Data networks IP and the internet

Data Networks, IP and the Internet


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Data Networks, IP and the Internet
Protocols, Design and Operation

Martin P. Clark
Telecommunications Consultant, Germany


Copyright  2003

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Library of Congress Cataloging-in-Publication Data
Clark, Martin P.
Data networks, IP, and the Internet : networks, protocols, design, and operation / Martin
P. Clark.
p. cm.
Includes bibliographical references and index.
ISBN 0-470-84856-1
1. Computer networks. 2. TCP/IP (Computer network protocol) 3. Internet. I. Title.
TK5105.5 .C545 2003
004.6–dc21
2002191041
British Library Cataloguing in Publication Data
A catalogue record for this book is available from the British Library
ISBN 0-470-84856-1
Typeset in 9.5/11pt Times by Laserwords Private Limited, Chennai, India
Printed and bound in Great Britain by Biddles Ltd, Guildford and King’s Lynn
This book is printed on acid-free paper responsibly manufactured from sustainable forestry
in which at least two trees are planted for each one used for paper production.

F¨ur Ruth,
in Erinnerung an Wade
in dessen Gesellschaft dieses Buch entstanden ist.


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Contents

Preface

xiii

Acknowledgements
Foreword

xvii

1 The Internet, Email, Ebusiness and the Worldwide Web (www)
1.1

xv

In the beginning — ARPANET

1
1

1.2

The emergence of layered protocols for data communication

2

1.3

SNA (systems network architecture)

3

1.4

DECnet

5

1.5

Other mainframe computer manufacturers

5

1.6

X.25 (ITU-T recommendation X.25)

5

1.7

DTE (data terminal equipment), DCE (data circuit-terminating equipment),
line interfaces and protocols

7

1.8
1.9

UNI (user-network interface), NNI (network-network interface) and INI
(inter-network interface)

10

Open systems interconnection (OSI)

11

1.10

EDI (electronic data interchange)

17

1.11

CompuServe, prestel, minitel, BTx (Bildschirmtext) and teletex

18

1.12

The role of UNIX in the development of the Internet

20

1.13

The appearance of the PC (personal computer)

20

1.14

Local area networks (LANs)

20

1.15

LAN servers, bridges, gateways and routers

22

1.16

Why did IP win through as the standard for ‘open’ communication?

24

1.17

The development and documentation of IP (Internet protocol) and the
Internet

24

1.18

Electronic mail and the domain name system (DNS)

24

1.19

html, WindowsNT and the Worldwide Web

26

1.20

Internet addresses and domain names

26

1.21

What are ISPs (Internet service providers) and IAPs (Internet access

providers)?

27

The emergence of ebusiness

27

1.22


viii

Contents

2 Fundamentals of Data Communication and Packet Switching

29

2.1

The binary code

29

2.2

Electrical or optical representation and storage of binary code numbers

30

2.3

Using the binary code to represent textual information

31

2.4

ASCII (American standard code for information interchange)

31

2.5

EBCDIC and extended forms of ASCII

34

2.6

Use of the binary code to convey graphical images

35

2.7

Decoding binary messages — the need for synchronisation and for avoiding
errors

36

2.8

Digital transmission

37

2.9

Modulation of digital information over analogue media using a modem

38

2.10

Detection and demodulation — errors and eye patterns

44

2.11

Reducing errors — regeneration, error detection and correction

48

2.12

Synchronisation

51

2.13

Packet switching, protocols and statistical multiplexing

55

2.14

Symmetrical and asymmetrical communication: full duplex and all that!

