Telecom Concepts

7/29/2019 Telecom Concepts

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General Telecom

Telecom Concepts 2000

Handout


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Status Released

Change Note

Short Title Telecom Concepts 2000

All rights reserved. Passing on and copying of thisdocument, use and communication of its contentsnot permitted without written authorization from Alcatel.


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Contents

Preface 11. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .

1 Introduction 13. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .

1.1 Voice 14. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .

1.2 Data 15. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .

1.3 Comparison 15. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .

2 Text Structure 17. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .

3 The Core Network 19. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .

3.1 Network Structures and Topologies 19. . . . . . . . . . . . . .3.1.1 Point to Point 20. . . . . . . . . . . . . . . . . . . . . . . .

3.1.2 Star Network 20. . . . . . . . . . . . . . . . . . . . . . . .3.1.3 Meshed Network 21. . . . . . . . . . . . . . . . . . . .3.1.4 Ring Network 21. . . . . . . . . . . . . . . . . . . . . . .3.1.5 Tree Network 23. . . . . . . . . . . . . . . . . . . . . . . .

3.2 Links : Transmission 27. . . . . . . . . . . . . . . . . . . . . . . . . . .3.2.1 Plesiochronous Digital Hierarchy (PDH) 28.3.2.2 Synchronous Digital Hierarchy (SDH) 30. . .3.2.3 Wavelength Division Multiplexing (WDM) 333.2.4 Media 34. . . . . . . . . . . . . . . . . . . . . . . . . . . . . .3.2.5 Repeaters 35. . . . . . . . . . . . . . . . . . . . . . . . . . .

3.3 Nodes : Switching 36. . . . . . . . . . . . . . . . . . . . . . . . . . . . .3.3.1 Switching Techniques 36. . . . . . . . . . . . . . . . .3.3.2 Cross Connection (XC), Add Drop Mux

(ADM) 40. . . . . . . . . . . . . . . . . . . . . . . . . . . . .3.3.3 Circuit Switching 41. . . . . . . . . . . . . . . . . . . . .3.3.4 Packet Switching 44. . . . . . . . . . . . . . . . . . . . .3.3.5 Signalling 54. . . . . . . . . . . . . . . . . . . . . . . . . .

4 Access Networks 59. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .

4.1 Objectives 59. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .

4.2 Media 64. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .

4.3 Analogue Line Access 69. . . . . . . . . . . . . . . . . . . . . . . . . .

4.4 ISDN Access 69. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .

4.5 Concentrating Remote Users 72. . . . . . . . . . . . . . . . . . . .

4.6 Digital Subscriber Line, ADSL 73. . . . . . . . . . . . . . . . . . .

4.7 Hybrid Fiber Coax (HFC) 79. . . . . . . . . . . . . . . . . . . . . . .

4.8 Fiber in the Loop (FITL) 81. . . . . . . . . . . . . . . . . . . . . . . . .

4.9 Microwave 83. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .

4.10 GSM 85. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .


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6.3 Intelligent Networks 127. . . . . . . . . . . . . . . . . . . . . . . . . . .6.3.1 Introduction 127. . . . . . . . . . . . . . . . . . . . . . . . .6.3.2 Services Overview 127. . . . . . . . . . . . . . . . . . .6.3.3 Implementation 130. . . . . . . . . . . . . . . . . . . . .

6.4 Internet Services 133. . . . . . . . . . . . . . . . . . . . . . . . . . . . . .6.4.1 Internet Access Provider 134. . . . . . . . . . . . . . .6.4.2 FireWall 136. . . . . . . . . . . . . . . . . . . . . . . . . . . .6.4.3 Proxy Server 137. . . . . . . . . . . . . . . . . . . . . . . .6.4.4 Internet Telephony 138. . . . . . . . . . . . . . . . . . .

6.5 Mobile Telephony Services 139. . . . . . . . . . . . . . . . . . . . . .

6.6 Quality of Service (QOS) 141. . . . . . . . . . . . . . . . . . . . . . .

7 Network Management 143. . . . . . . . . . . . . . . . . . . . . . . . . . . . . .

7.1 Introduction 143. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .7.1.1 Telecom Network Management (TMN) 143. .7.1.2 Simple Network Management Protocol

(SNMP) 146. . . . . . . . . . . . . . . . . . . . . . . . . . . . .

Abbreviations 149. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .

Glossary 155. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .

Appendix A Analog versus Digital 163. . . . . . . . . . . . . . .

Appendix B Analog to Digital 165. . . . . . . . . . . . . . . . . . .

Appendix C Multiplexing - Concentration 166. . . . . . .

Appendix D Time - Frequency - Code 167. . . . . . . . . .

Appendix E Modulation & Coding 169. . . . . . . . . . . . . . .

Appendix F Asynchronous, Plesiochronous,Synchronous 171. . . . . . . . . . . . . . . . . . . . . . .

Appendix G Real Life Networks and their Features 172

Appendix H Connection-Oriented vs.Connectionless 173. . . . . . . . . . . . . . . . . . . . .

Appendix I Standards 175. . . . . . . . . . . . . . . . . . . . . . . . .

Appendix J The Race for Bandwidth 178. . . . . . . . . . . .

Appendix K a Protocol Stack 179. . . . . . . . . . . . . . . . . . . .

Appendix L A Call Scenario 181. . . . . . . . . . . . . . . . . . . .

Appendix M the Frequency Spectrum 182. . . . . . . . . . . .

Appendix N MultiMedia 183. . . . . . . . . . . . . . . . . . . . . . . .

Appendix O Links to Further Information 186. . . . . . . .


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Figures

Figure 1 Text structure maps on 5 aspects of Telecom Networks 14. . . . .Figure 2 Point to Point network 20. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .

Figure 3 Star network 20. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .Figure 4 Meshed Network 21. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .Figure 5 Ring Network 22. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .Figure 6 Backbone Network : example 22. . . . . . . . . . . . . . . . . . . . . . . . . .Figure 7 Protection Switching 23. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .Figure 8 Self Healing Ring 23. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .Figure 9 Hierarchical Networks 24. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .Figure 10 Hierarchical Networks : example 24. . . . . . . . . . . . . . . . . . . . . . .Figure 11 example 1 : the first Internet plan 25. . . . . . . . . . . . . . . . . . . . . .Figure 12 example 2 : todays complex networks 25. . . . . . . . . . . . . . . . . . .Figure 13 example 3 : Map with European Fiber links 26. . . . . . . . . . . . . .

Figure 14 Basic E1 structure 27. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .Figure 15 Multiplexing Hierarchy 28. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .Figure 16 Multiplexing Hierarchy 30. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .Figure 17 Coupling PDH and SDH 31. . . . . . . . . . . . . . . . . . . . . . . . . . . . . .Figure 18 Back to Back versus Add-Drop Multiplexer 33. . . . . . . . . . . . . .Figure 19 Combining WDM and TDM 34. . . . . . . . . . . . . . . . . . . . . . . . . . .Figure 20 Space Switching : 4*5 space-switch 37. . . . . . . . . . . . . . . . . . . .Figure 21 Time Switching : 4*4 time-switch 38. . . . . . . . . . . . . . . . . . . . . .Figure 22 Time Space Switch 38. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .Figure 23 Cascading switching elements 39. . . . . . . . . . . . . . . . . . . . . . . . .Figure 24 Folded View, Reflection Point 40. . . . . . . . . . . . . . . . . . . . . . . . . . .Figure 25 User Information versus Signalling Information 41. . . . . . . . . . .Figure 26 Hierarchical Structure of the Telephone Network 42. . . . . . . . . .Figure 27 Hierarchical Structure of the Telephone Network :

Multi-Carrier 43. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .Figure 28 ATM Cell 48. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .Figure 29 STM versus ATM 48. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .Figure 30 IP Packet (IPv4) 51. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .Figure 31 ATM versus IP 52. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .Figure 32 IP, ATM and SDH 53. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .Figure 33 Separation of Signalling Network and User Data Network 56.

Figure 34 Modular Structure of CCS #7 57. . . . . . . . . . . . . . . . . . . . . . . . . .Figure 35 Network evolution with Access Nodes 61. . . . . . . . . . . . . . . . . . .Figure 36 Network evolution with Access Nodes : view from the sky 61. .Figure 37 Access to Several Networks 62. . . . . . . . . . . . . . . . . . . . . . . . . . . .Figure 38 MultiService Access Networks 63. . . . . . . . . . . . . . . . . . . . . . . . . .Figure 39 (Unshielded) Twisted Pair, 4 pairs 65. . . . . . . . . . . . . . . . . . . . . . .Figure 40 Coaxial Cable 65. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .Figure 41 Optical Fiber (8 fibers) 66. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .Figure 42 Radio : Microwave Dish and other antenna's 68. . . . . . . . . . . .Figure 43 Analogue Access 70. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .


