

214
spare

operator, language French

operator" language English

operator, language German

operator, language Russian

operator" language Spanish

36 TELEPHONE SIGNALS

I)

Bits 9-12: spare (reserved for regional and/or national use)

Bits 13-16: calling-party'Is-category indicator

0000

0'001

0010

001 I

0100

010 I

oI 101 available to Administration

oI I I for selecting a particular language provided by mutual agreement

1000

I 0 0 I reserved (see Recommendation Q.I 04)

I 0 I 0 ordinary calling subscriber

I 0 I I calling subscriber with priority

I I 0 0 data call

I I 0 I test call

I I I 0 spare

I I I I spare (reserved for regional and/or national use)

Bits 17-20: spare (reserved for regional and/or national ~se) ))

c) Subsequentsignal units (numbers 2-5) - telephone call

The heading code 0 0 is used.

The length indicator is coded as appropriate (see Recommendation Q.25 7" 3.1.3.4).

The four 4-bit parts of the signal information field contain address signals in seq uence" bits 5-8. bits

9-12" etc." and coded as follows:

0000 filler (no information)

000 I digit I

00 I 0 digit 2

00 I I digit 3

o100 digit 4

o I 0 I digit 5

oI I 0 digit 6

o I I I digit 7

I 000 digit 8

100 I digit 9

I 0 I 0 digit 0

I 0 I I code II

I 100 code 12

I I 0 I spare

I I I 0 spare

I I I I ST

. ~he filler .c~~e 0 0 0 0 is used where needed to complete the signal information field of the last subsequent

signal unIt of the Initial address message. .

VOLUME VI.2 - Ree. Q.2S8

215
TELEPHONE SIGNALS 37

d)Subsequentsi,gnal ,U"nit(num,ber 2) -tes.tcall

The heading rode ...is used..

The length indicator is coded as appropriate (see Recommendation Q.251 3. 1.1.4).

ATME 2 -signalling che'ck and transmission test

ATME 2 -sign,aUingchecK only

\

. 6 .... t.. k \ s:ystem No.. :. con,llfiuitycuec..

sp:are

spare

spare

spare

'spare

sp.are

:spare

spare

spare

spar:e

spare

sp,are

'spare

The fiBt 4-bit part (bits 5-'8) of the signal information field contains an address signal ,coded as

follows:

.,eo

..,.

8810

•• 11

8180

.101

8110

8111

to:8.

t.,.

.8.1..

••tl

III'

.110 I

III.

111 1

The codes used to complete the .signal information fteld of the .subsequent 'signal unit (number 2) test cail

are the end-of-'putsing (ST) and fillers.

l.2" 1.3 Exampleofan initialaddressmesS4ge

An example of a three-unit initial address ,message is shown in Figure 7/Q..258.

ISU 1 () 0 () ,0 1 () 00 0 1 X X X X X xxxxx X /x x x xxx X X

Heading Signal Label Check

code infQI'·

matron

code

l:stSSU 40/01/XXXOOOOOXXXXOOOO/XXXXXXXX

** Otber routing information

2n4,SSU OOjOl/X.XXX XXXX X.XXX XXXX/XXXXXXXX

*** 1st 2nd Jed 4th Check

Address:signals

·Su~uent 'Signal u'nit heading code

·-Length indicator

FIGURE 7/Q~258 - ExampJeof a thre~~unjt initial address message

VOLUME VI.2 - Rec.Q.2S'8

216
38

3.2.2 Subsequent address message (SAM)

TELEPHONE SIGNALS

A subsequent address message (SAM) is used to transmit additional address signals not available when

the initial address message is formed.

A subsequent address message may be either a one-unit message or a multi-unit message.

3.2.2.1 Formats ofsubsequent address messages

a) Lone signal unit

The format of the lone signal unit is shown in Figure 5/Q.25 7.

b) Multi-unit message

The format ofthe initial signal unit is shown in Figure 5/Q.257.