60

2.15

Serial and parallel communication

62

2.16

The problem of long lines — the need to observe the maximum line length

62

3 Basic Data Networks and Protocols

67

3.1

The basic components of a data network

67

3.2

Layer 1 — physical layer interface: DTE/DCE, line interfaces and protocols

70

3.3

Layer 2 — data link layer

96

3.4

Layer 3 — network layer and network layer addresses

103

3.5

Layer 4 — transport layer protocol

111

3.6

Layers 5–7 — higher layer protocols

114

3.7

Protocol stacks and nested protocol control information (PCI)

117

3.8

Real networks and protocol stack representations

119

Protocol encapsulation

119

3.10

3.9

Control and management protocols

120

3.11

Propagation effects affecting protocol choice and network design and
operation

122

4 Local Area Networks (LANs)

125

4.1

The different LAN topologies and standards

4.2

Ethernet (CSMA/CD; IEEE 802.3)

126

4.3

Ethernet LAN standards (IEEE 802.3 and 802.2)

128

4.4

Ethernet LAN datalink layer protocols — LLC and MAC

129

4.5

Ethernet physical layer — basic functions of the physical layer signalling
(PLS)

135

Ethernet hubs (half duplex repeaters)

136

4.6

125


Contents

ix

4.7

Alternative physical layers — ethernet, fast ethernet and gigabit ethernet

138

4.8

LAN segments and repeaters — extending the size of a single collision
domain

142

4.9

LAN switches — extending coverage and managing traffic in LAN networks

145

4.10

Other types of LAN (token ring and token bus)

149

4.11

LAN operating software and LAN servers

156

4.12

Interconnection of LANs — bridges, switches, VLANs, routers and
gateways

157

5 WANs, Routers and the Internet Protocol (IP)

165

5.1

WANs (wide area networks), routers, Internet protocol (IP) and IP addresses

165

5.2

Main functions of routers

167

5.3

Unicast, broadcast, multicast and anycast forwarding

172

5.4

Routing table format — static and dynamic routing

173

5.5

Routing table conventions

176

5.6

Simple IP routing control mechanisms: time-to-live (ttl) and hop limit fields

177

5.7

Internet protocol version 4 (IPv4)

178

5.8

ICMP (Internet control message protocol)

184

5.9

Internet addressing (IPv4)

187

5.10

Differentiated services (Diffserv and DS field)

193

5.11

Internet protocol version 6 (IPv6)

198

5.12

ICMP for IPv6

204

5.13

IPv6 addressing

205

5.14

Multicasting

209

6 Routing Tables and Protocols

215

6.1

Routing tables: static and dynamic routing — a recap

215

6.2

Choosing the best route by comparing the routing distance or cost of the
alternatives

216

6.3

Storage, updating and recalculation of the routing table and routing database

218

6.4

The accuracy and stability of routing tables

219

6.5

Representation of destinations in a routing table

222

6.6

Routing protocols and their associated algorithms and metrics

223

6.7

Distributing routing information around an internetwork

223

6.8

Distance vector and link state protocol routing methodologies

227

6.9

Initiating router protocols: neighbour discovery and the hello procedure

229

6.10

Routing protocols and their relationship with the Internet protocol (IP)

229

6.11

The different internetwork routing protocols — when to use them

230

6.12

RIP (routing information protocol)

232

6.13

OSPF (open shortest path first)

237

6.14

BGP4 (border gateway protocol version 4)

259


x

Contents
6.15

Problems associated with routing in source and destination local networks

266

6.16

Routing management issues

274

7 Transport Services and Protocols

277

7.1

Transport services and end-to-end communication between hosts

277

7.2

User datagram protocol (UDP)

282

7.3

Transmission control protocol (TCP)

283

7.4

Resource reservation protocol (RSVP)

299

7.5

MPLS (multiprotocol label switching)

305

8 IP Networks in Practice: Components, Backbone
and Access

317

8.1

The components and hierarchy of an IP-based data network

317

8.2

The Internet, intranets, extranets and VPN

320

8.3

Network technologies typically used in IP-backbone networks

323

8.4

Access network technologies

332

8.5

Link establishment and control

338

8.6

Wireless technologies for Internet access

350

8.7

Host functionality and software for communication via IP

354

9 Managing the Network

357

9.1
9.2

Managing and configuring via the console port
Basic network management: alarms, commands, polling, events
and traps

