telecommunication and computer networks

7/24/2019 Telecommunication and Computer Networks

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Telecommunication and Computer Networks

Edition 7.0

Pieter Kritzinger

July 2001

Department of Computer ScienceUniversity of Cape Town


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ForewordMan has always had a desire to exchange information amongst fellow man. Not so long ago, people gatheredon the town square to gossip about the affairs of the town and its inhabitants. The Internet has now turnedthe World into one big Electronic Town Square. As in the early days, the value of the information thusdisseminated should be taken with a grain of salt, or at least should be veried before taken seriously.Amazingly some individuals seem to forget this age old wisdom.

This version 7.0 constitutes a major rewrite of the previous version and as the new title indicates, reectsthe merging of telephone and computer and other networks into a single telecommunication network whichwould carry video, data and voice.

No single person can invent the knowledge reected in these pages and so the content is compiled frommany articles, textbooks and my own work. There are many good textbooks on the topic and the one which

comes closest to covering the material in these notes is Understanding Data Communications and Networksby W.A. Shay and published by the PWS Publishing Company (ISBN 0-534-20244-6). Another good text,albeit somewhat academic in nature, is the Third edition of Computer Networks by Andrew Tanenbaum(ISBN 0-13-394248-1). The one I would recommend you to buy (as of July 9, 2001) is Computer Networksand Internets (Second Edition) by Douglas E Comer, published by Prentice-Hall International, ISBN 0-13-084222-2.

As with all elds of Computer Science there is a great deal of theoretical work associated with computernetworks and their protocols. Here I think in particular of concepts such as formal specication methods,stochastic modelling and models such as Petrinets and Process Algebras to model concurrent systems. Inthis book we have been able to cover only a tiny fraction of this and then only in the later chapters.

The subject matter of (tele-)communications has not only become vast but much more signicant in theworld of computing than it was a mere decade ago. Recognition of this is the introduction of the C in theprevious IT of Information Technology to turn it into Information and Communication Technology . No textcan expect to cover all of it unless one wants to leave the reader with a feeling of knowing nothing abouteverything. If the reader discovers that his or her favourite network topic is therefore not even mentioned:that is inevitable.

This seventh edition of Telecommunication and Computer Networks has four parts. Part I is intended to laythe foundation for a course in the subject and as such could serve as reading material for a few introductorylectures the subject. Part II covers the fundamentals of data communication. It introduces the engineer-ing concepts required to understand both electrical and optical data channels and their properties. Part IIIconcerns the basic protocols required in any network. Starting with the Data Link layer it also considers

Packet Switching and Media Access Control (MAC) techniques used by the common local networks. Thematerial covered to this point are required reading for a good undergraduate course in computer networks.The last part covers advanced topics such as network security, wireless networks, network management andnetwork middleware topics such as CORBA. This part contains sufcient material for a second course intelecommunications and computer networks.

Many generations of students have worked on these notes: The latest is Andy Shearman who was responsiblefor the chapters on Security, Wireless Communication and Network Middleware. Thank you Andy.

Any constructive critique about the content of these notes will always be most welcome.

Pieter Kritzinger Cape Town July 2001


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Contents

I Overview of Telecommunication and Computer Networks 3

1 Basic Concepts 5

1.1 Objectives of this Chapter . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 5

1.2 Introduction . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 6

1.3 Evolution of Computer Networks . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 6

1.4 Electrical Signals . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 9

1.5 Optical Signals . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 11

1.6 A Matter of Protocol . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 11

1.7 Network Architecture . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 12

1.7.1 Physical layer . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 141.7.2 Data link layer . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 15

1.7.3 Network layer . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 15

1.7.4 Transport layer . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 16

1.7.5 Session layer . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 16

1.7.6 Presentation layer . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 17

1.7.7 Application layer . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 17

1.7.8 Conclusion - Standards and ISO . . . . . . . . . . . . . . . . . . . . . . . . . . . . 17

1.8 International Standards Bodies . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 17

1.9 Exercises . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 18

II Transmission Fundamentals 21

2 Electrical Signals 23


2.2 Fourier Analysis . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 26

2.3 Transmission Media . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 26

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2.3.1 Twisted pair . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 27

2.3.2 Coaxial cable . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 28

2.4 Channel Characteristics . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 28

2.4.1 Attenuation . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 29

2.4.2 Noise Distortion and Shannons Law . . . . . . . . . . . . . . . . . . . . . . . . . 29

2.4.3 Delay distortion . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 31

2.4.4 Electrical noise . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 31

2.4.5 Bandwidth limitation . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 31

2.5 Data encoding . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 34

2.5.1 Digital Data, Digital Signals . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 34

2.5.2 Digital Data, Analog Signals . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 37

2.5.3 Analog Data, Digital Signals . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 39

2.6 Exercises . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 41

3 Optical Signals 43


3.2 Optic Fibres . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 443.3 Method of Transmission . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 45

3.4 Optical Media . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 45

3.4.1 Multimode and single-mode bres . . . . . . . . . . . . . . . . . . . . . . . . . . . 46

3.4.2 Optical Signal Generation . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 47

3.5 Signal Loss in Optic Fibre . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 48

3.6 Advantages and Disadvantages of Optical Fibres . . . . . . . . . . . . . . . . . . . . . . . 48

3.7 Exercises . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 49

4 Wireless Communication 51


4.2 Frequency Spectrum . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 52

4.3 Radio Transmission . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 53

4.4 Terrestial Microwave Signals . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 54

4.5 Satellite Communication . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 54

4.6 Exercises . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 56


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5 Channels and Their Properties 57


5.2 Introduction . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 585.3 Simplex, Half-duplex and Full-Duplex Communication . . . . . . . . . . . . . . . . . . . . 58

