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Packet Switching
and X.25 Networks
Simon Poulton
Pitman
PITMAN PUBLISHING

128 Long Acre, London WC2E 9AN
A Division of Longman Group UK Limited
© S.Poulton 1989
First published in Great Britain 1989
This edition published in the Taylor & Francis e-Library, 2003.
British Library Cataloguing in Publication Data
Poulton, Simon

Packet Switching and x.25 networks
1. Computer systems. Networks. Data
transmission. Packet switching systems
I. Title
004,6 6¢
ISBN 0-203-16884-4 Master e-book ISBN
ISBN 0-203-26412-6 (Adobe eReader Format)
ISBN 0-273-02986-X (Print Edition)

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otherwise, without either the prior written permission of the
Publishers or a licence permitting restricted copying in the
United Kingdom issued by the Copyright Licencing Agency Ltd,
33–34 Alfred Place, London WCIE 7DP. This book may not be
lent, resold, hired out or otherwise disposed of by way of trade in
any form of binding or cover other than that in which it is
published, without the prior consent of the publishers.

Acknowledgements

There are a number of people who have provided invaluable assistance in the
preparation of this book, and to whom I wish to give my thanks.

Chief amongst these is Neil Matthew. Neil spent a great deal of time and effort
on a detailed reading of the manuscript, and found many places where my brain
had gone offline without telling the pen. His sheer hard work is much appreciated.
Neil also made numerous suggestions, both stylistic and technical, which have made
the book much better than it otherwise would have been.

I am very grateful to my wife Judith, who spent many hours on the word
processor deciphering my handwriting and turning it into a presentable document.
She also suffered my preoccupation in this project without complaint, and offered
much needed support and encouragement.

Thanks are also due to Anselm Waterfield one of my colleagues, and to Ian
Campbell of Exeter University, who both read through the finished work and
gave me valuable reassurance.

Finally to Racal-Milgo Ltd., my employers, who loaned the equipment for
the photograph, and Camtec Electronics Ltd. who gave permission for me to
describe their Network Management System.

Contents

1 The packet switching network 1
1.1 People and computers 1
1.2 Networking solutions 4
1.3 Packet switching 6
1.4 The layered network model 7
1.5 The network users 23
1.6 PAD location 26
1.7 Host interface 26
2 The PAD and the switch 29
2.1 Introduction 29
2.2 The two ends of the call 29
2.3 Reverse PAD 37
2.4 PAD commands 38
2.5 Call redirection and call reestablishment 43
2.6 Switch configuration 44
2.7 Parallel routes 45
2.8 The PAD switch 46
2.9 X.3 (1980) parameters 49
2.10 X.28 commands and service signals 57
2.11 Differences in 1984 recommendations 60
3 Topology and components 65
3.1 Introduction 65
3.2 Ring topologies 65
3.3 Bus topologies 70
3.4 Protocols 72
3.5 Modems and line drivers 75
3.6 Types of network 81
3.7 Topologies 83
4 Details of X.25 87
4.1 Introduction 87
4.2 Other types of frame 88
4.3 Other types of packet 97
4.4 Differences between X.25 (1980) and X.25 (1984) 122
5 Network management 127
5.1 Introduction 127
5.2 Information from the protocols 127
5.3 More examples of statistics 130
5.4 Collecting the statistics 132
5.5 Remote collection 133
5.6 Running the network 133
5.7 Managing the network 135
5.8 Network management systems 136
6 Going beyond X.25—the seven-layer model 149
6.1 Introduction 149
6.2 Layering 149
6.3 Interconnection of open systems 156
6.4 Sorting out the X.25 numbering 157
6.5 Connecting networks together 160
6.6 The IBM Systems Network Architecture 162
7 Plugs and wires 167
7.1 Introduction 167
7.2 Physics 167
7.3 Transmission types 168
7.4 Serial and parallel transmission 173
7.5 Simplex and duplex 173
7.6 Signalling rate and data rate 174
7.7 The two ends of the circuit 174
7.8 V.24 178
7.9 ISO 2110 185
7.10 V.28 187
7.11 Cable length 190
7.12 Some example connections 192
7.13 Constructing the cable 199
7.14 The breakout box 200
7.15 Inside the devices 204
7.16 RS-232-C 207
7.17 X.21 208
Appendix A Physical interchange circuits 211
Appendix B X.25 (1980) frame and packet formats 212
Appendix C International alphabet five (IA5) character set 232
Index 234

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