ICE & TECHNOLOGY:

COMPUTER

NETWORK

IIMTERCONNECTIOIM

PROBLEMS AND

PROSPECTS

NBS Special Publication 500-6

U.S. DEPARTMENT OF COMMERCE

National Bureau of Standards
NATIONAL BUREAU OF STANDARDS

The National Bureau of Standards^ was established by an act of Congress March 3, 1901. The Bureau's overall goal is to strengthen and advance the Nation's science and technology and facilitate their effective application for public benefit. To this end, the Bureau conducts research and provides: (1) a basis for the Nation's physical measurement system, (2) scientific and

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" Located at Boulder, Colorado 80302.
1 1377 <\CL, COMPUTER SCIENCE & TECHNOLOGY:

1 Computer Network Interconnection;

t,oo-4 Problems—and Prospects

Ira W. Cotton

Computer Systems Engineering Division

Institute for Computer Sciences and Technology

National Bureau of Standards

Washington, D.C. 20234

Sponsored by the

National Science Foundation

1800 G Street, N.W.

Washington, D.C. 20006

U.S. DEPARTMENT OF COMMERCE, Juanita M. Kreps, Secretary

Dr. Betsy Ancker-Johnson, Assistant Secretary for Science and Technology

Vjv,C national bureau of standards, Ernest Ambler, Acting Director

Issued April 1977
Reports on Computer Science and Technology

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NBS Institute for Computer Sciences and Technology are designed to provide ADP

standards, guidelines, and technical advisory services to improve the effectiveness of

computer utilization in the Federal sector, and to perform appropriate research and

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series will report these NBS efforts to the Federal computer community as well as to interested specialists in the academic and private sectors. Those wishing to receive

notices of publications in this series should complete and return the form at the end

of this pubhcation.

National Bureau of Standards Special Publication 500-6

Nat. Bur. Stand. (U.S.), Spec. Publ. 500-6, %3 pages (Apr. 1977) CODEN; XNBSAV

Library of Congress Cataloging in Publication Data

Cotton, Ira W.

Computer science & technology.

(National Bureau of Standards special publication ; 500-6) Bibliography: p. Includes index. Supt. of Docs, no.: C 13:10:500-6.

1. Computer networks. 2. Data transmission systems. I. Institute for Computer Sciences and Technology. Computer Systems Engineering Division. II. United States. National Science Foundation.

III. Title. IV. Series: United States. National Bureau of Standards.

Special publication ; 500-6. QC100.U57 no. 500-6 [TK5 105.5] 602Ms [001.6'44'04] 77-608026

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WASHINGTON: 1977

For sale by the Superintendent of Documents, U.S. Government Printing Office Washington, D.C- 20402 - Price $1.45

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TABLE OF CONTENTS

INTRODUCTION

1 . 1 Environment

1.2 The Problem

1.3 Organizations

1.3.1 Communications Carriers

.3.1.1 Common Carriers

.3.1.2 Specialized Common Carriers

.3.1.3 International Record Carriers

.3.1.^ Value-Added Carriers

1.3.2 Regulatory Organizations

3.2.1 Postal Telephone & Telegraph

Administrations

3.2.2 Federal Communications Commission

3.2.3 State Public Utility Commissions

1.3.3 Standards Organizations

3.3.1 International Standards Organization

3.3.2 Consultative Committee For International

Telegraph and Telephone

3.3.3 American National Standards Institute

3.3.^ Electronic Industries Association (EIA)

3.3.5 Federal Government Standards Organizations

1.3.3.5.1 National Bureau Of Standards

1.3.3.5.2 National Communications System

1.3.^ Hardware Manufacturers

1.3.5 User Groups

2. TECHNOLOGICAL BACKGROUND

2.1 New Data Networks

2.1.1 Circuit Switching

2 Virtual Circuit Switching

3 Message Switching

H Packet Switching

1.4.1 Datagram Service

1.4.2 Virtual Circuit Service

Value Added Networks

2.1.5.1 Additions To Basic Communications

2.1.5.2 New Services

2.1.5.3 Non-Technical Benefits

2.2 Interface Components

2.2.1 Physical

2.2.2 Electrical

2.2.3 Logical

2.2.4 Procedural

2.3 Protocol

2.3.1 Link Control

2.3.2 Process To Process

2.4 Control Requirements

2.4.1 Addressing

2. 1 2. 1 2. 12

2

2.1.5

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12

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4

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20

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20

21

21

22

m
Page

2.4.2 Signaling 22

2.4.3 Flow Control 23

2.4.4 Error Control 23

3. CIRCUIT SWITCHED NETWORK TO PSN INTERCONNECTION 23

3.1 Access 24

3.2 Inter-node Links 24

3.3 Integrated Data Networks 24

4. STAR NET TO PSN INTERCONNECTION 25

4.1 Emulating Known Devices 26

4.2 Special Host Interfaces 27

4.3 Standard Host Interface — X.25 27

4.3.1 Overview 27

4.3.1.1 Packet Size 29

4.3.1.2 Addressing 29

4.3.1.3 Flow Control 30

4.3.2 Criticisms Of X.25 30

4.3.3 Future Developments 31

5. SIMPLE TERMINAL TO PSN INTERCONNECTION 32

5.1 Requirements 32

5.1.1 Buffering 32

5.1.2 Flow Control 33

5.1.3 Signaling 34

5.2 Additional Services 34

5.2.1 Speed Recognition 34

5.2.2 Code Conversion 35

5.2.3 Special Terminal Parameters 35

5.3 Alternatives 35

5.3.1 Host Computer 36

5.3.2 Intelligent Concentrator 36

5.3.3 Expanded Switch 37

5.4 Standards 37

6. PSN TO PSN INTERCONNECTION 38

6.1 Gateway Functions 38

6.1.1 Packet Size 38

6.1.2 Addressing 39

6.1.3 Flow Control 39

6.2 Gateway Alternatives 40

6.2.1 Common Host 40

6.2.2 Common Switching Node 40

6.2.3 Internode Device 41

6.3 Gateway Standards 41

7. BARRIERS TO INTERCONNECTION 42

7.1 Technical Barriers 42

7.1.1 Standards 43

7.1.2 Host Interface 43

7.1.3 Application Level Protocols 44

iv
Page

7.1.3.1 User To User Protocols 44

7.1.3.2 Simple-Terminal Access 45

7.1.M Gateways 45

7.2 Non-Technical Barriers 46

7.2.1 National Barriers 46

7.2.2 International Barriers 46

7.3 Mixed Barriers 47

8. CONCLUSIONS 47

8.1 Standards Development 47

8.2 Use of Service 48

BIBLIOGRAPHY 49

GLOSSARY 54

LIST OF ABBREVIATIONS . 60

APPENDIX: EXISTING DATA COMMUNICATIONS STANDARDS 62

INTERNATIONAL RECOMMENDATIONS (CCITT) 64

INTERNATIONAL STANDARDS (ISO) 63

AMERICAN NATIONAL STANDARDS (ANSI) 68

ELECTRONIC INDUSTRIES ASSOCIATION 69

FEDERAL INFORMATION PROCESSING STANDARDS (NBS) 70

FEDERAL TELECOMMUNICATIONS STANDARDS (NCS) 71

INDEX
ACKNOWLEDGEMENTS

The assistance of the following people in reviewing

the manuscript for this report and offering numerous

helpful suggestions is gratefully acknowledged:

Marshall D. Abraras, National Bureau of Standards

Derek L. A. Barber, European Informatics Network

George E. Clark, National Bureau of Standards

Kenneth T. Coit, Honeywell Information Systems Inc.

Harold C. Folts, National Communications System

Joseph 0, Harrison, Jr., National Bureau of Standards

Edwin J, Istvan, National Bureau of Standards

Stephen R. Kimbleton, National Bureau of Standards

Peter T. Kirstein, University College London

John L. Little, National Bureau of Standards

Raymond T. Moore, National Bureau of Standards

Joseph P. Podvojsky, Burroughs Corporation

Louis Pouzin , Insitut Recherche d 'informatique et

d ' Automatique

Gerald C. Schutz, U. S. Department of Transportation

Jack L. Wheeler, Xerox Corporation

vi
COMPUTER NETWORK INTERCONNECTION:

PROBLEMS AND PROSPECTS

Ira W. Cotton

ABSTRACT

This report examines the current situation

regarding the interconnection of computer

networks, especially packet switched networks

(PSNs). The emphasis is on identifying the

barriers to interconnection and on surveying

approaches to a solution, rather than recommending

any single course of action.

Sufficient organizational and technical

background is presented to permit an understanding

and appreciation of the problem. Four major types

of interconnections are then surveyed:

1 . Circuit Switched Network to PSN

2. Star Network to PSN

3. Simple Terminal to PSN

4. PSN to PSN

The major barriers to interconnection are

then outlined. The report concludes with some

comments on the prospects for overcoming these

barriers.

An extensive bibliography, glossary with list

of abbreviations, and listing of existing data

communications standards relevant to

interconnection are also included.

Key Words: Communications networks; computer

networks; data communications; interconnection;

networks; packet switching; standards.

1.0 INTRODUCTION

The data communications world has begun to undergo a major change in recent years, from an ad hoc array of "home

brew" systems that worked more in spite of, than due to,

publicly available facilities, to a more planned and

integrated set of facilities designed both for the

communications of data and for public use. At the same

time, the data communications needs of users have been

1
growing, measured in terms of sophistication, volume and

geographic coverage. In an age of transcontinental and

multinational business, users require access to a full range

of services and coverage. Thus, the same user demands that

lead to the development of high capability public data

networks will inevitably also lead to the interconnection of

these data networks.

Although it is clear that data networks, particularly

public data networks, will be interconnected, the way in

which this will occur is less clear. There are a variety of

factors that will affect the way in which interconnection is

accomplished, including technical issues, political and

regulatory issues, and market power. This report will focus

primarily on the technical issues involved in

interconnection, though some of these other issues,

particularly the regulatory issues, will also be touched

upon. The intent, however, is to clarify the problem and

illustrate the alternatives, not to presume to solve the

problem nor to choose any alternative. The search for an

optimum solution is currently an active pursuit of many

organizations

,

1 . 1 Environment

The current data communications networking environment

is characterized by unprecedented innovation and growth.

New technologies are moving from the laboratory to

commercial application in record time, and a variety of new

service offerings are being presented to customers. The

result is both widened opportunities for users to avail

themselves of these new services and confusion on the part

of carriers and customers alike as to the service

effectiveness and economic viability of all the new

offerings

.

The environment, in the U. S. at least, is also one of

increased competition in the data communications

marketplace. Barriers to en>;ry have been lowered in many

formerly monopolistic service areas. Consequently, many new

firms are entering these markets. On the other hand, it is

at least questionable whether all of these firms will

succeed financially, and it is worth noting that legislation

has been introduced in the Congress to reverse the

pro-competitive policies of the FCC.

2
1.2 The Problem

The basic problem is to devise a strategy for the

interconnection of computer communications networks that is

workable and acceptable to users, carriers and regulatory

authorities. Users seek solutions that are technically

efficient, easy to use, and which enable them to take full

advantages of all facilities on any of the networks that are

so interconnected. Carriers also seek solutions that are

efficient, though they are loathe to alter the internal

operation of their systems. Any system for interconnection

must also permit them to protect the integrity of their own

network as well as to account for and charge for all

services supplied. Regulatory authorities seek methods that

are understandable, controllable, and for which tariffs can

be devised.

Not all of these goals are congruent. Obviously,

compromises may need to be made along the way. However, it

is not clear that all the various groups are presently

willing to compromise. Indeed, it has been suggested that

the opposite is true:

"We are again witnessing the saga of the

large manufacturers attempting to dominate the

customer community through standards of their

own design while the users try to formulate less

constraining industry-wide standards." [SA76]

Others looking at the data communications environment

see a "jungle" [MCC7^] or a "snarl" [P075c]. Hyperbole

considered, the data communications area is certainly a most

"turbulent" [D074] one at present.

1.3 Organizations

A variety of different groups are involved with network

interconnection. In this section, we seek to identify the

more significant of these groups, and to indicate their

particular role and/or views.

1.3.1 Communications Carriers - Carriers are organizations that actually provide

"basic" data communications services, including telephone

and telegraph, voice and data services, switched and point

to point services, etc. Carrier organizations include both

3
government agencies and recognized private operating

agencies (RPOAs).

In most of the world, data communications is a state

monopoly, run by a Postal Telephone and Telegraph (PTT)

ministry or authority. Only in a relatively few countries

do private companies provide domestic communications

services, and in those countries they do so under government

regulation. The United States and Canada are examples of

such countries where communications have been provided by

regulated monopolies, with only limited (but growing)

competition between carriers.

International communications services are provided by

private organizations that have been chartered to provide

these services by a series of bilateral agreements between

the national authorities of the countries they interconnect.

The international carriers only provide service between the

national carriers of the respective countries.

1.3 'I.! Common Carriers -

The common carriers are the government agencies (PTTs)

and authorized private companies that provide the local loop

and the bulk of the backbone, long haul telecommunications

services. In most of the world there is one common carrier

per country, the PTT. In the U. S. and Canada there are

many (far fewer in Canada than in the U. S.), providing

either or both local distribution and long haul services.