5.4 Asynchronous and Synchronous Transmission . . . . . . . . . . . . . . . . . . . . . . . . . 58

5.5 Multiplexing Signals . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 60

5.5.1 Synchronous Time Division Multiplexing . . . . . . . . . . . . . . . . . . . . . . . 61

5.5.2 Statistical Time Division Multiplexing . . . . . . . . . . . . . . . . . . . . . . . . . 62

5.5.3 Wavelength Division Multiplexing . . . . . . . . . . . . . . . . . . . . . . . . . . . 63

5.5.4 Combining WDM and STDM . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 63

5.6 Digital Subscriber Line (DSL) . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 64

5.7 Switching Techniques . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 66

5.7.1 Circuit Switching . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 66

5.7.2 Packet Switching . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 67

5.7.3 Cell Switching . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 68

5.8 Exercises . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 68

6 Physical Data Communication Interfaces, Standards and Errors 71

6.1 Objectives of this Chapter . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 716.2 Electrical Signal Interfaces . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 72

6.2.1 Serial Analog Interface . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 72

6.2.2 Serial Digital Interface . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 74

6.3 Optical Signal Interfaces . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 77

6.3.1 Synchronisation and SONET/SDH . . . . . . . . . . . . . . . . . . . . . . . . . . . 78

6.3.2 The relationship between SONET and SDH . . . . . . . . . . . . . . . . . . . . . . 79

6.4 Transmission Errors . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 80

6.4.1 Sources of Transmission Errors . . . . . . . . . . . . . . . . . . . . . . . . . . . . 80

6.4.2 Error Rates . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 81

6.4.3 Error Codes . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 81

6.4.4 Hamming Distance . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 82

6.4.5 Parity . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 83

6.4.6 Error-Correcting Codes . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 83

6.4.7 Block check characters . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 85

6.4.8 Cyclic Redundancy Coding . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 85

6.5 Exercises . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 89


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7 Data Link Protocols 91


7.2 Principles of Protocol Design . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 93

7.2.1 Acknowledgements . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 94

7.2.2 Flow Control . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 94

7.2.3 Sequence Numbers . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 95

7.2.4 Piggybacking . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 96

7.2.5 Sliding Window Protocols . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 97

7.2.6 Pipelining . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 99

7.2.7 Positive Acknowledgements . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 101

7.3 Synchronous Data Link Control (SDLC) . . . . . . . . . . . . . . . . . . . . . . . . . . . . 101

7.3.1 Flag eld . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 102

7.3.2 Address eld . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 103

7.3.3 Control eld . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 104

7.3.4 Acknowledgement and Retransmission. . . . . . . . . . . . . . . . . . . . . . . . . 105

7.3.5 Supervisory Format . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 106

7.3.6 Nonsequenced format . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 107

7.4 The Data Link Layer on the Internet . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 1077.4.1 Serial Line IP (SLIP) . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 107

7.4.2 Point-to-Point Protocol (PPP) . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 108

7.5 Exercises . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 109

III Network Protocols 111

8 Local Area Networks 113

8.1 Objectives of this Chapter . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 1138.2 Introduction . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 114

8.3 Classication of LANs . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 115

8.3.1 Topologies . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 115

8.3.2 Common Bus Topology . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 115

8.3.3 Star Topology . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 116

8.3.4 Ring Topology . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 116

8.3.5 Media Access Control . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 117

8.4 Random Access Techniques . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 117


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8.4.1 1-persistent CSMA . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 118

8.4.2 Nonpersistent CSMA . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 118

8.4.3 CSMA/CD . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 118

8.4.4 CSMA/CA . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 119

8.4.5 IEEE LAN Classication . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 119

8.5 Logical Link Control (IEEE 802.2) . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 120

8.6 Ethernet . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 121

8.6.1 Ethernet Performance . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 124

8.6.2 Fast Ethernet . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 126

8.7 Deterministic or Token Passing Protocols . . . . . . . . . . . . . . . . . . . . . . . . . . . 127

8.7.1 Token Ring (IEEE 802.5) . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 127

8.7.2 Ring Maintenance . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 130

8.7.3 Token Ring Performance . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 131

8.7.4 Token Passing Bus (IEEE 802.4) . . . . . . . . . . . . . . . . . . . . . . . . . . . . 132

8.8 Wireless LAN (IEEE 802.11) . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 133

8.9 Network Interconnection . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 135

8.10 Exercises . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 136

9 Wide Area Networks 137


9.2 Overview . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 138

9.3 Datagram Service, Virtual Circuit Service . . . . . . . . . . . . . . . . . . . . . . . . . . . 138

9.4 Internal Structure of the Network . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 140

9.5 Routing Techniques . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 143

9.6 Congestion Control . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 145

9.6.1 Preallocation of Buffers . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 1469.6.2 Discarding Packets . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 146

9.6.3 Choke Packets . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 147

9.7 Flow control . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 148

9.8 Deadlock Prevention . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 149

9.9 Frame Relay Networks . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 150

9.9.1 Circuit Connections . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 152

9.9.2 Committed Information Rates . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 152

9.9.3 Local Managment Interface (LMI) . . . . . . . . . . . . . . . . . . . . . . . . . . . 154


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9.10 Asynchronous Transfer Mode (ATM) Networks . . . . . . . . . . . . . . . . . . . . . . . . 155

9.10.1 Quality of Service (QoS) . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 156

9.10.2 ATM Cells . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 1589.10.3 ATM Service Classes . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 159