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Figure 44 ISDN Basic Rate Access 71. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .Figure 45 ISDN Primary Rate Access 71. . . . . . . . . . . . . . . . . . . . . . . . . . . . .Figure 46 ADSL Frequency Spectrum 74. . . . . . . . . . . . . . . . . . . . . . . . . . . .Figure 47 Discrete Multi-Tone 74. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .

Figure 48 Internet Access Provider, ADSL 75. . . . . . . . . . . . . . . . . . . . . . . . .Figure 49 Different DSL techniques 77. . . . . . . . . . . . . . . . . . . . . . . . . . . . . .Figure 50 IP on ATM over ADSL 78. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .Figure 51 ADSL Access on ATM core Network 78. . . . . . . . . . . . . . . . . . . . .Figure 52 ADSL Network Termination 79. . . . . . . . . . . . . . . . . . . . . . . . . . . .Figure 53 Hybrid Fiber Coax evolution : Full Coax Star Network 80. . . . .Figure 54 Hybrid Fiber Coax evolution : Optical Backbone, Coax Tail 80Figure 55 Typical spectrum allocation for Cable-Access 81. . . . . . . . . . . .Figure 56 Fiber in the Loop 82. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .Figure 57 Local Multipoint Distribution Services 84. . . . . . . . . . . . . . . . . . . .Figure 58 Example of Frequency Planning : groups of 7 frequencies 86.

Figure 59 GSM Frequency/TDM Structure 87. . . . . . . . . . . . . . . . . . . . . . . .Figure 60 GSM Network Structure 90. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .Figure 61 Network Elements for Short Message Service 91. . . . . . . . . . . . .Figure 62 Network Elements for WireLess Access Protocol 92. . . . . . . . . . .Figure 63 GPRS Network Structure 93. . . . . . . . . . . . . . . . . . . . . . . . . . . . . .Figure 64 DECT Frequency/TDM Structure 96. . . . . . . . . . . . . . . . . . . . . . . .Figure 65 Positioning of Bluetooth in Bandwidth versus Distance 97. . . . .Figure 66 Satellite Systems 99. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .Figure 67 Parabolic dish to GeoStationary Satellite 100. . . . . . . . . . . . . . . .Figure 68 Globalstar logo 101. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .Figure 69 SkyBridge logo 102. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .Figure 70 Iridium logo 103. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .Figure 71 Internet Access via combination Modem / Satellite 105. . . . . . . .Figure 72 Internet Access : Terminology 107. . . . . . . . . . . . . . . . . . . . . . . . . .Figure 73 Alcatel 21XX, Analog Telephone 110. . . . . . . . . . . . . . . . . . . . . . .Figure 74 Alcatel 2810, ISDN Telephone 111. . . . . . . . . . . . . . . . . . . . . . . . .Figure 75 Mobile Phone (Alcatel One Touch Easy db-W@p 113. . . . . . . .Figure 76 Ethernet LAN without / with a Hub 117. . . . . . . . . . . . . . . . . . . . . .Figure 77 Token Ring LAN without / with a Hub 118. . . . . . . . . . . . . . . . . . .Figure 78 3COM Palm V and Alcatel One Touch Com 119. . . . . . . . . . . . .Figure 79 Alcatel 'Web Touch' 121. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .

Figure 80 Centrex 127. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .Figure 81 Intelligent Network Structure 131. . . . . . . . . . . . . . . . . . . . . . . . . . .Figure 82 Internet Access Provider, Analog or ISDN Dial-Up 135. . . . . . . .Figure 83 Internet Access Provider, ADSL 136. . . . . . . . . . . . . . . . . . . . . . . . .Figure 84 Internet Access Provider, Cable 136. . . . . . . . . . . . . . . . . . . . . . . . .Figure 85 FireWall 137. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .Figure 86 Proxy Server 138. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .Figure 87 Voice over IP, all functionality in the terminals 139. . . . . . . . . . . .Figure 88 Voice over IP, functionality in Gateways 139. . . . . . . . . . . . . . . . . .Figure 89 Management Network 145. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .


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Figure 90 SNMP Protocol Stack 148. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .Figure 91 32 QAM : Quadrature Amplitude Modulation 170. . . . . . . . . . . .Figure 92 ITU logo 175. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .Figure 93 ETSI logo 175. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .

Figure 94 Bellcore logo 176. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .Figure 95 ATM Forum logo 176. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .Figure 96 ISO logo 176. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .Figure 97 Bandwidth growth predictions 178. . . . . . . . . . . . . . . . . . . . . . . . .Figure 98 Protocol Stack 179. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .Figure 99 ISDN Local Call Scenario 181. . . . . . . . . . . . . . . . . . . . . . . . . . . . .Figure 100 Frequency Chart 182. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .Figure 101 One Touch Com 184. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .Figure 102 Overlapping Businesses, MultiMedia 185. . . . . . . . . . . . . . . . . . . .


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Tables

Table 1 Voice versus Data 16. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .Table 2 Plesiochronous Digital Hierarchy 29. . . . . . . . . . . . . . . . . . . . . . .

Table 3 SDH Containers 31. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .Table 4 SDH Multiplex Signals, STMn 32. . . . . . . . . . . . . . . . . . . . . . . . . .Table 5 PDH versus SDH 33. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .Table 6 Types of Switching 36. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .Table 7 Routing/Connection Table Contents : 39. . . . . . . . . . . . . . . . . . .Table 8 STM versus ATM 50. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .Table 9 Types of Access Networks 60. . . . . . . . . . . . . . . . . . . . . . . . . . . . .Table 10 Active/Passive Optical Networks 83. . . . . . . . . . . . . . . . . . . . . . . .Table 11 Satellite Frequency Bands, a comparison 101. . . . . . . . . . . . . . . .Table 12 Analogue versus Digital 164. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .Table 13 Connection Oriented versus Connection-Less 174. . . . . . . . . . .


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Preface

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Preface

The purpose of this Telecom Concepts 2000 course is to introduceyou to the basic principles and techniques used in the Telecombusiness. As this area is quite large, the introduction goes wide,rather than deep : all relevant concepts are explained, principlesare generalized, overviews and comparisons are made. For moredetails, the course points you, where-ever possible, to additionalmore in-depth information.

As the Telecom business undergoes a rapid evolution, not to say arevolution, this material needs constant updating. If you'd like tocontribute a comment, suggestion or anything else, you are

welcome to send it to : [email protected]


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1 Introduction

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1 Introduction

Telecom systems provide a service to their users which is all abouttransporting information. In the past there were mainly twodistinct types of information :

" voice, also called speech

" data

As we are heading towards a multi-media communicating world,the difference between these two types becomes blurred or nolonger relevant. Therefore this text will no longer categorize alltelecom technologies in either the voice-world or the data-world.Rather this text will try to describe these technologies or principlesin the order in which you would act if you were to build such asystem or network yourself :

a) the Core Network

b) theAccess Network

c) the Terminals

d) the Services

e) the Network Management

However, some basic understanding of the specifics and

differences of voice and data communication is important.Therefore both are introduced here.


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1 Introduction

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amount is called the Bit-rate. As a result a single digital voiceconnection is often referred to as a 64 kbps(kilo-bits-per-second). This continuous stream lasts as long asthe phone conversation lasts, typically a few minutes.

Note As a comparison, an audio CD signal uses 44100 samples persecond, each 16 bit precision, and two channels (left+right) toprovide stereo. You will agree that CD sound quality is muchbetter than a phone-line, but the price for this is that you need totransport 1.411 Mbps for the CD (mega-bits-per-second),where only 64 kbps for a telephone line.

Note From this explanation, it is clear why telephone calls are usuallycharged as a function of the duration : each second 64000 bits

need to be handled by the telecom system, the longer the calllasts, the more total bits need to be handled.

1.2 Data

When computers or machines communicate with each other, theyusually don't send a continuous stream of information. Typically acomputer needs some limited input data, then processes this, andthen responds with a limited amount of result data. Therefore

data communication is using the concept of information packets: a group of information bits. So one computer sends apacket ofinput to the other computer, which processes it, and then returns apacket with the results.

1.3 Comparison

The two above communication mechanisms are fundamentallydifferent, and it is important to understand this in order tounderstand the future evolution of the telecom industry.