In this country, AT&T is a common carrier, as are each of

the regional operating companies. In addition, there are

many smaller independent telephone companies in various

parts of the U.S.

Common carriers generally have a monopoly position in

many, if not all, of their markets. Their pricing policies

are based on long term depreciation of extensive (and

expensive) plant and equipment, and may also be influenced

by internally or externally (say from a regulatory agency)

generated policies regarding cross subsidization among the

various markets served. The carriers, while dedicated to

providing high quality service to the public, are motivated

to maintain their monopoly positions and to pace the

introduction of new service offerings so as not to unduly

disrupt their long term financial positions.

4
1.3.1 "2 Specialized Common Carriers - Specialized common carriers are those American

companies authorized by the Federal Communications

Commission to provide limited telecommunications services of

particular types or in particular markets, on a competitive

basis. The first such specialized carrier was Microwave

Communications, Inc. (MCI), which was authorized in 1971,

after a long inquiry, to construct transmission facilities

and operate data communications services between St, Louis

and Chicago. Additional companies have since entered

various regional markets on a competitive basis.

The specialized common carriers are accused by the

established common carriers of "cream skimming," i. e.,

selecting only the high volume or most profitable markets to

enter and ignoring the low volume markets where the

established common carriers are required to provide service,

often for revenues set at less than full costs. It is

certainly true that specialized common carriers do have this

flexibility to select their markets. However, this

selectivity permits them to introduce new technologies and

facilities more rapidly than the established common carriers

would be wont to do.

1.3.1.3 International Record Carriers -

The International Record Carriers are those companies

authorized by bilateral international agreements to provide

data communications services between countries. Generally,

they do not provide any service within each of the countries

concerned, but simply provide a connection between two or

more intra-national carriers. Also, in most cases the

physical facilities and the revenue for international

service is shared by a consortium of companies from each of

the two countries.

The major U. S. based international record carriers

are the International Telephone and Telegraph Company, RCA

Global Communications, and Western Union International. The

foreign partner sharing the facilities and the revenue in

most cases is the PTT of the country concerned.

5
1.3.1.^ Value-Added Carriers - Value-added network carriers are a type of carrier

peculiar to the United States. These are private companies

who have received formal approval from the Federal

Communications Commission to lease basic communications

services from common and specialized common carriers, to

augment these basic services through the use of additional

physical facilities (such as intelligent switching

processors) , and to resell the "higher value" service to the

public. Currently, only two value-added carriers are in

operation. Telenet Communications Corporation and Graphnet

Systems, Inc., but other companies, including the

International Telephone and Telegraph Corporation (ITT) and

Tymshare, have filed with the FCC for similar authority.

The first company to obtain authority to offer value-added

services. Packet Communications, Inc. (PCI), was unable to

secure sufficient financing to implement its planned system,

and is now essentially defunct.

In countries where communications is a state monopoly,

the organization of a value-added carrier does not exist.

Tne same services may be offered to the public by the

national carrier organization itself without the necessity

for purchase and resale of the basic services.

1.3.2 Regulatory Organizations -

For most kinds of service, telecommunications has been

felt to be a natural monopoly by governments worldwide.

Free competition, it has been feared, could lead either to

wasteful duplication of facilities or monopoly exploitation

of the public. Furthermore, governments have sometimes felt

that control over their national communications systems was

a prerequisite to internal security. Consequently,

telecommunications have traditionally been highly regulated

or kept as a state monopoly.

1.3.2.1 Postal Telephone & Telegraph Administrations -

In most countries outside the U. S. and Canada,

telecommunications is a state monopoly operated by a Postal

Telephone & Telegraph Administration (PTT). These

Administrations intrinisically perform the regulatory

function, since they determine what facilities are to be

constructed, what services are to be offered, and what

tariffs are to be set.

6
1.3.2.2 Federal Communications Commission (FCC) -

The Federal Communications Commission was created by

Congress through the Communications Act of 193^ to regulate

interstate and international communications by wire and

radio in the public interest. Through the years, the FCC

has in large part determined the structure of the U.S.

telecommunications industry.

No area has proved more perplexing to the FCC than that

of data communications. Data processing and data

communications are becoming increasingly interrelated; the

FCC faces the increasingly difficult task of formulating

regulatory policy for communications that leaves the data

processing portion unaffected. FCC rulings have

extraordinary impact on the data communications marketplace.

Two fledgling industries owe their very existence to the

FCC: specialized carriers and value-added carriers.

Martin [MA76] has reviewed the relevant actions related

to data communications taken by the FCC in recent years,

along with major issues currently facing it (with the

exception of the recent reopening of the Computer Inquiry

[BU76b] subsequent to the publication of his article). We

cannot discuss all of these here, except to note that recent

rulings have evidenced the Commission's desire to encourage

somewhat greater competition in providing communications

services, especially of new types and to large volume

customers

.

1.3.2.3 State Public Utility Commissions -

For telecommunications systems whose extent is purely

intrastate, the particular state Public Utility Commission

(or equivalent) has jurisdiction, rather than the FCC. In

California, for example, a packet switched system is

currently being constructed for the state university system

by Pacific Bell. Since this is to be a purely intrastate

system, no FCC application was required or was filed.

On the other hand, the FCC has been aggressive in

defining the limits of exclusive state authority. Martin

describes a case in which the FCC rejected the telephone

companies' contention that interconnection to the local

telephone exchanges was within the control of state utility

commissions [MA76]. The FCC reasoned that such facilities,

though located entirely within one state, are nevertheless

used as links in interstate communications.

7
1,3.3 Standards Organizations -

A number of different organizations, including

professional societies, trade associations, government

agencies at both the state and Federal levels, and national

and international standards bodies, are involved in the

development of standards related to data communications and

computer networking. An appreciation of the different roles

of these various organizations is helpful in understanding

the various cross currents in standards development,

particularly as they affect strategies for the

interconnection of networks. Additional detail on the

standards organizations discussed here may be found in

[SC74, HA75] or be obtained from the organizations

themselves. Standards related to data communications and

network interconnection produced by these organizations are

listed in the Appendix.

1.3.3.1 International Standards Organization (ISO) -

The International Standards Organization is the primary

international organization for the development of voluntary

standards. Within ISO, Technical Committee (TC) 97,

Subcommittee (SO) 6 is concerned with digital data

transmission. Membership in this committee, as in all ISO

activities, is provided by the n^^tional standards

organizations of the participating countries. The U, S.

member is ANSI X3S3 (see section 1.3.3.3).

Over the years, the ISO has been involved in a wide

range of data communications standards activities, primarily

directed at codifying existing practices. In contrast, one

of the most significant ISO activities in data

communications over the past few years has been the

development of a draft international standard for a

bit-oriented High Level Data Link Control Procedure (HDLC)

where no common industry practice exists.

1,3.3.2 Consultative Committee For International - Telegraph & Telephone (CCITT)

The Consultative Committee on International Telegraph

and Telephone is a major operating unit of the International

Telecommunications Union (ITU) an intergovernmental

organization concerned with establishing agreements,

treaties and standards with respect to telecommunications.

The CCITT was established within the ITU to examine and make

8
recommendations on technical, operational and tariff matters

relating to telegraphy, facsimile and telephony. The United

States is officially represented on CCITT by the Department

of State, with domestic common carriers, the FCC and the

Federal Office of Telecommunications Policy serving as its

principal technical consulting agents.

Three groups within the CCITT program of work are of

prime interest to the computer networking community:

* Special Study Group A, Data Transmission, is the

principal point for the data processing community's

contacts with CCITT. The work program of this group

includes all matters relevant to data transmission,

including interfaces, modems and maintenance procedures,

* Joint Working Party ALP, Alphabet, is responsible

for work on alphabets, codes and related matters

pertaining to character transmission structure and

keyboards

.

* Study Group VII, New Data Networks, is responsible

for working on the new types of data networks that are

being introduced, with the emphasis on digital data

networks

.

It seems obvious that there is overlap between the

interests of the CCITT and ISO committees just identified.

This is true, but, as Vaughn explains [VA75], duplication of

effort has been avoided through cooperation of the two

groups. A CCITT administrative recommendation, A20, lays

down the basis for this cooperation and the division of

responsibilities between CCITT and ISO. Briefly, ISO is

responsible for standards relating to customer-provided

equipment such as computers and terminals (DTE) which

connect to telecommunications facilities, while CCITT is

responsible for standards for the telecommunications network

interfaces and for the telecommunications networks

themselves, including transmission channels, switching,

signaling, etc. Naturally, in the current changing

environment for data communications, the refinement of the

division of responsibilities is a continuing process.

1.3.3.3 American National Standards Institute (ANSI) -

The American National Standards Institute is the

primary organization in the United States through which

voluntary national standards are promulgated and through

which American positions on international standards and

9
recommendations are expressed. Within ANSI, under Sectional

Committee X3 on Computers and Information Processing,

Technical Committee X3S3 on Data Transmission is responsible

for defining the characteristics of digital data generating

and receiving systems that function with communications

systems and for developing and recommending standards for

data communications. The Committee is organized into a

number of task groups with specific responsibilities within

this general program:

X3S31, Planning, is responsible for planning and

coordinating the activities of the other task groups.

X3S32, Glossary, is responsible for developing a common

vocabulary for use by all the task groups.

X3S33, Data Communications Headings and Formats, is

responsible for defining formats (for data

communications) of bits within characters and of

characters within a hierarchy of groups, and for

outlining functional control requirements and

procedures for data systems other than those required

for control of a data link.

X3S3^, Data Communications Control Procedures, has been

engaged in the development of a bit-oriented Advanced

Data Communications Control Procedure (ADCCP).

X3S35, System Performance, is responsible for standardizing

data communication system performance determination

techniques and criteria.

X3S36, Digital Data Transmission Rates, is responsible for

establishing standard signaling speeds for use at the

interface between data terminal and data communication

equipments

,

X3S37, Public Data Networks, is focusing its activities on

the development of standards for digital interfaces to

public data networks, including those operating in the

circuit switched and packet switched modes.

1.3.3.4 Electronic Industries Association (EIA) -

The Electronic Industries Association is a non-profit

organization representing over three hundred manufacturers

of electronic products. The organization has been involved

in the promulgation of voluntary industry standards through

its technical engineering committees. The responsibility

10
for data communications standards in the United States is

shared by the technical committees of EIA and ANSI.

Two primary committees in EIA relate to data

communications standards [HIL72]:

TR30, Data Transmission Systems and Equipment, is

responsible for developing and maintaining standards

for the interface between data communication and data

terminal equipment, including work on the interface

between digital data terminal equipment and signal

converters, data sets and modems. TR30 is the advisory

body to ANSI X3S3 for these standards areas, both for

domestic and internatinal activity.

TR37, Communications Interfaces, is responsible for

developing and maintaining standards for the interface

between common carrier-provided communications

equipment and systems. This includes work on data,

graphic and voice communication systems.

1.3. 3. 5 Federal Government Standards Organizations -

As the world's largest single user of computers and

communications, the Federal Government is understandably

concerned with the development of standards as a means of

reducing long term costs and improving the effectiveness of

operations. Consequently, a number of programs have been

set up in Government to formally address the issue of

standards. Efforts relating to interconnection have been

addressed by programs administered by two principal

organizations: the National Bureau of Standards and the

Office of the Manager, National Communications System.

1.3.3.5.1 National Bureau Of Standards -

Through a combination of Congressional action (PL

89-306) and Executive Orders, the major responsibility for

ADP standards in the Federal Government rests with the

National Bureau of Standards. As outlined in [US74], the

NBS role in ADP standards includes:

Providing guidance and leadership in the requirements

determination, development and testing of ADP

standards

;

Participating in national and international voluntary

11
standards activities;

Monitoring and coordinating all Federal participation

in these voluntary activities;

Preparing recommendations for standards to be adopted

for Federal implementation;

Monitoring the implementation of Federal standards and

assessing their impact on computer services; and

Carrying out the necessary research and analysis in

support of the development, implementation and

management of ADP standards.

The Federal Information Processing Standards

Publications (FIPS PUB) Series is the official publication

medium for information relating to standards developed under

these programs.

1.3»3.5.2 National Communications System -

The National Communications System has the

responsibility, delegated by the General Services

Administration, to administer the Federal Telecommunications

Standards Program, The objectives of this program are:

To develop, coordinate and promulgate the technical and

procedural standards required to achieve interoperability among functionally similar

telecommunication networks of the National

Communications System;

In concert with the National Bureau of Standards, to

develop and coordinate technical and procedural

standards for data transmission and the computertelecommunications interface; and

Increase cohesiveness and effectiveness of the Federal

telecommunication community's participation in

national/international standards programs and on the

Federal Information Processing Standards Program.