9.10.4 Real-Time Services . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 159

9.11 The X.25 Packet Interface . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 161

9.12 Internet Network Layer Protocol (IP) . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 167

9.13 Exercises . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 170

IV Advanced Topics 173

10 Advanced Network Technologies 175


10.2 Fibre Distributed Data Interchange . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 176

10.2.1 FDDI Layered Structure . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 176

10.2.2 MAC Functional Operation . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 178

10.2.3 FDDI Frame and Token Formats . . . . . . . . . . . . . . . . . . . . . . . . . . . . 179

10.2.4 Data Encoding . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 180

10.2.5 Physical Layer (PHY and PMD) Operation . . . . . . . . . . . . . . . . . . . . . . 180

10.2.6 Timed Token Rotation (TTR) Protocol . . . . . . . . . . . . . . . . . . . . . . . . . 182

10.2.7 Ring Initialization . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 182

10.2.8 Data Transmission . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 183

10.2.9 FDDI Token Timing . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 185

10.2.10 Token Rotation Time Sequence Diagram . . . . . . . . . . . . . . . . . . . . . . . 186

10.3 Distributed Queue Dual Bus (DQDB) Networks . . . . . . . . . . . . . . . . . . . . . . . . 187

10.3.1 Background . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 187

10.3.2 DQDB Architecture . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 188

10.3.3 DQDB Virtual Queue . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 190

10.3.4 DQDB Medium Access Control (MAC) Algorithm . . . . . . . . . . . . . . . . . . 191

10.3.5 Performance of DQDB Protocol . . . . . . . . . . . . . . . . . . . . . . . . . . . . 193

10.3.6 Advantages of DQDB . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 193

10.3.7 Disadvantages of DQDB . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 193

10.3.8 Protocol Enhancements . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 194

10.3.9 The Future of DQDB . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 195

10.4 Exercises . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 195


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11 Network Security 197


11 .2 In t roduc t ion . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 198

11.3 Access Control and Passwords . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 199

11.3.1 Passwords . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 200

11.4 Conventional Encryption . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 201

11.4.1 Introduction and Historical Overview . . . . . . . . . . . . . . . . . . . . . . . . . 201

11.4.2 DES . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 204

11.5 Public Key Crpytography . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 207

11.5.1 Dife Hellmen Key Exchange . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 20711.5.2 RSA . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 209

11.6 Encryptions Place in a Network . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 210

11.6.1 End-to-End Encryption . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 211

11.6.2 Link Encryption . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 211

11.7 Digital Signiatures & Authentication . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 212

11.7.1 Cryptographic Checksums . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 212

11.7.2 Digital Signatures . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 21411.8 Network Based Attacks . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 216

11.8.1 TCP/IP Connection Set-up Primer . . . . . . . . . . . . . . . . . . . . . . . . . . . 217

11.8.2 The SYN Attack . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 218

11.8.3 ICMP Attacks . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 219

11.8.4 Simple Attackers : Smurf and Fraggle . . . . . . . . . . . . . . . . . . . . . . . . . 219

11.8.5 Coordinated and Distributed Network Based Attacks . . . . . . . . . . . . . . . . . 220

11.8.6 Countering DDOS Attacks . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 222

11.9 Firewalls . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 224

11.9.1 What Constitutes a Firewall . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 225

11.9.2 Different Congurations of a Firewall . . . . . . . . . . . . . . . . . . . . . . . . . 228

11.9.3 Dual-Homed Gateway Firewall . . . . . . . . . . . . . . . . . . . . . . . . . . . . 228

11.9.4 Screened Host Firewall . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 229

11.9.5 Screened Subnet Firewall . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 229

11.9.6 Summary of Advantages . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 231

11.10 Exercises . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 231


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xii CONTENTS

12 Cellular Networks 233


12 .2 In t roduc t ion . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 234

12.3 Cellular Concept . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 234

12.3.1 Frequency Reuse . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 234

12.3.2 Overview of the Air Interface . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 237

12.3.3 Roaming Mobile Users . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 237

12.4 How a Call is Made . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 239

12.5 GSM . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 240

12.5.1 Radio Information and Channel Types . . . . . . . . . . . . . . . . . . . . . . . . . 241

12.5.2 Channel Types . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 241

12.6 3G Networks Mobile Networks . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 243

12.6.1 General Packet Radio Service . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 243

12.6.2 Roaming in a GPRS Network . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 243

12.6.3 Data Transmission . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 246

12.7 Universal Mobile Transmission System . . . . . . . . . . . . . . . . . . . . . . . . . . . . 246

12.7.1 Overview . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 247

12.8 Exercises . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 251

13 Network Middleware 253


13 .2 In t roduc t ion . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 254

13.2.1 Design Approaches . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 254

13.3 Common Object Request Broker Architecture (CORBA) . . . . . . . . . . . . . . . . . . . 254

13.4 The Object Request Broker (ORB) . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 256

13.5 Distributed Component Object Model (DCOM) . . . . . . . . . . . . . . . . . . . . . . . . 25913.6 Java Distributed Computing - RMI . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 260

13.6.1 Java Distributed Object Model . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 261

13.6.2 RMI Implementation . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 261

13.6.3 Marshalling . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 263

13.7 Exercises . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 264


14/292

List of Figures

1.1 Mainframe computer ca. 1970 . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 6

1.2 One typical conguration of a terminal-oriented network. . . . . . . . . . . . . . . . . . . . 7

1.3 A typical conguration of a terminal-oriented network using Statistical Multiplexers. . . . . 81.4 A typical conguration of a terminal-oriented network using a FEP. . . . . . . . . . . . . . 8

1.5 Examples of analog and digital signals . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 10

1.6 Manchester encoding of the byte 1111 0000. . . . . . . . . . . . . . . . . . . . . . . . . . . 11

1.7 The ISO OSI Reference Model. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 13

1.8 Layers, protocols, and interfaces. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 15

1.9 TCP/IP protocol suite . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 18

2.1 Schematic of a conducting coil rotating at speed

radians per second through a eld of magnetic ux, generating a current in the conductor as it does so . . . . . . . . . . . . . . . 24

2.2 Illustrating phase differences in a sinusoidal signal . . . . . . . . . . . . . . . . . . . . . . 25

2.3 Shielded twisted-pair line . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 27

2.4 A typical coaxial cable. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 28

2.5 Sources of attenuation and distortion of a digital signal . . . . . . . . . . . . . . . . . . . . 30

2.6 A binary signal and its Fourier components. . . . . . . . . . . . . . . . . . . . . . . . . . . 32

2.7 Two binary signals: with double the capacity (number of bits per second) of . . . 33

2.8 Illustrating the effect of bandwidth on a digital signal . . . . . . . . . . . . . . . . . . . . . 352.9 Some digital encoding techniques . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 36

2.10 A binary signal (a) amplitude modulated (PAM) (b) frequency modulated (MFSK) (c) phasemodulated (PSK) . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 38

