Computers in Communication, Gordon Brebner

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Cuốn sách "Máy tính Truyền thông" của Gordon Brebner ban đầu được xuất bản bởi McGraw-Hill International (Anh) Limited vào năm 1997. Phiên bản này có được làm sẵn có ở dạng PDF điện tử của tác giả, bản gốc phiên bản đi ra ngoài in ấn và bản quyền quay trở lại cho tác giả. Tất cả được xác định lỗi đánh máy trong phiên bản ban đầu đã được sửa chữa, nhưng nếu không đã có không cập nhật nội dung. Đặc biệt, tất cả các số trang là tương tự như trong phiên bản gốc. ...

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Nội dung Text: Computers in Communication, Gordon Brebner

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 3 Time 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
CONTENTS xi 10.6 Exercises and further reading 386 11 Case Study 3: Making a video telephone call 388 11.1 Introduction 388 11.2 The problem: information, time and space issues 391 11.3 Management of an ATM connection 397 11.4 Using an ATM connection 402 11.5 Overall B-ISDN implementation 405 11.6 Chapter summary 408 11.7 Exercises and further reading 408 12 Standardization 411 12.1 Introduction 412 12.2 Standards bodies 414 12.2.1 International Organization for Standardization (ISO) 415 12.2.2 International Telecommunication Union 417 12.2.3 Internet Society 418 12.2.4 Professional and manufacturer bodies 420 12.2.5 Summary of standardization bodies 423 12.3 ISO standards 424 12.3.1 Structure 424 12.3.2 Services and protocols 430 12.3.3 Management 433 12.3.4 Other standards 434 12.4 Internet standards 435 12.4.1 Structure 435 12.4.2 Protocols 438 12.5 ITU-T standards 441 12.6 IEEE 802 standards 444 12.7 Proprietary standards 445 12.7.1 IBM System Network Architecture 446 12.7.2 Digital Network Architecture 447 12.7.3 AppleTalk 448 12.7.4 Xerox Network Systems and descendents 449 12.8 Chapter summary 449 12.9 Exercises and further reading 450 Index 454
PREFACE 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 computer-based 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 general xiii
xiv PREFACE approach 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 cross-references 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.
PREFACE xv For a reader who wants to start with a large practical example, Chapter 9 is a good place to begin. It contains a case study of accessing a World Wide Web page. This is likely to be familiar from ﬁrst-hand experience, and studying the communication problems involved gives an introduction to most of the themes of the book. It also allows a look at the workings of the Internet, which is the largest, and best-known, example of computer networking in the world. For a complete beginner, many of the technical details, and general issues, of Chapter 9 can be ignored at a ﬁrst reading. There will still be enough accessible ‘meat’ left in the case study. For a reader who seeks a basic understanding of the techniques of computer communications, without surrounding context, then Chapters 2 to 8 are enough, possibly illuminated by one of the three case study chapters. For a more rapid read, the many examples included in these chapters can be read selectively. To see why Chapters 1 and 12 are dispensible for such a reader, it is worth explaining their purposes. Chapter 1 is mainly concerned with constraints placed on computer commu- nications: by the activities that are using the computer communications; and by the physical communication media that are available to link computers. That is, it presents computer communications as an activity that lies between the physical world of communication media and the logical world of computer users. If these external constraints are taken on trust, the details may be omitted by the reader. Chapter 12 is also concerned with constraints, but of a different and more specialized type. If one computer is to communicate with one or more others, then there must be agreement on the way in which communication will take place, otherwise the computers will not understand one another. Humans are responsible for putting agreements in place, and this exposes computer communications to the process of international standardization. The result is further constraints on what can be done and, if these are taken on trust, the details may be omitted by the reader. The material on basic principles contained in Chapters 2, 3 and 5 (Chapter 4 expands on the ideas in Chapter 3) forms the core of the book. Thus, it is essential reading. There is some scope for omission, especially by a reader who has some prior experience of computer communications. Each basic principle introduced is ﬁrst motivated by examples of the principle occurring in human communications. Some readers may not need this motivation, or will be happy with fewer examples. Also, each basic principle is illustrated with examples of the principle occurring in computer communications. Readers may already be familiar with some examples, or will need fewer examples to understand the point. The material on 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 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 the aim is to include all of the important network types. Thus, there are more examples than strictly necessary to illustrate the general principles. A reader may choose to omit some of these examples, perhaps