357

9.3

Management information base (MIB) and managed objects (MOs)

361

359

9.4

Structure of management information (SMIv1 and SMIv2)

364

9.5

Management information base-2 (mib-2 or MIB-II)

365

9.6

Remote network monitoring (RMON)

366

9.7

MIB for Internet protocol version 6 (ipv6MIB)

369

9.8

Simple network management protocol (SNMP)

376

9.9

The ISO management model: FCAPS, TMN, and CMIP/CMISE

393

Tools for network management

397

9.10

10 Data Networking and Internet Applications
10.1

Computer applications and data networks: application layer protocols

407
407

10.2

Telnet

411

10.3

FTP (file transfer protocol)

416

10.4

TFTP (trivial file transfer protocol)

425

10.5

Secure shell program and protocol (SSH or SECSH)

428

10.6

RTP/RTPC: real time signal carriage over IP networks

444

10.7

Applications, protocols and real networks

448

10.8

Other network/application protocols of note

450


Contents

11 The Worldwide Web (www)

xi

453

11.1

The emergence of the Worldwide Web (www)

453

11.2

Domain name system (DNS)

454

11.3

Internet cache protocol (ICP)

464

11.4

WINS; Windows2000 ADS; Novell NDS

464

11.5

Hypertext transfer protocol (http)

465

11.6

Hypertext markup language (html)

476

11.7

Web browsers

479

11.8

Web-based applications

480

12 Electronic Mail (email)

483

12.1

A typical electronic mail

483

12.2

The benefits of electronic mail (email)

484

12.3

The principles of the Internet mail transfer system (MTS)

485

12.4

Operation of the Internet mail system

487

12.5

The Internet message format

489

12.6

Simple mail transfer protocol (SMTP)

494

12.7

Internet mail access protocol (IMAP4)

499

12.8

The post office protocol version 3 (POP3)

503

13 Data Network Security

507

13.1

The trade-off between confidentiality and interconnectivity

507

13.2

Data network protection: the main types of threat and counter-measure

508

13.3

Destination access control methods

512

13.4

Firewalls

516

13.5

Path protection

524

13.6

Network entry or access control

541

13.7

Encryption

550

13.8

Application layer interface for security protocols

560

13.9

Other risks and threats to data security and reliable network operations

560

14 Quality of Service (QOS), Network Performance
and Optimisation

565

14.1

Framework for network performance management

565

14.2

Quality of service (QOS) and network performance (NP)

566

14.3

Quality of service (QOS), type of service (TOS) and class of service (COS)

569

14.4

Data network traffic theory: dimensioning data networks

575

14.5

Application design factors affecting quality of service

585

14.6

Network design for efficient, reliable and robust networks

586

14.7

Network operations and performance monitoring

595

14.8

Network management, back-up and restoration

596

14.9

Performance optimisation in practice

606


xii

Contents

15 Challenges Ahead for IP

611

15.1

Financing the network

611

15.2

Network architecture, interconnection and peering

612

15.3

Quality of service (QOS) and network performance (NP)

612

15.4

Scaling and adapting the network for higher speeds and real-time
applications

612

Network management

613

15.5

Appendix 1

Protocol Addresses, Port Numbers, Service Access
Point Identifiers (SAPIs) and Common Presentation
Formats

615

Internet Top-Level Domains (TLDs) and Generic
Domains

633

Internet Country Code Top-Level Domains
(ccTLDs — ISO 3166-1)

635

Internet Engineering Task Force (IETF) Request for
Comment (RFC) Listing

639

Appendix 5

IEEE 802 Standards for LANs and MANs

657

Appendix 6

IEEE 802.11: Wireless Local Area Networks
(WLANs)

661

Appendix 7

Interfaces, Cables, Connectors and Pin-outs

667

Appendix 8

X.25 Packet Switching (ITU-T Recommendation
X.25)