2.11 Illustrating pulse code modulation . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 40

2.12 A transmitted byte . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 41

3.1 The Optical Telegraph . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 44

3.2 Components of an optical bre cable . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 45

3.3 Multimode and single-mode optical bres . . . . . . . . . . . . . . . . . . . . . . . . . . . 46

xiii


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xiv LIST OF FIGURES

4.1 Satellite communication . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 55

5.1 Simplex, Half-duplex and Full-duplex Communication . . . . . . . . . . . . . . . . . . . . 58

5.2 Asynchronous and Synchronous Transmissions . . . . . . . . . . . . . . . . . . . . . . . . 59

5.3 Multiplexing . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 61

5.4 Synchronous Time Division Multiplexing . . . . . . . . . . . . . . . . . . . . . . . . . . . 61

5.5 Statistical Time Division Multiplexing . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 62

5.6 Wavelength Division Multiplexing . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 63

5.7 Asymmetric Digital Subscriber Line (ASDL) technology using FDM only . . . . . . . . . . 64

5.8 Asymmetric Digital Subscriber Line (ASDL) technology using Echo-suppression . . . . . . 65

5.9 Circuit switching . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 66

5.10 Conceptual view of packet switching . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 67

6.1 RS-232C/V.24 interface . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 72

6.2 RS-232C Connector . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 73

6.3 Sending and Receiving over an RS-232 Connection . . . . . . . . . . . . . . . . . . . . . . 73

6.4 Signal lines used in the X21 serial digital interface . . . . . . . . . . . . . . . . . . . . . . 74

6.5 X21 protocol state transition graph . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 76

6.6 SONET conguration. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 78

6.7 An STS-1 SONET frame. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 79

6.8 Mapping basic signals onto an STS-N channel. . . . . . . . . . . . . . . . . . . . . . . . . 80

6.9 Error burst examples. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 82

6.10 Example of block sum parity checking . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 86

7.1 Conceptual model of Layer 2, 3 and 4 protocols . . . . . . . . . . . . . . . . . . . . . . . . 93

7.2 The Alternating Bit (AB) protocol . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 96

7.3 A sliding window of size 1, with a 3-bit sequence number. Part (a) initially (b) after the rstframe has been sent (c) after the rst frame has been received (d) after the rst acknowl-edgement has been received page 85 . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 97

7.4 Pipelined ARQ protocols . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 100

7.5 The SDLC frame format. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 102

7.6 An example of bit stufng . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 103

7.7 Three formats for the basic SDLC control eld . . . . . . . . . . . . . . . . . . . . . . . . 105

7.8 The PPP full frame format for unnumbered mode operation . . . . . . . . . . . . . . . . . . 109

8.1 LAN Bus topology . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 115


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LIST OF FIGURES xv

8.2 LAN Star topology . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 116

8.3 Ring topology. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 117

8.4 Correspondence between IEEE 802 layers and the OSI model . . . . . . . . . . . . . . . . . 120

8.5 (a) Position of LLC and (b) Protocol formats . . . . . . . . . . . . . . . . . . . . . . . . . . 120

8.6 Ethernet 10BASE5 topology . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 122

8.7 CSMA/CD frame format . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 123

8.8 CSMA/CD MAC process . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 124

8.9 Popular networking conguration . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 125

8.10 Ethernet states: contention, transmission, or idle. . . . . . . . . . . . . . . . . . . . . . . . 125

8.11 Token-Ring frame format . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 128

8.12 Token-Ring medium access algorithm . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 128

8.13 Token-Ring frame format and content . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 129

8.14 Token bus network . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 133

8.15 Wireless LAN communication . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 134

9.1 One directional Virtual Circuits showing the order on which they are set up . . . . . . . . . 141

9.2 NODE tables corresponding to the Virtual Circuits set up Fig. 9.1 . . . . . . . . . . . . . . 142

9.3 Congestion in the Service Provider network. . . . . . . . . . . . . . . . . . . . . . . . . . . 145

9.4 Congestion in a NODE can be reduced by putting an upper bound on the number of buffersqueued on an output line . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 147

9.5 Store-and-forward lockup: (a) direct, (b) indirect. . . . . . . . . . . . . . . . . . . . . . . . 149

9.6 Message re-assembly lockup. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 150

9.7 Packet Level (top) and Frame Relay (bottom) PDU formats . . . . . . . . . . . . . . . . . . 151

9.8 Packet and Frame Relay processing ow diagrams. . . . . . . . . . . . . . . . . . . . . . . 153

9.9 Illustrating Committed Information Rate (CIR) and discard eligibility of frames . . . . . . . 154

9.10 The ATM principle . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 155

9.11 ATM connection paths . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 156

9.12 ATM call establishment . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 157

9.13 ATM cell format at (a) User-Service Provider interface and (b) internal to the network . . . . 158

9.14 The DTE-DCE X.25 ITU-T interface standard for Wide Area packet switched networks. . . 162

9.15 X.25 CALL REQUEST packet format. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 163

9.16 Sequence diagram of X.25 call establishment, data transfer and call clearing phases . . . . . 163

9.17 X.25 Control packet format . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 165

9.18 X.25 data packet format . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 166

9.19 The IP (Internet Protocol) header format . . . . . . . . . . . . . . . . . . . . . . . . . . . . 168


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xvi LIST OF FIGURES

9.20 Source and destination address formats in the IP header . . . . . . . . . . . . . . . . . . . . 169

9.21 Virtual circuit conguration. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 170

10.1 FDDI relationship to OSI model . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 177

10.2 Typical FDDI topology . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 178

10.3 FDDI Physical Layer . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 182

10.4 The FDDI Timed Token protocol . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 184

10.5 FDDI token rotation timing diagram . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 186

10.6 DQDB architecture . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 188

10.7 Open (top) and Closed (bottom) DQDB architecture . . . . . . . . . . . . . . . . . . . . . . 189

10.8 DQDB slot format . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 18910.9 State diagram of the DQDB MAC Protocol without BWB . . . . . . . . . . . . . . . . . . . 191