Although this is no strict rule, today the majority of voice istransported usingsynchronous circuit switching, whereas themajority of data usespacket switching. (These terms will beexplained in more detail later) Given this simplification, circuitswitching (voice) and packet switching (data), can be compared asfollows :


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Table 1 Voice versus Data

Circuit Switching : Voice Packet Switching : Data

Fixed time between samples Undefined time between

packetsBitrate is multiple of 64 kbps Bitrate can be anything

Bitrate is fixed during 'the call' Bitrate can vary dynamically

Note The aspect of transporting voice or data is called theService : thefunction you deliver to the end-user. The aspect of using circuitor packet switching to accomplish this is called the Transport Modeor also Bearer Capability. Strictly speaking transport mode andservice are independent of each other : a service can be realized

using different transport modes, and different services can berealized using the same transport mode.


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Chapt

2 Text Structure

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2 Text Structure

This text is divided into five chapters, and a set of appendices.See also Figure 1.

The chapters cover the most important telecom technologies andconcepts in the following order :

" the Core Network: this is the part of the network thatprovides information transfer at high speeds over longdistances. It is shared by all users of the network. Whencomparing with the roads network, the core network mapsonto the Highways. When thinking about the classicaltelephone network, the Core Network is the internationaltelephone network, whereby all telephone exchanges are

interconnected. The Core Network is also called theBackBone.

" theAccess Network: this is the part of the network thatallows the user to get him onto the core network. Compare itto the roads network where you have smaller access roadsbetween you and the highways. Highways don't pass next toeach house. Most of the access network is only used by alimited number of users, some of it is dedicated to just asingle user. When thinking of the classical telephone network,the Access Network would be your telephone connection fromyour home to your local exchange.

" the Customer Premises Equipment : this is the equipmentwhich the end user uses. Simple examples are a telephone, amobile phone, a computer with modem, a local network ofcomputers, etc.

" the Network Services : when we have a network (byinstalling a core network, access network and terminals) thisnetwork can be used forseveral different functions. As anexample, when you have the classical telephone network, youcan use it (a.o.) to :

D make voice communications between two personsD let the network wake you up, by making a call to you at a

specified time.

So differentservices can be offered, using the same networkinfrastructure. In this chapter the services made available tothe end-user are described. The services or features whichare important for the network provider, are described togetherwith the description of each of the technologies.


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" the Network Management : when we have a network (byinstalling a core network, access network and terminals) thisnetwork must also be operated : you must constantly checkfor errors, add new customers, increase capacity, reconfigure,

etc. The network operator is the one who is directly involvedin Network Management, however for the end user, awell-managed network provides services such as reliability,low signal distortion, low information loss, etc.

In addition to the five main chapters, there is a set of Appendices.They each cover a specific item which is more generic and is usedin several different telecom systems. Therefore they are groupedat the back, and the chapters will point to them.


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3 The Core Network

The core network refers to a (public or private) infrastructure thatprovides the connections between communicating users. Thisinfrastructure contains network nodes and transmission links.

Examples of core networks" are :

" the Public Switched Telephone Network (PSTN),

" Public (and Private) Data Networks,

" the Broadband Integrated Services Digital Network (B-ISDN),

" the Internet.

The main purpose of the Core Network is to :

" transport information fast over long distances,

" transport information at low cost,

" transport information with minimal amount of errors.

As for the low cost aspect, note that Core Networks are alwaysshared by many users. So the cost of these networks is alsoshared by many users and therefor, even if the networksthemselves are typically very expensive,per user they areeconomical, due to the large number of users.

3.1 Network Structures and Topologies

Because networks typically grow in an organic way, each networkis different. However, some basic principles for the structure ofnetworks can be sorted out :


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The advantages of a star network are :

" simple routing, addressing in the center of the star.

" simple next step from a point-to-point network : add the

network center device, then add terminals.A disadvantages of a star network is its reliability : if the device inthe center of the star fails, all terminals are out of service.

3.1.3 Meshed Network

In a meshed network, everynode is connected to all the othernodes. It is a collection of point to point links between a collectionof nodes. You can easily see that when the number of nodesgrows, the number of links grows quadratically (n*(n-1)/2). Sofully meshed networks, are only found when there are a limitednumber of nodes in the network.

A big advantage of a meshed network is its reliability : even whensome links would break, there are always alternativeinterconnections between a pair of nodes. A price for thisreliability or redundancy is that the nodes are more complex :each node needs to be able to route information further to thedestination.

Figure 4 Meshed Network

3.1.4 Ring Network

Ring networks simplify the interconnection of a large number ofnodes : each node interconnects to only two neighbor nodes,resulting into onlyn links. This is not only reducing equipment


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Node NodeActive fibre

Spare fibre

Figure 7 Protection Switching

Node

Node

Node Node

Fiber cut

Restoration

Figure 8 Self Healing Ring

3.1.5 Tree Network

With a very large number of nodes, each network becomescomplex to manage. To simplify a network, a hierarchical treeconcept with different levels can be used : the highest level is thebackbone, on the backbone the nodes are actually subnetworks.Each network, consists again of subnetworks, etc.

Examples :

" Telephone network

" Internet


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RegionalNetwork 1

Backbone Network

RegionalNetwork 3

RegionalNetwork 2

Figure 9 Hierarchical Networks

Figure 10 Hierarchical Networks : example


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3.2 Links : Transmission

All telecom technologies which interconnect nodes with links,deploy some form ofMultiplexing. Multiplexing is a technique totransport several information-flows over the same carrier.Multiplexing reduces the cost, because all individual data-flows,share the cost of the common carrier. There exist a number ofdifferent techniques to do multiplexing, and they are explainedand compared in Appendix D :

" Time Division Multiplexing (TDM)

" Frequency Division Multiplexing (FDM)

" Code Division Multiplexing (CDM)

Furthermore these multiplexing techniques can be combined into

more complicated schemes : for example the Global System forMobile Communications (GSM) uses a combination ofFDM-TDM.

The building block for building a multiplex is a Channel (CH). Asequence of channels is combined into a so-called Frame. A verypopular multiplex structure is shown in Figure 14 : 32 channelsare combined into a frame. The frame repeats itself 8000 timesper second.

CH 0 CH 1 CH 16 CH 31CH 17CH 15

1 Frame = 125 sec

frame synchronization (optional) signalling

Figure 14 Basic E1 structure

Multiplexing is different from Concentration, this difference isexplained in Appendix C.

Today's multiplexing systems can send a huge number ofinformation-channels over a single medium. In order to keep itmanageable, a hierarchy is used :

" basic information channels are multiplexed into a so-calledfirst-order multiplex, sometimes also called lower-order.

" a number of the first order multiplex signals are againmultiplexed into asecond-order multiplex.


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When the timing references are all slightly different,interconnection becomes more complicated, just like making anexact appointment becomes more difficult when all wrist-watchesindicate different times.

This complexity limits the PDH bit-rates to the fourth order 140Mbps. It also makes it impossible to extract the E1 out of an E4,without first extracting the E3..E2. It results in the so calledBack-to-Back multiplexing.

BasicSignals

First OrderMultiplex

Second OrderMultiplex

Third OrderMultiplex

Figure 16 Multiplexing Hierarchy

A lot of PDH is still installed and operated today. However giventhe limitations, todays new networks are all using the moreadvanced SDH system.

3.2.2 Synchronous Digital Hierarchy (SDH)

From the experience of PDH, and the push of new technologicaldevelopments, a new multiplex system was defined :SynchronousDigital Hierarchy (SDH). SDH improves PDH in the followingways:

" It is standardized as a worldwide standard. As a matter offact Europe (SDH) and North-America (SONET) had differentstandards, but they were defined in a more compatible way.This allows easy connection of SDH and SONET.

" SDH simplifies interconnection of nodes, by using a singlereference clockfor a whole group of nodes, a so-calledSDH-'island'. This clock is generated by a Primary ReferenceClockand distributed through the whole SDH-network.

" given the simpler interconnection, SDH allows to multiplex tohigher bitrates.


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" given the large installed base of PDH equipment, SDH ismade in such a way that it is backward compatible withexisting PDH. This means that existing PDH can beinterconnected with new SDH :

D PDH signals can be sent 'inside' a higher speed SDHsignal,

D SDH islands can be interconnected using PDH links.

" Synchronous : no introduction of additional overhead.

SDH Network

SDH Network 1 SDH Network 2

PDH Source PDH Sink

PDH Link

Figure 17 Coupling PDH and SDH

" SDH added more network management functions : networksgrew bigger and bigger, and decent network managementbecame a necessity :

D detecting and measuring bit errors

D labelling and naming the multiplex components

D alarm indications

D protection mechanisms, eg. Automatic ProtectionSwitching (APS)

Table 3 SDH Containers

SDH Containers bitrate

VC11 1.544 Mbps

VC12 2.048 Mbps

VC2 8 Mbps

VC3 34 Mbps

VC4 140 Mbps

In SDH, the signals are named Synchronous TransportModule (STM). For example, the Unit signal is the STM-1. This


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signal is then multiplexed into higher orders STM-n. Thecomponents of the STM-1 are called Virtual Container (VC). Themost important Virtual Containers contain the standard PDHsignals.

