information and conveyance

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PART 1FUNDAMENTALS OF

TELECOMMUNICATIONSNETWORKS

Networks and Telecommunications: Design and Operation, Second Edition.Martin P. Clark

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


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IZnformation andits Lonveyance

The world about us brims with information.ll the time our ears,yes, fingers, mouths and nosessense theenvironmentaround us, continually ncreasing our awareness, intelligence andinstructive knowledge. Indeed these last two phrases aret the heartof theOxfordDictionarysdefinition of theword nformation.Communication,on heotherhand, is defined as theimparting, conveyance or exchange of ideas, knowledge or information. t might be done byword, mage, nstruction, motion, smell -or maybe ust a wink! Telecommunication is com-munication by electrical, radio or optical (e.g. laser) means. We introduce the basic capabilitiesand terminology of telecommunications and networking in this chapter.

As hybrid words go, telecommunications wins no prizes, but it is all we have to workwith. Thegreek tele prefix means distant, and nothinglse; communication, in the enseof information passed to and fro etween human beings, and animals, s an activity thatgoes back beyond recorded times. With a broad view, long distance communicationsbrings to mind Armada beacons, heliographs flashing between Frontier posts, empiresheld together by relays of post-houses, whales singing to one another in the deep, andthe family dog which conducts its socialife by laying and following scent trails.For thenarrower purposes of this book telecommunications is going to mean the transfer ofinformation by electromagneticmeans, andwith his will goacertainamount ofaccepted jargon). All systems have much in coqnon, whatever their age. In principleeach requiresa transmitter, acarrying device or transmission medium, a receiver, and asupply of information which ill be equally comprehensible at both ends. For lessons intechnique nothing should be disregarded, however ancient: for a cheap, speedy andcomprehensive message, what is there to beat a human wink?In the science and business of telecommunications, a structured framework has beencreated for conveying certain types of nformation across ong distances, with ittlerespect for the barriers of geography. In this book we study this framework; first weunderstand how the forcesof electricity, light and radiowaves may be tamed to providea basis for such communication; then we focus on the pragmatic operationof networksand the quest for solutions to the business needs of information flow.

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Networks and Telecommunications: Design and Operation, Second Edition.Martin P. Clark

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


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4 INFORMATION AND ITSONVEYANCEFigure 1.1 illustrates a simple but powerful model for understanding and categorizingvarious different means of communication. The model illustrates a number of differentways in which a business may communicate either within tself, or with ts externalenvironment of suppliers andcustomers.Thus we introduceheconcept of aninformationenvironment,acrosswhich information flows inone of anumber of

different forms.The simplest form of information flow (illustrated by Figure 1.1) might be directlyfrom one person to another, by word of mouth or by a visual signal. Alternatively theinformation could have been conveyed on paper orelectrically. The advantage of eitherof the latter two methods is that the information in paper or electronic form may alsobe readily stored for future reference.In this book we shall use the model of Figure 1.1 twice. Here we use it to illustratehow different methods of communication may be categorized into one of the threebroad types, and as a basis for explaining the prerequisite components of a telecom-munications system. In Chapter43 it is used to illustrate the analysis, simplificationndplanning of business information flows. This double theme runs throughout the book:understanding elecommunications echnology, and explaining tsexploitation in apragmatic business-oriented manner. Well-known examples of communications met-hods that fall into the three categories of paper, person-to-person and electronic aregiven 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 overthe telephone network and recreating them at a distant location. This would appear asquite a complex information path on our model of Figure 1.1, as Figure 1.3 shows.Firstapersonmustrecord herelevant nformationonpaper shownas a)onFigure 1.3), then he must feed it into the facsimilemachinewhichconverts t into

nusiness Paper PaperprocessI 4 4

Per s&2 . r . r f

Person

storage

Figure 1.1 Categorizinginformation flows

axternal


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INFORMATION AND ITS CONVEYANCE 5

Mode

Paper

Person - to - person

Electronic

Examples

One - t o - one BroadlyimedSending letterdvertisingoardLeavingnoteTalkingWinking

Act ingTelevisionRadio

c-- Computernetworking

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 nformation to paper (d) andthe receiver reads it (e).Theexample we havechosen is ratherconvoluted, equiringseveral successiveconversions to take place, changing the format of the information from personal topaper to electronic and backagain. All these onversions makehe processinefficient, and as we shall find out later, companieswho have recognized this fact havealready etabout convertingallheir key businessnformationntoelectronic(computer) ormat,notonly orconveyance,butalso orstorageandprocessingpurposes.