The format of the subsequent signal units is shown in Figure 6/Q.257. In this case. however. the signal

information fields of every subsequent signal unit are sub-divided into four 4-bit parts.

3.2.2.2 Codes used in subsequent address messages

a) Heading

Heading codes in the range I 0 0 0 -I - 1 0 1 I I are used in the lone signal unit or initial signal unit

depending on the sequence number of the subsequent address message concerned. The first subsequent address

message of a call uses heading 1 000 I, the second I 0 0 I 0, the third 1 0 0 I t etc. While it is preferred to limit

the number of subsequent address messages, if more than seven are sent, the sequence is recycled so that the

eighth uses heading code I 000 I.

Subsequent signal units ofsubsequent address messages use the heading code 0 o.

b) Signal information

L one signal unit

In the case of a one-unit subsequent address message~ the signal information field (bits 6-9) contains one

ofthe address signals which are coded as follows:

0001 digit

0010 digit 2

001 I digit 3

0100 digit 4

010 I digit 5

oI I 0 digit 6

oI I I digit 7

1000 digit 8

1001 digit 9

101 0 digit 0

I I I 1 ST

Codes I 0 I I, I I 00, I I 0 I, I I I 0 and 000 0 are not used in the signal information field of a

one-unit subsequent address message.

Multi-unit message

The signal information field of the initial signal unit is coded as 0 0 0 O.

VOLUME VI.2 - Ree. Q.258

217
TELEPHONE SIGNALS 39

The signal information field of the subsequent signal units contains the address signals which are coded

as follows:

0000

0001

0010

001 I

0100

010 1-

o I I 0

oI II

1000

100 I

101 0

I II I

filler (no information)

digit

digit 2

digit 3

digit 4

digit 5

digit 6

digit 7

digit 8

digit 9

digit 0

ST

Signal information codes I 0 1 I, 1 1 00, . 1 1 Oland 1 I I 0 are not used in multi-unit subsequent

address messages.

The filler code 0 0 0 0 is used, where needed, to complete the signal information field of the last

subsequent signal unit of the 'subsequent address message.

c) Label

The assigned label code is used.

3.2.3 Other telephone signals

3.2.3.1 Telephone signals with heading code 1 0 0 0 O.

The following signal information codes, in conjunction with heading code I 0000, arc allocated:

000 0 initial signal unit of an initial address message (see Recommendation Q.258, 3.2.1.2)

o0 0 I spare (reserved for international use)

001 0 spare

00 I I spare

o100 spare

o I 0 I spare

o I I 0 spare

oI I I spare

1 0 0 0 spare (reserved for regional and/or national use)

100 I spare

1 0 I 0 spare

I 0 II spare

I 100 spare

II 0 I spare

I 1 I 0 spare

I I I I spare

The formats for messages using signal information code 0 0 0 I have not yet been decided. The formats

for messag~s using signal information codes in the range 0 0 I 0 -11 I I will be determined by regional

organizations and/or national Administrations.

VOLUME VI.2 -- Ret. Q.258

218
40 TELEPHONE SIGNALS

3.2.3.2 Telephonesignals with heading code I 1 0 0 0

The format of one-unit telephone signals using heading code I I 0 00 is shown in Figure 5/Q.257.

Signals~ sent in the backward direction, in lone signal units using heading code I 1 000" are allocated

signal inforl11ation codes as follows:

o0 0 I release-guard

00 I 0 answer~ charge (priority)

o0 I I answer, no charge (priority)

01 0 o clear-back No. I

010 I reanswer No. I

o I 10 clear-back No.2

01 I I reanswer No.2

I 0 0 0 clear-back No.3

10 0 I reanswer No.3

101 0 spare

I 0 I I spare

I 100 spare

1 1 0 I spare

_ 1 I 0 spare

• I • I· spare

Signal information code 0 0 0 0 indicates that the signal unit is the initial signal unit ofamulti-unit

message. This facility is reserved for possible future expansion.