Table 4 SDH Multiplex Signals, STMn

Multiplex order bitrate

STM-1 / STS-3 155.52 Mbps

STM-4 / STS-12 622.08 Mbps

STM-16 / STS-48 2.488 Gbps

STM-64 / STS-192 10 Gbps

STM-256 / STS-768 40 Gbps

Note The different multiplex orders are also referred to as OC-n,where n is the order-number of the STS multiplex (this is thetypical naming in the USA) :

" STM-1 = OC-3

" STM-16 = OC-48

Note Multiplexing a number of STM-1 into higher levels (STM-4,STM-16, ..) introduces no additional overhead.

Another important advantage of SDH is the possibility to directlyaccess lower-order signals within a high-order multiplex : theSDH overhead includes a number of Pointers and by applying thepointer-processing, the multiplexer can find any contributingsignal (tributary) in the multiplex signal. This feature allows thento make simpler Add Drop Multiplexers.

As a result from this, SDH networks are often deployed in

ring-topologies : a boackbone ring, with at each station, anAdd-Drop Multiplexer contributing traffic to the ring.


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Back to Back Multiplexer (PDH)

Add-Drop Multiplexer (SDH)

Figure 18 Back to Back versus Add-Drop Multiplexer

Table 5 PDH versus SDH

PDH SDH

Plesiochronous Synchronous

Different Hierarchies One Higher order Hierarchy

Fixed Hierarchy Flexible Hierarchy

Limited management features Extended management features

Stuffing Bits -> complicatedmultiplexing

Very limited bit-stuffing ->simpler

No pointers to frame-boundary -> back-to-back multi

plexers

Pointers to frame-boundary-> Add-Drop multiplexing

" http://www.alcatel.com/products

" search in categories SDH", Sonet", Backbone ServiceProvider"

3.2.3 Wavelength Division Multiplexing (WDM)

Wavelength Division Multiplexing (WDM) is in fact a form of

frequency multiplexing : wavelength = reciprocal of frequency. Itcombines several colors of light onto the same optical fiber. Eachof the individual colors can be any signal, but typically the signalcomponents are SDH signals, eg. STM-16. Therefor, WDM is nota direct replacement or successor of SDH or PDH, but rather acomplementary new technology which can be combined withexisting ones to further increase bandwidth capacity.

Example of combination of SDH and WDM :

" SDH : STM-16 = 2.488 Gbps


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" WDM : 40 wavelengths, also called 40 's

total capacity : 40 * 2.488 = 100 Gbps

WDM is one of the first functions of what is becoming a whole

new technological area : the Optical Networking. Its rapiddevelopment being fueled by the exponential increasing demandfor bandwidth, next to multiplexers, other optical functions likecross connects, add-drop muxes, etc. are being developed.

" WDM Products : http://www.alcatel.com/products

" ITU-T Standards : G.692

" Flash animation explaining DWDMhttp://www.alcatel.com/telecom/snd/keytech/wdm/index.htm

The bandwidth carried by a WDM signal is theproduct of the

number of lambdas and the bandwidth of each lambda signal. Assuch, the WDM signal can increase its capactity in two ways :increasing the number of lambdas, and increasing the bitrate ofeach member-signal. This results in an even faster increase ofbandwidth capacity. In the next figure, each a*b bps refers to alambdas, each at b bps.

1984 1988 1992 1996 2000 2004

0.1

1

10

100

1000

FiberCapacity[Gbps]

Year140 Mbps

565 Mbps2.5 Gbps

2*2.5 Gbps4*2.5 Gbps

8*2.5 Gbps16*2.5 Gbps

8*10 Gbps40*10 Gbps 128*2.5 Gbps

96*2.5 Gbps

Figure 19 Combining WDM and TDM

see Appendix J for an article about the race for bandwidth.

3.2.4 Media

Different media can be used for the high-speedinterconnection-links :

" Fiber


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3.3 Nodes : Switching

When sending information through the core network, it typicallyhas to traverse several nodes. Inside the nodes, the incoming link

has to be connected to the outgoing link. Several techniques existto do this. Below these concepts are listed, ordered by thelife-time of the connection they make.

Table 6 Types of Switching

Connection Life Time* Switching Concept

typically more than 1 hour Cross Connect

Add Drop Multiplexer

between 1 hour - 1 second Circuit Switchtypically less than 1 second Packet Switch

Note Connection Life Time refers to the duration of the connectioninside the switching node. Even for packet switched systems, anend-to-end connection may exist for longer times, eg. hours.

3.3.1 Switching Techniques

Switching information is physically performed usingSpace-switching, Time-switching, or a combination of Both.

In Space switching, a number of physically distributed inputs areconnected to a number of physically distributed outputs by themeans ofSwitches. (hence the origin of the name 'switching').When there are m inputs and n outputs, you need m*n switches.Typically each user has an input and an output, and then m=nand equals the number of users. To make a bidirectionalconnection, usuallytwo switches need to be closed.


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Space Switch

Figure 20 Space Switching : 4*5 space-switch

Space switches are limited by the physical density of the input andoutputports.

Time switching occurs in the dimension of time : the individualusers are all on the same physical medium, but multiplexed intime. Typically the Time Division Multiplex (TDM) structure consistsof a number of channels, occurring in a cyclic pattern. In manytelecom systems this pattern repeats at 8000 times per second.

When doing time switching, information received on one

particular time-slot or channel is sent out on another time-slot/ channel.


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CH 1 CH 4CH 3CH 2 CH 1 CH 4CH 3CH 2 CH 1 CH 4CH 3CH 2

Time Division Multiplex

CH 1 CH 4CH 3CH 2 CH 1 CH 4CH 3CH 2 CH 1 CH 4CH 3CH 2

In

Out

TimeSwitch

In Out

Figure 21 Time Switching : 4*4 time-switch

Time switches are limited by the frequencies of the TDMinput/outputs.

Both mechanisms described above are usually combined in orderto maximize the density in bothphysical and frequency domain.As an example, the A1000 S12 switch element uses 16*16 Spaceswitching, combined with 32 channels TDM signals in order to

build a 512*512 channels switch element.

Any time-slot/channel (of 32) from any port (of 16) can beswitched to any other time-slot/port.

Time-SpaceSwitch

CH 1 CH 4CH 3CH 2

In

PORT1

Out

CH 1 CH 4CH 3CH 2PORT3

CH 1 CH 4CH 3CH 2PORT

2

CH 1 CH 4CH 3CH 2PORT4

CH 1 CH 4CH 3CH 2 PORT1

CH 1 CH 4CH 3CH 2 PORT

2



Figure 22 Time Space Switch


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Table 7 Routing/Connection Table Contents :

Port Channel Connect to :

1 1 Port 2, Ch 3

2 4 Port 4, Ch 2

4 1 Port 4, Ch 4

Even with a combination of Space- and Time-switching, a singleswitching element cannot handle all ports from a large exchange.The solution to this is to interconnect several switching elements ina cascade. These are calledSwitching Stages. In figure 23, it isshown how a 4*4 switching element can be made, using four 2*2switch elements. In order to set up a connection from input to

output, a connection in 2 switch elements must be made. Foreven bigger capacities, the concept can be applied recursively, the'new' 4*4 element can be combined to form (eg.) a 16*16element. As such by increasing the number of stages, an arbitrarylarge switch can be built, of course at the cost of more and more'basic' switch elements.

Resulting 4*4 Switch Element

Second StageFirst Stage

2*2

SwitchElement

2*2

SwitchElement

2*2

SwitchElement

2*2

SwitchElement

Figure 23 Cascading switching elements

Because in most systems Inputs and Outputs are paired, thesystem is representedsymmetrical in a so-called Folded view.The folding line of such a folded system is called the ReflectionPoint : information travels into the switch up to the reflection point,and then travels back out to the destination output.


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3.3.3 Circuit Switching

When a connection is only needed for a duration from (typically)seconds to hours, it becomes too much a burden to have the

connections set up manually through an operator. In that case aCircuit Switch can do that automatically. This kind of switching iscalled Call-by-Call switching".

One extra function required for a switch, is to communicatedirectly with the end-user, calledSignalling. Signalling is ameta-communication, that controls the real user to usercommunication : through signalling, the user instructs the networkwhat kind of connection to set up and to what destination.