An agreement between the National Bureau of Standards

and the National Communications System identified areas of

exclusive and of joint responsibility. As outlined in

[US7^], NBS is exclusively responsible for standards in the

area of teleprocessing, NCS is exclusively responsible for

standards in the area of data transmission, and the two
agencies share the responsibility for interface standards

1.3.^ Hardware Manufacturers -

The major computer manufacturers are still the

principal source of supply for data communications software

for their systems. As such, they have both a strong

interest in and a strong influence on the design of

commercial telecommunications systems. However, what is in

the best interests of the computer hardware manufacturers

may not always be most effective for the telecommunications

system. At least one industry critic suggests that the

popularity of IBM proprietary link control procedures is

primarily due to "IBM's ability to significantly influence a major segment of the data processing community, rather than

by operational advantages" [BU76a].

With IBM moving to enter the data communications area

more aggressively, such as through its Satellite Business

Systems venture, its impact on telecommunications systems

design can only be expected to increase. Though not so far

reaching as IBM's, the influence of the other major

mainframe manufacturers will also be felt, particularly

within their user communities. Both Digital Equipment

Corporation and Burroughs, for example, have announced

proprietary approaches to network systems and are proceeding

with implementation.

1,3.5 User Groups -

Users are presumably the ones most affected by

decisions on telecommunications facilities, services and

standards. However, users have not had any significant role

in recent decisions regarding any of these issues and have

been "conspicuously absent" from major meetings [FE75],

This may be due to the lack of a suitable mechanism through

which user views may be expressed.

It is almost a cliche to say that user views are

ultimately expressed in the marketplace. What is not always

recognized is that this presupposes a freely competitive

marketplace, which is not quite the case in

telecommunications. Users can only express their

preferences by selecting from among the available goods and

services, or by refraining from consumption altogether. It

would seem preferable to be able to influence in advance the

selection of goods and services to be offered.

13
Telecommunications users lack any organized forum for

expressing their preferences. Unlike the major computer

hardware manufacturers, telecommunications companies have

not fostered the development of user groups as a means of

communicating with their customers. Only quite recently has

a fledgling Data Communications Users' Group sought to

attract such users under a common umbrella; it remains to

be seen how successful such a group will be.

The only notable success users have experienced in the

data communications area have been the joint user networks

which have been developed in such specialized areas as air

traffic information interchange [HIR74] and the

over-the-counter stock exchange [MI72]. In these areas,

private networks have been designed and implemented by a consortium of users. Because the systems were designed by

users themselves, they reflected user needs rather than

carrier or manufacturer convenience. Also, compared to the

strength of an individual user, the influence of the joint

group on carriers and manufacturers could be significant.

This common approach to data communications needs in

specialized industries may become more popular in the

future

.

2.0 TECHNOLOGICAL BACKGROUND

A clear understanding of basic networking concepts is

necessary to appreciate the issues in network

interconnection. In this chapter we present some of these

basic concepts, including a review of the types of public

data networks, an unraveling of the components of an

interface, and a discussion of the various levels of

protocol. Additional information of a tutorial nature may

be found in [PY73, DA73] and elsewhere.

2,1 New Data Networks

The continuous and rapid decline in the cost of

computer hardware has been called "the most dominant force

over the past twenty years in both computer and

communications architecture" [R0B7^]. This continued

decline in the cost of computation relative to the cost of

communications has led to the introduction of intelligent

computing components into communications systems in an

effort to economize on communications bandwidth. This

trend, which began with message switching systems and

concentrator networks, has led most recently to the

14
development and successful operation of packet switched

networks, which employ computers as integral components of

the communications system itself.

In the following sections we briefly review the various

switching techniques employed in most computer networks.

2.1.1 Circuit Switching - Circuit switching, as illustrated by the voice

telephone network, is a system whereby a complete physical

path is established from sender to receiver that remains in

effect for the duration of the conversation. In most

telephone exchanges, this route is actually established by

mechanical means (such as relays), though the selection of

the route may be made electronically. The process of

selecting a route, or call establishment, may take on the

order of seconds for a complex network. Once the route is

established, however, data transfer is continuous through

the network in the sense that there are no delays added to

data by the switches. End to end transmission time through

the network is limited only by the propagation times of the

various circuit media employed (e,g., cable, microwave,

satellite )

.

Circuit switched service customarily has been

engineered for analog transmission, such as for voice

telephony. The use of such systems for data necessitated

the use of modems to convert the digital signals to analog

form. Recently, with the growth in demand for data

transmission service, both point-to-point and switched

digital communications services have been designed and

implemented. Such services are expected to become more

widely available in the future.

2.1.2 Virtual Circuit Switching - Virtual circuit switching as implemented in Tymnet

[TY71] is the logical analog of physical circuit switching.

The network consists of some number of point-to-point

circuits connected by switching computers at the nodes.

During the call establishment phase a complete path through

the network is established from sender to receiver. This

path is not changed for the duration of the call. In

contrast to physical circuit switching, the definition of

this path is accomplished by routing tables in each of the

node computers. These tables in aggregate define all the

15
virtual or logical circuits through the network. Actual

data transfer is accomplished by forwarding the data from

node to node towards the destination. The data are

forwarded in blocks, and is stored briefly at each node

while it is forwarded to the next node. Thus, virtual

circuit switching employs a "store and forward" procedure to

accomplish data transfer through the network and does

introduce a finite delay at each node.

2.1.3 Message Switching - Message switched networks also contain a number of

circuits with switching computers at the nodes. Message

switching is similar to virtual circuit switching in that

data are transmitted through the network in blocks on a store and forward basis from node to node. Message

switching differs from virtual circuit switching in that

there is no fixed route followed by all the data blocks.

Instead, each block contains a destination address which is

examined by each node through which the block passes. The

switch at each node determines the best output line on which

to forward the block based on the destination address and

the status of the network at the time of processing.

Because each block is handled individually, there is no call

establishment phase so far as the communications system is

concerned

.

2.1.4 Packet Switching -

Packet switching is a special type of message switching

distinguished by a number of different characteristics, each

of which may not seem too critically different, but which in

aggregate serve to define a quite different type of

communications system [C0T75]. First of all, packet

switched systems employ transmission blocks of a maximum

length which is generally significantly smaller than most

message switched systems. Next, packet switched systems

make no attempt to store blocks for any prolonged period of

time while attempting delivery. Rather, transmission

blocks, or packets as they are called, are discarded if

difficulties are encountered in their delivery, in which

case they must be retransmitted by the sender. Packet

switched systems are designed to rapidly forward the packets

towards their destinations with minimal delay in the node.

Finally, packet switched systems will refuse to accept input

when necessary to prevent saturation.

16
With this basic internal network capability, a number

of different types of services may be offered to customers.

Two principal (and different) types that have been

identified are designated as datagram and virtual circuit

service

.

2.1.4.1 Datagram Service -

Datagram service is the most fundamental service that

could be offered by a packet switched network. Under this

service, individual packets are accepted by the network from

source terminals and are delivered independently to the

destination terminals, with the order of delivery bearing no

fixed relationship to the order of entry into the network.

Any sequencing of packets that is required in order to

transmit a stream of meaningful information must be done by

the receiving terminal. All packets must be fully addressed

when they are entered into the network.

2.1.4.2 Virtual Circuit Service - Virtual circuit service undertakes to provide a communications service more nearly resembling an actual

physical circuit. A call establishment phase is required,

during which sending and receiving terminals are prepared to

exchange information and during which network resources may

be reserved for the data transfer. Data accepted by the

network as a stream of packets will be delivered in the

order entered without further customer responsibility.

Since the destination of a stream of information is known to

the network after the call establishment phase, a highly

abbreviated addressing scheme, identifying the virtual

circuit rather than the receiving terminal, may be employed

with data packets. A single channel terminal would not even

have to provide this much identification with data packets,

since there could be no ambiguity as to the destination of

the data. A "permanent virtual circuit" service could also

be provided wherein a single access terminal determines at

subscription time the destination to which all data from the

subscribing terminal would be sent. This would even

eliminate the need for call establishment.

17
2,1.5 Value-Added Networks -

As identified in section 1,3. value-added networks

are a special type of new data network that are independent

of the switching system with which they are implemented. A

value-added carrier obtains basic communications services on

a point to point basis from established common carriers,

provides special equipment such as switching computers at

the nodes to improve or "add value" to the service, and

sells the new service to end users.

2.1,5,1 Additions To Basic Communications -

Value-added networks improve the quality of the basic

communications service in a number of ways. Through

adaptive multiplexing, high speed lines can be shared by

large numbers of users and more efficient use can be made of

the available bandwidth. Alternate routing permits the

network to maintain continuity of operations in the event of

selective internal component failures. Powerful error

detection techniques combined with mechanisms for the

network to automatically retransmit data in which errors are

detected provides an end to end error rate to users that is

many orders of magnitude better than basic communications

services could provide.

2.1.5.2 New Services -

The intelligent components in a value-added network can

be employed to provide new services to customers not

provided by other types of communications services. Among

these services are speed recognition and conversion, code

conversion, and the imposition of a set of standard

protocols network wide. Speed and code conversion permit a wide variety of different terminals to communicate with each

other. Standard protocols provide a means for user

processes on different systems to do likewise.

2.1.5.3 Non-Technical Benefits -

Value-added networks also offer a number of

non-technical benefits. Among these are the total

management of a multi-vendor communications system and the

ability to obtain service on a metered basis. Many users

would welcome the opportunity to obtain an end to end

18
service rather than operating their own private networks.

The ability to obtain service in continuously varying volume

is of great value to organizations whose demands are not

constant

.

2.2 Interface Components

An interface, simply enough, is something between two

systems, devices or components that serves to connect them.

The interface may be a system, device or component itself,

or a set of specifications to which the connected things

adhere

,

Computer communications interfaces are generally

composed of both devices or components and specifications.

There are many different aspects or ways of describing a computer communications interface. One possible breakdown,

and the one chosen here, is to distinguish between different

levels of function, viz., physical, electrical, logical and

procedural. These different functions or levels of an

interface are outlined following, A more detailed overview

of computer-communications interfaces may be found in

[NE74b]

.

2.2.1 Physical -

The physical portion of the interface specifies the way

in which the two devices are actually connected

mechanically. This includes the number of wires and the

dimensions of the physical connectors (generally a male and

female connector are specified) in which the wires

terminate. For example, a 25-'Pin connector is currently in

widespread use for computer-communications interfaces; new

15 and 37-pin connectors will be standardized for use with

the new interfaces now under development.

2.2.2 Electrical -

The electrical portion of the interface specifies the

voltage levels and duration (or for some interface

specifications, the current flow) to be used for signaling

on the various leads. The basic capability provided by

adherence to the standards at this level is the transfer of

data bits across the interface. These bits may be

identifiable as parts of characters and/or be used for

19
higher level signaling functions within the interface

2.2.3 Logical -

The logical portion of the interface specifies how the

data bits and/or characters are grouped into fields for the

purposes of signaling and data transfer. The use of certain

characters for communications control functions such as

synchronization (SYN) and start of header (SOH) is also

specified at this level. In a sense, the logical

specification of a computer-communications interface

provides a language that may be employed for controlling and

effecting data exchange across the interface. Standards at

this level are sometimes called "elements of procedure."

2.2.^ Procedural -

If the logical level of the interface is viewed as

specifying the syntax of the data flow across the interface,

then the procedural specifications should be viewed as

providing the semantics. Specifications at this level

determine the legal sequences of communications control

characters, or the legal contents of various fields, or the

valid commands and responses in controlling data flow. The

same basic set of control characters or fields may be used

in a variety of different ways according to the procedural

specifications. Standards for these different ways are

sometimes called "classes of procedure."

2.3 Protocol

The term "protocol" is generally used to refer to the

logical and procedural aspects of an interface. Thus, a protocol specification includes both syntax and semantics.

Semantics also include relative timing information — not

just the legal commands, but who can issue them and when.

A complex interface may contain several levels of

protocol. An appreciation of this aspect of protocols is

quite important in designing and evaluating network

interconnections

,

It has been suggested* that the term "protocol" be used

to designate communications conventions between entities at

* Louis Pouzin , private communication.

20
the same level while "interface" be used to designate

conventions between entities at adjacent levels. This is an

attractive suggestion, and some confusion may be eliminated

if the use of these terms in this way becomes widespread.

2.3. 1 Link Control -

Although they could be implemented in software and

therefore be considered as a type of process to process

protocol, in most cases link control disciplines or

protocols are at least capable of or intended to be

implemented in hardware and should therefore be discussed

separately. The link control protocol is simply part of the

transport mechanism to get data reliably and in a controlled

fashion from one end of a link to the other.

Link control protocols are either bit-oriented or

character-oriented. In both cases, there is a way to frame

discrete units of information, a set of commands and a way

to convey them, and an error detection mechanism. Some

examples of link control procedures are American National

Standard (ANS) X3.28 (character-oriented) and the ISO

High-Level Data Link Control Procedures (bit-oriented).

2,3.2 Process To Process - In computer communications systems, most protocols are

for the exchange of information between processes, where a process may loosely be viewed as a program. (Multiple use

of a re-entrant program is viewed as multiple processes).

In the case of a simple terminal, the actions of the human

operator are considered to constitute the process.

Many different processes may reside in the same

physical device. For example, a host computer may have a communications handler process, various operating system

processes, and many user (application level) processes.