3.2.3.3 Telephone signals with heading code 1 1 0 0 1

The format of one-unit telephone signals using heading code I I 0 0 • is shown in Figure 5IQ.257.

Signals, sent in the backward direction, in lone signal units using heading code I I 8 0 I.. are allocated

signal information codes as·follows:

000 I spare

0010 spare

001 1 switching-equipment-congestion

01 0 0 circuit-group-eongestion

o I 0 I national-network-congestion

o• I 0 spare

oI • I s,pare

I 800 call-failure

I 001 spare

18 I 0 spare

I 01 I spare

•• 00 spare

II 0 I spare

I I I o confusion

I II I spare

Sign~1 in~o~m~tion code 0000.indicates that the signal unit is the initial signal unit of. a multi-unit

message. ThiS facility IS reserved for possible future expansion.

VOLUME VI.2 - Rec. Q.258

219
TELEPHONE SIGNALS 41

3.2.3.4 Telephone signals with heading code I I 0 I 0

The format of one-unit telephone signals using heading code I I 0 lOis shown in Figure 5/Q.~57.

Signals" in lone signal units using heading code I I 0 I 0, are alloc~ted signal information. codes as

follows:

o0 0 I continuity

o0 I 0 clear-forward sent in the forward direction

o0 I I forward-transfer

o100 spare

o I 0 I spare

oI I 0 spare

oI I I spare

100 0 spare

100 I spare

I 0 I 0 spare

I 0 I I blocking

I I 0 0 unblocking

I I 0 I blocking-acknowledgement sent in either direction

I I I 0 unblocking-acknowledgement

I I I I message- refusal

Signal information code 0 0 0 0 indicates that the signal unit is the initial signal unit of a multi-unit

message. This facility is reserved for possible future expansion.

3.2.3.5 Telephone signals with heading code I I 0 I I

The format of one-unit telephone signals using heading code I I 0 I I is shown in Figure 5/Q.257.

Signals, sent in the backward direction, in lone signal units using heading code ,I 1 0 1 I, are allocated

signal information codes as follows: .

o0 0 I address-complete, subscriber-free, charge

o0 I 0 address-complete, subscriber-free, no charge

00 I I address-complete, subscriber-free, coin-box

oI 0 0 subscriber-busy (electrical)

oI 0 I vacant-national number

oI I 0 line-out-of-service

oI I I subscriber-transferred (changed number)

100 0 spare

100 I spare

I 0 I 0 address-complete, charge

I 0 I I address-complete, no charge

I I 0 0 address-complete, coin-box

I I 0 I address-incomplete

I I I 0 spare

I I I I spare

Signal information code 000 0 indicates that the signal unit is the initial signal unit of a multi-unit

message. This facility is reserved for possible future expansion.

VOLUME VI.2 - Rec. Q.2S8

220
42 TELEPHONE SIGNALS

3.2.3.6 Reserved heading codes

The signal information codes under the heading codes 0 • 0 0 0.. 0 I 0 0 t 0 I 0 • 0.. 0 I 0 I I.. I I I 0 0"

.0 ••. 0 and ••••• are reserved for regional and/or national use.

Signal information code 000 0 indicates that the signal unit is the initial signal unit of a multi-unit

message. This facility is reserved for possible future expansion.

3.2.4 Exaomples ofaddress messages

Examples of address messages are given below to elucidate the formats and codes adopted for address

messages. As there is no telephone signal information contained in the check fields of the signal units.. °these fields

are not shown in the examples.

3.2.4.1 Transit call from USA (international exchange New York) to the Netherlands (international

exchange Amsterdam) via the United Kingdom (transit exchange London).

Assumptions: - Semi-automatic traffic.. English language.

- The signalling links New York-London and London-Amsterdam are both associated

with their respective speech circuit groups.