10.10Logical states of a DQDB station . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 192

11.1 Access matrix . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 200

11.2 Entering a password for a user . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 200

11.3 Verifying a password for a user . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 200

11.4 The encryption decryption mechanism . . . . . . . . . . . . . . . . . . . . . . . . . . . 201

11.5 The Engima machine . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 204

11.6 Diagramatic view of a rotor machine such as engima . . . . . . . . . . . . . . . . . . . . . 204

11.7 DES encryption mechanism . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 205

11.8 DES operation . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 207

11.9 End-to-end encryption at the application layer . . . . . . . . . . . . . . . . . . . . . . . . . 211

11.10End-to-end encryption at the transport layer . . . . . . . . . . . . . . . . . . . . . . . . . . 211

11.11Link encryption . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 212

11.12Cryptographic checksum in use . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 213

11.13Using public key encryption for digital signatures . . . . . . . . . . . . . . . . . . . . . . . 215

11.143way handshake process . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 217

11.15Smurf reector type attack . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 220

11.16A DDOS ready network . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 222

11.17Ingress lterning . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 223

11.18A rewall resides between external networks and internal networks . . . . . . . . . . . . . . 226

11.19A dual homed rewall with a proxy . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 229

11.20A screened subnet rewall, notice the two routers and the subnet between . . . . . . . . . . 230

12.1 Cellular concept . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 235


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LIST OF FIGURES xvii

12.2 Cellular network . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 235

12.3 Frequency reuse concept . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 236

12.4 Handoff scenario . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 238

12.5 Catering for high speed trafc with umbrella cells . . . . . . . . . . . . . . . . . . . . . . . 239

12.6 Timing diagram for a mobile handset . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 239

12.7 GSM schematic showing interfaces between functional entities . . . . . . . . . . . . . . . . 240

12.8 GSM TDMA frame . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 241

12.9 Logical architecture of a GPRS network . . . . . . . . . . . . . . . . . . . . . . . . . . . . 244

12.10Distiction of RAs and LAs in a GPRS network . . . . . . . . . . . . . . . . . . . . . . . . 245

12.11GPRS functional state model . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 245

12.12UMTS elements . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 247

12.13UTRAN connection . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 248

12.14UTRAN connection with new core network (CN) . . . . . . . . . . . . . . . . . . . . . . . 248

12.15Spreading of channels in a CDMA type access system . . . . . . . . . . . . . . . . . . . . . 249

13.1 Organisation of a distributed system . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 254

13.2 OMG reference model architecture . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 255

13.3 A request being sent through the ORB . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 256

13.4 The object request broker . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 257

13.5 The overall DCOM architecture . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 259

13.6 How DCOM works . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 260

13.7 RMI stubs and skeletons . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 262

13.8 Expanded view of RMI architecture . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 263


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xviii LIST OF FIGURES


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List of Tables

4.1 Electro magnetic spectrum and its uses . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 53

6.1 Typical payloads. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 77

6.2 Transmission speeds of SONET and SDH. . . . . . . . . . . . . . . . . . . . . . . . . . . . 80

6.3 The Hamming code explained . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 84

6.4 The Hamming code bit positions . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 84

6.5 Calculation of the polynomial code checksum. . . . . . . . . . . . . . . . . . . . . . . . . . 87

8.1 IEEE 802.3 networks with 512 bit slot times, 96 bit gap times and jam signals lasting 32 48 bits . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 121

8.2 Token ring control frames. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 130

9.1 Summary of the major differences between Virtual Circuit and Datagram service. . . . . . . 140

9.2 LAN and WAN characteristics compared . . . . . . . . . . . . . . . . . . . . . . . . . . . 155

9.3 X.25 PDU format . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 162

10.1 FDDI Frame and Token format . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 179

10.2 FDDI 4B/5B codes . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 181

11.1 Different methods of arbiter working . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 216

1


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2 LIST OF TABLES


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Part I

Overview of Telecommunication andComputer Networks

3


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Chapter 1

Basic Concepts

1.1 Objectives of this Chapter

There is a great deal to know about telephone and computer networks, or more correctly telecommunicationnetworks sin ce with the advent of the digital age and cellular networks, the distinction is getting increasinglyfuzzy.

1. We start off by explaining the origins of computr networks and why the telephone network has alwaysplayed a role in computer networks from the very start.

2. Without understanding the difference between analog and digital signals, one will never comprehend

why networks are playing such an increasingly important role in the world of computing, so weexplain those fundamentals. In some countries of the world, this is the sort of information that istaught in the 13th grade at school. Maybe it is better than learning Shakespeare, but I doubt it.

3. Computers, like humans need rules when communicating. These are called protocols as we shallexplain.

4. The software that makes communication between computers possible is some of the most complicatedthat can be written. In order to understand how it works, we divide it into layers which group thefunctions; hence network architecture.

5. Since computers communicate across continents and national boundaries, we need international or-

ganisations to make and approve the rules or protocols. There are many as we describe in the last partof this chapter.

A reader who understands everything discussed in this chapter will be very aware of how much there stillis to learn in telecommunication and computer networks, but will have a clear understanding of the veryfundamental concepts.

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6 CHAPTER 1. BASIC CONCEPTS

1.2 Introduction

A decade or two we associated the word communication with the use of the telephone or radio. Whilethose applications remain we nowadays think almost exclusively of the Internet and probably wireless,cellular telephones. It has become so pervasive that we simply take it for granted. One of the greatestimpacts that networking has had on society is to bring together vast sources of information, be they humanminds or computer systems: the emergence of a global village is changing the way society interacts anddoes business.

An understanding of the principles behind computer networks is therefore something that every studentshould have. Communication and Information Technology is advancing at an exponential rate, and we areliving in an exciting age where we are shaping the future of this technology. This introduction seeks toprovide the student with the core concepts behind communication networks.

