Mạng và viễn thông P1

Chia sẻ: Hug Go Go | Ngày: | Loại File: PDF | Số trang:14

lượt xem

Mạng và viễn thông P1

Mô tả tài liệu
  Download Vui lòng tải xuống để xem tài liệu đầy đủ

Znformation and its Lonveyance The world about us brims with information. the time our ears, All eyes, fingers, mouths and noses sense theenvironmentaround us, continuallyincreasing our ‘awareness’,‘intelligence’ and ‘instructive knowledge’. Indeed these last two phrases are the heart of the Oxford Dictionary’s at definition of thewordinformation.

Chủ đề:

Nội dung Text: Mạng và viễn thông P1

  1. 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) PART 1 FUNDAMENTALS OF TELECOMMUNICATIONS NETWORKS
  2. 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) I Znformation and its Lonveyance All The world about us brims with information. the time our ears, eyes, fingers, mouths and noses sense theenvironmentaround us, continuallyincreasing our ‘awareness’,‘intelligence’ and ‘instructive knowledge’. Indeed these last two phrases are the heart of the Oxford Dictionary’s at definition of thewordinformation.Communication,ontheotherhand, 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. Thegreek ‘tele’ prefix means distant, and nothing communication, in the else; 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 by laying and following scent trails.For the life narrower purposes of this book telecommunications is going to mean the transfer of information by electromagneticmeans,(andwiththis will goacertainamount 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 be equally comprehensible at both ends. For lessons in will 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 forcesof electricity, light and radiowaves may be tamed to provide a basis for such communication; then we focus on the pragmatic operationof networks and the quest for solutions to the business needs of information flow. 3
  3. 4 CONVEYANCE INFORMATION AND ITS 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 concept the of an ‘informationenvironment’,acrosswhich information flows inone of anumber 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 orelectrically. 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 Chapter43 it is used to illustrate the analysis, simplification and planning of business information flows. This double theme runs throughout the book: understandingtelecommunicationstechnology, and explainingitsexploitation 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. Firstapersonmustrecordtherelevantinformationonpaper(shownas(a)on Figure 1.3), then he must feed it into the facsimilemachinewhichconvertsit into n Business I Per s& Paper process 4 4 4 Paper m Person a External 2. r. r f storage Figure 1.1 Categorizing information flows
  4. INFORMATION AND ITS CONVEYANCE 5 Mode Examples One - t o - one Broadly aimed Paper Sending a Advertising letter board Leavingnote a Person - t o - person Tal king Acting Winking Television Radio Electronic 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 information to paper (d) and the receiver reads it (e). Theexample we havechosen is ratherconvoluted,requiringseveral 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, companieswho have recognized this fact have already about set converting their all key businessinformation electronic into (computer)format,notonlyforconveyance,butalsoforstorageandprocessing purposes. Paper process (a) Information typed (b)Paper (dl Receiving fed into facsimile Per son facsimile machine Person ( sender ) machine Ireceiver) telephone Figure 1.3 Information conveyance by facsimile
  5. 6 CONVEYANCE INFORMATION AND ITS 1.1 TYPES INFORMATION OF 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 intoan electricalsignal in orderto beconveyed 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 electricalwaveformwhichhasashapedirectlyanalogous 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 notexpect 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 atransmitting device 0 atransport mechanism e areceivingdevice 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
  6. A 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 ineffective due to the incompatibility is 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 Figure1.4illustratesthephysicalelements of asimpletelecommunicationssystem including the transmitter, the receiver and the transport mechanism. As alreadydiscussed,thephysicalelementshowninFigure 1.1 mustbecomple- mented by the use of a compatible protocol between transmitter and receiver. Together with such a protocol,we have all the means communication from point A to point for 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 manymore examples of communication, two way, or duplexoperation is normallyrequired. For duplexoperation,atransmitter and a receiver must be provided at both endsof the connection, as shown in Figure 1.5. A telephonehandset, for example,contains bothamicrophoneand anearphone. Duplex operation allows both parties to talk at once and both to be able to (and have to)listen.Thisallowsthe human listener tointerrupt,or twocomputerstosend 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. Information f l o w * Receiver Transport transmitter mechanism A*0 Figure 1.4 Basic physical elements of a telecommunications system (simplex operation)
  7. 8 CONVEYANCE INFORMATION AND ITS Information flow 4 1 1 Transmitter 4Receiver A 0 + Receiver + Transmitter Figure 1.5 A basic duplex telecommunications system There is also halfduplex operation, inwhichcommunication is possiblein 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 forconveyance over electrical transmission media. Chapters 2 to 5 describehow many of thecommonforms of information(e.g.speech, TV, telex, computer data,facsimile, etc.) are converted into a transmittable signal carried either in ‘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. 1.4 COMMON TYPES OF TELECOMMUNICATIONSSYSTEMS Inorderto meetdifferingcommunicationsneeds,anumber of differenttypes of telecommunications equipment have been developed time. over 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 ( WANs)
  8. NETWORKS 9 0 integrated voice and data networks 0 multimedia networks This book coversthe principles involved in each of theabovetelecommunications 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 thevarious 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. - Telephone ‘circuit’ or ‘line’ A Telephone . Telephone B Figure 1.6 A simple two station telephone system Telephone Telephone A B c Telephone . ~ Telephone Telephone Telephone C Figure 1.7 Three stations interconnected by independent telephone lines