Messages entering the host through the same physical

interface may be intended for any of these processes. In

most cases, there is a hierarchy of control for delivering

messages to the proper recipient. The communications

handler must examine all messages; the operating system

must examine all messages intended for user processes.

Thus, the basic structure for all messages must have some

mechanism for identifying the level and identity of the

proper recipient. Frequently, information for recipients at

multiple levels may be conveyed in the same physical

21
message

.

For example, the various levels of communication in a

packet switched network have been identified, and a mechanism recommended to distinguish between them [OH74].

The method is simply to nest messages within messages,

according to relative position in the hierarchy. Each

recipient then "peels off" the part intended for it and

passes the remainder to the next level recipient. This

approach works quite well, and is implemented in most packet

networks (e.g., [CA70]).

2.M Control Requirements

There are a variety of control-related functions that

must be performed by all computer networks. When networks

are interconnected, these functions must be addressed in

some way by the interface.

2.4.1 Addressing -

In a communications system, some means of addressing is

required whenever sender and receiver are not directly

connected. With a multi-channel terminal such as a host

computer, which also contains different levels of addresses

(such as operating system level, user program level), the

question of addressing the proper recipient is non-trivial.

2.4.2 Signaling - Signaling refers to the means by which control

information is exchanged between the network and its users.

The two primary classes of signaling techniques are in-band

and out-of-band signaling. In the former, control

information is exchanged in the same way that data are

exchanged, with some special identification marking it as

control; in the latter, some other means is provided for

exchanging contr: information, separately from data. Where

nested protocols are concerned, control information sent at

a different level of protocol may be considered as

out-of-band signaling, even though all the data at all

levels of protocol are transported by the same physical

means

.

22
2.4.3 Flow Control - Flow control refers to mechanisms for throttling the

rate of data exchange between any two points in order to

prevent overload. With probabilistic message generation and

fixed capacity in network components (such as buffers),

overload would be inevitable without such mechanisms to

temporarily stop or slow down the rate of message arrivals.

Sequencing and acknowledgments of messages can be

considered as part of flow control or can be treated

separately. The question of sequencing is generally

considered as part of the service to be offered (e.g.,

datagram or virtual circuit) and not as a mechanism that can

be implemented in a variety of ways. Message

acknowledgement, on the other hand, can be accomplished in a number of ways, and is frequently integrated with flow

control mechanisms.

Just as message exchange can occur between parties at

various levels in a hierarchy, so too can flow control be

implemented for any of these levels. Of course, the actions

of a flow control mechanism at lower levels in the hierarchy

(at the communications handler, for example) would also be

effective for all higher levels. Pouzin [P075d] discusses

some objectives of flow control and some alternative

implementations

.

2.4.4 Error Control -

While lower levels of an interface may adequately

address the question of error detection and possibly even

retransmission, somewhat higher levels may nave to become

involved with the detection of duplicate data occuring due

to retransmissions after timeouts and reinitialization in

the event of catastrophic failure. Message acknowledgement

may also be implemented as part of an error control scheme,

and in fact, error and flow control can be based on the same

mechanisms

.

3.0 CIRCUIT SWITCHED NETWORK TO PSN INTERCONNECTION

Packet switched networks commonly use the switched

telephone network for local distribution to terminals.

Switched connections are less frequently used between the

nodes of a packet net. Integrated data networks designed

for both message-oriented and continuous stream data may

23
eliminate the need for interconnection.

3 . 1 Access

Circuit switched networks are commonly used today as a means of local access to packet switched networks. A local

dial connection can readily be made between both simple

terminals and packet-mode terminals (terminals that are

capable of dealing in packets, including host computers) and

the access node of the packet network. Modems at the

terminal and the packet node permit the dial network to be

used in tnis way.

The connection may be initiated by either the terminal

or the packet node. Connection establishment from simple

terminals is generally manual (the terminal operator dials

the call). Connection establishment from a host computer

may be manual or automatic (the computer itself can dial the

call if equipped with an automatic calling unit).

Connection establishment from a packet node would normally

be automatic, as there is not generally an operator for such

nodes

,

3.2 Inter-node Links

The communications links between nodes in a packet

switched network are generally provided by dedicated

(point-to-point, unswitched) circuits. However, the

switched network could be used as a means of providing

backup or extra capacity, and could even be used instead of

dedicated facilities. At least one network, MERIT, has

taken tnis approacn [BEC72], although even they concluded

after some experience that point to point facilities would

be more suitable. Use of the circuit-switched facility is

accomplished through automatic calling units, tne software

for which "nas proven to be much more complex than

originally envisioned" [AU73].

3.3 Integrated Data Networks

This section on the interconnection of circuit switched

networks and packet switched networks nas been based on the

premise that separate backbone networks of each type will

exist due to the inherent suitability of each for a particular type of traffic. At the present time this

24
appears to be true; nowever, attention is beginning to be

directed to the design of "integrated" or hybrid data

networks capable of efficiently transporting both voice (or

real time data) and message traffic [COV75, DE76].

Discussion of the details and tradeoffs in the design of

integrated data networks is beyond the scope of this

Technical Note. However, it does seem evident that such

networks will be built in the not too distant future.

Obviously, providing a single network for both types of

traffic would eliminate the necessity to interconnect two

separate networks of different type.

^.0 STAR NET TO PSN INTERCONNECTION

A star network is one in which the computing resources

are centralized at a single node. The canonical model is a timesharing system with a large number of user terminals

connected through a possibly quite complex communications

system. However complex the communications system, all

terminals effectively have direct access to the central

facility

.

The existence of this central point of access for all

users greatly facilitates the interconnection of such a network with distributed packet switching systems. The

entire star network can be modelled as a multi-channel data

terminal equipment (DTE) capable, with suitable interfaces,

of operating in the packet mode. Thus, a star network can

be interconnected with a packet switched network in the same

way that an ordinary host computer is interfaced to the PSN.

There are a variety of ways to interface host computers

to packet switched networks. One alternative is for the

network itself to undertake to simulate some (array of)

device(s) that the host already supports. Thus, some

networks interface to hosts through multiple asynchronous

low speed lines or through a single multiplexed synchronous

line into the host's normal front end or communications

controller

.

The other alternative is to specify a new interface to

which the host must conform and whose procedures the host

must support. This was the approach taken in the ARPA

network [HE70]. Based on experiences with this and other

experimental systems, considerable work has been done

towards standardizing the interface between multi-channel

(sometimes called "multiaccess") user terminals and packet

switched networks.

25
4,1 Emulating Known Devices

Perhaps the easiest approach to interfacing from the

host point of view is for the network to undertake to

emulate devices that the host (or host front end) already

supports. There are two main categories under this

approach:

1. The PSN can appear to the host as an array of

low speed asynchronous terminals and interface through

a number of terminal ports on the host or the host

front end,

2. The PSN can appear to the host as a terminal

network and interface through a multiplexed port on the

host or the host front end (according to some protocol

or multiplexing discipline that is already supported).

Essentially, the difference between these two

categories is the difference between space division

multiplexing (by separate ports) and time division

multiplexing (through a single port).

Each of these categories has been implemented in Tymnet

[TY71, BER72]. This is not surprising, since a commercial

network would be expected to seek to minimize the

implementation effort required on the part of (potential)

customers. The primary advantage of this approach is

precisely that it does not impose any new requirements on

the host. By emulating known devices, the network can be

employed interchangeably (or simultaneously) with those

devices, and cutover is greatly facilitated.

The disadvantages of this approach is that only those

facilities which were supported by the old interface will be

usable with the new network. This may force the utilization

of the PSN in an inefficient manner. For example, single

asynchronous terminal access lines, and even protocols for

multiplexed access lines, generally lack any provision for

flow control. This might force the line to be used at a lower rate than possible, or alternately introduce the

possibility of data loss. Another disadvantage is that a network supporting many different hosts with multiplexed

interfaces might need to implement many different protocols

for the various hosts. Unfortunately, the standardization

of communications disciplines has not progressed to the

point where all hosts can be supported with a single

discipline. Even for interfaces employing multiple

asynchronous ports, two different families of terminals

(ASCII and EBCDIC) might need to be emulated for different

hosts

.

26
4.2 Special Host Interfaces

If changes to the host hardware and/or software are

permitted, then more efficient interfaces can be devised

which take full advantage of all network capabilities and

services. The problem of designing an efficient host

interface is not unlike that of designing an access method.

Looking towards the network, the interface must provide for

reliable data communications with the network device and

must support a multiplexed link. Looking into the host, the

interface must have the ability to accept and deliver

messages from individual processes, either through

appropriate operating system service routines or by its own.

In terras of tne required control functions discussed

earlier, we have just identified the error control and

addressing function. In consequence of the finite capacity

of both the host and (especially) the switch, the interface

must also provide some form of flow control. In the absence

of flow control, data would likely be lost at times due to

overflow, or the data transfer rate across the interface

would have to be limited to some low value. Even then tnere

would be some finite probability for data loss.

4.3 Standard Host Interface — X.25

At the beginning of the current four year CCITT study

period, a Special Rapporteur was appointed within Study

Group VII to deal with issues relating to packet sv>?itching

and to prepare draft Recommendations in this area, if

possible. The Rapporteur convened a number of international

meetings over the past few years, and, with the active

assistance of interested PTTs and RPOAs as well as national

standards organizations from various countries, succeeded in

producing a draft Recommendation X.25 which was adopted by

the CCITT Plenary Assembly in the fall of ^9^6, This

section will outline the details of the draft

Recommendation. More detailed summaries may be found in

[RY76, ri076J.

4.3,1 Overview -

CCITT Recommendation X.25 is an international standard

for tne "interface between data terminal equipment and data

circuit-termination equipment for terminals operating in the

packet mode on public data networks." Simply stated, tnis

interface is intended for use by host computers and other

27
types of customer terminals which are intelligent enough to

format data in the ways specified by the interface

specification and to respond in the proper ways to the

complex control signaling requirements of the interface.

The interface is designed for multi-channel use; that is,

it provides for communications with multiple independent

user processes within a single user terminal (computer). It

is expected that a simpler interface specification will be

developed for single channel terminals (e.g., remote batch

terminals or single user intelligent terminals).

The X.25 Recommendation is structured into three

independent parts as follows:

Level 1 - the physical, electrical, functional, and

procedural characteristics to establish, maintain

and disconnect the physical link between the DTE

and the DCE.

Level 2 - the link access procedure for data interchange

across the link between the DTE and the DCE.

Level 3 - the packet format and control procedures for the

exchange of packets containing control

information and user data between the DTE and the

DCE.

For Level 1, the X.25 Recommendation specifies the use

of CCITT Recommendation X.21, a new general-purpose

interface for synchronous operation on public data networks.

For an interim period, the use of Recommendation X.21 bis

(essentially the same as EIA RS-232) is approved.

For Level 2, the X.25 Recommendation uses the

principles and terminology of the High Level Data Link

Control Procedure (HDLC) specified by the International

Standards Organization (ISO). This is a bit-oriented line

discipline suitable for use in a variety of environments.

(It is expected that this level will be totally specified by

reference to ISO standards once they are completed for

HDLC).

For Level 3, the X.25 Recommendation specifies a set of

packet formats and elements of procedure developed for this

use by the various carrier organizations planning to offer

public packet switching services. The type of service

supported by the current X.25 Recommendation is the

so-called "virtual circuit" service (as described in section

2.1.4.2), though other services such as "datagrams"

(described in section 2.1.4.1) are identified as areas for

future study.

28
4.3.1.1 Packet Size -

The current X.25 interface recommendation specifies

that all user data shall be passed to the network in packets

with a maximum data field length of 128 octets. Thus, no

packet assembly/disassembly capability is assumed by the

interface. (Networks supporting the X.25 recommendation are

free to disassemble the 128 octet data fields into still

smaller packets for internal switching, if this is

required). The X,25 recommendation also permits

Administrations to offer additional maximum data field

lengths from the following list: 16, 32, 64, 256, 512, and

1024 octets, or exceptionally 255 octets. The maximum data

field length applying to a given DTE is fixed at

subscription time.

4.3,1,2 Addressing -

The current X,25 Recommendation specifies a multiplexed

interface between the packet terminal and the network. Up

to 4095 "logical channels" can be in operation

simultaneously between the terminal and the network. Each

of these channels identifies an independent data connection

between a user process in the packet terminal and a user

process or single access terminal elsewhere in the network.

Call Request packets are used during the call

establishment phase to set up a virtual circuit between the

originating and destination stations. The originating

station selects the logical channel number that it will use

to identify the virtual circuit and indicates the network

address of the called DTE. Network addresses can be of

variable length up to a maximum of eight octets. There are

no standards in the X,25 Recommendation for network

addresses; however, work is proceeding in other CCITT

groups on a standard network numbering scheme.

Data packets sent during the data transfer phase will

only be identified by the logical channel number. The

network will associate the logical channel numbers at each

end with the appropriate virtual circuit until Clear packets

are exchanged to terminate the circuit.

29
4.3.1.3 Flow Control -

Tne flow control specified by the current X.25

Recommendation at tine packet level applies during the data

transfer pnase for data and reset packets on each logical

channel used for a switched or permanent virtual circuit.