685

Frame Relay

691

Appendix 2
Appendix 3
Appendix 4

Appendix 9

Appendix 10 Asynchronous Transfer Mode (ATM)

699

Glossary of Selected Terms

717

Abbreviations and Standards Quick-Reference

737

Bibliography

809

Index

815


Preface

The business world relies increasingly upon data communications, and modern data networks
are based mainly on the Internet or at least on the IP (Internet Protocol). But despite these
facts, many people remain baffled by IP and multiprotocol data networks. How do all the
protocols fit together? How do I build a network? And what sort of problems should I expect?
This book is intended for experienced network designers and practitioners, as well as for
the networking newcomer and student alike: it is intended to provide an explanation of the
complex jargon of networking: putting the plethora of ‘protocols’ into context and providing
a quick and easy handbook for continuing reference.
Even among experienced telecommunications and data-networking professionals, there is
confusion about how data network components and protocols work and how they affect the
performance of computer applications. I have myself bought many books about the Internet, about IP and about multiprotocol networks, but found many of them ‘written in code’.
Some have the appearance of computer programmes, while others perversely require that you
understand the subject before you read them!
Putting the pieces of knowledge and the various components of a network

together — working out how computers communicate — can be a painstaking task requiring
either broad experience or the study of a library full of books. The experience has spurred me
to write my own book and handy reference and this is it. My goal was a text in plain language,
building slowly upon a solid understanding of the principles — introducing a newcomer slowly
and methodically to the concepts and familiarising him or her with the language of data
communications (the unavoidable ‘jargon’) — but always relating new topics back to the
fundamentals:
• relating to the real and tangible;
• sharing experiences and real examples;
• not only covering the theoretical ‘concepts’; but also
• providing practical tips for building and operating modern data networks.
The book covers all the main problems faced by data network designers and operators: network
architecture and topology, network access means, which protocol to use, routing policies,
redundancy, security, firewalls, distributed computer applications, network service applications,
quality of service, etc.
The book is liberally illustrated and written in simple language. It starts by explaining the
basic principles of packet-data networking and of layered protocols upon which all modern
data communications are based. It then goes on to explain the many detailed terms relevant
to modern IP networks and the Internet. My goal was that readers who only wanted to ‘dip
in’ to have a single topic explained should go away satisfied — able to build on any previous
knowledge of a given subject.
The extensive set of annexes and the glossary of terms are intended to assist the practising
engineer — providing a single reference point for information about interfaces, protocol field


xiv

Preface

names and formats, RFCs (Internet specifications) and acronyms (the diagrams and some of

the appendices are also available for download at: With so
many acronyms and other terms, protocols, code-fields, and technical configuration information
to remember, it is impossible to expect to keep all the details ‘in your head’! And to distinguish
where jargon and other special ‘telecommunications vocabulary’ is being used in the main text,
I have highlighted terms as they are being defined by using italics.
The book is intended to provide a complete foundation textbook and reference of modern
data networking — and I hope it will find a valued position on your bookshelf. Should you
have any suggestions for improvement, please let me know!
Martin Clark


Acknowledgements

No book about the Internet can fail to recognise the enormous contribution which has been
made to the development of the Internet by the Internet Engineering Task Force (IETF) and
its parent organisation, the Internet Society. Very many clever and inspired people have contributed to the process and all those RFC (request for comments) documents — unfortunately
far too many to allow individual recognition.
I would also like to thank the following organisations for contribution of illustrations and
granting of copyright permission for publication:
• Apple Computer;
• Black Box Corporation and Black Box Deutschland GmbH;
• France T´el´ecom;
• IBM;
• International Telecommunications Union;
• Microsoft Corporation/Waggener Edstrom;
• RS Components Ltd.
The media departments of each of the organisations were both kind and helpful in processing
my requests, and I would like to thank them for their prompt replies. The experience leads
me in particular to recommend the online IBM archive (www.ibm.com/ibm/history) as well as
the cabling and component suppliers: Black Box Corporation, Black Box Deutschland GmbH