1.3 Evolution of Computer Networks

The evolution of computer networks is best traced by following the development of computing resourcesused by large organisations. The earliest commercially available computers were characterized by expen-sive hardware and relatively primitive software. Typically, an organisation would purchase a single computerwhich would then be centrally located in a large air-conditioned room. It would consist of a central process-ing unit with a limited quantity of primary memory, some secondary disc storage, a printer, a punch-cardreader and an operator console. Something looking somewhat like that in Fig. 1.1.

Figure 1.1: Mainframe computer ca. 1970

Users normally prepared their data off-line on a card punch located in a different room, and the computeroperator would then load and run the prepared programs sequentially.

As computer hardware became faster and its operating software advanced, fast secondary storage largemagnetic drums and later magnetic disks and multi-programming operating systems came into existence.This made it possible to timeshare the central processing unit between a number of active programs, thereby


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1.3 Evolution of Computer Networks 7

allowing several users to run their programs interactively and to access stored data simultaneously via theirown individual terminal. The terminals were normally electromechanical teletypewriters (TTYs) similar tothose already in use at the time in international telex networks. They were designed, therefore, to transmitand receive one cahracter at a time over long distances in serial (or sequential) mode.

The computers then became known as multi-access systems providing on-line access to stored programs anddata. Initially, the terminals were all located in close proximity to the main computer complex with a simpleserial line connection between the computer and the terminal.

Users increasingly found it useful though to have a computer terminal on their desk. Within a building oron a campus that posed no problem. However, over longer distances the expense became important and,above all, communication (of voice at rst, but now data as well) was the monopoly of a central publicorganisation: Telkom!

Technically there was the additional issue that telephone networks carried analogue signals (explained in thenext section) and computers used binary or digital signals. Hence modems were invented to carry computercommunications nationally over wide geographical areas. An operational computer system typical at thattime is shown in Fig. 1.2 where the letters PSTN stand for Public Switched Telephone Network .

T

CENTRAL COMPUTER

= Terminal = Modem

PSTN

T

T

Figure 1.2: One typical conguration of a terminal-oriented network.

The use of a switched (switched in that connections could be changed from time to time to connect twoindividual users, the computer and the terminal in this instance) telephone network as the basic data commu-nication medium meant that the communication line costs could no longer be regarded as insignicant and,indeed, soon constituted a substantial proportion of the system operating costs. To minimize these costs,therefore, devices such as statistical multiplexers and cluster controllers were introduced. Essentially, theseallowed a single communication line, often on permanent lease from the telecommunications authorities, tobe shared between a number of simultaneous users all located, for example, at the same remote site. Anexample of such a system is shown diagrammatically in Fig. 1.3.

In addition, the increasing level of usage of computers soon gave rise to systems of hundreds of computers

a modem or modulator-demodulator

is used to convert the digital signals of a computer into equivalent analogue signals. Theseconcepts are explored later in this chapter


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T

CENTRAL COMPUTER

= Terminal = Modem

PSTN

Stat.MUX

Stat.MUX

T

T

T T

T

T

T

T

TT

Stat.MUX

Stat.MUX

Figure 1.3: A typical conguration of a terminal-oriented network using Statistical Multiplexers.

within an organisation. The effect of this was that the central computer could no longer cope with theprocessing over heads associated with servicing the various communication lines in addition to its normalprocessing functions. Consequently a special device, called a Front End Processor (FEP) was introducedfor the job of controlling the communication lines. Such a system is shown diagrammatically in Fig. 1.4.

T

T

T T

T T

T T

T T

T

FEP PSTN

CC

CC

= Terminal = Modem CC = Cluster controller

Figure 1.4: A typical conguration of a terminal-oriented network using a FEP.

The structures shown in Figs 1.3 and 1.4 were particularly prevalent in organizations normally holding largeamounts of data at a central site, such as the major clearing banks and airlines.

In many other organisations, it is not necessary and in fact desirable not to hold all the data centrally, andhence it became common place for organisations to have a number of autonomous computer systems locatedat different geographical locations. Typically, these systems provided a local computing function, but therewas often a requirement for them to communicate with each other to share resources and data. In sucha network, the internal message units used may be long. This can result in a signicant delay between amessage entering the network and the time it leaves the network (known as the response time ), due to longmessages waiting upon each other at the various resources along the way.


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1.4 Electrical Signals 9

It then became economically feasible for TelCos to provide a separate, autonomous computer commu-nication network . Such communication networks that carry mostly computer data normally operate usingsmaller units of data known as a data packet . It is therefore commonly referred to as a Packet Switched Network. Increasingly the term IP network is also used for reasons that will become clear only muchlater. Also, since the interconnected computers are normally geographically widespread, such a network isalso called a Wide Area Network (WAN) . .

The next obvious development was that computers of different manufacturers (IBM, SUN Microsystems,Microsoft, etc) with their different architectures and applications need to communicate with one another.The need for open (for everybody to see)interface standards became all important. This was the status at thebeginning of the 1980s since being overtaken by the event of the Internet in the middle of the last decade.

What was to become the Internet, was rst proposed in 1967 in the United States of America as an experi-mental computer network. For a number of years before 1967, DARPA, the (Defense) Advanced ResearchProjects Agency of the United States Department of Defense had been funding the growth and development

of many multiclass timeshared computer systems at a number of university and industrial research centresacross the United States. By 1967, many of these had shown themselves to be valuable computing resourcesand it was recognized that the Department of Defense as well as the scientic community could benetgreatly if there were to be made available a communication service providing remote access from any termi-nal to all of these systems. In early 1969 a contract was awarded for the implementation of the ARPANETto Bolt, Beranek and Newman (BBN) an engineering rm based in Massachusetts.

Simultaneously DARPA funded the research which resulted in a set of network standards that specify thedetails of how computers communicate, as well as a set of conventions for interconnecting networks androuting trafc. These are now known as the Transmission Control Protocol/Internet Protocol or TCP/IPwhich can be used to communicate across networks which represent a wide variety of underlying network technologies. What started as ARPANET is now known as the Internet, literally a network that (Inter-)connects many worldwide networks. The evolution has however not ended and is been fueled further bythe popularity of wireless, cellular communication. In order for the Internet to reach the very edge of thenetwork the last bastion of traditional voice transmission has to be broken: In other words, transmittingvoice over data (or IP) networks, including the wireless link.