User Information Connection

Signalling Information Connection

Figure 25 User Information versus Signalling Information

An important aspect of a circuit switched connection is that itconsists of a dedicated, end-to-end path. A channel is reservedstrictly for the end-user during the whole duration of the call.This is sometimes referred to as aphysical connection in contrastto a virtual connection from a packet switch.

For large public networks, it is impossible to connect allsubscribers to a single exchange. So regional areas will becovered by several exchanges, and all of these must beinterconnected to provide a worldwide network. This

interconnection can be a fully meshed network, but from a certainsize, this becomes also unmanageable, and a hierarchicalnetwork is used. In this hierarchy, there are the following levels :

" Local : at the 'lowest' level, the Local Exchange collects thesubscribers, using access networks (see Section 4). The localexchange can be recognized by the fact that it has aSubscriber Database with all information about its subscribers.


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able to use any of the transit networks. In this case, there is anadditional level in the hierarchy :

" InterExchange Carrier : all Carriers access the localexchanges via this special transit exchange.

Note In the US, the names Transit, toll and Tandem are used in a lessstrict and interexchangeable way.

InterExchange

Carrier

Local

Tandem

Carrier 1 Carrier 3Carrier 2

Figure 27 Hierarchical Structure of the Telephone Network :Multi-Carrier

Some critiques of the current Public Switched TelephoneNetwork (PSTN) are the following :

" It is Narrowband (NB), i.e. it can deliver bitrates up to a fewkbps.

" It is half-integrated, i.e. different services must be providedby different networks, e.g. PSTN, Datanetworks, CableTelevision Network (CATV), ...

" It is 64 kbps based. Although this bitrate is quite adequate toprovide telephony, it can not support new services likevideophony.

Therefore, the tendency of the future (which is progressing veryslowly) is to come to 1 unified, multi-service B-ISDN


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Some examples of Packet Switching techniques are :

" X.25

" Frame Relay

" Asynchronous Transfer Mode (ATM)

" Internet Protocol (IP)

Their key characteristics are worked out here in a bit more detail.

X.25

X.25 is the name of an International TelecommunicationUnion (ITU) standard describing a packet switching system.

The X.25 system offers :" Error-Free delivery of data packets to the destination

" Delivery of data packetswithout loss.

" In-sequence delivery of data packets.

" Flow-Control Procedures

" Bit-rates up to 64 Kbit/sec.

" Packets up to 4096 bytes, however, typical packets will be 128bytes. (each network specifies the maximum supported packetlength)

" The packet switching protocol is connection-oriented.

X-25 is a mature technology. The X.25-related protocols areamong the most used packet protocols and offer worldwideinterconnectivity.

The provider of a network requires that you pay for the usage ofhis network infrastructure. This is called Charging. For differenttypes of networks, there are different ways of charging developed.For example, for the telephone network, you will have to pay as afunction of :

" duration of the call

" distance between originator and receiver

However for packet switched networks, this is not the best way tocharge : because of the 'packet' nature of the information, youmay be connected for several hours, only exchanging packets


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during some minutes. In this case, the charging will be on Volume: the amount of packets sent.

Note In fact, the X-25 protocol describes the interface between the user

and the network, the so-called User-to-Network Interface(UNI). In addition, the X.75 protocol has been defined forcommunication on the Network-to-Network Interface (NNI).


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Frame Relay

As the quality of the network improved, it became a burden to doall the error checking, retransmitting, etc. in each node, as therewas hardly any error in the network. A first simplification led toFrame Relay (FR). In fact, no more processing is performedanymore, on the packet level, which explains the name FrameRelay.

Frame Relay was originally designed in1981 as a NB ISDN packetmode bearer service, but the main driving force was the demandfor the interconnection of Local Area Networks. Although thefunctionality of the FR network, and consequently its services, ismore limited than in a packet switching network, this is more thancompensated for by the higher bit rates.

The most important aspect of the reduced functionality of FR is thelack of error correction. Error detection is performed anderroneous frames are discarded, with the result that not all theinformation is delivered to the destination.

Thus, what the FR network offers is :

" In-sequence delivery of data frames.

" Higher Throughput

" Lower Delay

" bit rates of 45 Mbps (US) or 2 Mbps (Europe).

" The Frame Relay protocol is connection-oriented.

Asynchronous Transfer Mode

ATM is a form of connection-oriented cell-switching. ATM-cellsare Fixed-size packets, 53 bytes long. The cell is divided into aHeader and a Payload part. The header is standard, the payloadpart can contain whatever information : speech, video, text, data,graphics, ... ATM offers a uniform method for the transport ofthese multiple services, i.e. the contents of the payload completely

transparent to the network (there is even no error control).Therefore, ATM is recommended by ITU-T as the technology tobe used in the future B-ISDN.


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User PayloadHeader

VPI

VCI

HEC

CLP

PTI

48 Bytes5 Bytes

Figure 28 ATM Cell

On an ATM link, a continuous stream of cells is transmitted. Aspecific user can send at any moment in time, which explains theterm asynchronous". Therefore, the receiver can not distinguishinformation (coming from different users ) by means of thetimeslots. To solve this, the ATM cell header carries a Virtual PathIdentifier (VPI) / Virtual Channel Identifier (VCI).

Asynchronous Transfer Mode (ATM)

Synchronous Transfer Mode (STM)

ch 1 ch 2 ch 3 ch 4 ch 1 ch 2 ch 3 ch 4 ch 1 ch 2 ch 3 ch 4

AA ACBA B

Idle cells

Figure 29 STM versus ATM

ATM operates in the connection-oriented mode, i.e. a setup of avirtual connection must be performed first.

" An important effect of this setup phase, is that it is possible tohave some guaranty of service, the minimum performance ofthe connection can be negotiated. (for example the Cell LossRatio (CLR) )

" Another effect of the connection-oriented mode is that thesequence of ATM cells will be preserved.

ATM allows extreme fast switching, up to the Gbps range, becauseof

" the connection-oriented nature,

" the simple protocol stack (no error control, no flow control, noreassembly in intermediate nodes),

" the fixed size of the cells.

However the connection setup/release requires some time, somesignalling, and is due to the large feature-set quite complex.


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User PayloadHeader

0..65535 Bytes20 Bytes

Figure 30 IP Packet (IPv4)

As an IP-network is connection-less, it is more computingintensive for the nodes to route the packets. On the other handthere is no need to set up a connection. Routing decisions will bebased upon the destination IP address carried in the IP header.This IP address has to be unique in the network. Today (IPv4) theIP address is 32 bits wide, allowing a theorethical 4 billionpossible addresses. IP addresses are represented in a so-called

dotted notation, which splits the 32 bits into 4 bytes, and eachbyte has a value from 0 to 255. Example : 138.203.048.001

An IP network operates in the best effort" mode : no guarantee isgiven ifand when an IP packet will reach its destination. Thismakes it somewhat more difficult for the terminals.

Most IP networks are charged as flat fee (fixed monthly price), orconnection time.

IP is 30 years old. Nevertheless, there is a fast evolution of theprotocol, fueled by the succes of internet applications. The

following are fast developing areas in the IP world :

" IPSEC : a mechanism to improve security in the Internet. Asecurity protocol will provide cryptographic security servicesthat support combinations of authenticication, integrity, accesscontrol and confidentiality.

" IP Multicast : Multicast is a protocol that enables the sameinformation to be sent from a server to a number of clients.By avoiding duplicate sending of the same information, itimproves efficiency. This makes the Internet more acceptablefor applications that are similar to conventional TV broadcast.

" IPv6 : The current Internet addressing scheme is beingupdated. The growth in the Internet has led to a shortage ofaddresses. IPv6 increases the length of the addresses.

" IP QoS : A lot of effort is going on to provide some quality ofservice in IP networks, thus also allowing real-time services,e.g. Voice over IP (VoIP). 2 models are defined :


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D IntServ : Integrated Services providing QoS in anATM-like way, reserving bandwidth to a particulardestination, by means of the Resource ReservationProtocol (RSVP).

D DiffServ : The difficulties of large-scale implementationof RSVP have led to alternative suggestions for solving theproblem of securing sufficient bandwidth. TheDifferentiated Services model is based on the idea oftagging individual packets with an indication of theirpriority.

ATM versus IP

ATM and IP are to some extent overlapping (and as such

competing) technologies, as well as complementary. A lot ofresearch is going on which technology is the best answer to ourcurrent and future needs.

ATM and IP originate from a different Business : ATM fromTelecom, IP from computer networks. Both these businesses havetheir own history, legacy and practices.

