Tài liệu Mạng và viễn thông P1 ppt

Figure 1.1 illustrates a simple but powerful model for understanding and categorizing various different means of communication.. The simplest form of information flow illustrated by Figu

PART 1

TELECOMMUNICATIONS

NETWORKS

Copyright © 1991, 1997 John Wiley & Sons Ltd ISBNs: 0-471-97346-7 (Hardback); 0-470-84158-3 (Electronic)

I

Znformation and

The world about us brims with information All the time our ears, eyes, fingers, mouths and noses sense the environment around us, continually increasing our ‘awareness’, ‘intelligence’ and

‘instructive knowledge’ Indeed these last two phrases are at the heart of the Oxford Dictionary’s definition of the word information Communication, on the other hand, is defined as ‘the imparting, conveyance or exchange of ideas, knowledge or information’ It might be done by word, image, instruction, motion, smell - or maybe just a wink! Telecommunication is com- munication by electrical, radio or optical (e.g laser) means We introduce the basic capabilities and terminology of telecommunications and networking in this chapter

As hybrid words go, ‘telecommunications’ wins no prizes, but it is all we have to work with The greek ‘tele’ prefix means distant, and nothing else; communication, in the sense

of information passed to and fro between human beings, and animals, is an activity that goes back beyond recorded times With a broad view, long distance communications brings to mind Armada beacons, heliographs flashing between Frontier posts, empires held together by relays of post-houses, whales singing to one another in the deep, and the family dog which conducts its social life by laying and following scent trails For the narrower purposes of this book telecommunications is going to mean the transfer of information by electromagnetic means, (and with this will go a certain amount of accepted jargon) All systems have much in c o q n o n , whatever their age In principle each requires a transmitter, a carrying device or transmission medium, a receiver, and a supply of information which will be equally comprehensible at both ends For lessons in technique nothing should be disregarded, however ancient: for a cheap, speedy and comprehensive message, what is there to beat a human wink?

In the science and business of telecommunications, a structured framework has been created for conveying certain types of information across long distances, with little

respect for the barriers of geography In this book we study this framework; first we understand how the forces of electricity, light and radiowaves may be tamed to provide

a basis for such communication; then we focus on the pragmatic operation of networks and the quest for solutions to the business needs of information flow

3

Networks and Telecommunications: Design and Operation, Second Edition.

Martin P Clark Copyright © 1991, 1997 John Wiley & Sons Ltd ISBNs: 0-471-97346-7 (Hardback); 0-470-84158-3 (Electronic)

Figure 1.1 illustrates a simple but powerful model for understanding and categorizing

various different means of communication The model illustrates a number of different

ways in which a business may communicate either within itself, or with its external

environment of suppliers and customers Thus we introduce the concept of an

‘information environment’, across which information flows in one of a number of

different forms

The simplest form of information flow (illustrated by Figure 1.1) might be directly

from one person to another, by word of mouth or by a visual signal Alternatively the

information could have been conveyed on paper or electrically The advantage of either

of the latter two methods is that the information in paper or electronic form may also

be readily stored for future reference

In this book we shall use the model of Figure 1.1 twice Here we use it to illustrate

how different methods of communication may be categorized into one of the three

broad types, and as a basis for explaining the prerequisite components of a telecom-

munications system In Chapter 43 it is used to illustrate the analysis, simplification and

planning of business information flows This double theme runs throughout the book:

understanding telecommunications technology, and explaining its exploitation in a

pragmatic business-oriented manner Well-known examples of communications met-

hods that fall into the three categories of paper, person-to-person and electronic are

given in Figure 1.2

Some types of communication are hybrids of the three basic methods Modern fac-

simile machines, for example, are capable of relaying images of paper documents over

the telephone network and recreating them at a distant location This would appear as

quite a complex information path on our model of Figure 1.1, as Figure 1.3 shows

First a person must record the relevant information on paper (shown as (a) on

Figure 1.3), then he must feed it into the facsimile machine which converts it into

Per s&

2 r r f

Person

m

4

storage

Figure 1.1 Categorizing information flows

INFORMATION AND ITS CONVEYANCE 5

Mode

Paper

Electronic

Examples

One - t o - one Broadly aimed Sending a letter Advertising board

Leaving a note

Tal king

Winking

A c t i n g Television Radio

Figure 1.2 Categorizing simple communication methods

electronic format (b) Next the telephone network conveys the electronic information (c), before the distant facsimile machine reconverts the information to paper (d) and the receiver reads it (e)

The example we have chosen is rather convoluted, requiring several successive conversions to take place, changing the format of the information from ‘personal’ to

‘paper’ to ‘electronic’ and back again All these conversions make the process inefficient, and as we shall find out later, companies who have recognized this fact have already set about converting all their key business information into electronic