Paperprocess(a) Information

typed

P er son( sender )

( b) Paperfed ntofacsimilemachine

(dl Receivingfacsimilemachine

telephoneFigure 1.3 Information conveyance by facsimile

PersonI eceiver)


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6 INFORMATION AND ITSONVEYANCE1.1 TYPESOFNFORMATION

Put specifically in the context of telecommunications, information might be a page ofwritten text, a conversation or a television picture. The information usually requiresconversion intoan electricalsignal in order o beconveyed by telecommunicationmeans. However, there are many different types of information, so can they all betreated identically? The answer to this is no, because each type of information makesslightly different demands on the telecommunication system.Information conveyed over telecommunications systems is usually classed as eitheran analogue signal information or as data (digital information). An analogue signal isan electricalwaveformwhichhasashapedirectlyanalogous to the nformation itrepresents (e.g. speech or a television picture). Data, on the other hand, is the wordgiven to describe information in the form of text, numbers or coded computer or videoinformation.Different forms of data and analogue signal information require different treatment.For example, when conversing with someone we expect their reply to follow shortlyafter our own speech, but when we send a letter we do notexpect a reply for some days.The analogy runs directly into telecommunications. Thus, an electrical representationof conversation must allow the listener to respond instantly. However, in the case ofdata communication, slightly more leeway exists, as a computer is prepared to acceptresponse times of several seconds. A human would find this length of delay intolerablein everyday speech. Another difference between electrical representations designed fordifferent applications will be the speed with which information can be transferred. Thisis normally referred to as the information rate, the bandwidth or the bitrate. A speechcircuit requires more bandwidth to carry the different voice tones than a telegraph wireneeds simply to carry the same information as text. Later chapters in this book discussthe various methods of electrical representation and the technical standards used.

1.2 TELECOMMUNICATIONS YSTEMSThere are four essentials for effective information transfer between two points, all ofwhich are provided in well-designed telecommunications systems:0 a ransmitting device0 a ransport mechanisme a eceivingdevicee the fourth requirement s that the conveyed information is coded in such a way as tobe 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 oneanother, the transmitting device is the mouth, the transport mechanism s the soundthrough the air, and the receiving device is the other persons ear. Provided that bothpeople talk the same anguage, then the fourth requirement has also been met, and


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A 7conversation can continue. However, if the talker speaks English, and the listener onlyunderstands French, then, despite the availability of the physical components of thesystem (i.e. mouth, ear and air), communications ineffective due to the incompatibilityof the information.The coding and method of transfer of the information over the transport mechanismis to said to be the protocol. In our example the protocol would be either the Englishor the French anguage: the fact that the talker is English and the istener French isan example of protocol incompatibili ty. P rotocol also defines the procedure to be used.An example of the procedural part of protocol is the use of the word over to signifythe end of radio messages (for example Come in Foxtrot, Over). The protocol in thiscase prompts a reply and prevents both parties speaking at once. The hardest part oftelecommunications system design is often the need to ensure the compatibility of theprotocol. In some cases, this necessitates the provision of interworking devices. In ourexample, the interworking device might be a human English/French interpreter.

1.3 A BASIC TELECOMMUNICATIONS SYSTEMFigure1.4 llustrates hephysicalelements of asimple elecommunicationssystemincluding the transmitter, the receiver and the transport mechanism.As alreadydiscussed,thephysicalelementshown nFigure 1.1 mustbecomple-mented by the use of a compatibleprotocol between transmitter and receiver. Togetherwith such a protocol,we have all the means or communication from point A to pointin 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 iscalled, may suffice for some purposes. For manymore examples of communication, twoway, or duplexoperation is normallyrequired. For duplexoperation,atransmitterand a receiver must be provided at both endsof the connection, as shown in Figure.5.A telephonehandset, for example,contains bothamicrophoneand anearphone.Duplex operation allows both parties to talk at once and both to be able to (and haveto) isten.Thisallows he human listener to nterrupt,or twocomputers osendinformation to one another in both directions at the same time. Not all devices arecapable of talking and listening at the same time as required for duplex operation.