and RS Components Ltd.
The copyright extracts drawn from ITU-T recommendations were chosen by the author,
but reproduced with the prior authorisation of the ITU. All are labelled with their source
accordingly. The full texts of all ITU copyright material may be obtained from the ITU Sales
and Marketing Division, Place des Nations, CH-1211 Geneva 20, Switzerland, Telephone: +41
22 730 6141 (English) / +41 22 730 6142 (French) / +41 22 730 6143 (Spanish), Telex: 421
000 uit ch, Fax: +41 22 730 5194, email: or Internet: www.itu.int/publications.
Finally I would like to thank my ‘personal assistants’ — who assisted in wading through
the voluminous drafts and made suggestions for improvement:
• my brother, Andrew Clark;
• my close friend and data networking colleague, Hubert G¨artner;
• Jon Crowcroft of Cambridge University — who spent many hours patiently reviewing the
manuscript and explaining to me a number of valuable suggestions;


xvi

Acknowledgements

• Susan Dunsmore (the poor copy editor) — who had to struggle to correct all the italics,
‘rectos’ and ‘decrements’ — and not only that, but also had to make up for what my English
grammar teacher failed to drill into me at school;
• the production and editorial staff at John Wiley — Zo¨e Pinnock, Sarah Hinton and Mark
Hammond.
Martin Clark


Foreword

Before we start in earnest, there are three things I would like you, the reader, to keep in mind:

1 The first part of the book (Chapters 1–3) covers the general principles of data communications. This part is intended to introduce the concepts to data communications newcomers.
Chapters 4–15 build on this foundation to describe in detail the IP (Internet protocol) suite
of data communications protocols and networking procedures.
2 Terms highlighted in italics on their first occurrence are all telecommunications vocabulary
or ‘jargon’ being used with their strict ‘telecommunications meaning’ rather than their
meaning in common english parlance.
3 Although the book is structured in a way intended to ease a reader working from ‘cover to
cover’, you should not feel obliged to read it all. The extensive index, glossary and other
appendices are intended to allow you to find the meaning of individual terms, protocols
and other codes quickly.


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1
The Internet, Email, Ebusiness
and the Worldwide Web (www)
Nowadays every self-respecting person (particularly if a grandparent!) has a personal email address. And many modern companies have encompassed ebusiness.
They have prestigious Internet ‘domain names’ (advertised with modern lower case
company names) and run Worldwide Web (www) sites for advertising and ordertaking. What has stirred this revolution? The Internet. But when, why and how did
data networking and interworking start? And how did the Internet evolve? Where
will it lead? And what does all that frightful jargon mean? (What are the acronyms
and the protocols?). In this chapter we shall find out. We shall talk about the emergence of computer networking, the Worldwide Web (www), about ISPs (Internet
service providers) and about where the Internet started — in the US Defense Department during the 1970s. We discuss the significance of the Internet Protocol (IP)
today, and where it will lead. And most important of all — we start ‘unravelling’
the jargon.

1.1 In the beginning — ARPANET
The beginnings of the Internet are to be found in the ARPANET, the advanced research project

agency network. This was a US government-backed research project, which initially sought to
create a network for resource-sharing between American universities. The initial tender for a 4node network connecting UCLA (University of California, Los Angeles), UCSB (University of
California, Santa Barbara), SRI (Stanford Research Institute) and the University of Utah took
place in 1968, and was won by BBN (Bolt, Beranek and Newman). The network nodes were
called Internet message processors (IMPs), and end-user computing devices were connected
to these nodes by a protocol called 1822 (1822 because the Internet engineering note (IEN)
number 1822 defined the protocol). Subsequently, the agency was increasingly funded by the
US military, and consequently, from 1972, was renamed DARPA (Defense Advanced Research
Project Agency).
These beginnings have had a huge influence on the subsequent development of computer
data networking and the emergence of the Internet as we know it today. BBN became a
leading manufacturer of packet switching equipment. A series of protocols developed which
are sometimes loosely referred to either as TCP/IP (transmission control protocol/Internet
protocol) or as IP (Internet protocol). Correctly they are called the ‘IP-protocol suite’. They
are defined in documents called RFCs (request for comment) generated under the auspices

