Computers in communication

OBJECTIVES The building of ‘information superhighways’, in the first instance exemplified by the massive growth of the Internet, has introduced the general public to computers in communication. The end result is to allow the sharing of computerbased resources that are physically located in all parts of the world. From the point of view of most users, the mechanisms by which computers communicate are best kept hidden. However, without these mechanisms, no sharing of resources could happen. This book aims to reveal the basic principles of computer communications, and show how these underpin the practical communication mechanisms that are actually used. It is intended both for readers who will be involved in getting computers to communicate, and also those who just have a general interest in what happens under the tarmac of the information highway. The book is deliberately focused tightly on matters concerned with communic ation. It avoids excursions into other areas of computer science, such as computer programming, modelling or simulation, not to mention electronics, photonics or mathematics. Such matters are covered in other specialized texts. The reader re quires no background in computer communications, only a basic familiarity with computers in general. A knowledge of computer programming is not required although, in a very few places, a little knowledge would aid full understanding of points made. However, this is not crucial to the main plot. Conversely, the author hopes that the book can also offer something to the reader who does have practical experience of computer communications, by setting this experience in an overall context, and revealing alternative possibilities. In several respects, this book differs from other general textbooks on computer communications. This is not likely to trouble the beginner, but a little explanation is in order for a reader who is not new to the subject area. First, the generalapproach is to identify the basic principles of computer communication (strongly motivated by how human communication works), and then apply these principles in increasingly practical settings. This contrasts with a more conventional approach that catalogues practical examples of computer communication systems, noting principles (sometimes repeatedly) as the cataloguing proceeds. One important benefit of this book’s approach is to give coverage that should stand the test of time. The basic principles underpinning the book have endured through a period when massive changes and developments in practical computer communications have come about. The second difference concerns the presentation of the material. Most text books on computer communications become bogged down, both by technical details and by technical terminology. This obstructs understanding of the import ant issues. This book tackles the problem in two ways. One is to omit the lowest levels of technical description of virtually all practical mechanisms included. There is sufficient detail for a reader to understand how the mechanisms work, but a reader who needs enough detail to actually implement the mechanisms will need to seek extra specialist information elsewhere. The author feels that the benefits of comprehension for all far outweigh the benefits of encyclopaedic status for a few. The other policy is to use a uniform technical vocabulary throughout the book. Rather than describe practical mechanisms in terms of their particular jar gon and acronyms, each is described using consistent terminology that refers back to the basic principles of communication. The author feels that this is desirable both to aid comprehension and also to stress that there are only a few basic ideas underpinning an apparent plethora of different mechanisms. USING THE BOOK The book has five main parts: Chapter 1: introduction Chapters 2–5: basic principles Chapters 6–8: computer networking Chapters 9–11: three case studies Chapter 12: standardization The recommended way to use the book is to read the five parts in this order, that is, to read the book from beginning to end. Forward and backward crossreferences in the text have been designed to be optimal for a sequential reader. However, some readers might want to vary the order, either because they are already familiar with some of the material or because they prefer to see complete practical examples before tackling the basic principles. This is feasible, making use of the index to check up on any unfamiliar terms that are defined in earlier, unread, chapters.