- Speech path New York-London is a satellite circuit equipped with echo suppressors..

speech path London-Amsterdam is a cable circuit not equipped with echo suppressors

(due to bilateral agreement between the Administrations concerned).

Dialled information: 31 215043551.

- En bloc operation.

a) Address message New York-London

10000/0000/ 000 0101/0011

00/11/1110 OO~O 0010 0000

00/11/0011/0001/0010/0001

00/11/0101/1010/0100/0011

00/11/0101/0101/0001/1111

b) Address message London-Amsterdam

10000/0000/ 000 0000/1010

00/11/0100 0000 0010 0000

00/11/0010/0001/0101/1010

00/11/0100/0011/0101/0101

00/11/0001/1111/0000/0000

The intermediate CT London serves as a transit exchange.

3.2.4.2 Direct call from the Netherlands (international exchange Amsterdam) to USA (international

exchange New York).

Assumptions: Automatic traffic, ordinary subscriber.

Speech path Amsterdam-New York is a cable circuit equipped with echo suppressors.

Speech circuit group Amsterdam-New York has no associated signalling link. Signal

information will be transferred via the two signalling links Amsterdam-London and

London-New York in tandem, thus using a quasi-associated mode of operation.

Dialled information: 1 20 I 949 5813.

- Overlap with subscribers' dialling operation.

VOLUME VI.2 - Rec. Q.258

221
TELEPHONE SIGNALS

a) Address messages Amsterdam-London

43

I nitial address message

•••••'.00./ 0018000/1001

'8/10/0010 0000 1010 0000

O"IO/Olle/lIIO/OOOI'IO.1

.0/.0/0.0./1011/'.00/0000

I•• 0 I' 0 I • II • 0 I 0 • 0 0 II 0 0 I First subsequent address message

I • 0 I 0/1 0 0 0/ 0 0 I 0 0 0 0II 0 0 I Second subsequent address m,essage

I • 0 I 1/ 0 0 0 II 8 0 I 0 0 0 0 II 0 0 I Third subsequent address message

10100/0011/ .81 0000/1001 Fourth subsequent address message

I 0 I 0 III I I II 0 0 I 0 0 0 0/1 0 0 I- Fifth subsequent address message

• ST-signal. sent ifthe end ofthe address has been recognized.

b) Address messages London-New York

Exactly the same messages are sent as under a).

The London exchange serves as signal transfer point only. It is assumed that by agreement between the

Administrations concerned there is no need for a change of label at this signal transfer point.

Recommendation Q.259

3.3 SIGNALLING-SYSTEM-CONTROL SIGNALS

3.3.1 (;ener~

The signalling-system-control signals are not related to telephone signal information. They are necessary

rorthe proper functioning ofthe signalling system.

All signaJling-system-control signals specified (see Recommendation Q.255) are transferred by means of

lone sign,aI units:

- acknowledgement signal unit,

- synchronization signal unit, and

- system-control signal unit.

3.3.2 ..4 cknowledgementsignal unit(A CU)

The function ofthe acknowledgement signal unit (ACU) is described in Recommendation Q.25 1.

3.3.2.1 Formato/the ACU

The format ofthe ACU is given in Figure 8/Q.259.

Oll/XXXXXXXXXXX/XXX/XXX/XXXXXXXX

1·3 4-14 15-17 18-20 21-28

Heading

code

Acknowledgement

indicators

• •• Check

• Sequence Dumber of block. being acknowledged

•• Sequence number of block completed by this ACU

fIGURI:: 81Q.2S9 - Format of the acknowledaement silnal unit

VOLUME VI.2 - Rec. Q.259

222
44 SIGNALLING-SYSTEM-CONTROL SIGNALS

3.3.2.2 Codesfor the ACUparts

a) Heading

The heading code 0 I 1 is used.