Transmission is controlled separately for each direction of

each logical circuit and is based on authorizations from the

receiver. Packets are numbered modulo 8 (or optionally

modulo 128 when extended) for the purpose of flow control

and positive/negative acknowledgements.

Three commands are provided for the purposes of flow

control. A receive ready (RR) packet acknowledges sequenced

packets up to some given number (modulo the appropriate

base) and approves the sending of the next one in sequence.

A receive not ready (RNR) packet temporarily disallows the

transmission of further packets on a given logical channel.

An acknowledgement for packets previously received can also

oe conveyed by the RNR packet. Finally, a reject (REJ)

packet may be sent to request transmission or retransmission

of sequenced data packets beginning from a given number.

Note that all these commands are exchanged between the host

and the packet network, and not end to end between

subscribers. The ultimate effect of a command from the host

to the network, however, might be to cause tne network to

issue a similar command to the source customer station.

4.3.2 Criticisms Of X.25 -

The X.25 Recommendation has been criticized on the

basis that it is too complex, fails to support datagrams,

and duplicates functions at several levels [ P076a , P076b]

.

It is indeed true that the X.25 Recommendation is a complex specification, intended for implementation in a host

computer or intelligent front end. For simpler, single

access terminals (such as remote job entry terminals), a simpler interface, perhaps a subset of X.25 would be more

appropriate. This subject has been raised in the

appropriate standards bodies, and is expected to be

addressed in tne next study period.

It is also true that X.25 only supports virtual circuit

service. Datagrams were left for further study by the

designers of the Recommendation. Acceptance of the X.25

Recommendation does not rule out its extension or the

development of an additional Recommendation to cater for

datagrams. It is understood that the subject of Datagrams

30
will also be addressed during the next study period.

The question of duplication of function is perhaps the

most intriguing one. It may well be that in the zeal to

identify independent levels that could be standardized

separately, some duplication of function has been kept in

the total Recommendation. If this is the case, experience

with implementation and use of the interface should reveal

the necessity for change. The Recommendation is recognized

as likely to evolve somewhat during its early years of use.

4,3.3 Future Developments -

The area of packet network interface standards is still

a very dynamic one, and Recommendation X,25 should not be

viewed as the final word in the matter. In fact, a variety

of developments can be foreseen in this area:

1, Adoption by ANSI - It is likely that ANSI will

adopt the Recommendation as an American National Standard

now that it has been adopted by CCITT, It is possible

that modifications would be made prior to such adoption,

but it does not now seem likely that significant

modifications will be made,

2, Modification to X.25 - The details of X.25 will

undoubtedly continue to be analyzed during the next four

year CCITT study period, and modifications to the

Recommendation will likely be proposed based on experience

with its implementation and use,

3. Additions to X.25 - As distinct from

"modifications" to the current version of the

Recommendation, a number of study points are identified,

for example, datagram service and access by character-mode

terminals. It can be expected that these points will be

addressed during the next study period, and that the

Recommendation may be expanded or new Recommendations

developed to address these points,

4. "Simplification" of X.25 - A simplified version

of X.25, intended for use by single-access DTEs (e.g., RJE

terminals with limited intelligence), can be expected to

be developed within ANSI, if not within CCITT.

31
5.0 SIMPLE TERMINAL TO PSN INTERCONNECTION

"Simple" terminals are defined as those character-mode

user terminals which do not contain the logic to format

information into packets before transmission. Rather,

information is transmitted asynchronously as it is entered.

The canonical model of a simple terminal is a teletypwriter

.

As Davies and Barber recognize [DA73], the problems

relating to the connection of simple terminals fall into

three main areas: 1) the basic physical connection, 2) the

procedural interface for controlling the terminal, and 3)

the interfaces between the user and remote computing

services as well as with other users. The physical

connection problem involves the hardware interface between

the terminal and the network; the procedural interface

covers the special signals and control information that has

to be interchanged between the terminal and the network;

the user interface involves network command languages,

service command languages, user languages, and a variety of

man-machine interaction considerations.

5 . 1 Requirements

Simple terminals lack any internal processing

capability to format or disassemble packets, or to respond

to any but the lowest level of communications control. In

modern networks that are designed for process to process

communications, a terminal handler must be provided

somewhere as a surrogate process for the terminal. So far

as the network is concerned, the terminal handler process,

also referred to as "packet assembly/disassembly" (PAD),

communicates in the same way as all other processes in the

network (in conformance with whatever network protocols have

been established). So far as the terminal is concerned, the

terminal handler communicates with it in the character

asynchronous mode.

5.1.1 Buffering -

The buffering function in support of simple terminals

is primarily to provide for the conversion (in both

directions) between character asynchronous and synchronous

data. From the terminal to the network, the accumulation of

input characters in a buffer permits packets to be assembled

which contain more than a single character of data, thus

greatly improving the efficiency of line utilization within

32
tne PSN (and probably lowering the customer's charges as

well). From the network to the terminal, multicharacter

packets must be held in a buffer while they are disassembled

and delivered to the terminal one character at a time.

5.1.2 Flow Control -

As discussed previously, flow control includes

mechanisms for throttling input so as to prevent the

receiving device from overflowing and losing data. This is

not a support function that customarily has been provided

for simple terminals, (Sequencing has customarily been

provided for such terminals). Flow control is, however,

more critical in a networking environment, where device

capacities are designed with average needs in mind, yet in

which the peak to average ratios for resource demands may be

quite high,

Tne flov/ control problem arises on input from simple

terminals and on output to simple terminals where the data

source is operating at a higher rate than the receiving

terminal. In the latter case, the data source is normally a computer, so previously discussed flow control mechanisms

can be applied. The problem, then, is principally one of

throttling input from simple terminals.

The EBCDIC family of terminals is probably superior to

the ASCII family in the ease of implementing flow control,

since most EBCDIC terminals are built to respond to keyboard

lock/unlock control sequences. Thus, on approaching

overload, it is a relatively simple task for the terminal

support device to temporarily lock the keyboard. Most

terminal operators are sufficiently attuned to the

characteristics of their keyboard to recognize at once what

has been done.

ASCII fam.ily terminals, however, lack the lock/unlock

facility. The most common approach for such terminals has

been to ring the terminal's audible alarm (which is provided

on most ASCII terminals) to indicate overflow. This has the

disadvantage of occurring only after an overflovj has

occurred, and frequently leaves the operator confused as to

which characters have been accepted and which not.

33
5.1.3 Signaling -

Signaling refers to the exchange between the terminal

and the network of a variety of different control

information necessary to establish, maintain and terminate

calls. With intelligent devices, much of this control

information is passed at lower levels in the interface, but

the nature of simple terminals requires that it be conveyed

at the highest level when they are used. In general, all

signaling functions between simple terminals and the network

have to be accomplished, with the operator's cooperation, by

means of what characters are keyboarded and what characters

are printed (or displayed).

Conventions need to be established for signaling

service requests, such as the address of a destination

station, or a request to disconnect. All this signaling

normally occurs "in-band", when the terminal is recognized

as authorized to transmit. One exception is the "break"

key, provided on most simple terminals, which can interrupt

network output to the terminal. Use of the audible alarm or

"wiggling" the keyboard by successively locking and

unlocking it could, in a sense, be considered as out-of-band

signaling to the operator.

5.2 Additional Services

Additional services can be provided in support of

simple terminals where logic in the supporting device is

used to advantage. These services may be viewed as

"value-added" services as discussed previously. When they

are provided, they may be viewed as integral to the

interface

.

5.2.1 Speed Recognition

A useful function for the terminal support or PAD

device to provide is speed recognition for a variety of

speeds on the same access line. Algorithms for speed

recognition upon the input of a known initial character are

well understood and have been in use for years. The

advantage to users is that a single access line can be used

by all terminals within the range of recognition. (This

feature is most commonly used to identify terminals which

may be operating at burst speeds of 10, 15 or 30 characters

per second.)

34
Adequate buffering and flow control facilities in the

PSN make it possible for terminals operating at different

speeds to communicate directly with each other.

5.2.2 Code Conversion -

The code employed by a terminal may be viewed as one of

the parameters describing that particular terminal;

however, the significance of this parameter relative to all

the others warrants individual discussion.

Unfortunately, not all terminals employ the same

character set, even for the basic alphabetic and numeric

characters, thus necessitating some translation capability

in the terminal support device if a wide variety of

terminals are to be supported. Fortunately, the great

majority of simple terminals do fall into one of two main

classes according to the character set employed: ASCII and

EBCDIC. Thus, only one translator is generally required,

since one of the two codes is generally "built into" the

software of the terminal handler.

The particular code used by a terminal can be detected

as in 5.2.1 to provide for automatic code conversion.

5.2.3 Special Terminal Parameters -

Even among terminals within the same character set

family there may be individual differences that would

interfere with interworking if not handled properly. Some

examples of such differences are requirements for different

timing delays and/or different requirements for the

transmission of padding characters after such functions as

carriage return or line feed. The wide variety of

requirements in such areas among different terminals may

require that tables be kept of the characteristics of

various terminals or that each terminal user have the

capability to change parameters from the initial set.

5.3 Alternatives

Given the above requirements for simple terminal

support, there are two primary ways that a network can

undertake to provide it. The network can finesse the

problem by requiring all terminals to access through host

35
computers which provide the support. If the network does

provide support for direct access, it can be through a separate device specifically for this function, or by adding

capability to the network switch. Each approach naturally

has its own advantages and disadvantages.

5.3.1 Host Computer -

The simplest approach to terminal support on a network

is not to provide any at all. Terminal users can be

required to connect to a local host which provides all

terminal support functions and access to the network. So

far as the communications subnet is concerned, terminal

traffic is no different from the intercomputer traffic it is

designed to carry. This approach has been implemented in

the MERIT network [BEC72].

The advantage of this approach is that it greatly

simplifies design of the subnet. The primary disadvantage

is that communications functions are forced on host

computers that may be ill suited to perform them. Such

functions as buffering characters from asynchronous

terminals and performing speed and code conversion are

better performed by specially designed communications

processors rather than general purpose hosts. In addition,

two computers must be employed (and paid for) whenever a terminal wishes to access a remote computer through the

network. Even if terminal support at the local computer is

provided by a front end processor, there is some reduction

in the capacity of the local system that could be avoided if

the terminals could be provided access directly to the

network

.

5.3.2 Intelligent Concentrator -

An alternative to the use of a general purpose host to

provide terminal support and access to a network is to

install a separate device exclusively for that function.

Such a device could be called an intelligent concentrator,

since it is generally based on a stored-program minicomputer

and since its function is to concentrate the traffic from

ra.any individual terminals. As mentioned previously, this

function nas also been referred to as "packet

assembly/disassembly" , since the concentrator will generally

communicate with the packet network according to the

standard packet level interface (such as X.25, for example),

Rosenthal [ROS76] has reviewed some advanced uses of

36
minicomputer-based systems for the interconnection of

terminals and networks.

5.3.3 Expanded Switch -

A final alternative configuration for providing

terminal support and network access is to integrate the

functions of an intelligent concentrator with a network

switch. This is the approach that was taken in the

development of the ARPA network TIP [0R72, MI73]. A TIP is

an IMP (Interface Message Processor) expanded with the

addition of a terminal multiplexer and code to support it.

Tne code includes the minimal set of functions of a small

host computer, since all communications in the ARPANET are

between processes in host computers.

This integrated approach has the advantage of

eliminating the need for separate devices just for terminal

support. On the other hand, it greatly complicates the

design of the switching node and results in a situation in

which failure of the terminal support hardware or software

may adversely impact the capability of the node to perform

its switching function, and thus adversely impact the

network as a whole.

5.4 Standards

A small number of sets of standards exist at the

physical, electrical and logical level for the connection of

simple terminals to packet switched networks. (Folts [F076]

reviews recent and planned developments in this area.)

At the procedural level, however, no widely accepted

standard exists. The ARPA network developed one standard

for use network wide which has been in use for over five

years now [0R72, MI73]. Telenet, as the commercial sequel

to the ARPA network, learned from that experience and

developed a new standard for use within its network [TE75].

The CCITT is now planning to consider developing

recommendations for the procedural level of the interface

between simple terminals and packet switched networks. One

proposal has already been developed by the CEPT group

[CEP76].

37
6.0 PSN TO PSN INTERCONNECTION

The packet switched networks that have already been

built or that are currently under construction all differ

quite considerably in the internal details of their

implementation. Consequently, it does not appear possible,

even if it were desirable, to effect interconnection through

standardization of internal operation. However, as with the

attachment of multi-channel DTEs to PSNs (discussed in

Chapter 4), interconnection can be effected through

standardization of a common interface. For network to

network interconnection, such an interface is called a

gateway

,

6.1 Gateway Functions

The primary function of a gateway is to resolve

differences between the two interconnected networks in such

a way as to permit meaningful communications between users

on each of the networks. For packet networks, Cerf and Kahn

[CER74a] have identified the major differences between

networks that must be resolved

1. Each network may have distinct ways of addressing

the receiver of a message.

2. Each network may accept data and employ internal

transmission blocks of different maximum size.