Computers in Communication Gordon Brebner iii The book “Computers in Communication” by Gordon Brebner was originally published by McGraw-Hill International (UK) Limited in 1997. This edition has been made available in electronic PDF form by the author, following the original version going out of print and the copyright reverting to the author. All identified typographical errors in the original edition have been corrected, but otherwise there has been no updating of the content. In particular, all page numbering is the same as in the original version. Copyright c 2002 Gordon Brebner. All rights reserved. You may save a local copy or print a copy of this book, or parts of this book, for your own personal use, provided that identification of the title and author, and this copyright notice, is not removed. Any commercial exploitation is expressly prohibited. All trademarks are acknowledged. To Rosemary CONTENTS Preface xiii 1 Introduction 1 1.1 Background 1 1.2 Uses of computer communications 4 1.2.1 Computer-oriented communication 4 1.2.2 Telecommunications 8 1.2.3 Radio and television broadcasting 10 1.2.4 Summary of uses of computer communications 11 1.3 Physical links 12 1.3.1 Physical media and their properties 14 1.3.2 Physical communication services 20 1.4 How computers communicate 26 1.4.1 Information, time and space 26 1.4.2 Agreement and implementation 27 1.4.3 Human influences 29 1.5 Chapter summary 31 1.6 Exercises and further reading 32 2 Information 35 2.1 Introduction 35 2.2 Agreement on information types 37 2.2.1 Absolute and relative information types 38 2.2.2 Examples of communicated information types 38 2.3 Implementation of required information types 45 2.3.1 Transformation of information 45 vii viii CONTENTS 2.3.2 Representation of information 65 2.4 Chapter summary 74 2.5 Exercises and further reading 75 3Time 80 3.1 Introduction 80 3.2 Agreement on time periods 83 3.2.1 Absolute and relative time measurement 83 3.2.2 Examples of communication time periods 85 3.3 Implementation of required time periods 92 3.3.1 Segmentation and concatenation 92 3.3.2 Flow control 97 3.3.3 Acknowledgement and error handling 103 3.4 Chapter summary 107 3.5 Exercises and further reading 108 4 Time Packages 110 4.1 Introduction 110 4.2 Unsegmented time package 112 4.2.1 Connectionless services 112 4.2.2 Examples of unsegmented time packages 114 4.3 Simple handshake time package 116 4.3.1 Examples of simple handshake time packages 118 4.4 Multi-stage handshake time package 120 4.4.1 Examples of multi-stage handshake services 121 4.5 Connection-oriented time package 125 4.5.1 Connection-oriented services 126 4.5.2 Examples of connection-oriented time packages 129 4.6 Chapter summary 134 4.7 Exercises and further reading 135 5 Space 138 5.1 Introduction 138 5.2 Agreement on spaces 140 5.2.1 Identifier schemes 140 5.2.2 Absolute and relative spaces 147 5.2.3 Examples of communication spaces 149 5.3 Implementation of required spaces 150 5.3.1 Filtering and switching 150 5.3.2 Splitting and multiplexing 159 5.4 Chapter summary 165 CONTENTS ix 5.5 Exercises and further reading 166 6 Message Broadcasting Networks 168 6.1 Introduction 168 6.1.1 Information basics 169 6.1.2 Time basics 171 6.1.3 Space basics 172 6.2 Multipeer channel implementation 173 6.2.1 Common medium 174 6.2.2 Chain or ring 174 6.2.3 Star or tree 176 6.3 Communication multiplexing 178 6.3.1 Isolated 179 6.3.2 Permission-based 181 6.3.3 Reservation-based 182 6.3.4 Physical division 183 6.4 Examples of guided technology network implementations 184 6.4.1 Ethernet 185 6.4.2 Token ring 190 6.4.3 FDDI and FDDI-II 194 6.4.4 Token bus 200 6.4.5 DQDB 204 6.4.6 100 BASE VG-AnyLAN 208 6.4.7 Discussion of examples 211 6.5 Unguided technology network implementations 212 6.6 Chapter summary 218 6.7 Exercises and further reading 218 7 Message Switching Networks 222 7.1 Introduction 222 7.1.1 Information basics 224 7.1.2 Time basics 225 7.1.3 Space basics 227 7.2 Channel implementation 229 7.3 Communication multiplexing 235 7.3.1 Isolated 237 7.3.2 Permission-based 238 7.3.3 Reservation-based 238 7.4 Examples of physical switching network implementations 239 7.4.1 Switched ethernet 240 7.4.2 Telephone system circuit switching 240 7.4.3 X.25 packet switching 241 7.4.4 Frame relay 244 x CONTENTS 7.4.5 ATM cell relay 248 7.5 Examples of public switching network services 253 7.5.1 PSTN 254 7.5.2 CSPDN 254 7.5.3 PSPDN 254 7.5.4 N-ISDN 255 7.5.5 SMDS 255 7.5.6 B-ISDN 256 7.6 Example of a high-level switching network implementation 258 7.7 Chapter summary 263 7.8 Exercises and further reading 263 8 Inter-networks 267 8.1 Introduction 267 8.1.1 Information basics 269 8.1.2 Time basics 269 8.1.3 Space basics 271 8.2 Inter-network switching 274 8.3 Examples of inter-network implementations 276 8.3.1 IEEE 802 LAN bridging 276 8.3.2 The Internet 280 8.3.3 ISO inter-networking 302 8.4 Chapter summary 306 8.5 Exercises and further reading 307 9 Case Study 1: Accessing the World Wide Web 311 9.1 Introduction 311 9.2 The problem: information, time and space issues 314 9.3 Reliable end to end communication using TCP/IP 324 9.4 Home computer and the Internet 334 9.5 Within the Internet 340 9.6 WWW server and the Internet 347 9.7 Overall communications 349 9.8 Chapter summary 355 9.9 Exercises and further reading 355 10 Case Study 2: Controlling a manufacturing device 359 10.1 Introduction 360 10.2 The problem: information, time and space issues 362 10.3 FullMAP implementation 367 10.4 MiniMAP simplified implementation 384 10.5 Chapter summary 385 [...]