Data Networks, IP and the Internet: Protocols, Design and Operation
 2003 John Wiley & Sons, Ltd ISBN: 0-470-84856-1

Martin P. Clark


2

The Internet, email, ebusiness and the worldwide web (www)

of the Internet Engineering Task Force (IETF). The current most-widely used version of the
Internet protocol (IP) — version 4 or IPv4 — is defined in RFC 791. The current version of
TCP (transmission control protocol) is defined in RFC 793.

1.2 The emergence of layered protocols for data communication
In parallel with the development of the ARPANET, a number of standardised layered protocol ‘stacks’ and protocol suites for simplifying and standardising the communication between
computer equipment were being developed independently by various different computer and
telecommunications equipment manufacturers. Most of these protocols were ‘proprietary’. In
other words, the protocols were based on the manufacturers’ own specifications and documentation, which were kept out of the public domain. Many manufacturers believed at the
time that ‘proprietary’ protocols gave both a ‘competitive advantage’ and ‘locked’ customers
into using their own particular brand of computer hardware. But the principles of the various
schemes were all similar, and the ideas generated by the various groups of developers helped
in the development of the standardised protocols which came later.
All data communications protocols are based upon packet switching, a form of electronic
inter-computer communication advanced by Leonard Kleinrock of MIT (Massachusetts Institute of Technology — and later of UCLA — University of California in Los Angeles) in his
paper ‘Information flow in large communication networks’ (July 1961). The term packet
switching itself was coined by Donald Davies of the UK’s National Physical Laboratory
(NPL) in 1966.
Packet switching is analogous to sending letters through the post — the data content, analogous to a page of a letter is the user content (or payload ) of a standard packet. The user
content is packed in the packet or frame (analogous to an ‘envelope’) and labelled with the
destination address. When the size of a single packet is too small for the message as a whole,
then the message can be split up and sent as a sequence of numbered packets, sent one after
another (see Figure 1.1). The networking nodes (which in different types of data networks have
different names: routers, switches, bridges, terminal controllers, cluster controllers, front-end

Figure 1.1

Post Office analogy illustrating the principles of packet switching.


SNA (systems network architecture)

3

processors, etc.) all essentially work like a postal sorting office. They read the ‘address’ on
each packet (without looking at the contents) and then forward the packet to the appropriate
next node nearer the destination.
The best-known, most successful and widely used of the 1970s generation of packetswitching protocols were:
• SNA (systems network architecture) — the networking protocols used for interconnecting
IBM (International Business Machines) computers;
• DECnet — the networking protocols used for interconnecting computers of the Digital
Equipment Corporation (DEC);
• X.25 (ITU-T recommendation X.25) and its partner protocol, X.75. This was the first
attempt, coordinated by the International Telecommunications Union standardisation sector
(ITU-T), to create a ‘standard’ protocol — intended to enable computers made by different
manufacturers to communicate with one another — so-called open systems interconnection (OSI).