3. The success or failure of a transmission and its

performance in each network is governed by different time

delays in accepting, delivering and transporting the data,

4. Recovery procedures for lost or altered data may

differ between networks.

5. Status information, routing, fault detection and

isolation are typically different in each network.

6.1.1 Packet Size - Differences in packet size is one of the first

obstacles with which a global strategy for network

interconnection must deal. The problem is basically one of

coping with the fragmentation that must inevitably occur

when the two interconnected networks employ different

internal maximum packet sizes.

38
Maximum packet sizes are selected by different networks

for maximum internal operating efficiency in consideration

of the implemen tational details of the particular netv/ork.

Such factors as propagation delay and anticipated frequency

and distribution of errors go into the analysis. The

optimal values for different networks may differ widely,

especially between systems which use terrestrial links

exclusively and those which use satellite links.

Some systems also require user terminals to enter all

data in terras of packets, while other systems can accept

data in longer blocks and provide packet

disassembly/assembly functions.* An interconnection strategy

must make minimal demands on the design of the internal

operation of the networks.

6.1.2 Addressing - Addressing across network boundaries requires either a standard network numbering scheme or a means of address

translation in the gateway. For public networks, a standard

numbering scheme seems likely, given the success of this

approach in the public circuit-sv-zitched (telephone) netv;ork.

Such a project is currently underv/ay in CCITT.

6.1.3 Flow Control -

Flow control is necessary across network boundaries

even more strongly than between host computer and a netv/ork.

The interface between two networks is ultimately between

tneir switching nodes, each of which has limited capacity.

Flow control also provides each network with a mechanism for

protecting itself from disorderly operation on the part of

the otner. If not performed by the error control mechanisms

between tne networks, means must be provided for the

detection of duplicate and missing packets. Flow control

may be accomplished in ways similar to that employed for

host to network interfaces.

* Note that there is a dual use of the term "packet" in many

references. The same term is frequently used to refer to

the block of user data entered into the network as is used

for the transmission block sent between nodes within the

network. The two need not be the same.

39
6.2 Gateway Alternatives

There are a variety of different ways in which the

gateway between two packet switched networks may be

implemented. In the following sections, we review the major

alternatives

.

6.2.1 Common Host -

One of the simplest and most straight forward

approaches to interconnecting two networks is to do it

through a host that is attached to each network. Under this

approach, the individual subnetworks do not even have to be

aware that they are interconnected. All messages which need

to be sent from one network to the other are simply

transmitted to a special process in the gateway host which

then retransmits them into the other network. The host is

responsible for any reformatting and signaling conversion

that may be necessary between networks.

Because of the relative ease of implementing this

approach (especially if the host were already connected to

each network, and ^.t is only the interconnection procedure

that needs to be developed), it has been a quite popular way

of interconnecting networks. As the most intelligent

component in the network, hosts are certainly capable of

performing whatever translations are necessary. However,

this does involve the host in communications processing,

with its high overhead, and so may not be a suitable long

term solution to the problem. General purpose computers are

generally not the most cost-effective means of accomplishing

communications processing, and few hosts could justify

performing this function except in experimental or limited

volume situations.

6.2.2 Common Switching Node - Another approach to interconnecting packet networks

would be to have a switching node which is common to each of

them, Kirstein and Lloyd [KI75] argue that this is the most

complex method to implement, since it involves implementing

in the same device all the switching capabilities of each

network as well as the mappings from each low-level protocol

to the other. If the networks are very different, these

mappings could be quite difficult (and even more difficult

than mapping at higher levels).

40
6.2.3 Internode Device -

The final possible approach, and also a very popular

one up to the present, is to provide a separate device

performing only gateway functions between each of the

networks to be interconnected. This gateway is generally

designed to appear as a special host to each of the networks

(rather than seeking to perform the gateway functions at the

subnet level). By interconnecting at the host level, the

"sovereignty" of each of the networks involved is preserved

[WA75]. Under this approach, the gateway will not seek to

convert between the host-to-host protocols of the two

networks (which Walden and Rettberg believe would be

impossible) , but will communicate with each network in the

lowest form of host/network protocol supported by the

network. Experiments with such a gateway are currently

being performed [HIG75].

As Cerf and Kahn [CERT^a] recognize, in practice a gateway between two networks may be composed of two halves,

each associated with its own network. Each half-gateway

would only be responsible for translating between the

internal packet format of its own network and some common

internetwork format. The internetwork format and signaling

conventions for internetwork data exchange could be

standardized in much the same way that the host to network

interface is standardized.

6.3 Gateway Standards

While international recommendations have not yet been

developed for completely specifying the means of

interworking between public packet switching networks, the

approach to be taken for interconnection is clear. The "two

half-gateway" model will be followed, with each network free

to deal with the required translations in whatever way it

chooses. The international recommendations will specify the

nature of the interface between the networks, without making

any statements about functions performed in the nodes that

are interconnected. It seems likely that the circuit will

be specified to be high speed, full duplex, and that low

level signaling will be according to X,21 and data transport

will be according to a procedure from HDLC, A new set of

internetwork packets will be defined for signaling and data

exchange between networks, similar to those specified in

X,25. Roberts [ROB76] also suggests a few optional features

that would be desirable in X.25 for internetting (though he

stresses that these features would also be desirable for the

host interface ) :

41
1. Extended numbering at the link level, to

permit a larger number of unacknowledged packets to be

outstanding (particularly important for long delay

channels such as satellite links);

2. Extended numbering at the packet level or

variable packet size per call, addressing the same

problem as #1, but at the packet level;

3. Accounting information on call setup and

clearing packets, in order to permit international

tariffs to be established.

Common host-to-host protocols are essential to do

anything useful after networks are interconnected. While at

least one candidate for a standard host-to-host protocol has

been suggested [CERT^a], the widespread adoption of any such

standard does not appear imminent. On the other hand, there

are a number of interconnection experiments currently in

progress [GI75] that may lead to a better understanding of

the way in which an acceptable standard should be

structured

.

7.0 BARRIERS TO INTERCONNECTION

Tnere are a number of barriers that have to be overcome

in order to interconnect two networks successfully.

Technical problems must be overcome so that data exchange

and control signaling can be performed efficiently and

reliably between the two systems. Solving the technical

problems may be sufficient for an experimental system.

Interconnecting for commercial purposes raises additional

problems of a legal and regulatory nature.

7.1 Technical Barriers

The primary technical barriers to network

interconnection have been identified in the previous four

chapters for each type of interconnection discussed. In the

following sections, we summarize the situation and prospects

for advances in the major technical areas. Before doing so,

however, it is appropriate to comment on the role of

standards in overcoming technical barriers.

42
7.1.1 Standards -

Tne development of any particular standard or group of

standards in a particular area can assume one of two primary

roles

(1) Development of the standard can serve to codify

existing practice; or

(2) The standard can pace innovation in the field by

providing for compatibility in advance of

implementation, or where implementations are not yet

frozen.

The second role, that of standardizing in advance of

implementation, is somewhat riskier, but the payoff is

potentially much greater in terms of the benefits that may

be gained. Such an approach is frequently criticized on the

grounds tnat it may stifle, rather than simply pace,

innovation. This argument can only be answered when the

alternatives are well understood.

With respect to packet network interface standards, it

has Deen argued that "any" standard will do. This may be an extreme position, but with logic so cheap and getting still

cheaper, tnere is also some validity to this view. There

are strong benefits to be gained from standards in this

area, and the extra cost or effort to modify a suboptimal

standard later may not be very significant.

7.1.2 Host Interface -

It has been suggested that the intense technical

debates of the past few years has resulted in "a remarkable

unanimity regarding tne basic structure of a standard method

for accessing data communication networks" (i.e., a standard

host interface) [SA76J. An essential similarity, from the

"logical structure point of view", is noted for IBM's system

network architecture (SNA), tne CCITT X,25 Recommendation,

and protocols recently announced by Digital Equipment

Corporation, Burroughs, Honeywell, and others. Based on

tnis similarity, it is argued that there is "little

technical justification for each of tne manufacturers to

support network access protocols which differ only in

details .

Lack of technical justification notwithstanding, it is

not clear when, and indeed if, the various proprietary

access protocols will be abandoned in favor of a common

43
standard. The lack of manufacturer support (in the real

sense of software maintenance) for a common access protocol

may be a serious barrier to network growth.

7.1.3 Application Level Protocols - Application level protocols for use in a network

environment have not been seriously addressed by any formal

standards group. This appears likely to change as work is

completed on lower level issues. However, the lack of

suitable protocols at the application level will continue to

be a barrier for the near future. For this reason, most

network utilization will tend to be terminal-to-host or

among private subgroups with homogeneous hosts.

7.1.3.1 User To User Protocols -

Standard end-to-end protocols have been nearly

universally identified as a critical factor in accomplishing

useful tasks in a computer networks [CER7^a, CER74b, COT75,

KI75, MCK74, P075a, P074J. Unfortunately, this is a problem

that has plagued computing since the first few machines were

built with different architectures. The gleam of hope in

the networking environment is that networks may provide the

means as well as the incentives for developing portable

and/or standard software and means of representing data.

A great deal of knowledge has been gained from such

experimental networks as the Arpanet about ways of

standardizing information interchange between dissimilar

systems [CR72]. It remains to build upon this basic

knowledge to design commercially acceptable systems and

standards for today's and tomorrow's public data networks.

Johnson, in a projection on the future roles for packet

switching networks [J076], suggests that "it is reasonable

to hope that the 1980 session of the CCITT will be able to

agree on standards for process-to-process communication

between host computers using a network up to quite a high

level." However, he feels that "even these standards will

have to allow a great deal of choice and flexibility," and

that "it is doubtful if process-to-process communications

between differing systems will ever be a straightforward

matter .

44
7.1.3.2 Simple-Terminal Access -

While host systems differ more widely than do simple

terminals, it was possible to employ their built-in

intelligence to conform to a given interface standard. This

is not directly possible with unintelligent simple

terminals. There are at least two major families of

asynchronous terminals and a number of families of

synchronous terminals all of which are unable to directly

communicate across family boundaries. It appears that a major function of public networks will be to cater for these

different terminals to allow them to intercommunicate. This

will unfortunately necessitate the support of a number of

different interfaces by the network switch or surrogate

device for terminal handling.

At the highest procedural level of signaling according

to what characters are typed, it does seem feasible to

standardize. Work along this line has been identified and

is continuing.

As the cost of logic continues to decline, it may

become feasible at some point to devise a simple and

inexpensive black box to convert lower level terminal

interface functions to a standard. Present approaches of

accomplishing this conversion in switch or terminal support

software does not appear suitable for commercial use.

7.1.^ Gateways -

The lack of a recognized international standard for a gateway between public packet networks cannot presently be

cited as a major obstacle to the interconnection of such

networks, given the early stage of developments and small

number of networks. However, if development of an

international standard is prolonged beyond the point when

the networks are ready to interconnect, then the prospect of

the standard being completed might delay that

interconnection. Administrations might be reluctant to

interconnect according to bilateral standards knowing that

international standards afterwards might obsolete their own

design

.

45
7.2 Non-Technical Barriers

It may well be that the most severe barriers to network

interconnection are of a political/legal nature rather than

of a technological nature. There are, as section 1.3

indicated, a number of quite different vested interests that

are affected in various ways by the prospect and reality of

interconnection. An extensive discussion of all the

pol itical/ legal issues is beyond the scope of this technical

note; we can do no more than illustrate some of the

problems in order to impart an appreciation for this type of

barrier

,

7.2.1 National Barriers -

The prospects for the interconnection of U. S.

domestic networks are influenced both by competitive

considerations and the regulatory policies of the FCC. In

the latter case, the policy has been to encourage, and in

some cases require, networks to provide interconnections

requested by other networks. This is most commonly the case

when a specialized or packet-switched network wishes to use

the telephone system for local delivery. In many cases AT&T

nas been quite reluctant to provide such in teconnection

until required by the FCC,

The situation may be somewhat different for two

value-added networks with roughly the same geographic

coverage. These networks may both not wish to interconnect

due to competitive considerations, though it may be in the

public interest for such interconnection to be available for

customer use. It remains to be seen how this question will

be resolved.

7.2,2 International Barriers - Interconnection of networks across national boundaries

requires (minimally) bilateral agreements between the

nations concerned. Such agreements are generally entered

into quite cautiously, and the services to be offered are

generally limited to those that are available

intranationally in both countries. Multilateral agreements

are even more difficult to obtain than bilateral agreements,

though they may be necessary for effectively coordinating

international networks.

46
7.3 Mixed Barriers

A number of barriers to interconnection cannot easily

be classified as totally technical or totally non-technical.

Such questions as internetwork accounting and the security

provisions to be included in public networks might be

classed as "mixed" barriers to interconnection. Kuo [KU74]

identifies some of the major issues that must be addressed

to overcome these barriers.

8.0 CONCLUSIONS

It is possible now to interconnect widely dispersed

data processing systems and terminals through public

networks. It will soon be possible to do this in a standardized way. Access to systems in Canada seems assured

within the near future; access to systems in Europe and

possibly Japan can also be expected. It has been forecast

that by I98O, international and intercontinental links

between networks will be routine, and most major computer

users in industralized countries will have reasonably easy

access to a worldwide communications utility [J076].