*

A * 0

Informat ion f l o w

t r a n s m i t t e r Transporteceiver mechanism

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


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8 INFORMATION AND ITSONVEYANCEInformat ion f low

4

1 1

Transmit ter Receiver

A 0

+ +eceiver TransmitteFigure 1.5 A basic duplex telecommunications system

There is also halfduplex operation, inwhichcommunication is possible n bothdirections, but not at theame time, as only one communications path s available. Firstthe talker must stop speaking, then the listener can reply.The transport mechanism can be one of a range of different media, ranging fromsound waves passing through air to laser ight pulses passing down the atest tech-nology, optical fibre. Furthermore the transport mechanism may or may not comprisean element of switching, as we describe later in the chapter.Most transport mechanisms demand an encoding of the information or data into asignal form suitable forconveyance over electrical transmission media. Chapters2 to 5describehow many of thecommon orms of information e.g.speech, TV, telex,computer data, acsimile, etc.) are converted into a transmittable signal carriedn eitheranalogue or digital form. In Chapters6 and9we discuss various methods of switch-ing and in Chapter 8we discuss a range of different transmission media (cables, radiosystems, etc.), describing how different ones provide the optimum balance of low costand good transmission performance for individual cases of application.

1.4 COMMON TYPES OF TELECOMMUNICATIONSSYSTEMSInorder o meetdifferingcommunicationsneeds,anumber of different ypes oftelecommunicationsequipmenthave been developedover ime.These nclude, inchronological order:e telegraphe telephonee telexe data networks using either circuit-, packet-,frame- or cell-switched conveyancee computer local urea networks (LANs),metropolitan ureu networks ( M A N S )and idearea networks (WANs)


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NETWORKS 9

0 integrated voice and data networks0 multimedia networksThis book coversthe principles involved in each of theabovetelecommunicationstypes; it also aims to give adequate background to enable the reader to tackle basicnetwork planning of any of these types. The book covers networking from the simpleinterconnection of two telephones right up to complex, globally spread, telecommu-nications networks.

1.5 NETWORKSLet us nowonsiderhe ideal properties of theariousomponents of thetelecommunications system illustrated in Figure 1.5. If both stations A and B areprovided with telephones, then the transport mechanism need be no more than a singletransmission line, as illustrated in Figure 1.6.The system can also be extended to include further parties. For example, if a thirdstationCwishes to be interconnected for private interconnection with either or both ofthe other two stations(A and B) then this can be achieved by duplication of the simplelayout. In this way a triangular network between A , B and C is created, as shown inFigure 1.7.

-A Belephoneelephone

Telephone circuit or line

Figure 1.6 A simple two station telephone system

Te l ephone Telephone

A Bc

Telephone Telephone.

Telephone Te l ephone

CFigure 1.7 Three stations nterconnected by independent telephone lines


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10 INFORMATION AND ITS CONVEYANCEThe configuration of Figure 1.7 is used today by some companies n their privatenetworks,whereadedicated private ine elephonemayoperateoveraspecialtelephone line, leased from a telecommunications administration, to connect differentpremises.However, foranetwork nterconnectinga argenumber of stations, theconfiguration is uneconomical nequipment. In the hree-station (A , B, C) case

illustrated, six telephones and three lines are needed to interconnect the stations, butonly two telephones and one line can ever be used at any one time (unless one of thepeople is superhuman and can talk and listen on more than one telephone at a time).As even more stations are ntroduced to the configuration, the relative inefficiencygrows. In a systemof N stations in which each has a direct link o each other, a totalofi N ( N - 1) telephone ines will be needed, together with N(N - 1) telephone sets. Ifconfigured in the manner shown in Figure 1.7, the linking of 100stations would need5000 links (and 10000 telephones) and a 10000station system would need 50 millionlines and 100million 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 possiblewe install a switching deviceat each station to enable appropriate ine selection,so thatconnection to the desired destination may be achievedn demand.This is now a simpleswitchednetwork, asFigure 1.8 shows. Now he ransport mechanism (stylised inFigure 1.5) is no longer just a single line, but s a more complex switch and linearrangement.Let us develop Figure 1.8 further, by permitting more stations (telephones in thiscase) to be connected to each of the three switches. Three more stations, A, B and Care shown in Figure1.9.The new configuration allows the idle linesf Figure 1.S (A-Cand B-C) to be put touse.