1.3 SNA (systems network architecture)
The systems network architecture (SNA) was announced by IBM in 1974 as a standardised
communications architecture for interconnecting all the different types of IBM computer hardware. Before 1974, transferring data or computer programs from one computer to another
could be a time-consuming job, sometimes requiring significant manual re-formatting, and
often requiring the transport of large volumes of punched cards or tapes. Initially, relatively
few IBM computers were capable of supporting SNA, but by 1977 the capabilities of the third
generation of SNA (SNA-3) included:
• communication controllers (otherwise called FEPs or front end processors) — hardware
which could be added to mainframe computers for taking over communication with
remote devices;
• terminal controllers (otherwise called cluster controllers) — by means of which, end-user
terminals (teletypes or computer VDUs, video display units) could be connected to a remote
host computer;
• the possibility to connect remote terminal controllers to the mainframe/communication
controller site using either leaselines or dial-in lines;
• the possibility of multi-host networks (terminals connected to multiple mainframe
computers — e.g., for bookkeeping, order-taking, personnel, etc. — by means of a single

communications network).
Figure 1.2 illustrates the main elements of a typical SNA network, showing the typical star
topology. Point-to-point lines across the wide area network (WAN) connect the front end
processor (FEP or communications controller) at the enterprise computer centre to the terminals
in headquarters and remote operations offices. The lines used could be either leaselines, pointto-point X.25 (i.e., packet-switched ) connections, frame relay connections or dial-up lines.
During the 1980s and 1990s, SNA-based networks were widely deployed by companies
which used IBM mainframe computers. At the time, IBM mainframes were the workhorse
of the computing industry. The mainframes of the IBM S/360, S/370 and S/390 architectures
became well known, as did the components of the SNA networks used to support them:


4

The Internet, email, ebusiness and the worldwide web (www)

Figure 1.2

A typical SNA network interconnecting IBM computer hardware.

• Front end processor (FEP or communication controller) hardware: IBM 3705, IBM 3725,
IBM 3720, IBM 3745;
• Cluster controller hardware: IBM 3174, IBM 3274, IBM 4702, IBM 8100;
• VTAM (virtual telecommunication access method) software used as the mainframe communications software;
• CICS (communication information control system) mainframe management software;
• NCP (network control program) front end processor communications control software;
• NPSI (NCP-packet switching interface) mainframe/FEP software for use in conjunction
with X.25-based packet-switched WAN data networks;
• TSO (time sharing option) software allowing mainframe resources to be shared by many
users;
• NetView mainframe software for network monitoring and management;

• APPN (advanced peer-to-peer networking) used in IBM AS-400 networks;
• ESCON (enterprise system connection): a high-speed ‘channel’ connection interface
between mainframe and front-end processor;
• Token ring local area network (LAN).
Due to the huge popularity of IBM mainframe computers, the success of SNA was assured. But
the fact that SNA was not a public standard made it difficult to integrate other manufacturers’
network and computer hardware into an IBM computer network. IBM introduced products
intended to allow the integration of public standard data networking protocols such as X.25
and Frame Relay, but it was not until the explosion in numbers of PCs (personal computers)
and LANs (local area networks) in the late 1980s and 1990s that IBM lost its leading role in


X.25 (ITU-T recommendation X.25)

5

the data networking market, despite its initial dominance of the personal computer market.
LANs and PC-networking heralded the Internet protocol (IP), routers and a new ‘master’ of
data networking — Cisco Systems.

1.4 DECnet
The Digital Equipment Corporation (DEC) was another leading manufacturer of mainframes
and computer equipment in the 1980s and 1990s. It was the leading force in the development
of mini-computers, workstations and servers and an internationally recognised brand until it
was subsumed within COMPAQ (which in turn was swallowed by Hewlett Packard). DEC
brought the first successful minicomputer (the PDP-8) to the market in 1965.
Like IBM, DEC built up an impressive laboratory and development staff. The main philosophy was that software should be ‘portable’ between the various different sizes of DEC
hardware platforms and DEC became a prime mover in the development of ‘open’ and public
communications standards.
DECnet was the architecture, hardware and software needed for networking DEC computers. Although some of the architecture remained proprietary, DEC tended to incorporate public

standards into DECnet as soon as they became available, thereby promoting ‘open’ interconnectivity with other manufacturers’ devices. The technical legacy of DEC lives on — their
very high performance alpha servers became the basis of the server range of COMPAQ. In
addition, perhaps the oldest and best-known Internet search engine, Alta Vista, was originally
established by DEC. Unfortunately, however, the commercial management of DEC did not
match its technical prowess. The company overstretched its financial resources, largely through
over-aggressive sales, and was taken over by COMPAQ in 1998 (and subsequently subsumed
by Hewlett Packard in 2002).