8.1 Standards Development

The development of effective standards for data network

interconnection may continue to be hampered by the large

number of organizations participating. Coordination between

all of these groups is not always as good as it might be,

and the lines dividing areas of responsibility are not

always clearly drawn. The dangers in such a situation are

that so many non-standard and incompatible systems will be

constructed that adoption of an eventual standard would be

difficult and costly; or that a single-company standard

would be imposed on the industry through market power rather

than technical merit.

Fortunately, it appears that both of these dangers can

be avoided for data network interconnection, since the pace

of activity in the formal standards bodies is quickening.

Consensus has been reached for the first stage of X.25, and

work is in progress on extensions. The significance of this

development is likely to be far reaching:

"Before 1976 it was commonly assumed that each

country's packet network would be significantly

different from all others... However, now that CCITT

47
has agreed on a standard interface protocol, X.25, this

potential nightmare is firmly behind us. Perhaps more

than users realize, the adoption of a standard network

access protocol constrains not only this interface, but

also the entire range of network services,, . The

result will undoubtedly (and fortunately) be a remarkable degree of similarity of both structure and

services between the packet networks evolving

throughout the world." [R076j

8.2 Use Of Service

The availability of standardized service from public

networks will obviate the need for many current private data

networks. In the U.S. at least, the computer network

environment promises to be a very competitive one. Some

degree of competition already exists for providing the

communications service nationally, and other companies have

indicated their intention to enter that market. With

respect to the data processing services offered through

networks, a major effect of the network is to increase the

scope of competition. Through networks, data processing

service vendors can compete for clients nationally and

internationally. The effect of network interconnection is

to widen the scope of this competition even more.

The benefits to customers from widened competition are

(1) an increased range of alternatives from which to choose

in solving their problems, and (2) reduced price and/or

improved service resulting from competition in either of

these areas. On the other hand, the availability of

additional alternatives may make the users' task of

selection more difficult. Frisch [FR75] sees a "potential

for disaster" when users try to configure the best network

to meet requirements. Still, the problems arising from an

abundance of alternatives seem preferable to problems

arising from a paucity of them.

48
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53
GLOSSARY*

ASYNCHRONOUS TRANSMISSION - Transmission in which time

intervals between transmitted characters may be of

unequal length. Transmission is controlled by start and

stop elements at the beginning and end of each character,

BLOCK - A group of characters or digits transmitted as a unit, over which a coding procedure is usually applied

for synchronization or error control purposes, Syn

FRAME, TRANSMISSION BLOCK, See also: PACKET.

CENTRALIZED (COMPUTER) NETWORK - A computer network

configuration in which a central node provides computing

power, control, or other services,

CHANNEL - (1) That part of a communications system that

connects a message source to a message sink, (2) A means

of one way transmission.

CIRCUIT - In communications, the complete electrical path

providing one or two way communication between two points

comprising associated send and receive channels,

CIRCUIT SWITCHING - A method of communications, where an

electrical connection between calling and called stations

is established on demand for the exclusive use of the

circuit until the connection is released.

COMMON CARRIER - In telecommunications, a public utility

company that is recognized by an appropriate regulatory

agency as having the authority and responsibility to

furnish communication services to the general public.

COMMUNICATIONS COMPUTER - A computer that acts as the

interface between another computer or terminal and a network, or a computer controlling data flow in a network

,

COMPUTER NETWORK - An interconnection of assemblies of

computer systems, terminals and communications

facilities

,

CONTROL PROCEDURE - The means used to control the orderly

communication of information between STATIONS on a DATA

LINK. Syn: LINE DISCIPLINE. See also: PROTOCOL.

* See also [NE74a].

54
DATA COMMUNICATION - The interchange of data messages from

one point to another over communications channels.

DATA COMMUNICATION EQUIPMENT - The equipment that provides

the functions required to establish, maintain and

terminate a connection, the signal conversion, and coding

required for communications between DATA TERMINAL

EQUIPMENT and data CIRCUIT. The DCE may or may not be an

integral part of the DTE or of a computer, e.g., a MODEM.

Abbr: DCE.

DATA LINK - An assembly of terminal installations and the

interconnecting circuits operating according to a particular method that permits information to be

exchanged between terminal installations. Note: the

method of operation is defined by particular transmission

codes, transmission modes, direction and control.

DATA TERMINAL EQUIPMENT - (1) The equipment comprising the

data source, the data sink, or both. (2) Equipment

usually comprising the following functional units:

control logic, buffer store, and one or more input or

output devices or computers. It may also contain error

control, synchronization, and station identification

capability. Abbr: DTE.

DATA TRANSMISSION - The sending of data from one place for

reception elsewhere.

DECENTRALIZED (COMPUTER) NETWORK - A computer network, where

some of the network control functions are distributed

over several network nodes.

DISTRIBUTED NETWORK - A network configuration in which all

node pairs are connected either directly, or through

redundant paths through intermediate nodes.

FULL DUPLEX OPERATION - Two way simultaneous operation.

GATEWAY - An interface device between two networks.

HALF DUPLEX OPERATION - Two way alternate operation.

HOST COMPUTER - A computer attached to a network providing

primarily services such as computation, data base access

or special programs and programming languages. Compare:

COMMUNICATIONS COMPUTER.

IDENTIFICATION - (1) The process of providing personal,

equipment or organizational characteristics or codes to

gain access to computer programs, processes, files or

55
data. (2) The process of determining personal,

equipment, or organizational characteristics or codes to

permit access to computer programs, processes, files or

data

.

INFORMATION PATH - The functional route by which information

is transferred in a one-way direction from a single data

source to a single data sink,

INFORMATION (TRANSFER) CHANNEL - (1) The functional

connection between the source and the sink data terminal

equipments. It includes the circuit and the associated

data communications equipments. (2) The assembly of data

communication equipment and circuits including a backward

channel if it exists.

INTERFACE - (1) A shared boundary defined by common physical

interconnection characteristics, signal characteristics,

and meanings of interchanged signals. (2) A device or

equipment making possible interoperation between two

systems, e.g., a nardware component or common storage

register, (3) A shared logical boundary between tv/o

software components,

LINK - The logical association of two or more data

communications stations interconnected by the same data

communications circuit for the purpose of transmitting

and receiving data using prescribed data communication

control procedures.

MESSAGE S^;iTCilING - A method of handling messages over

communications networks. The entire message is

transmitted to an intermediate point (i.e., a switching

computer), stored for a period of time, and then

transmitted again towards its destination. The

destination of each message is indicated by an address

integral to tne message.

MODEM - Modulator-demodulator. A device that modulates and

aemoaulates digital signals so that they may be

transmitted over an analog transmission medium, Syn

Data Set.

NETWORK CONTROL PROGRAM - That module of an operating system

in a host computer v^hich establishes and breaks logical

connections, communicating with the network on one side

and witn user processes within the host computer on the

other, Abor: NCP,

56
NODE - (1) An end point of any branch of a network, or a junction common to two or more branches of a network,

(2) Any station, terminal, terminal installation,

communications computer, or communications computer

installation in a computer network,

ONE-WAY ONLY OPERATION - A mode of operation of a data link

in which data are transmitted in a preassigned direction

over one channel. Syn : Simplex Operation,

PACKET - A group of binary digits including data and control

elements which is switched and transmitted as a composite

whole. The data and control elements and possibly

control information are arranged in a specified format.

PACKET SWITCHING - A data transmission process, utilizing

addressed packets, whereby a channel is occupied only for

the duration of transmission of the packet. Note: In

certain data communication networks, the data may be

formatted into a packet or divided and then formatted

into a number of packets (either by the data terminal

equipment or by equipment within the network) for

transmission and multiplexing purposes.

PROCESS - (1) A systematic sequence of operations to produce

a specified result. (2) A set of related procedures and

data undergoing execution and manipulation by one or more

computer processing units.

PROTOCOL - A formal set of conventions governing the format

and relative timing of message exchange between two

communicating processes.

SIMPLEX OPERATION - (1) Two way alternate operation. (2)

One way only operation.

SINK - (1) The point of usage of data in a network. (2) A

data terminal installation that receives and processes

data from a connected channel.

SOURCE - (1) The point of entry of data into a network. (2)

A data terminal installation that enters data into a connected channel,

STAR NETWORK - A computer network with peripheral nodes all

connected to one or more computers at a centrally located

facility

.

STATION - That independently-controllable configuration of

data terminal equipment from or to which messages are

transmitted on a data link. It includes those elements

57
which serve as sources or sinks for the messages, as well

as those elements which control the message flow on the

link by means of data communication control procedures,

STORE AND FORWARD - Pertaining to communications where a message is received, stored and forwarded (as in message

and packet switching).

SYNCHRONOUS TRANSMISSION - A transmission process such that

between any two significant instants there is always an

integral number of unit intervals,

TARIFF - (1) A published rate for services provided by a regulated communications carrier. (2) The means by which

regulatory agencies approve communications services. The

tariff is part of the contract between customer and

carrier

.

TERMINAL - (1) A point in a communications network at which

data can either enter or leave. (2) A device that

permits data entry into or data exit from a computer

system or computer network.

TERMINAL INSTALLATION - (1) The totality of equipment at a user's installation, including data terminal equipment,

data communication equipment, and necessary support

facilities. (2) A set composed of a data terminal, a signal converter, a possibly intermediate equipment;

this set may be connected to a data processing machine or

may be part of it.

TRANSPARENCY - A property of a communications medium to pass

within specified limits a range of signals having one or

more defined properties, e.g., a channel may be code

transparent, or an equipment may be bit pattern

transparent

.

TWO WAY ALTERNATE OPERATION - A mode of operation of a data

link in which data may be transmitted in both directions,

one way at a time (but not simultaneously), Syn

Half-Duplex Operation.

TWO WAY SIMULTANEOUS OPERATION - A mode of operation of a data link in which data may be transmitted simultaneously

in both directions over two channels. Note: one of the

channels is equipped for transmission in one direction

while the other is equipped for transmission in the

opposite direction.

58
VALUE-ADDED SERVICE - A communications service utilizing

communications common carrier networks for transmission

and providing added data services with separate

additional equipment. Such added service features may be

store and forward message switching, terminal

interfacing, and host interfacing.

59
LIST OF ABBREVIATIONS

ADCCP - Advanced Data Communications Control Procedure

ANSI - American National Standards Institute

ARPA - Advanced Research Projects Agency

(U. S. Department of Defense)

ASCII - American Standard Code for Information Interchan

AT&T - American Telephone and Telegraph Company

CCITT - Consultative Committee on International

Telegrapn and Telephone

CEPT - European Conference of Postal and

Telecommunications Administrations

DCE - Data Communications Equipment

DEC - Digital Equipment Corporation

DTE - Data Terminal Equipment

EBCDIC - Extended Binary Coded Decimal Interchange Code

ElA - Electronic Industries Association

FCC - Federal Communications Commission

HDLC - High-level Data Link Control [Procedure]

IBil - International Business Machines Corporation

IMP - Interface Message Processor

ISO - International Standards Organization

ITT - International Telephone and Telegraph Corporation

ITU - International Telecommunications Union

HCI - Microv;ave Communications, Inc.

MERIT - Michigan Educational Research Information Triad

NBS - National Bureau of Standards

NCS - National Communications System

60
NCP - Network Control Program

PAD - Packet Assembly/Disassembly

PCI - Packet Communications, Inc.

PSN - Packet Switched Network

PTT - Postal, Telephone and Telegraph [Authority]

RJE - Remote Job Entry

RPOA - Recognized Public Operating Agency

SDLC - Synchronous Data Link Control [Procedure]

SNA - Systems Network Architecture

TIP - Terminal Interface [Message] Processor

VAN - Value-Added Network

WUI - Western Union International

61
APPENDIX

EXISTING DATA COMMUNICATIONS STANDARDS AND RECOMMENDATIONS

International Standards (ISO)

International Recommendations (CCITT)

American National Standards (ANSI)

Electronic Industry Association Standards (EIA)

Federal Information Processing Standards (NBS)

Federal Telecommunications Standards (NCS)

62
INTERNATIONAL STANDARDS (ISO)

ISO 646 — 1973 7-Bit Coded Character Set for Information

Processing Interchange [cf. CCITT V.3,

ANSI X3.4J

ISO 1 155 —1969 The use of Longitudinal Parity to Detect

Errors in Information Messages

ISO 1177 —1970 Character Structure for Start/Stop and

Synchronous Transmission

ISO 1745 —1975 Basic Mode Control Procedures for Data

Communication Systems

ISO 2022 —1973 Code Extension Techniques for Use With the

ISO 7-Bit Coded Character Set

ISO 21 10 —1972 Data Communication-Data Terminal and Data

Communication Equipment Interchange

Circuits Assignment of Connector Pins

ISO 21 1 1 —1972 Data Communications-Basic Mode Control

Procedures — Code-Independent Information

Transfer

ISO 2375 — 1974 Data Processing — Procedure for

Registration of Escape Sequences

ISO 2593 —1973 Connector Pin Allocations for the High

Speed Data Terminal EQuipraent

ISO 2628 — 1973 Basic Mode Control Procedures—Complements

ISO 2629 —1973 Basic Mode Control Procedures —Conversational Information Message

Transfer

ISO 3309 —1976 Data Communications — High Level Data Link

Control — Frame Structure

ISO DP 4335 High Level Data Link Control Procedures

(Proposed Draft International Standard) —

Elements of Procedure (Independent

Numbering)

63
INTERNATIOWAL RECOMMENDATIONS (CCITT)

Series V Recommendations

(Data Transmission Over Telephone or Telex Networks)

V.I Equivalence between binary notation symbols and

the significant conditions of a two-condition

code

.