(computer) format, not only for conveyance, but also for storage and processing

purposes

Paper process (a) Information

typed

Per son

fed into facsimile machine

( d l R e c e i v i n g facsimile machine

telephone

Figure 1.3 Information conveyance by facsimile

Person

I r e c e i v e r )

1.1 TYPES OF INFORMATION

Put specifically in the context of telecommunications, information might be a page of written text, a conversation or a television picture The information usually requires

conversion into an electrical signal in order to be conveyed by telecommunication

means However, there are many different types of information, so can they all be treated identically? The answer to this is ‘no’, because each type of information makes slightly different demands on the telecommunication system

Information conveyed over telecommunications systems is usually classed as either

an analogue signal information or as data (digital information) An analogue signal is

an electrical waveform which has a shape directly analogous to the information it represents (e.g speech or a television picture) Data, on the other hand, is the word given to describe information in the form of text, numbers or coded computer or video information

Different forms of data and analogue signal information require different treatment For example, when conversing with someone we expect their reply to follow shortly after our own speech, but when we send a letter we do not expect a reply for some days The analogy runs directly into telecommunications Thus, an electrical representation

of conversation must allow the listener to respond instantly However, in the case of data communication, slightly more leeway exists, as a computer is prepared to accept response times of several seconds A human would find this length of delay intolerable

in everyday speech Another difference between electrical representations designed for different applications will be the speed with which information can be transferred This

is normally referred to as the information rate, the bandwidth or the bitrate A speech

circuit requires more bandwidth to carry the different voice tones than a telegraph wire needs simply to carry the same information as text Later chapters in this book discuss the various methods of electrical representation and the technical standards used

1.2 TELECOMMUNICATIONS SYSTEMS

There are four essentials for effective information transfer between two points, all of which are provided in well-designed telecommunications systems:

0 a transmitting device

0 a transport mechanism

e a receiving device

e the fourth requirement is that the conveyed information is coded in such a way as to

be compatible with, and comprehensible to, the receiver

All four components together form a telecommunications system

In the example of a communication system consisting of two people talking to one another, the transmitting device is the mouth, the transport mechanism is the sound through the air, and the receiving device is the other person’s ear Provided that both people talk the same language, then the fourth requirement has also been met, and

A BASIC TELECOMMUNICATIONS SYSTEM 7

conversation can continue However, if the talker speaks English, and the listener only understands French, then, despite the availability of the ‘physical components’ of the system (i.e mouth, ear and air), communication is ineffective due to the incompatibility

of the information

The coding and method of transfer of the information over the transport mechanism

is to said to be the protocol In our example the protocol would be either the English

or the French language: the fact that the talker is English and the listener French is

an example of protocol incompatibility Protocol also defines the procedure to be used

An example of the procedural part of protocol is the use of the word ‘over’ to signify the end of radio messages (for example ‘Come in Foxtrot, Over’) The protocol in this case prompts a reply and prevents both parties speaking at once The hardest part of telecommunications system design is often the need to ensure the compatibility of the

protocol In some cases, this necessitates the provision of interworking devices In our

example, the interworking device might be a human English/French interpreter

1.3 A BASIC TELECOMMUNICATIONS SYSTEM

Figure 1.4 illustrates the physical elements of a simple telecommunications system including the transmitter, the receiver and the transport mechanism

As already discussed, the physical element shown in Figure 1.1 must be comple-

mented by the use of a compatible protocol between transmitter and receiver Together

with such a protocol, we have all the means for communication from point A to point B

in Figure 1.4 We do not, however, have the wherewithal for communication in reverse (i.e from B to A) Such single direction communication, or simplex operation as it is

called, may suffice for some purposes For many more examples of communication, two

way, or duplex operation is normally required For duplex operation, a transmitter and a receiver must be provided at both ends of the connection, as shown in Figure 1.5

A telephone handset, for example, contains both a microphone and an earphone Duplex operation allows both parties to talk at once and both to be able to (and have to) listen This allows the human listener to interrupt, or two computers to send information to one another in both directions at the same time Not all devices are capable of talking and listening at the same time as required for duplex operation

*

A * 0

Information f l o w

transmitter Transport Receiver

mechanism

Figure 1.4 Basic physical elements of a telecommunications system (simplex operation)

Information flow

4

Figure 1.5 A basic duplex telecommunications system

There is also halfduplex operation, in which communication is possible in both

directions, but not at the same time, as only one communications path is available First

the talker must stop speaking, then the listener can reply

The transport mechanism can be one of a range of different media, ranging from

sound waves passing through air to laser light pulses passing down the latest tech-

nology, optical fibre Furthermore the transport mechanism may or may not comprise

an element of switching, as we describe later in the chapter

Most transport mechanisms demand an encoding of the information or data into a

signal form suitable for conveyance over electrical transmission media Chapters 2 to 5

describe how many of the common forms of information (e.g speech, TV, telex,

computer data, facsimile, etc.) are converted into a transmittable signal carried in either