b) A cknowledgement indicators

The ACU contains 11 acknowledgement indicators to acknowledge sequentially the corresponding

eleven signal units of a block received. That is, bit 4 refers to the first signal unit in the block being acknowledged..

bit 5 refers to the second" etc. Each indicator will be coded in the following way:

o no error detected,

I error detected

The error detected condition includes signals rejected by the terminal as covered in

Recommendations Q.277, Q.278 and Q.293, 8.6.1.

c) Block sequence numbers

Both the block being acknowledged and the block completed by the ACU are indicated by cyclic

sequence numbers from the series 0 0 0 , 00 1 , 0 1 0, 0 1 1 , I 0 0, I 0 I , 1 I 0, I I I, 0 0 0 ...

3.3.3 Synchronization signal unit (SYU)

The function ofthe synchronization signal unit (SYU) is described in Recommendation Q.251.

3.3.3.1 FormatoftheSYU

The format of the SYU is given in Figure 9/Q.259.

11101/1101/110001

1·16

Synchronization pattern

* Scqu~ncc number of signal unit in the block

/XXXX/XXXXXXXX

17-20 21-28

Check

FIGURE 9/0.259 Format of the synchronization signal unit

3.3.3.2 Codesfor theSYUparts

a) S)'nchronizationpattern

This pattern is coded as: 1 1 1 OJ 1 1 0 I 1 I 0 0 0 I 1.

The first nine bits of the synchronization pattern may be considered to contain the heading and signal

information fields which are coded I 1 1Oland 1 I 0 I respectively.

The heading eode 1 1 10 1 is used for signalling-system-control signals (except ACU) as well as for

management signals. The spare signal information codes can be allocated either to system-control signals or to

management signals. -

b) Signal unit sequence number

The sequence number may have any code ofthe 4-bit binary code 0 0 0 0,0 00 J , 0 0 10 up to J 0 I 0

inclusive. The number chosen for a synchronization signal unit is determined by the position of that

synchronization signal unit in the block ofsignal units.

The remaining codes J 0 I I to I I I 1 are not assigned.

VOLUME VI.2 - Ree. Q.2S9

223
SIGNALLING-SYSTEM-CONTROL SIGNALS

3.3.4 System-controlsignal units (SCU)

The function of the system control signal units is described in Recommendation Q.255.

3.3.4.1 FormatofanSCU

The format of an SCU is given in Figure 10/Q.259.

45

I I I 0 I

1-5

IIOO/XXX/XXXX/XXXX/XXXXXXXX

6-9 10-12 13-16 17-20 21-28

Heading

code

Signal

information

code

Control information Check

FIGURE 10/Q.259 ._- Fonnat of a system-control signal unit

3.3.4.2 Codesfor the SCU parts

a) Heading

The heading code I I I 0 I is used.

The heading code I I I 0 I is used for signalling-system-control signals (except ACU) as well as

management signals. The spare signal information codes can be allocated either to system-control signals or to

management signals.

b) Signal itiformation

The signal information code I I 0 0 is used.

c) Control itiformation

bits 10-12 are coded as 0 0 I. The other codes are spare.

bits 13-16 are coded as 0 0 0 I. The other codes are spare.

bits 17-20 system-control signals,defined in Recommendation Q.255, are coded as follows:

0000 spare

o0 0 I changeover

o0 1 0 manual-changeover

001 I spare

oI 0 0 standby-ready

oI 0 I spare

oI I 0 load-transfer

o I I I emergency-load-transfer

I 000 spare

100 I spare

I 0 I 0 manual-changeover-acknowledgement

I 0 I I spare

I I 0 0 standby-ready-acknowledgement

I I 0 I spare

I I I 0 load-transfer-acknowledgement

II I I spare

VOLUME VI.2 - Rec. Q.259

224
46 SIGNALLING-SYSTEM-CONTROL SIGNALS

3.3.5 Multi-block-synchronization signal units (MBS)

The function of the multi-block-synchronization signal units is described in Recommendation Q.255.