Clelephone CFigure1.8 A simple three station switched network


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CONNECTION-ORIENTED RANSPORT ERVICE AND CONNECTIONLESSNETWORKERVICE 11

6 Switchpoint apon 1+Swi tchpoi nt ac t i ve )C C

Figure 1.9 A simple telephone networkFigure 1.9 illustrates simultaneous calls involvingA and B, B and C, A and C. Inthis example each of the switches (which are now labelled as exchanges) is shared by anumber of stations, each of which is switched and connected to theexchange by a localline or local loop. Our examplenow resembles a publicswitched elephonenetwork(PSTN) .Because the lines between exchanges are correctly referred to as junctions ortrunks, they have been labelled accordingly.In a real telephonenetwork henumbers of exchanges and their ocationsaregoverned by the overall number and geographical density of stations (telephone users)requiring nterconnection.Similarly, henumber of junctions or trunks providedbetween the various exchangeswill be made sufficient to cater for the normal elephonecall demand. In this way, far fewer junctions than stations need to be provided. Thisaffords 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 anarray of individual switch points. This allows either of the telephones connected to the

exchange to access either of the junctions, and this isaso-calledfull availability switchas any one f the incoming ines may be connected to any one f the available junctions.We shall come back to circuit theory of switching and availability in Chapter 6.

1.6 CONNECTION-ORIENTED TRANSPORT SERVICE (COTS)ANDCONNECTIONLESS NETWORK SERVICE (CLNS)In the example of the ast section we justified on economic grounds alone the use ofswitched as opposed to transmission line only networks. The particular case that we


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12 INFORMATION AND ITSONVEYANCEhave developed is an example of a circuit-switched network. Other important switchednetwork types, especially used for data transmission, are those of packet switching,message switching and cell switching.Circuit-switched and most packet-switched and cell-switched networks are examplesof connection-oriented switching or connection-oriented transport service (COTS). n aconnection-oriented switching technique a circuit,virtualcircuit,connection or virtualconnection (VC) is established between sender and receiver before nformation isconveyed. Thus a telephone connection is first established by dialling, before the con-versation takes place. This ensures the readiness of the receiver to receive informationbefore it is sent. There is no point n alking if nobody is listening). In contrast,connectionless switching echniques or connectionless-networkservice (C ONS ) allowmessages to be despatched, maybe even without checking the validity of the address.Thus, for example, the postal service is analogous to aconnectionlessservice. The senderposts the entire message (envelope and contents) into the post ox and forgets about it.Sometime later, the receiver receives the message, either delivered directly through hisletter box or by picking it up from his local post office. Electronic mail , todays com-puterizedversionof thepostal service, is alsoa connectionlessnetworkservice forsending letters directly between computers.The advantage of connectionless service is that the sender need not wait or hereceiver to be ready and the network need not be encumbered with the extra effort ofsetting-up a connection. Thus neither sender nor networkeed bother to keep rediallingwhen either the receiver is already busy on another call, asleep on the other side of theworld, disconnected, switched-off or otherwise unable to answer the call. nstead themessage is lodged ina temporary store or postoffice-like device. The disadvantage isthat the sender has no clear guarantee or confirmation of message delivery. He is left indoubt: did the receiver not get the message or was he simply too lazy to reply?

Message switching networks are networks which deliver the message (e.g. letter ordocument) n one go. Most message switching networks (including perhaps the bestknown, Internet) are based on connectionless network service.As an aside, sometimes the end-to-end communication s connectionless even thougheach of the individual inks in thephysical communication chain is aconnection-orientedconnection. Thus both sender and receiver mighttelephone an electronic mail postofice to send and receive their mail. The connection between the two post offices maybe apermanentconnectionandboth elephone calls (to deliver and pick up hemessage) are connection-oriented, but because the sender and receiver do not bothneed to connect to their respective post offices at the same time, the end-to-end com-munication is connectionless.

1.7 CIRCUIT-, PACKET- AND CELL-SWITCHED NETWORKSThe distinguishing property of a circuit-switched connection is the existence throughoutthecommunicationphaseofthe call, of an unbroken physical and electrical pathbetween origin and destination points. The path s established at call set-upand clearedafter hecall.Thepathmay offer eitheronedirection (simplex) or two direction(duplex) use. Telephone networks are circuit-switched networks.