Now that we now a little about their evolution, we shall start from scratch to understand the exciting worldof telecommunication networks.

1.4 Electrical Signals

Signals to represent data, voice conversations or video transmissions are either electromagnetic radio waves,electrical signals owing in a conductor or optical (light) signals. Optical signals are being used more andmore but we shall discuss those later.

An electrical signal is transmitted along a conductor by varying some electrical property such as voltage orcurrent which could either be an analog or digital signal as illustrated in Figure 1.5.

Electrical signals can be either analog or digital as illustrated in Fig. 1.5. Digital signals are also calledbinary signals. An analog signal is continues in time whereas digital signals take on discrete values andchanges instantaneously from one voltage or current value to the other in time. The number of signal levelsare always

for

. Originally

but as technology improves it is normal for more thantwo levels to be represented. In practice the way in which the binary data are represented or coded by the

A general term for the Telecommunication Company Monopolies which alas exist in too many countries still.


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1 cycle

C u r r e n t o r v o

l t a g e


l t a g e

time

time

time time


l t a g e


l t a g e

Analogue SignalsDigital Signals

Figure 1.5: Examples of analog and digital signals

digital signal is all but simple. One method commonly used is known as Manchester Encoding illustrated inFigure 1.6.

Since analog signals vary continuously in time they can be represented by mathematical formulae and ma-nipulated accordingly. Analogue signals do not have to be cyclical in that they repeat themselves after axed time called the period. If they are, the number of complete cycles per second (also called Hertz)is the inverse of the period and is called the frequency , usually denoted by the symbol . Thus cycles/second, or seconds. The signals with periodic wave forms are the most useful to us intelecommunication since can modulate them to carry digital signals as we shall see in Chapter 2.

Analog signals not only represent current owing in a copper conductor, but most importantly may beelectro magnetic waves travelling through space in which case we call them radio waves . Depending ontheir frequency, radio waves can travel around the world, which means that everyone can receive them.Although this is exactly what radio waves were originally intended for, it is simultaneously a problem if toomany parties want to use the limited frequency spectrum.

When the frequency of the electro magnetic waves is very high we refer to them as microwaves which havethe advantage (for the intended purpose) that they are easily attenuated by obstacles like buildings in theirway (the same effect that chickens in a microwave experience) and are only really reliable when they aretransmitted to a receiver which is in direct line of sight. With this property it is possible to re-use the


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1.5 Optical Signals 11

1 0 0 01 1 1 0 1 0 0 01 1 1 0

Manchester encoded signalUnencoded binary signal

Figure 1.6: Manchester encoding of the byte 1111 0000.

scarce frequencies in geographically separated areas. If we transmit the microwave signals at low powerin addition, we can divide a geographical area into cells where the transmissions do not interfere witheach other provided the frequencies in adjacent cells are different. This is the way that a GSM (cell phone)

network works. We shall return to all these concepts in later chapters.

1.5 Optical Signals

Telecommunication Networks also use optical bre to transport data encoded as ON/OFF pulses of light.Depending on the type of bre, a transmitter at one end of the bre uses a light emitting diode (LED) or Laser to send pulses of light down the bre. A receiver at the other end uses a light detector such as a Photo Intrinsic Diode (PIN) or Avalanche Photo Diode (APD) to detect the pulses.

The number of bits per second (Bps) that can be transmitted along an optical bre is virtually unlimitedsince it is theoretically possible to send several beams (called modes ) of light of different frequencies downa bre, each carrying typically 2 GBps . We will return to optical signals and the associated standards inChapter 3.

1.6 A Matter of Protocol

Now that we understand a little about the signals travelling between two communicating parties in a telecom-munications network, we turn to the rules of communication or protocols which are needed for the partiesto exchange information without error.

Essentially, a computer network consists of one or more computer talking with each other. The rules that govern the exchange of information , and the exact structure of that information are known as a protocol . Asan introduction to the idea of a protocol, we consider an analogy from everyday life namely, a telephoneconversation between two people living in different cities in different countries. As we all know the processis something like the following:

First of all, the caller picks up the telephone receiver. If a dialing tone is heard, he will select the numberof the callee on the buttons of his telephone. The called number constitutes the address of the callee. If nodialing tone is heard, the caller would normally discontinue his attempts to place the call.

Once the call goes through, the remote instrument either rings and is answered or not. The callee could alsobe using the device. Should the callee answer the ringing telephone, a verbal handshake typically occurs.Note that in this case it is a name , rather than a number , that identies the caller and callee to each other.

A Gigabit is

bits


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Now that the connection has been established, the two parties conduct a conversation. Certain conventionsalso apply in this case. Two people do not (normally that is!) talk simultaneously; if they did, a process of re-synchronization would need to occur. If one person speaks too quickly, the other may ask him to slowdown.

If the line should fade of if there is noise on the line, messages like I did not hear your last words orWould you please repeat the last sentence? would ensure that the information passing between the twoparties remains error-free.

Should an abrupt cut-off occur, we know from experience that a frustrating situation of deadlock mightoccur, when both people try simultaneously to call each other. There are no rules in this case which is anerror in the specication of the (human) protocol.

At the conclusion of their conversation the caller and callee go through similar conventions for terminatingtheir call and would eventually put their respective telephone receivers down, thereby not only ending theirconversation, but also breaking the physical (electrical) communication link.

When two processes in separate computers communicate with each other, similar conventions for:

addressing

call establishment and call termination

error checking

information recovery

and ow control

must be decided upon.

Once again, the rules and conventions used in a session of communication are known as a protocol. Thespecication of a protocol also includes a specication of the format and content of the messages beingpassed. As computers are dumb, deterministic machines, they require a far more elaborate and exact protocolthan in the case of a conversation conducted in natural language between two human beings.