‘analogue’ or ‘digital’ form In Chapters 6 and 9 we discuss various methods of switch-

ing and in Chapter 8 we discuss a range of different transmission media (cables, radio

systems, etc.), describing how different ones provide the optimum balance of low cost

and good transmission performance for individual cases of application

In order to meet differing communications needs, a number of different types of

telecommunications equipment have been developed over time These include, in

chronological order:

e telegraph

e telephone

e telex

e data networks using either circuit-, packet-, frame- or cell-switched conveyance

e computer local urea networks (LANs), metropolitan ureu networks ( M A N S ) and ”ide

area networks ( W A N s )

NETWORKS 9

0 integrated voice and data networks

0 multimedia networks

This book covers the principles involved in each of the above telecommunications types; it also aims to give adequate background to enable the reader to tackle basic network planning of any of these types The book covers networking from the simple interconnection of two telephones right up to complex, globally spread, telecommu- nications networks

1.5 NETWORKS

Let us now consider the ideal properties of the various components of the telecommunications system illustrated in Figure 1.5 If both stations A and B are provided with telephones, then the transport mechanism need be no more than a single transmission line, as illustrated in Figure 1.6

The system can also be extended to include further parties For example, if a third station C wishes to be interconnected for private interconnection with either or both of the other two stations (A and B) then this can be achieved by duplication of the simple

layout In this way a triangular network between A, B and C is created, as shown in

Figure 1.7

-

A Telephone Telephone ‘circuit’ or ‘line’ Telephone B

Figure 1.6 A simple two station telephone system

c

C

Figure 1.7 Three stations interconnected by independent telephone lines

The configuration of Figure 1.7 is used today by some companies in their private networks, where a dedicated ‘private line telephone’ may operate over a special

telephone line, leased from a telecommunications administration, to connect different premises However, for a network interconnecting a large number of stations, the

configuration is uneconomical in equipment In the three-station (A, B, C) case

illustrated, six telephones and three lines are needed to interconnect the stations, but only two telephones and one line can ever be used at any one time (unless one of the people is superhuman and can talk and listen on more than one telephone at a time)

As even more stations are introduced to the configuration, the relative inefficiency grows In a system of N stations in which each has a direct link to each other, a total of

i N ( N - 1) telephone lines will be needed, together with N ( N - 1) telephone sets If configured in the manner shown in Figure 1.7, the linking of 100 stations would need

5000 links (and 10000 telephones) and a 10000 station system would need 50 million lines and 100 million telephones We need to find a more efficient configuration! Let us limit each station in Figure 1.7 to one telephone only To make this possible

we install a switching device at each station to enable appropriate line selection, so that connection to the desired destination may be achieved on demand This is now a simple switched network, as Figure 1.8 shows Now the transport mechanism (stylised in Figure 1.5) is no longer just a single ‘line’, but is a more complex ‘switch’ and line arrangement

Let us develop Figure 1.8 further, by permitting more stations (telephones in this case) to be connected to each of the three switches Three more stations, A’, B’ and C’

are shown in Figure 1.9 The new configuration allows the idle lines of Figure 1 S (A-C

and B-C) to be put to use

Cl Telephone C

Figure 1.8 A simple three station switched network

CONNECTION-ORIENTED TRANSPORT SERVICE AND CONNECTIONLESS NETWORK SERVICE 11

6 Switchpoint (apon 1

+ Switchpoint (active)

Figure 1.9 A simple telephone network

Figure 1.9 illustrates simultaneous calls involving A and B, B’ and C’, A’ and C’ In

this example each of the switches (which are now labelled as exchanges) is shared by a number of stations, each of which is switched and connected to the exchange by a local

line or local loop Our example now resembles a public switched telephone network

( P S T N ) Because the lines between exchanges are correctly referred to as junctions or

trunks, they have been labelled accordingly

In a real telephone network the numbers of exchanges and their locations are governed by the overall number and geographical density of stations (telephone users) requiring interconnection Similarly, the number of junctions or trunks provided

between the various exchanges will be made sufficient to cater for the normal telephone call demand In this way, far fewer junctions than stations need to be provided This affords a significant cost saving over the configuration of Figure 1.6

Before leaving Figure 1.9, note how each of the exchanges has been drawn as an array of individual switch points This allows either of the telephones connected to the exchange to access either of the junctions, and this is a so-calledfull availability switch

as any one of the incoming lines may be connected to any one of the available junctions

We shall come back to circuit theory of switching and availability in Chapter 6

1.6 CONNECTION-ORIENTED TRANSPORT SERVICE (COTS) AND CONNECTIONLESS NETWORK SERVICE (CLNS)

In the example of the last section we justified on economic grounds alone the use of

switched as opposed to ‘transmission line only’ networks The particular case that we