3.3.5.1 Format ofan MBS

The format of an MBS is given in Figure II/Q.259.

I I I 0 I

1-5

I 0 I I I X X X / X X X X X I X X X / X X X X X ,X X X

6-9 10-12 13-17 18-20 21-28

Heading

code

Signal

informatiQn

code

Control information Check

FIGURE 11/Q.259 - Format of a multi-block-synchronization signal unit

3.3.5.2 Codesfor the multi-block-synchronization signal unit parts

a) Heading

The heading code I I I 0 I is used.

The heading code I I I 0 I is used for signalling system control signals (except ACU) as well as

management signals. See 3.3.4.2.

b) Signal i'lformation

'The signal information code I 0 I I is used.

c) Control information

bits 10-12 are coded as follows:

000 multi-block monitoring signal

] 0 0 multi-block acknowledgement signal

The other codes are spare.

bits 13-17 indicate the sequence number of the multi-block in which the multi-block monitoring

signal is sent by a S-bit binary code from the series 0 0 0 0 0, 0 0 0 0 I, 0 0 0 I 0, ...,

11111,00000.

bits 18-20 indicate the sequence number of the block in which the multi-block monitoring signal is

sent (or placed into the output butTer) [see 3.3.2.2 c) above].

Recommendation Q.260

3.4 MANAGEMENT SIGNALS

3.4.1 General

Management signals may include:

network-management signals,

network-maintenance signals,

signalling-network-management signals,

Le. signals concerned with the management of the signalling network and of the speech circuit network.

These signals may be transferred by means of one-unit messages or multi-unit messages.

VOLUME VI.2 - Rec. Q.260

225
MANAGEMENT SIGNALS

3.4.1.1 Basicformat ofmanagement signals

The basic format of a one~unit management message is shown in Figure 12/Q.260.

IIIOI/XXXX/XXXXXXXXXXX/XXXXXXXX

)..S 6-9 10-20 21-28

47

Heading

code

Signal

information

Management

information

Check

fIGURE J2/0.260 Basie format of a Qne,.unit management message

The management information field, bits 1()"~0, may be subdivided as required. When a band number is

included in.the management signal unit, it is placed in bits 10-16.

The format of multi-unit management signals has not yet been decided.

3.4.1.2 Codesfor management signals

a) Heading

The heading code 1 I I 0 I is used. This code is also assigned to signalling~system-control signals (except

ACU). See Recommendation Q.2S9.

b). Signalt'lformotion

Signal information codes are assigned as follows:

0001

0010

00 II

0100

0101

0110

oI I I

1000

1001

1010

101 I

1100

I 101

J I I 0

I I II

network-management and network-maintenance signal units

spare

spare

spare

signalling-network-managementsignal unit

spare (reserved for regional and/or national use)

spare (reserved for regional and/or national use)

spare

spare

spare

spare

SCU (see Recommendation Q.2S9)

SYU (see Recommendation Q.2S9)

spare (reserved for regional and/or national use)

spare (reserved for regional and/or national use)

VOLUME VI.2 - Ree. Q.260

226
48 MANAGEMENT SIGNALS

Signal information code 0 0 0 0 indicates that the signal unit is the initial signal unit of a multi-unit

message. This facility is reserved for possible future expansion.

The spare international signal information codes may be assigned to either management signals or

signalling-system-control signals.

c) Management i'lformation

The codes used in the management information field are shown later in this Recommendation.

3.4.2 Ne/h'ork-managementsignals

As network-management signals have not yet been defined" no detailed format can be given.. except for

the heading and signal information fields" which are coded as I I I 0 I and 0 0 0 I respectively.

3.4.3 Network-maintenance signals

As network-maintenance signals have not yet been defined" no detailed format can be given.. except for

the heading and signal information fields" which are coded as I I I 0 I and 0 0 0 I respectively.