The telephone analogy also reveals another basic concept important in the study of computer communi-cation. This is the fact that there are a number of stages to the conversation between communicatingcomputer processes:

call-establishment phase

data (information) transfer phase

call-clearing phase

idle phase

1.7 Network Architecture

Computers have different architectures understand different languages, store data in different formats, andcommunicate at different rates. Consequently there is much incompatibility, and communication is difcult.In fact, how do computers manage to communicate at all ?? They communicate the same way that the


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1.7 Network Architecture 13

people in our analogy do. In the analogy, there are rules which apply at the physical level (number called,dialling tone, engaged signal, etc) and rules which apply at, shall we say logical level between the humansthemselves. The same is true for computers.

As long ago as 1976 the International Standards Organisation (ISO) recognised the need for some stan-dardisation of computer network interfaces. It was apparent even at that early stage, that for the reasonsmentioned already, it would be expedient to describe the architecture of the network software in layers inorder to facilitate the interconnection of unlike systems. In other words, to provide an open system that anyother system could connect to. ISO thus set out to standardise the Open System Interconnection Reference Model , or OSI model for short, intended as a reference architecture for network design. This model isillustrated in Fig. 1.7.

Data Link

Presentation

Application

Session

Transport

Network

Physical

Data Link

Presentation

Application

Session

Transport

Network

Physical bytes

frames

packets

message

message

message

message

Physical Medium

Host A Host B

Figure 1.7: The ISO OSI Reference Model.

Between each pair of adjacent layers in the OSI model there is an interface. The interface denes whichservices the lower layer offers to the upper one. When network designers decide how many layers to includein a network and what each one should do, one of the most important considerations is having clearly denedinterfaces between the layers. Having clearly dened interfaces, in turn, requires that each layer performs aspecic collection of well understood functions . In addition to minimizing the amount of information thatmust be passed between layers, clear cut interfaces also make it simpler to replace the implementation of onelayer with a completely different one (e.g., when all the telephone lines are replaced by satellite channels),because all that is required of the new implementation is that it offers exactly the same set of services to thenext layer higher up as the old implementation did.

The set of layers and protocols is called the network architecture. The specication of the architecture mustcontain enough information to allow an implementor to write the program for each layer so that the programwill correctly obey the appropriate protocol. Neither the details of the implementation nor the specication


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of the interfaces are part of the architecture.

When describing the operation of any of the protocol layers, it is important from the outset to discriminate

between:1. the services provided by the layer

2. the protocol (i.e., logical operation of the layer)

3. the services used by that layer

This is important because only then can the function of each layer be dened in the context of the otherlayers. This also has the effects that a person implementing one protocol layer needs only to have a knowl-edge of the services the layer is to provide to the layer above, the internal operation (protocol) of the layer,and the services that are provided by the layer below to transfer the appropriate items of information, calledProtocol Data Units or PDUs associated with the protocol to the corresponding layer in a remote computer.

Equally, the specication of each protocol layer comprises two sets of documents:

1. service denition document

2. protocol specication document .

The service denition document contains a specication of the services provided by that layer to the layerabove it that is the user services . These are in the form of a dened set of service primitives . These serviceprimitives are similar to procedure calls in a high level language, each with an associated set of service parameters . These primitives are invoked to and from the layer through service access points (SAPs) (seenin gure 1.8 e.g.. a request, indication, response and conrm primitive. As will be seen in later chapters, it isthrough the service primitives that the layer above achieves the transfer of information to the correspondentlayer in a remote system.

We need to clarify the rather cryptic notations N, N+1 and N-1, seen in Fig. 1.8. A layer in the OSI modelis generally as the (N)-layer. The layer above it (if one exists) is designated as the (N+1)-layer, similarlythe one below it is the (N-1)-layer. The peer process abstraction is crucial to all network design. Withoutthis technique, it would be impossible to partition the design of the complete network into several smaller,manageable, design problems, namely the design of the individual layers.

It was not intended that there should be a single standard protocol associated with each layer. Rather a setof standards is associated with each layer, each offering different levels of functionality. In each of the

following sections we briey discuss each layer of the architecture

1.7.1 Physical layer

The physical layer is concerned with transmitting raw bits over a communication channel. The design issueshave to do with making sure that when one side sends a bit, it is received by the other side as a bit, not asa

bit. Typical questions here are how many volts should be used to represent a and how many for a

, howmany micro seconds a bit occupies, whether transmission is simplex or duplex, how the initial connectionis established and how it is broken when both sides are nished, how many pins the network connector hasand what each pin is used for. In some cases a transmission facility consists of multiple physical channels,in which case the physical layer can make them look like a single channel, although higher layers can alsoperform this function.


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1.7.2 Data link layer 15

Layer N-1

SAP

SAP SAP

SAP

Peer protocolentity

Protocol

Correspondent

UserUser

(Logical) exchange path of PDUs

Used services

Service provider

User services

Service user

Layer N

(N-1) services

N-services

Layer N + 1

SAP = Service access point

entityPDUs

Figure 1.8: Layers, protocols, and interfaces.

1.7.2 Data link layer

The task of the data link layer is to take a raw transmission facility and transform it into a line that appears free of transmission errors to the network layer. It accomplishes this task by breaking the input data upinto data frames transmitting the frames sequentially, and processing the acknowledgement frames sentback by the receiver. Since Layer 1 merely accepts and transmits a stream of bits without any regard tomeaning or structure, it is up to the data link layer to create and recognize frame boundaries. This can beaccomplished by attaching special bit patterns to the beginning and end of the frame. These bit patterns canunintentionally occur in the data, so special care must be taken to avoid confusion. It is up to this layer tosolve the problems caused by damaged, lost, and duplicate frames, so that network layer can assume it isworking with an error-free (virtual) line.

1.7.3 Network layer

The network layer controls the operation of the subnet. Among other things, it determines the chief char-acteristics of the DCE-DTE interface, and how Network Protocol Data Units or NPDUs , more commonlyknown as packets , the units of information exchanged in the network layer, are routed within the network.A major design issue here is the division of labour between the DCEs and hosts, in particular who shouldensure that all packets are correctly received at their destinations, and in the pr

telecommunication and computer networks

Documents