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CIRCUIT-,ACKET- AND CELL-SWITCHEDETWORKS 13Conversely, althoughpacket-switched networks are alsoconnection-oriented, an entirephysical path from origin to destination will not generally be established at any timeduring communication. Instead, the total information toe transmitted is broken downinto a numberof elemental packets, eachof which is sent in turn. Ananalogy might besending the textof alarge book through the postn a large number f envelopes. In eachindividualenvelopemight be just a single page. The envelopes caneither be sentsequentially (say one each day), r numberedothat the eceiver can reassemble the pagesin order.Figure 1.10 illustrates hissimplepacket-switchedcommunicationsystem.Should the eceiver in Figure 1.10 not receive any given numbered envelope, he may writeback over the everse connection and re-request it. n this way,very accurate and eliablecommunication may be established.Packet-switched networks are usually connection-oriented. A connectionset-up phaseconfirms the readiness of the receiver to receive information and it determines the routethrough the network which will be used to carry the packets. The connection whichresults is actually termed a virtual connection, because though t appears to the two

end-users as though a dedicated path exists, the physical connection is actually sharedwith other users. By breaking the information intopackets, statistical multiplexingmaybe used to increase the network throughput. Statistical multiplexing is the techniqueof sendingpackets rom different users virtualconnections over hesame physicalconnection Figure 10). This is made possible by labelling eachpacket pages ofFigure 1.10) with the identity of the virtual connection to which it belongs (separatebooks could be sent simultaneously in Figure 1.10). The labelling allows packets to besent from any of the virtual connections, provided that the line is at that moment idle.If the line is already busy, it may be that the new packets must wait a fraction of asecond before transmission is possible. This possibility of slight delay leads to anotherdescription of packet-switching asastore-and-forward technique.Packet-switching is thetechnique behind X.25, frame relay and many other computer network techniques.Circuit-switched networksare generally necessary when very rapid or nstantan-eous nteraction is required (as is the case with speech). Conversely, packet-switched

Received so fa rPage number page numberr - - - - - - - - - l

-Figure 1.10 I Paae S IPage L I ITomorrows Todays iI posto st IL - - - ATransportmechanisml the postman )A simple message switched communicationsystem


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14 INFORMATION AND ITS CONVEYANCEnetworks are more efficient when instantaneous reaction is not required, but when verylow corruption (either through distortion or loss of information) is paramount.Cell-switching is a specialized form of packet-switching in which the packet lengthsare standardized at a fixed length. As we shall see in Chapters 25 and 26, cell switchingor cell relay switching is the basis of multimedia networks, the broadband ntegratedservices digital network (B-ZSDN)and AT M (asynchronous transfer mode).

1.8 CONSIDERATIONS FOR NETWORK PLANNERSWe have established the economic value of switched networks (as opposed to directwire systems), and briefly addressed the needs for basic communication (the physical,switching and protocol needs). What other factors need to be provided in order toenable networks to function in a manner fit for purpose? The following list s a briefsummary of some of the factors which require consideration.

Transmission and configuration planA plan laying out uidelines according to which appropriate transmission media mayearranged so that adequate end-to-end conveyanceof information is achieved. The planwill include safeguards to ensure that the eceived signal is loud enough, clear enoughand free of noise and interference.Numbering and routing plansThe numbering plan is crucial to the ability of the network to deliver communicationsto the appropriate destination. Much like reading an address on an envelope, it is the

inspection of the destination number that permits the determinationof destination andappropriate route within the network.Usage monitoring planA usage monitoring plan is needed to ensure continuing and future suitability of thenetwork. Adequate measurements of network performance are required so that earlysteps may be taken, as necessary, to adjust the network design, or expand ts overallthroughput capacity to meet demand.Charging and accounting plan

Public elecommunicationoperators (PTOs) need eimbursement or heir services.Specific equipment may be required to monitor individuals use of the network, so thatbills can be generated.Maintenance planA maintenance plan is needed to guarantee that the service level meets agreed targets,that routine maintenance is carried out, and that problems are quickly seen to. Any ofthese items may be appropriate to any type of telecommunications network,egardlessof what type of information it is carrying (for example, telephone, data or multimedianetwork). Each is discussed more fully in later chapters.


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TECHNICALTANDARDS FOR TELECOMMUNICATIONYSTEMS 151.9 TECHNICAL STANDARDS FORTELECOMMUNICATIONS SYSTEMS

As a way of promotinggreatercompatibility between various elecommunicationssystems in different parts of the world, a number of bodies that define technical stand-ards are active in various parts of the world. These are discussed n more detail nChapter 40. However, two of the most significant and influential bodies are worthmentioning now. These are:0 the nternational Organization for Standardization (ISO)0 the International Telecommunications Union (ITU), and specifically its sub-entities,thestandardizationsector (ITU-T, formerly CC I TT, consultativecommittee forinternational telephones and telegraphs) and the radiocommunication sector (I TU -R,formerly CC I R, consultative committee for international radiocommunication).

The standards and recommendations of thesewobodiesare ommonlyusedthroughout telecommunications, and are often referred to by this book.

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