IP is currently very successful, and extremely fast-growing. Thereis a lot of support for it, thanks to the (relatively) simple andproven protocol stack. 20 years of legacy do not seem to burdennew innovations, and the Internet Engineering Task Force (IETF)standardization body seems to work decent, steadily andefficiently.

ATM has been developed as a Broadband Multimedia technologyfrom scratch. This way it is not at all compromised by backwardcompatibility to legacy systems. On the other hand, thecomplexity (and cost) of many aspects of this new technology wereunder-estimated : policing, call acceptance control, OAM, ...

IP : virtuous circle

RapidInnovation

High Growth

ATM : vicious circle

Lack ofApplications

Slow Take-Up

High cost ofOwnership

Figure 31 ATM versus IP


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IP, ATM and SDH

For 'applications' to communicate to each other, there exist todayseveral possibilities. This can be represented in a protocol stackgraph as shown in Figure 32.

" The 'Optical/Physical' layer takes care of the actualtransmission of the bits at the lowest level. All systems needthis,

" The SDH layer can be included : it provides thestandardization and the management features of SDH(protection switching, bit error detection, signal labels, ...)

" The ATM layer can be included when high-speed switching isneeded : ATM can provide high-quality, high bandwidth'pipes' of information transfer. The pipes can be any

bandwidth with any Quality of Service." The IP layer can be included for the service integration : many

existing applications allow to exchange information via IPpackets.

Optical Layer (WDM) / Physical Layer

SDH

ATM

IP

Applications

1. 4.3.2.

Figure 32 IP, ATM and SDH

As a result of this, Figure 32 shows four ways to combine all these:

1. The application generates IP packets, and transmits themdirectly onto a physical medium. This is under development,as the IP and Optical layer need to be completed with somemaintenance features.

2. The application generates IP packets, and transmits them inSDH containers onto a physical medium. Very popular, butstill lacking a guaranteed real-time service.


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3. The application generates IP packets, packages a number ofstreams into ATM virtual connections. The ATM cells aretransmitted in SDH containers onto a physical medium. Quiteflexible, but also complex. Also there is some overlap

between the protocol stack layers, which leads to someinefficiency and higher cost.

4. The application generates ATM Cells. The ATM cells aretransmitted in SDH containers onto a physical medium. Thisis the stack proposed by the ATM community. Because it isstill quite complex, and because there is very little applicationsbeing able to interface to an ATM protocol stack, this solutionis not yet that successful.

Today, telecom observers estimate that the future could be 80% IPand 20% ATM. Most applications will use the IP protocol. Inside

the network, traffic can be optimized by using (a subset of) ATM :semi-permanent connections between subnetworks and routers.

3.3.5 Signalling

Signalling can be categorized in two types : Signalling inside thenetwork, and signalling between the user and the network.

The signalling between user equipment and the network is calledUser to Network Interface (UNI) signalling. The signalling insidethe network, i.e. between 2 exchanges is called the Network to

Network Interface (NNI) signalling. Different protocols aredefined because of a number of reasons :

" Different addressing identification for channels, because ofhigher degree of multiplexing for the NNI.

" NNI signalling is protected, versus UNI is not protected.

" NNI signalling is multi-service, versus UNI is single-service.

" Historical reasons

The older signalling systems can only communicate a limitednumber of events, states and digits. The newer signalling systemsare very flexible, because they use messages (packets ofinformation) between the network elements.

A sequence of these messages is called aScenario. See appendixL for a simple, typical call setup/release scenario.

User to Network Interface Signalling (UNI)

Following is a list of popular UNI signalling systems :


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" Analog Subscriber Signalling System (ASSS) : one of thefunctions of the analog signalling system, is to transport digitsdialled by the subscriber to the exchange. In the early daysthis was done using a Pulse system : short interruptions of the

line circuit. Today the more advanced Dual Tone MultipleFrequency (DTMF) system is used, where each digit isrepresented by a mix of two tone-frequencies.

" ISDN Digital Subscriber Signalling System 1 (DSS1)

" B-ISDN Digital Subscriber Signalling System 2 (DSS2)

Network Node Interface Signalling (NNI)

Following is a list of popular NNI signalling systems :

" Channel Associated Signalling (CAS)

" ISDN User Part (ISUP)

" B-ISDN User Part (B-ISUP)

Common Channel Signalling System #7

To transport ISUP signalling messages in a reliable way, atransport mechanism called Common Channel Signalling System#7 (CCS #7) was developed. In the mean time this transportmechanism has been extended with many other features, and hasbecome the dominant system for signalling and other controlcommunication. Therefore it is described in a little more detail in

this chapter.

User Payload Network

Signalling Network

Signalling Transfer Point

Figure 33 Separation of Signalling Network and User DataNetwork


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CCS #7 has become the dominant signalling system (and willmost probably stay it in the future) because of a number of itscharacteristics :

" Its modular structure, which makes it :

D multi-purpose, e.g. signalling for calls, for IN, forcharging and billing information, for TMN, ...

D expandable

D future-safe

" It uses a common channel for the signalling of different users,as is shown in figure 33. Moreover, the paths of thesignalling information and the user information can bedifferent.

This complete separation of the signalling network offerssome advantages :

D more efficient use of signalling links

D a much higher protection can be foreseen for thesignalling network, than for the user data, e.g. by usingSignalling Transfer Points (STPs)

D more advanced protocols can be implemented, e.g.look-ahead signalling.

Transport Layers

Application Layers

Message Transfer Part (MTP)

Signalling Connection Control Part SCCP)

Transaction CapabilitiesApplicationPart (TCAP)

B-ISDNUser Part(B-ISUP)

TelephoneUser Part

(TUP)

ISDN UserPart (ISUP)

BaseStation

SubsystemApplication

Part(BSSAP)

MobileApplicationPart (MAP)

IntelligentNetwork

ApplicationPart (INAP)

(MRVT)(SRVT)

OSI

Figure 34 Modular Structure of CCS #7


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MessageTransferPa

SignallingConnectionControlPa

Transac

Capabil

ApplicatiPa

TelephoneUserPa

ISDNUserPa

BISDNUserPa

MTPRouti

VerificatTe

SCCPRoutiVerificatTe

MobileApplication

Pa

BaseStationSubsystemApplicatiPa

IntelligentNetwork

ApplicatiPa

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The Message Transfer Part (MTP)is capable of sending messagesover the network. Error Correction and Flow Control functions areprovided to ensure reliable information transfer.

The Signalling Connection Control Part (SCCP) increases thefunctionality of the MTP. It supports a more efficient routingalgorithm (connection-oriented) as well as extended addressingcapabilities.

All user parts rely on the MTP, but not necessarily on the SCCP.

Recently a new common part was introduced to support newtelecom services, such as the Global System for Mobilecommunications (GSM) and the Intelligent Network (IN). Known asthe Transaction Capabilities Application Part (TCAP), it supportsremote operations in a real-time environment.

Telephone User Part (TUP): signalling for telephony.

ISDN User Part (ISUP): signalling in the ISDN network.

Broadband ISDN User Part (B-ISUP): signalling in the B-ISDNnetwork.

MTP Routing Verification Test (MRVT): procedures to test that datain the MTP routing tables is consistent.

SCCP Routing Verification Test (SRVT): various procedures to verifythe routing functions performed by the SCCP.

On top of the SCCP connection oriented an OSI stackcan run.Typically this is required for Operations and Maintenance and inthe TMN (Telecommunications Management Network).

TheMobile Application Part(MAP) is used to exchange call stateinformation not only between Mobile Switching Centers (MSC), butalso between a Mobile Switching Center and its associated VisitorLocation Register and Home Location Register.

TheBase Station Subsystem Application Part(BSSAP) is used forsignalling between the Base Station Subsystem and the MobileSwitching Center.

The Intelligent Network Application Part (INAP) is used toexchange messages between the Services Switching Point and theService Control Point during a service call.


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4 Access Networks

4.1 Objectives

Access networks are to solve the so-called Problem of the LastMile". The Access network must bridge the distance between theuser (at home, at work, on the move, etc.) and the core network.The main difference between core network and access network isthat the access part of the network, is used only for a single (orsmall number of) subscriber, where the core network is shared bythousands of subscribers. This makes the Access Network much

more cost-sensitive. It is for example still too expensive to bringoptical fiber to all homes directly.

Note Other names for Access Network are : subscriber loop, local loop,subscriber line.

As a result of this different costing, the main targets of an AccessNetwork are :

" Providing existing services, on new infrastructure, in a morecost-effective way. (a cost-improvement for existing services)

" Providing new services on existing infrastructure, thus for thesame cost. (re-using existing infrastructure to keep the costdown)

" Providing new services on new infrastructure.