3.4.4 Signalling-network-managementsignals

3.4.4.1 Format ofa signalling-network-managementsignal

The format of a one-unit signalling-network-management message is-given in Figure 13/Q.260.

IIIOI/OIOI/XXXXXXX/XXXX/XXXXXXXX

1·5 6-9 10-16 17-20 21-28

Heading

code

Signal

information

code

Band

number

Management

information

Check

FIGURE 13/Q.260 - Format of a one-unit signalling-network-management message

3.4.4.2 Codes/or the signalling-network-managemenl signal unil parIS

a) Heading

The heading code I I I 0 I is used.

b) Signali'fformanon

The signal information code 0 I 0 I is used.

c) Bandnumber

The band number (bits 10-16) indicates the group or sub-group of circuits to which the signal refers (see

Recommendation Q.257).

VOLUME VI.2 - Rec. Q.260

227
· MANAGEMENT SIGNALS

d)· Management itiformation

The codes used in the management information field are allocated as follows:

0000 spare

000 I spare

00 I 0 spare

00 I I spare

o100 spare

o I 0 I transfer-prohibited

oI I 0 transfer-allowed

oI I I spare

I 0 0 0 transfer-allowed-acknowledgement

100 I spare

I 0 I 0 spare

I 0 I I spare

I 100 spare

I I 0 I spare

I I 1.0 spare

I I I I spare

49

VOLUME VI.2 - Rec. Q.260

228
<

o

rrn

..<

N

I

."ft

,0

N

:

.1\.)

I\.)

\0

TABLE I - AUoeation of heading and signal information codes

NBits 1·5 OOOOX 0001X 0010X 00llX 01000 01001 01010 01011 011XX 1ססoo 10001 10010 10011 10100 10101 10110 10111 11000 11001 11010 11011 11100 11101 11110 11111 1.5 Bits 6·9 6·9

ISU ISU ISU ISU ISU ISU ISU ISU ISU ISU ISU ISU ISU ISU ISU ISU ISU ISU ISU ISU

ססoo of of of of of of of of of of of of of of of of of of of of ססoo

SSU MUM MUM MUM MUM lAM SAM 1 SAM 2 SAM 3 SAMl4 SAM 5 SAM 6 SAM 7 MUM MUM MUM MUM MUM MUM MUM MUM

ISU NOT LSU LSU LSU LSU of Lone Lone Lone Lone Lone Lone Lone LSU LSU LSU LSU LSU LSU LSU LSU NOT 0000 MUM SAM 1 SAM 2 SAM 3 SAM 4 SAM 5 SAM 6 SAM 7 0000

4~ , ססoo i ססoo

r--- I

0001 1 1 1 1 1 1 1 RLG COT AFC NMM : 0001

j

I

0010 i 2 2 2 2 2 2 2 ANC ClF AFN I 0010

0011 w w w w 3 3 3 3 3 3 3 ANN SEC FaT AFX 0011 en en en en w w w :::) :::) :::) :::) en to--- en en :::) :::) :::)

0100 ...J ...J ...J ...J 4 4 4 4 4 4 4 CB 1 CGC SSB ...J ...J ...J 0100 « « « « w z z z z en « « « >- :::) z z z 0 0 0 0 0 0 0 0101 C t= t= t= t= ...J 5 5 5 5 5 5 5 RA 1 NNC VNN t= SNM t= t= 0101 0

:e «z <z «z z« «z « « « 0 z z z 0110 .. .. .. .. t= 6 6 6 6 6 6 6 CB 2 lOS .. ~5~ .. .. 0110