1.5 Other mainframe computer manufacturers
In the 1970s and 1980s, there were a number of large computer mainframe manufacturers — Amdahl, Bull, Burroughs, DEC, Honeywell, IBM, Rockwell, Sperry, Sun Microsystems,
UNIVAC, Wang, etc. Each had a proprietary networking and operating system architecture,
or in the case of Amdahl and Wang, positioned their products as low cost alternatives to IBM
hardware. Where these companies have survived, they have been largely ‘reincarnated’ as service, maintenance, support and application development companies. Typically they sell other
people’s computer and networking hardware and specialise in system integration, software
development and support. Burroughs, Sperry and UNIVAC, for example, all became part of
the computer services company known today as UNISYS.

1.6 X.25 (ITU-T recommendation X.25)
ITU-T’s recommendation X.25 defines a standard interface for connecting computer equipment
to a packet switched data network (see Figure 1.3). The development of the X.25-interface
and the related packet-switched protocols heralded the appearance of public data networks
(PDN). Public data networks were meant to provide a cost-effective alternative for networking
enterprise computer centres and their remote terminals.
By using a public data network, the line lengths needed for dedicated enterprise-network
connections could be much shorter. No longer need a dedicated line stretch from the remote
site all the way to the enterprise computer centre as in Figure 1.2. Instead a short connection
to the nearest PSE (packet switch exchange) was adequate. In this way, the long distance


6

The Internet, email, ebusiness and the worldwide web (www)

Figure 1.3

A typical public packet-switched network.

lines between PSEs in the wide area network and the costs associated with them were shared
between different networks and users (see Figure 1.3). Overall network costs can thus be
reduced by using public data networks (assuming that the tariffs are reasonable!). In addition,
it may be possible to get away with fewer ports and connection lines. In the example of
Figure 1.3, a single line connects the front end processor (FEP) to the network where in
Figure 1.2. three ports at the central site had been necessary.
The X.25-version of packet switching, like SNA, DECnet and other proprietary data networking architectures, was initially focused on the needs of connecting remote terminals to
a central computer centre in enterprise computing networks. In commercial terms, however,
it lacked the success which it deserved. Though popular in some countries in Europe, X.25
was largely ignored in the USA. The X.25 standard (issued in 1976) had arrived late in comparison with SNA (1974) and did not warrant a change-over. On an economic comparison,
it was often as cheap to take a leaseline and use SNA than it was to use a public X.25
network to connect the same remote site. As a result, enterprise computing agencies did not
rush to X.25 and the computer manufacturers did not make much effort to support it. The
IBM solution for X.25 using NPSI (NCP-packet switching interface), for example, always
lacked the performance of the equivalent SNA connection. Only in those countries where
leaselines were expensively priced (e.g., Germany) did X.25 have real success. In Germany,
the Datex-P packet-switched public data network of the Deutsche Bundespost was one of the
most successful X.25 networks.
In the case where a remote dumb terminal is to be connected to a computer across a public
data network, a PAD may be used. A PAD (packet assembler/disassembler) is a standard
device, defined by the packet-switching standards in ITU-T recommendation X.3. Its function
is to convert the keystrokes of a simple terminal into packets which may be forwarded by
means of a packet-switched network to a remote computer. A number of different parameters

are defined in X.3 which define the precise functioning of the PAD. The parameters define
the linespeed to be used, the content of each packet and the packet flow control. Typically
the PAD would be adjusted to forward complete commands to the central computer. Thus a
number of keystrokes, as making up a series of command words, would first be collected by
the PAD, and only forwarded in a single packet once the human user typed the <return>