... into a model of interaction with computers that survives to the present day There are two major improvements One is in terminal technology, with video screens replacing printing, giving a consequent large increase in output speed, and also with terminals containing computers as their controllers (or, indeed, computers just emulating the behaviour of terminals) Beyond terminals, full WIMP (Window, Icon,... satisfactorily in the absence of any communication with other people 1 2 COMPUTERS IN COMMUNICATION Similar observations apply to computers too Individual computers are capable of gathering, processing, storing and distributing information, under the direction of humans They are not only found in distinctive boxes with keyboards, mice and screens attached, or in large cabinets bristlingwith flashing lights... Section 1.4 introduces the main themes of the book The features of communications are classified under three main headings: information: the type of information that is communicated; time: when, and how quickly, a communication takes place; and space: which computers, and inter-connecting channels between computers, are involved in a communication There are two main problems associated with communication: ... networking contained in Chapters 6, 7 and 8 introduces the topic in three stages, each stage involving more complex networking xvi PREFACE techniques The treatment combines and applies the basic principles of computer communications It is also essential reading, since networking is absolutely central to modern computer communications There are many examples in these chapters, some fairly detailed, since... achieving agreement between computers on the nature of communications; and implementing the required communications using available physical communication media The processes of achieving agreement and implementation require a lot of human decision making prior to communications being possible This entails human communications of a rather specialist kind The section ends by introducing the ways in which... special-purpose computers from a technical communications point of view The blending of computers and everyday devices will continue, to a point where, from a user point of view, explictly visible computers are not likely to be present A user will only see ‘intelligent’ devices; these will be internally underpinned by computers The devices might be examples of wearable computing, embedded in clothes or... positions, communication might take place using sound through the air, visible sign language, telephones, television or sending items through the postal system For computers, there is usually no movement involved in communication, robotics still being in its relative infancy Because of this, computers communicate using media that physically reach the computers, and that are capable of transmitting computer... and Pointer) interfaces can also be used, if the computer is sophisticated enough to interact via such an interface The other improvement is the one of interest to computer communications This is that the direct physical cable between terminal and computer can be replaced by any communication channel capable of passing information between a terminal and a computer Useable communication channels include... service is being provided by one single uniform computer system In distributed systems, it is fairly common for the computers involved to take on server and client roles Servers have special capabilities, for example, storing particular information, performing particular processing of information, or having particular input or output devices attached Clients can make use of servers by issuing requests,... of communicating computers can be placed into one box, with high speed electronic interconnections, to form a supercomputer: a new, high speed computer consisting of many individual computing elements This is the point at which computer communications meets the subject area of parallel computation 1.2.2 Telecommunications Human communication via a medium is long-established In the mid-nineteenth century, . satisfactorily in the absence of any communication with other people. 1 2 COMPUTERS IN COMMUNICATION Similarobservationsapplytocomputerstoo.Individualcomputersarecapable of gathering, processing, storing. reading 450 Index 454 PREFACE OBJECTIVES The building of ‘information superhighways’, in the first instance exemplified by the massive growth of the Internet, has introducedthe general publicto computers in. computer networking contained in Chapters 6, 7 and 8 in- troduces the topic in three stages, each stage involving more complex networking xvi PREFACE techniques. The treatment combines and applies