because of prior experience or lack of relevance. The three case studies in Chapters 9, 10 and 11 are included to show how complete computer communication systems ﬁt together, in order to bridge the gap between physical communication media and the demands of computer users. Readers are recommended to follow at least one of these case studies, otherwise the material in preceding chapters may seem rather disjointed. An alternative, for students aided by an instructor, is to choose a completely different problem that has ready familiarity, and then analyse it in a similar manner to the case studies presented in the book. EXERCISES AND FURTHER READING There is an ‘exercises and further reading’ section at the end of each chapter. In total, there are over 300 exercises in the book. None of these exercises is of the type that is just designed to test whether the reader has read and absorbed the material contained in a chapter. Instead, the exercises are of three main types, intended to encourage the reader to: relate topics covered to real-life human communication experience; think more deeply about less straightforward topics; and ﬁnd out more information about certain topics. The exercises vary widely in difﬁculty, from those that require a few moments’ thought, to those that have the potential to become term projects for a group of students. The instructor’s guide that is associated with the book gives further guidance on this matter, to assist in the selection of exercises for classroom use. The exercises, and further reading suggestions, have been carefully chosen to minimize the extra resources that are required by the reader. The author fully appre- ciates that many readers will not have access to well-stocked libraries containing specialist books, academic journals, technical reports and product documentation. Further, readers might not have access to state of the art computer communica- tion facilities that allow practical experimentation. Therefore, only one primary resource is particularly desirable to underpin extra activities of the reader: access to the Internet. The ability to access the Internet, and its facilities, allows the reader to experiment with many of the technical points covered in the book. Further, it makes a vast collection of information on computer communications readily and freely
PREFACE xvii available to the reader. It seems particularly appropriate for a book on computer communications to make practical use of the technology described, as much as possible. The most obvious Internet facility of interest for information gathering is the World Wide Web (WWW). Knowing well that the phrase ‘here today, gone tomorrow’ could have been invented to describe material on the WWW, the author has avoided giving explicit WWW references, except in a handful of very safe cases. The reader can use one of the WWW searching engines to track down useful material. The technical reasons behind the various frustrations of using the WWW are explained in the case study of Chapter 9. A key information resource is the ever-enlarging collection of Internet ‘Re- quests for Comments’ (RFCs), which can be accessed through the WWW. This is a splendid collection, combining folk history, technical discussion and speciﬁcation of Internet technology. Many of the end-of-chapter exercises involve looking at an RFC, either to skim or to digest. Of course, this introduces a potential danger of introducing an Internet bias to the material under consideration, but the author believes that the ready availability, and the well-written nature, of the RFC series make it a very worthy source. Aside from the electronic information resources, the author has been very parsimonious in recommending other printed sources. Fourteen other books are mentioned, covering various areas of computer communications in more depth than is possible (or desirable) in this book. Books still have a place in the electronic information age, to give overall coherence to a body of material culled from numerous sources. Departing from normal textbook tradition, this book does not include a lengthy section of worthy references to academic papers. This does not reﬂect laziness on the part of the author — rather, it is the product of experience and consideration. It would have been very easy to include references to the numerous papers consulted while writing the book, not to mention stitching on various ‘standard’ references. However, not only might many of the referenced papers be inaccessible to the average reader, but also the author knows that most references remain ﬁrmly unconsulted even if they are accessible. A further point is that, in this fast-moving subject area, many such references become rapidly out of date. Having said all this, the book has a few references to particularly relevant journal articles, but these are in journals most likely to be present in a com- puting library. For general reading, Computer Communication Review, published by the ACM SIGCOMM (Special Interest Group on Communication), including the proceedings of the annual SIGCOMM conference, is recommended for the reader who wants to track the major computer communications issues of the day. Other possible medium weight sources on topical communications issues are the IEEE Network and Communications magazines, or general purpose computing magazines such as Byte.