" " " " « " c: c: c: c: 2,,2 Z c: C)Ct- c: c: 0111 en en en ca ca ca ca 7 7 7 7 7 7 7 RA2 SST ca co co en en en 0111 en en ...J ...J ...J ...J :::) ...J ex: z ...J ...J W W a: « « « « (J " « « « > 0 w :::) Z Z Z z « c: z z z 1000 u: a: 0 0 0 0 0 ca 8 8 8 8 8 8 8 CB 3 CFl 0 0 0 1000 :I: LL. c:; c:; c:; c:; ...J a c:; c:; o' f- w w w w « w - w w a: a: a: a: z 9 9 9 9 9 9 9 RA3 a: a: a: 1001 :::) 0 1001

en

en a: a: a: a: c:; a: a: a: 0 0 0 0 w 0 - 0 0 W LL. LL. LL. LL. tt: 0 0 0 0 0 0 0 LL. LL. LL. 1010 z ADC 1010 0 0 0 0 0 w w w w a: 0 0 0 w w w > > > > 0 > > > 1011 a: a: a: a: LL.

w w w w BlO ADN a: MBS a: a: 1011 en en en en C w w w w w w w w en r--- en en > w w w

1100 a: a: a: a: a: UBl ADX a: SCU a: a: 1100 w

en r--- W

1101

1 1

a: BlA ADI SYU 1101 I !

:

1110

1 COF UBA .. 1110

o.go

ST ST ST ST ST ST ST MRF ai~ 1111 ~.~ I 1111

+ CCITT·54418

Note. - All unassigned codes are reserved for international use. The interpretation of the abbn~viationsfor signals is given in the List of abbreviations on the inside of the flap

of the cover at the end of the Volume.

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FORM OT-29

(3-73)

u.s. DEPARTMENT OF COMMERCE OFFICE OF TELECOMMUNICATIONS

BIBLIOGRAPHIC DATA SHEET

1. PUBLICATION OR REPORT NO.2. Gov1t Accession No.3. Recipient's Accession No.

NTIA-Report-79-13

4. TITLE AND SUBTITLE

CONTROL SIGNALING IN A MILITARY

SWITCHING ENVIRONMENT

7. AUTHOR(S)

R.F. Linfield

8. PERFORMING ORGANIZATION NAME AND ADDRESS

U.S. Dept. of Commerce

National Telecom. & Inform. Administration

Institute for Tel"ecomrnunication Sciences

325 Broadway, Boulder, CO 80303

11. Sponsoring Organization Name and Address

U.S. Army Communications Systems Agency

ATTN: CCM-RD-F

Ft. Monmouth, NJ 07703

14. SUPPLEMENTARY NOTES

5. Publication Date

~nuarY~.....;;I;;;;...;;..9-=-7-=9 ~_---I

6. Performing Organization Code

ITS

9. Project/Task/Work Unit No.

10. Contract/Grant No.

12. Type of Report and Period Covered

13.

15. ABSTRACT (A 200-word or less factual summary of most si~nificant information. If document includes a si~nificant

biblio~raphy ot literature survey, mention it here.)

A number of factors, including network topology, traffic characteristics, switching technology, and transmission media, affect the

choice of control signaling techniques to be used in networks of the

future. In this report these factors are considered from a military

standpoint. It is projected that, over the next two decades, all of

the elements of military networks will undergo a transition from

essentially all-analog to all-digital systems. It is also possible that

a unified integrated network capable of handling digital voice and

data could evolve. During the forthcoming period of trans~tion, as

well as for the ultimate network configuration, the control signaling

system will play a critical role in terms of both performance and

cost. This report examines that role by evaluating several advanced

signaling systems, particularly those using common control links.

Digital systems that have potential application on both the line and

16. Key Words (Alphabetical order, separated by semicolons) trunk sides of the dl.gl.tal SWl.tch

are examined.

17. AVAILABILITY STATEMENT

KJ UNLIMITED.

D FOR OFFICIAL DISTRIBUTION.

_~U.S._GOVEBNMENTPRINTINGOEEICE:t919~678~4191294

18. Securi ty Class (This report)

Unclassified

19. Security Class (This page)

Unclassified

20. Number of pages

250

21. Price: