Table Of ContentComputer Communications
Computer
Communications
Robert Cole
Department of Computer Science,
University College, London
Springer Science+Business Media, LLC
© Robert Cole 1982
Originally published by Springer-Verlag New York in 1982.
All rights reserved. No part of this publication may be reproduced or
transmitted, in any form or by any means, without permission.
First published 1982 in the United Kingdom by
The Macmillan Press Ltd
Library of Congress Cataloging in Publication Data
Cole, Robert.
Computer communications.
(Monographs in computer science)
Bibliography: p.
1. Computer networks. I. Title. II. Series.
TK5105.5.C58 1982 621.3819'592 81-21321
AACR2
ISBN 978-1-4757-3917-6 ISBN 978-1-4757-3915-2 (eBook)
DOI 10.1007/978-1-4757-3915-2
Contents
Preface vii
1 Introduction 1
2 Transmission in Wires S
2.1 Information Channel Theory 5
2.2 Channel Organisation 14
2.3 Character Framing 20
2.4 Send and Receive Equipment 25
2.5 Comparison of Character Framing Techniques 27
2.6 Summary 28
3 The Telephone Network as a Medium 30
3.1 The Telephone System 30
3.2 Modulation Techniques 35
3 .3 The MODEM 39
3.4 Digital Data Network 45
3.5 Standards 46
3.6 Summary 48
4 Character Terminal Networks 49
4.1 Character Codes 51
4.2 The Star Arrangement 52
4.3 Remote Multiplexers 56
4.4 Multiplexer Networks 64
4.5 Designing an Asynchronous Terminal Network 65
4.6 Summary 66
Computer Communications
S Simple Message-based Techniques 68
5.1 Multipoint (Multidrop) Lines 68
5.2 Applications 73
5.3 Remote Job Entry Stations 74
5.4 A Simple Message Protocol 75
5.5 Distributed Computing 80
5.6 Summary 82
6 Errors. Error and Flow Control 83
6.1 The Nature of Errors 83
6.2 Error Control 85
6.3 Echo Checking 87
6.4 Forward Error Correction (FEC) 88
6.5 Automatic Repeat Request (ARQ) 92
6.6 Sequence Numbering and Acknowledgement 99
6.7 Summary 106
7 Computer Network Technology 107
7.1 Wide Area Networks 108
7.2 Radio and Satellite Broadcast Networks 120
7.3 Local Networks 125
7.4 Summary 131
8 The X2S Network Access Protocol 133
8.1 Level 1 -The Physical Circuit 134
8.2 Level 2 - Link Level 136
8.3 Operation of Level 2 141
8.4 Level 3 - The Network Level 146
8.5 X25 Implementation 152
8.6 Summary of X25 155
9 End-to-end Network Protocols 156
9.1 Protocol Heirarchy 156
9.2 Datagrams and Virtual Calls 158
9.3 Transmission Control Protocol 160
9.4 Comparison of X25 and TCP Protocols 162
9.5 Network Applications Protocols 163
9.6 Summary 168
Computer Communications
10 The Network as a System 169
10.1 Resource Sharing 169
10.2 Throughput and Delay 172
10.3 Congestion Avoidance 174
10.4 Datagrams and Virtual Circuits 178
10.5 Connecting Networks 180
10.6 Summary 185
GLOSSARY 186
BIBLIOGUPIIY 192
195
Preface
The subject of computer communications is changing very
rapidly. Improvements in terminal access, aligned with
the development of timesharing, has brought hands-on
experience to a large number of non specialist users.
Computer networks have made available vast computing
resources and data banks to these users. This book is
for anyone familiar with using computers who wishes to
understand the techniques used in computer
communications. It is also an introduction to the
architecture of present day computer communication
systems.
I would like to thank Roland lbbett, Steve Treadwell,
Peter Kirstein and Del Thomas for their invaluable
advice and encouragement. My thanks also to Malcolm
Stewart and the staff at Macmillan. The late Gareth
Pugh encouraged my interest in computer communications
and provided the opportunity to develop the material for
this book. The text was formatted on a UNIX computer
system: I am grateful to Professor Kirstein for
permission to use this system. I am indebted to NEC
Telecommunications Europe for the use of a spinwriter
printer on which the master copy was produced.
Finally, no amount of words can express my debt to Jo
and Rosemary for patiently bearing with this project
over the last three years.
UNIX is a trademark of Bell Laboratories
1 Introduction
This text is concerned with the way computer-based
information is transferred over long distances, and with
the organisation of the communication system to meet
various needs. A computer system usually consists of a
CPU, memory, peripherals (on site) and communication
devices. These are normally connected together by one
or more short, high speed, data highways (figure 1.1).
Computer organisation around the data highway is one of
the topics of computer architecture. This book assumes
the reader has some knowledge of computer system
organisation and is familiar with using a computer. The
subject of computer communications involves the problems
and solutions of transporting data over distances longer
than the internal computer data highway. For practical
purposes we will treat a communications device as a
piece of equipment that is used to transfer computer
data to other equipment outside the computer room.
There are exceptions of course: for example the computer
console terminal may use communications techniques
covered in this book, and some devices such as tape
drives and large discs may be 'outside' the computer
room. Figure 1.2 shows the various types of
communication arrangements that will be covered. Each
type of organisation and the techniques used for the
device and the computer to exchange information will be
dealt with.
The increase in the organisational complexity of
computer communications reflects the trend in computing
overall. Increased computing power, the cost of which
is falling, is enabling the more efficient usage of
resources, such as communications devices, the cost of
which is r1s1ng. Even where communications costs are
falling the cost of computing power is falling faster.
This theme can be seen as a growing trend throughout the
book. As all communications uses limited bandwidth
channels, more and more complexity is added to the
2 Introduction
fast
memory peripherals
(discs)
fast
CPU ,~..---- system
bus
slow communi-
periphera Is cation
(CR, LP) interface
Figure 1.1 Computer System Components
communications system to obtain the maximum efficiency
from the communications channel. Increasing the system
complexity requires more computing power, but the cost
of extra processing is outweighed by the improved
efficiency of the system.
~
'"j remote
terminal
M=modem
..
C =concentrator ~ ~':;~,,
remote job entry
Figure 1.2 Communications Systems