V,2 Power levels for data transmission over telephone

lines.

V.3 International alphabet number 5.

V.4 General structure of signals of International

alphabet number 5.

V.IO Use of the telex network for data transmission at

the modulation rate of 50 bauds.

V.11 Automatic calling and/or answering on the telex

network

.

V.13 Answer-back unit simulators.

V.15 Use of acoustic couplers for data transmission.

V.21 200-baud modem standardized for use in the general

switched network.

V.22 Standardization of data-signaling rates for

synchronous data transmission in the general

switched telephone network.

V.22bis Standardization of data signaling rates for

synchronous data transmission on leased telephone

circuits

.

V.23 600/ 1 200-baud modem standardized for use in the

general switched telephone networks.

V.24 List of definitions for interchange circuits

between data terminal equipment and data circuit

terminating equipment.

V.25 Automatic calling and/or answering on the general

switched telepnone network including disabling of

echo-suppresors on manually established calls.

64
V,26 2400 bits per second modem standardized for use on

four-wire leased circuits.

V,26bis 2400/1200 bits per second modem standardized for

use in the general switched telephone network.

V.27 4600 bits per second modem standardized for use

on leased circuits.

V.28 Electrical characteristics for unbalanced

double-current interchange circuits.

V.30 Parallel data transmission modems standardized for

universal use in the general switched telephone

network

V.3I Electrical characteristics for single current

interchange circuits controlled by contact

closure

,

V.35 Data transmission at 48 kilobit per second using

60 to 108 kHz group band circuits.

V.40 Error indication with electromechanical equipment,

V.41 Code-independent error control system.

V.50 Standard limits for transmission quality of data

transmission

V.51 Organization of the maintenance of international

telephone-type circuits used for data

transmission.

V.52 Characteristics of distortion and error rate

measuring apparatus for data transmission.

V.53 Limits for the maintenance of telephone-type

circuits used for data transmission,

V.56 Comparative tests of modems for use over

telephone-type circuits.

V.57 Comprehensive data test set for high data

signaling rates.

65
Series X Recommendations

(Data Transmission Over Public Data Networks)

Section 1: Services and Facilities

X.I User classes of service for public data networks.

X.2 Recommended user facilities available in public

data networks.

Section 2: Data Terminal Equipment and Interfaces

X,20 Interface between data terminal equipment and data

circuit terminating equipment for start-stop

services in user classes 1 and 2 on public data

networks.

X.21 Interfaces between data terminal equipment and

data circuit terminating for synchronous operation

on public data networks.

X.24 List of definitions for interchange circuits

between data terminal equipment and data circuit

terminating equipment on public data networks.

X.25 Interface between data terminal equipment and data

circuit terminating equipment for terminals

operating in the packet mode on public data

networks.

X.30 Standardization of basic model page-printing

machine in accordance with International Alphabet

i\io . 5

.

X,31 Characteristics, from the transmission point of

view, at the interchange point between data

terminal equipment and data circuit terminating

equipment when a 200-baud start-stop data terminal

equipment with International Alphabet No. 5 is

used . X.32 Ansv/er-back units for 200 bauds start-stop

machines in accordance with International Alphabet

No . 5

.

66
standardization of international text for the

measurement of the margin of start-stop machines

in accordance with International Alphabet No. 5.

Section 2.' Transmission, Signaling and Switching

Standardization of frequency shift modulated

transmission systems for the provision of

telegraph and data channels by frequency division

of a primary group.

Fundamental parameters of a multiplexing scheme

for the international interface between

synchronous data networks.

Terminal and transit control signaling system for

start-stop services on international circuits

between anisochronous data networks.

Section 4: Network Parameters

Hypothetical reference connection for packet

switched data transmission services.

Network parameters in public data networks.

Call progress signals in public data networks,

services on international circuits between

anisochronous data networks.

Section 5: Charging Methods and Accounting

67
AMERICAN NATIONAL STANDARDS (ANSI)

X3.1 —1969 Synchronous signaling rates for Data

Transmission

X3.4 — 1968 American Standard Code for Information

Interchange

X3.15 —1966 Bit Sequencing of the American National

Standard Code for Information Interchange in

Serial-by-Bit Data Transmission

X3,16 —1966 Character Structure and Character Parity Sense

for Serial-by-Bit Data Communications in the

American National Standard Code for Information

Interchange

X3.24 — 1968 Signal Quality at Interface between Data

Processing Equipment and Synchronous Data

Communications Equipment for Serial Data

Transmission (EIA RS-33^)

X3.25 —1968 Character Structure and Character Parity Sense

for Parallel-by-Bit Data Communication in the

American National Standard Code for Information

Interchange

X3.28 —1975 Procedures for the Use of the Communication

Control Characters of the American National

Standard Code for Information Interchange in

Specified Data Communication Links

X3.32 —1973 Graphics for the Control Codes of ASCII

X3.36 —1975 High Speed Synchronous Data Signaling Rates

X3.'41 —1974 Code Extension Techniques for Use With ASCII

X3.44 — 1974 Determination of Performance of Data

Communications Systems

X3.57 —* Message Heading Formats for Information

Interchange Using the ASCII for Data

Communication System Control

X3. General Purpose Interface Between Data Terminal

Equipment and Data Circuit Terminating

Equipment for Synchronous Operation on Public

Data Networks

Currently out for ballot.

68
ELECTRONIC INDUSTRY ASSOCIATION STANDARDS (EIA)

RS232C Interface betv/een Data Terminal Equipment and Data

Communications Equipment Employing Serial BinaryData Interchange

RS334 Signal Quality at Interface between Data Terminal

Equipment and Synchronous Data Communications

Equipment for Serial Data Transmission

RS356 Interface between Data Terminal Equipment and

Automatic Calling Equipment for Data Communications

RS422 Electrical Characteristics of Balanced Voltage

Digital Interface Circuits

RS423 Electrical Characteristics of Unbalanced Voltage

Digital Interface Circuits

Under Development

RS-ABC General Purpose Interface Between Data Terminal

Equipment and Data Circuit Terminating Equipment

for Operation on Analog Public Data Networks

RS-XYZ Transitional Interface Between Data Terminal

Eqiupment and Data Circuit Terminating Eqiupment

for Operation on Analog Public Data Networks

69
FEDERAL INFORMATION PROCESSING STANDARDS (NBS)

FIPS PUB 1 Code for Information Interchange (ANSI

ISO 646)

FIPS PUB 16 Bit Sequencing of the Code for Information

Interchange in Serial by Bit Data Transmission

(ANSI X3. 14)

FIPS PUB 17 Character Structure and Character Parity Sense

for Ser ial-by-Bit Data Communication in the

Code for Information Interchange (ANSI X3.16)

FIPS PUB 18 Character Structure and Character Parity Sense

for Parallel-by-Bit Data Communications in the

Code for Information Interchange (ANSI X3.25)

FIPS PUB 22 Synchronous Signaling Rates Between Data

Terminal and Data Communication Equipment

(ANSI X3. 1

)

FIPS PUB 35 Code Extension Techniques in 7 or 8 bits (ANSI

X3.41

)

FIPS PUB 36 Graphic Representation of the Control

Characters of ASCII (ANSI X3.32)

FIPS PUB 37 Synchronous High Speed Data Signaling Rates

Between Data Terminal Equipment and Data

Communications Equipment (ANSI X3.36)

70
FEDERAL TELECOMMUNICATIONS STANDARDS (NCS)

Published Standards

1001 - Synchronous High Speed Signaling Rates Between DTE

and DTE (June 1975)

1002 - Time and Frequency Reference Information in

Telecommunication Systems (April 1974)

1020 - Electrical Characteristics of the Balanced Voltage

Digital Interface (September 1975)

1030 - Electrical Characteristics of the Unbalanced Voltage

Digital Interface (September 1975)

Proposed Standards

1003 - Data Link Control Procedures

1004 - Message Formats

1029 - General Purpose Interface Between Data Terminal

Equipment and Data Circuit Terminating Equipment for

Operation on Analog Telecommunication Networks

1031 - Functional and Mechanical Characteristics of Digital

Interface Circuits

1032 - Signal Quality for Digital Interface Circuits

1033 - Standard Performance Criteria for Assessment of

Telecommunication Systems Supporting Federal

Information Processing Systems

1037 - Glossary of Essential Telecommunication Terms and

Definitions

1040 - General Purpose Interface Between Data Terminal

Equipment and Data Circuit Terminating Equipment for

Synchronous Operation on Digital Telecommunication

Networks

71
INDEX

Page

ADCCP . . Addressing

ANSI . , . Arpanet

Asynchronous

AT&T . . . Block . . Burroughs

10

22, 29, 39

9, 31, 68

25, 37

54

4, 46

. 54

. 13

Carriers . CCITT . . CEPT . . . Channel . Circuit . Circuit switching

Common carrier

Computer network

Control procedure , * * , » Data communication equipment

Data terminal equipment . . Datagram

DCE

Digital Equipment Corporation

DTE

EIA

8, 27, 37, 39, 64

37

54

54

15, 54

4, 54

54

54

55

55

17

55

13

25, 55

69

FCC 7, 46

FIPS 12, 70

Full duplex . 55

Gateway 38, 45, 55

Graphnet 6

Half duplex

HDLC . . . Host . , .

55

8, 28

55

IBM 13, 43

Identification , 55

Interface 10, 19, 56

International record carrier , 5

ISO 8, 28, 63

ITT ....••.....«• 5*6

ITU ,8

72
Link 56

MCI 5

MERIT 24, 36

Message switching 16, 56

Modem 56

Monopoly 4

Multiplexing 18, 26

NBS 11, 70

NCS 12, 71

Network control program ... 56

Node 57

Packet 57

Packet size 29, 38

Packet switching 16, 57

PCI 6

Process 21, 57

Protocol 20, 57

PTT i|, 6, 27

RCA Globcom 5

SNA 43

Source , 57

Special Study Group A .... 9

Specialized common carriers . 5

Star network 57

Station 57

Store and forward 58

Study Group VII 9

Synchronous transmission ... 58

Tariff ..... 58

Telenet 6, 37

Terminal . 58

Transparency 58

Tymnet 26

Tymshare .6

Value-added 6, 18, 59

Virtual circuit 17, 28

Virtual circuit switching . • 15

WUI 5

X.25 27, 43

73
NBS-n4A (REV. 7-73)

U.S. DEPT. OF COMM. BIBLIOGRAPHIC DATA

SHEET

1. PUBLICATION OR REPORT NO. NBS SP-500-6

2. Gov't Accession

No.

3. Recipient's Accession No.

4. TITLE AND SUBTITLE COMPUTER SCIENCE & TECHNOLOGY:

Computer Network Interconnection;

Problems and Prospects

5. Publication Date April 1977

6. Performing Organization Code

7. AUTHOR(S)

Ira W. Cotton

8. Performing Organ. Report No.

9. PERFORMING ORGANIZATION NAME AND ADDRESS

NATIONAL BUREAU OF STANDARDS

DEPARTMENT OF COMMERCE

WASHINGTON, D.C. 20234

10. Project/Task/Work Unit No.

/" f~ O O ~7 O 6502372

11. Contract/Grant No. DCR72-01206 A05

12 Sponsoring Organization Name and Complete Address (Street, City, State, ZIP)

National Science Foundation

1800 G Street, N.W.

Washington, D.C. 20006

13. Type of Report & Period Covered Final

14. Sponsoring Agency Code

15. SUPPLEMENTARY NOTES

Library of Congress Catalog Card Number: 77-608026

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

bibliography or literature survey, mention it here.) This report examines the current situation regarding the interconnection of computer

networks, especially packet switched networks (PSNs). The emphasis is on identifying

the barriers to interconnection and on surveying approaches to a solution, rather than

recommending any single course of action.

Sufficient organizational and technical background is presented to permit an understanding and appreciation of the problem. Four major types of interconnections are

then surveyed: (1) Circuit Switched Network to PSN, (2) Star Network to PSN, (3)

Simple Terminal to PSN, and (4) PSN to PSN. The major barriers to interconnection,

both political/legal and technical, are then outlined. The report concludes with

some comments on the prospects for overcoming these barriers.

An extensive bibliography, glossary with list of abbreviations, and listing of

existing data communications standards relevant to interconnection are also included.

17. KEY WORDS (six to twelve entries; alphabetical order; capitalize only the first letter of the first key word unless a proper name; separated by semicolons)

Communications networks; computer networks; data communications; interconnection;

networks; packet switching; standards.

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