Table 9 groups some access network concepts according to thesecriteria. They will be explained in more detail below.


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NetworkStructure

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Table 9 Types of Access Networks

Existing Service New Service

Existing

Infrastructure

XDSL

HFC

New

Infrastructure

V5.x

HFC

GSM

DECT

LMDS

An important factor when dimensioning an access network, is thephysical distribution of the users in space. Density can be high,low, users can be clustered or evenly spread. Following is a list of

'design-parameters' for deploying an access network :" operator's existing infrastructure,

" service(s) to be provided,

" traffic requirements,

" estimated growth and new services,

" user density,

" user clustering,

" accessibility of the terrain,

TheAccess Node is an additional network element in the networkhierarchy. (see Figure 35) On the other hand, network builderswant tosimplify the networks, to reduce the operation cost. TheAccess Node can achieve this because it reduces the number ofLocal exchanges. This results in a trend where networks evolvefrom a large number of small locals, to a small number of largelocals, using access nodes to collect the users. An Access Node issmaller and simpler than an Exchange. It requires lessmaintenance.


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100 LocalExchanges,2.000subscribers

10 LocalExchanges,20.000Subscribers

AccessNodes

Users

to Transit Level

Figure 35 Network evolution with Access Nodes

Exchange with 5k subscribers

Traditional MultiExchange topology

20

10

20

5

10

5

3

20

10

5

5

Exchange Serviced Area

Access Node with 5k subscribers

Access Nodes based topologyy

78

30

5

Access Node Serviced Area5

15

0.5

51

0.5

5 1

0.5

5

1

0.5

5 1

0.5

5

1

0.5

51

0.5

1

Figure 36 Network evolution with Access Nodes : view from thesky


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Access Nodes also simplify the network design in aMulti-operator market. Each user should be free to choosewhichever telecom-service-provider he wants. On the otherhand, the access to the user is so expensive, that it is certainly not

economical to provide a separate access from each serviceprovider to each user.

Access

Nodes

Users

Voice NetworkProvider 1 Data Network

Provider 1Voice Network

Provider 2

Transit Level

Figure 37 Access to Several Networks

With Access Networks, different aspects of Telecom Services canbe splitted over several providers :

" one provider operates the access.

" one or more other providers operate the core network.

In some countries the government-owned network provider is toprovide the access network, where as the network behind that isliberalized. In the US. different companies are licensed to servicedifferent parts of the network :

" regional Bells : Access

" AT&T : Core Transit Network

Some time later, additional core network companies, called LongDistance Carriers have been added : MCI-Worldcom, Sprint, ...

Which service provider is choosen by the user can be staticallyconfigured in the Access Node. It can also be choosendynamically, by dialling a carrier prefix. Even if no Access Nodeexists, long distance calls can still be made via a long-distanceoperator, selected via a prefix.


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4.2 Media

In the access networks, a multitude of different media are used.On one hand because there is existing infrastructure from the

past, on the other hand because new services install additionalnew media. Most access networks use a Combination of :

" twisted pair

" coaxial cable

" optical glass-fiber and plastic fiber

" radio

The Combination of media results from the fact that the twosegments are very different, and thus optimized differently :

" the part between the user and the Access Node, also calledthe Tail.

" the part between the Access Node and the Core Network.

Twisted pair is the oldest and cheapest medium. Originally usedfor telephone lines, it was reused for data-networks, and XDSLtechnologies create an extra life-cycle for Twisted Pair. The pair istwisted, in order to reduce the interference with the environment.Usually more than one pair is combined into a single cable : from2 to hundreds of pairs. Twisted pair comes in an Unshielded andShielded variant. For the latter, the whole bunch of pairs, (or each

pair individually) is additionally shielded from interference with ametal cover. Th unshielded variant is called Unshielded TwistedPair (UTP), sometimes with an additional number indicating thetype of wire used. Example UTP-5 is a common type of twistedpair wire used for local area networks.

The advantage of Twisted Pair is that it is cheap, and most of all,that it is already installed to all of our houses. Re-using theexisting Twisted Pair infrastructure, as XDSL does, avoids the costof routing new media to the subscriber.

Typical throughputs of twisted pair are in the range 100 kbps(ISDN) to 100 Mbps (Ethernet).


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Figure 39 (Unshielded) Twisted Pair, 4 pairs

Coaxial Cable (nicknamed Coax) is a single wire, protected frominterference and signal loss by a metal cover. This allows higherfrequency signals to be transported compared to twisted pair.

Coax is more expensive than Twisted pair, but allows more

bandwidth as well. As with Twisted Pair, many Coax is alreadyinstalled, and can be re-used, avoiding installation costs. Forexample the Cable-TV network, brings a Coax to most of thehomes in cities.

Typical throughputs of coaxial cable are in the range 10 Mbps(Ethernet) to 1 Gbps (digital TV broadcast).

Figure 40 Coaxial Cable

Optical fiber has many advantages as a signal carrier (for thatreason it is widely deployed in core networks) :

" very high bandwidth capability. (Terabits/s or more)

" resistant to interference, noise, crosstalk, etc." it is made from cheap materials (sand), although the handling

is more expensive.

Fiber is also an interesting medium for the network builderbecause its bandwidth is currently only limited by the Terminations.This means that a given Fiber carries today (eg.) 2.5 Gbps,because the Laser-transmitter, and receiver-diode are limited tothis speed. The Fiber actually allows much more than 2.5 Gbps.As soon as better transmitters/receivers are developed, the


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" no medium to be installed, (it works in vacuum) so lowerinstallation costs. This can be a particularly importantadvantage in hostile environments like mountains, swamps,etc.

" short installation time, short reconfiguration time.

" in the broadcasting case, terminals can beMobile.

Radio has however some severe limitations : (Therefor radio isusually only deployed for applications really requiring mobility orfor environments where wiring is more expensive)

" Point-to-Point radio-links require Line of Sight : thesender- and receiver-antenna must be able tosee eachother. Because the earth is a sphere, for a long link thisrequires relay-stations every 50 km or so. Objects blocking

the line-of-sight also block the communication : newbuildings, trees, flying craft, ...

" the radio-medium is shared by everyone :

D it is not so secure, everyone can listen-in on yourtransmission,

D it is susceptible to interference. Everyone can send at thesame time in the same frequency band,

D its bandwidth is limited. Given the fact that there is onlyone spectrum for everyone, governments have regulated

the usage of this. They define the spectrum into severalbands, each for a particular application. Although thereis no real upper-limit to the spectrum, these bands arelimited. For example, a typical SDH point to point link islimited to STM-1, 155 Mbps.


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Figure 42 Radio : Microwave Dish and other antenna's

Note MCI is an abbreviation forMicrowave Communications

Incorporation : started as a microwave radio based networkprovider, they have expanded into many other telecom markets.http://www.wcom.com/

Note Negroponte switch : Professor Negroponte (MIT MediaLab) notedthat concerning wired and wireless, the world seems to evolve in aremarkable way : information which used to be broadcastedthrough radio (TV, Radio) is now being distributed through Wires(Cable TV). On the other hand, information that used to be'wired' like telephone, is becoming more and more wireless via

radio (Mobile phones). This observation is called the Negroponteswitch. This observation may not stand for the long term : whendemand for bandwidth grows to more than just telephony, wiredprovides a much more cost-effective solution. Also low-cost maybe more important to users than mobility. http://www.media.mit.edu/


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4.3 Analogue Line Access

A simple, low-cost twisted pair from the local exchange (or fromthe Access Node) to the subscriber is all you need for analogue

telephony. Millions of these lines are installed worldwide, some ofthem operating for many decennia. See figure 43 and 44 forcomparison with ISDN

4.4 ISDN Access

The Integrated Digital Services Network (ISDN) is a conceptdefined in the late 70's as a first attempt to define a singleuniform network for all services. Part of the standards describethe Access part of the network, not only the physical layer, but up

to the protocols. ISDN defines two types of Access :" Basic Rate Access (BRA), in the US also called Basic Rate

Interface (BRI). It consist of 2 user payload channels, each 64kbps, and 1 signalling channel of 16 kbps. The userchannels are know in the standards as B-channels, and thesignalling channel as D, so the BRA is sometimes referred toas 2B+D.

" Primary Rate Access (PRA), in the US also called Primary RateInterface (PRI). It consists of 30 user payload channels of 64kbps, and 1 signalling channel of 64 kbps. (another

channels is used for synchronization and is not available tothe user). The PRA is also referred to as 30B+D. The physicallayer bandwidth is 2.048 Mbps, the user has 1.92 Mbpsavailable.