xviii PREFACE ACKNOWLEDGEMENTS The basic framework of this book has been in development for many years. A key inﬂuence was Mike Padlipsky, Old Network Boy of the Internet, and Scotch whisky buff. On a memorable visit to Edinburgh in 1986, he foresaw the rise of Internet standards and the demise of ISO standards — near-heresy at that time. This helped to reinforce my own feelings of dissatisfaction with the traditional way of teaching computer communications: a combination of ‘encyclopaedic catalogue’ and ‘ISO reference model’ (the latter term is explained in Chapter 12). The fourth year computer communications class at Edinburgh University acted as unwitting guinea pigs in 1987, when I tried out a radically different approach — an approach that has largely survived to underpin this book. The students of that class must be thanked for their patience with frequent retuning of the course agenda. Many thanks are due to Fred King, for acting as a sounding board on numerous occasions throughout this period. My general approach to teaching computer communications has matured since this ﬁrst experiment, assisted by a fertile environment at Edinburgh Uni- versity. In the length of a single corridor, one can ﬁnd expertise ranging from the underlying theory of concurrency to the practical operation of major state of the art computer networks. To single out just two people of many, Robin Milner’s work on concurrency has been a signiﬁcant inﬂuence on the basic principles distilled in this book, and Sam Wilson has been an invaluable source of information on the operational details of advanced computer networks. During the production of this book, various people have been of assistance. At McGraw-Hill, Rupert Knight enticed me into beginning to write, and sustained the tricky early stages with his relentless enthusiasm. Alfred Waller has seen the project into port, sharing his invaluable experience and sage insights into publish- ing. Throughout, Ros Comer was a continuous source of guidance on the ﬁner points of language and layout. Several anonymous McGraw-Hill reviewers have contributed many helpful suggestions at various stages. At Edinburgh University, Steven Haeck was kind enough to read the near-ﬁnal manuscript, and make many constructive comments that improved the ﬁnal manuscript. However, at the end of day, the translation of my thoughts on computer communications into camera- ready copy was my responsibility — aided, abetted and sometimes frustrated by A L TEX— and so I take the blame for anything that is not as it should be. Finally, but most importantly, huge thanks are due to my wife Rosemary. She encouraged me to embark on the book in the ﬁrst place, and then put up with a husband who not only disappeared to commune with the computer keyboard most weekends but also insisted on giving her regular progress reports on esoteric topics outside her areas of expertise. This book is dedicated to Rosemary. Gordon Brebner Edinburgh January 1997
CHAPTER ONE INTRODUCTION The main topics in this introductory chapter are: beneﬁts of communication history of human and computer communication mechanisms present-day uses for computer communications physical underpinning of computer communications the main principles of computer communications 1.1 BACKGROUND Communication is beneﬁcial for the human race. By communicating with one another, information can be shared — past experience, current affairs, predictions of the future — from here, there and everywhere. Also, resources and expertise can be shared, by communicating with the right people. Only hermits are noted for their ability to live 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 and whirling tape drives, as seen in ageing science ﬁction ﬁlms. Their basic information-handling capabilities can be harnessed for controlling other machinery, and so they are also hidden inside things like wristwatches, microwave ovens, central heating systems, factory production line equipment and nuclear power plant safety systems. There are three main areas where beneﬁts can be expected if one computer is able to communicate with others: it can get information that is stored by other computers; it can get other computers to do specialized work; and it can communicate with humans that use other computers. The beneﬁts need not only be in one direction — this computer can also export its own information, its specialized abilities and access to its human users. The outcome is a beneﬁcial sharing of resources. At present, it is usually the case that computers inside personal items like wristwatches, or inside domestic equipment like microwave ovens, do not com- municate with others. However, this situation is on the point of changing, given that communication between the more recognizable types of computers has proved to be very useful, and that appropriate communication technologies are becoming available. It might also seem that a conventional home computer, or a single com- puter tucked into a musty ofﬁce, is island-like, cut off from the world community of computers. However, this is a delusion, since such computers usually have an obliging communication mechanism: human beings transferring the latest fruits of the computing trade on ﬂoppy disk or compact disk. This book is concerned with computer communications where there are no human middlemen, so that computers can converse directly with one another. Although human participation in the role of intermediary is being eliminated, it should not be forgotten that computers only communicate because humans have instructed them to and, moreover, that this is possible because humans have instructed the computers how to communicate. The techniques covered arise from communications between conventional types of computer. However, they are equally applicable to the world of the future, where there will be things like intelligent houses with communication not just between domestic appliances but also with the fabric of the building itself. The act of communication is not always easy for humans. For example, it is not feasible for every person in the world to communicate effectively with any other person whenever desired. Differences in culture, availability and physical location cause problems. Things get even tougher if one broadens communication to include other species of animal or plant, never mind any alien life forms that might visit or be visited.