Note On top of the B and D channels, ISDN also specifies anS-channel, used for synchronization.

The Basic Rate Access requires a Network Termination (NT) at thesubscribers premises. A number of terminals can be connected to

this box, allowing several devices (Telephones, Fax, Computer, ..)to share the access. Existing analog line terminals (telephone,fax, modem) can still be connected through a so-called TerminalAdapter (TA).


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Users requiring more bandwidth, such as computers, or privateexchanges, will connect through a Primary Rate Access, as shownin figure 45.

Note

The D channel of Basic Rate Access and Primary Rate Access aredifferent in bandwidth (16 resp. 64 kbps). This bandwidth issomewhat proportional to the amount of user channels (B) thathave to be controlled, and is found to be sufficient in both accesstypes.

Note ISDN is more than just an Access Network : it is also aboutservices. This services aspect will be described in chapter 6.

Local Exchange OfficeSubscriber Premises

AnalogTelephone

Analogue Line

LocalExchange

Figure 43 Analogue Access


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ISDNNT

LocalExchange

U Interface

S-Bus

AnalogTelephone

ISDN DigitalTelephone(s)

TerminalAdapter

Owned by Network ProviderOwned by Subscriber

ISDN Fax

Figure 44 ISDN Basic Rate Access


LocalExchange

Primary Rate Access

LocalExchange

Primary Rate Access

ISDNPrivate

Exchange

Figure 45 ISDN Primary Rate Access

Using ISDN as Access has the following advantages :

" Multi-Service : voice, data, fax, ...


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ISD

AlcatelProduc&

Lin

RTS

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" Higher speeds than analogue : 64 kbps, 128 kbps, 1920Kbps.

" High speed Signalling : 16 Kbps D-channel.

"

SubAddressing : several terminals onto a single access." Easy implementation of Supplementary Services (see also

chapter 6.1)

With the above features, ISDN can be well employed as an Accessfor voice and data. In the so-called Always-On / DynamicISDN (AOD ISDN) the user is connected fulltime using only the 16Kbps D-channel. This is sufficient for low-bitrate data-services,such as email, chat, etc. When you need more bandwidth (for adata-transfer, or for a voice call) one or more B-channels areopened. Using the fast signalling possibilities of the D-channels,

this can happen very fast. This is an efficient way of using theISDN network, and with the right charging strategy could becomean interesting way to provide internet access.

" http://www.alcatel.com/products search in ISDN

" ISDN : http://www.alumni.caltech.edu/~dank/isdn/

4.5 Concentrating Remote Users

There is a trend in the telecom networks, going from a network

with many smaller exchanges to a network with a few bigexchanges. (see also Figure 35) This is (among others) to reducethe operation costs.

When collecting more subscribers to a local exchange, they mustbe collected from a larger radius around the exchange, whichgives rise to more cabling. As a solution to this there are accessnodes, locally collecting/concentrating users, and then bringingthem to the exchange in an efficient way.

In some cases, the number of twisted pairs available wasexhausted, and network providers searched for a solution toservice more subscribers over the available twisted pairs.

The Accesses behind the Access Node are typically analogue lines,or Basic Rate Accesses, or a combination of them.

The first systems were proprietary solutions :

" the Alcatel Remote Terminal Subscriber Unit (RTSU)

" the Alcatel Remote Concentrator Unit (ARCU)


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More and more, operators demand standard products with openinterfaces One of the most successful products of this openstandard type is the V5.x access node. There are two types :

"

V5.1 : multiplexed non-concentrated. 30 subscribers on a 2Mbps access.

" V5.2 : concentrated, multiplexed : typically 200 subscriberson a 2 Mbps access.

4.6 Digital Subscriber Line, ADSL

Digital subscriber lines apply modern digital techniques on twistedpair medium to deliver new services over existing infrastructure.

The bandwidth and quality of a typical analog telephone line isrelatively low (300..3400 Hz). This is mainly because there is awide variety of types, lengths, qualities, etc of twisted pairs used,and an analog line must assume the worst case commondenominator of all.

However, today's more powerful signal processing and computingtechniques allow to build equipment that adapts to each particulartwisted pair, optimizing the use of it case by case, and resulting inmuch higher throughputs.

For the Telecom operator, the advantages are :

" no additional cable-cost : uses existing telephone line." telephone network is not used for data-services, like

accessing the Internet. Telephone networks are dimensionedfor phonecalls, not for accessing the Internet. for example theaverage phonecall duration is 100 seconds, when 'surfing'the Internet this is much longer, resulting in congestion in thetelephone network.

Advantages for the end-user :

" high throughput : up to Mbps

" telephone is still available when surfing the Internet,telephone and data-services can be used at the same time.


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Upstream

800 kbps

POTS

band

ADSL

Frequency[Hz]

Downstream

8 Mbps

Figure 46 ADSL Frequency Spectrum

Discrete Multi-Tone (DMT) is an advanced form of FrequencyMultiplexing. The total bandwidth of the twisted pair cable, isdivided into a large set of small bands. For each band the lineterminals determine what is the quality of each individualsub-band, by testing it. Better quality means less noise andinterference, and that results into less bit-errors. Another way tolook at this is that for the same amount of bit-errors, agood-quality frequency sub-band allows to transmit more bitsper second.

After measuring the quality of all the sub-bands, it is decided foreach band how much bits-per-second can be sent over this. Forthe total twisted pair, this results in a total bandwidth capacity,

much higher than the classical analog telephone line. As eachtwisted pair is different, its possibilities are also different, resultinginto typical capacities from 1 Mbps to tens of Mbps.

For example, for ADSL, the DMT technique partitions thefrequency band 5 KHz to 1.1 MHz in to 255 sub-bands of 4.3kHz. Each sub-band uses its own modulation scheme. SeeAppendix E for more info on modulation.

POTSband

sub-bands

Frequency[Hz]

Quality,

Throughput[

bps]

Figure 47 Discrete Multi-Tone


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ADS

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XDSL is a family name for a number of similar techniques. The xis a placeholder for several variants of Digital SubscriberLine (DSL). The first one which was deployed was High SpeedDigital Subscriber Line (HDSL) It is asymmetrical technique : the

same bandwidth is available in both directions. HDSL is typicallydeployed in the network where 2.048 Mbps are needed, but onlytwisted pair (no coax or fiber) is available.

Asymmetrical Digital Subscriber Line (ADSL) is the best knownvariant of XDSL : The main principles are the same, but thebandwidths are divided Asymmetrical : more bandwidth is madeavailable from network to user (Downstream) then from network touser (Upstream). This matches with typical residential applicationssuch as :

" Video-on-Demand (VOD) : video, typically a few Mbpsgoing downstream, with the user control (selecting the video,play, stop, rewind, etc) only a few kbps going upstream.

" Internet : WEB-contents going downstream is megabytes,user requests are only a few hundred bytes.

ADSL does not make changes to use of the twisted pair as anaccess to an analog phone : The new services come on top of theexisting Plain Old Telephony Service (POTS), or, in the newerversion, on top of ISDN.

ADSLModem(NT)

Computer Internet

Telephone Network(PSTN)

Telephone

RouterADSLModem(LT)

Local

Exchange

Figure 48 Internet Access Provider, ADSL

A future evolution will beAdaptive ADSL. In the current system,the capabilities of the twisted pair are measured once, beforetaking the ADSL-line into service. This is called Automatic RateAdaptive at Startup. Should the capacity of the line changedrastically after it is put into service, the ADSL Modem will restart


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and then adapt to the new line conditions. This will result in a 15seconds outage.

Further improvement is possible, when these measurements aredone continuously, resulting in the maximum capacity of the line

at that time to be made available. This is called Automatic RateAdaptive at On-Line. An additional complexity is that is requiresapplications aware of flow-control : the applications must adaptto the bandwidth which is available.

Todays ADSL modems are Rate Adaptive at Startup, not yet RateAdaptive On-Line. However, each time the modem starts up, hereserves a small amount of spare capacity. This margin can beused to adapt tosmall changes in the line conditions. Thistechnique is called Bit Swapping.

First generations ADSL provide a large bandwidth on top of astandardAnalog Line Access. New developments have extendedthis to ADSL on ISDN Basic Rate Access.

ADSL on ISDN Access is very similar to ADSL on analog lines.However, since IDSN uses more bandwidth for the Telephony, lessbandwidth is available for the ADSL-part. This results in a 15%lower throughput for the ADSL data.

The throughput of ADSL is directly determined by the 'quality' ofthe twisted pair. For this reason, ADSL positions the Splitter andADSL NT where your telephone line enters your house. This

however required then that you build two networks in your house :a telepho

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