  9. 10 INFORMATION AND ITS CONVEYANCE The configuration of Figure 1.7 is used today by some companies in their private networks, wherededicated a ‘private telephone’ operate a line may overspecial telephone line, leased from a telecommunications administration, to connect different premises.However, foranetworkinterconnectingalargenumber of stations, the configuration is uneconomical equipment. in In thethree-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 systemof N stations in which each has a direct linkto each other, a total of i N ( N - 1) telephone lines willbe 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 each station to enable appropriateline selection, so that at connection to the desired destination may be achieved demand.This is now a simple on switchednetwork, asFigure 1.8 shows. Nowthetransport 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 Figure 1 .S (A-C of and B-C) to be put to use. Cl Telephone C Figure 1.8 A simple three station switched network
  10. CONNECTION-ORIENTED TRANSPORT SERVICE AND CONNECTIONLESS NETWORK SERVICE 11 6 Switchpoint(apon 1 + Switchpoint(active) C C‘ 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 theexchange by a local line or local loop. Our examplenow resembles a publicswitchedtelephonenetwork ( 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 telephonenetworkthenumbers of exchanges and theirlocationsare governed by the overall number and geographical density of stations (telephone users) requiring interconnection. Similarly, number the of junctions or trunks provided between the various exchangeswill 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 the incoming lines may be connected to any one the available junctions. of of 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
  11. 12 CONVEYANCE INFORMATION AND ITS have developed is an example of a circuit-switched network. Other important switched network 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 examples of connection-oriented switching or connection-oriented transport service (COTS). In a connection-oriented switching technique a circuit,virtualcircuit,connection or virtual connection ( V C ) is established between sender and receiver beforeinformation is conveyed. Thus a telephone connection is first established by dialling, before the con- versation takes place. This ensures the readiness of the receiver to receive information before it is sent.(There is no pointintalking if nobody is listening). In contrast, connectionless switchingtechniques or connectionless-networkservice ( C O N S ) allow messages to be despatched, maybe even without checking the validity of the address. Thus, for example, the postal service is analogous to a connectionless service. The sender posts the entire message (envelope and contents) into the post box and forgets about it. Sometime later, the receiver receives the message, either delivered directly through his letter box or by picking it up from his local post office. Electronic mail, today’s com- puterizedversionof thepostal service, is alsoa connectionlessnetworkservice for sending letters directly between computers. The advantage of connectionless service is that the sender need not waitforthe receiver to be ready and the network need not be encumbered with the extra effort of setting-up a connection. Thus neither sender nor network bother to keep redialling need when either the receiver is already busy on another call, asleep on the other side of the world, disconnected, switched-off or otherwise unable to answer the call. Instead the message is lodged in a temporary store or ‘postoffice’-like device. The disadvantage is that the sender has no clear guarantee or confirmation of message delivery. He is left in doubt: 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 or document) in one go. Most message switching networks (including perhaps the best known, Internet) are based on connectionless network service. As an aside, sometimes the end-to-end communication is connectionless even though each of the individuallinks in thephysical communication chain is a connection-oriented connection. Thus both sender and receiver mighttelephone an electronic mail post ofice to send and receive their mail. The connection between the two post offices may be apermanentconnectionandbothtelephone calls (to deliver and pick upthe message) are connection-oriented, but because the sender and receiver do not both need 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 NETWORKS The distinguishing property of a circuit-switched connection is the existence throughout thecommunicationphaseofthe call, of an unbroken physical and electrical path between origin and destination points. The pathis established at call set-up and cleared afterthecall.Thepathmay offer eitheronedirection (simplex) or two direction (duplex) use. Telephone networks are circuit-switched networks.
  12. PACKET- CIRCUIT-, AND NETWORKS CELL-SWITCHED 13 Conversely, although packet-switched networks are alsoconnection-oriented, an entire physical path from origin to destination will not generally be established at any time during communication. Instead, the total information be transmitted is broken down to into a numberof elemental ‘packets’, each of which is sent in turn. Ananalogy might be sending the textof a large book through the post a large number envelopes. In each in of individualenvelopemight be just a single page. The envelopes caneither be sent sequentially (say one each day), numberedso that the or receiver can reassemble the pages in order.Figure 1.10 illustratesthissimplepacket-switchedcommunicationsystem. Should thereceiver in Figure 1.10 not receive any given numbered envelope, he may write back over thereverse connection and re-request it. In this way,very accurate andreliable communication may be established. Packet-switched networks are usually connection-oriented. A connection set-up phase confirms the readiness of the receiver to receive information and it determines the route through the network which will be used to carry the packets. The connection which results is actually termed a virtual connection, because though it appears to the two end-users as though a dedicated path exists, the physical connection is actually shared with other users. By breaking the information into packets, statistical multiplexing may be used to increase the network throughput. Statistical multiplexing is the technique of sendingpacketsfrom different users’ virtualconnections overthesame physical connection(Figure 10). This is made possible by labelling eachpacket(pages of Figure 1.10) with the identity of the virtual connection to which it belongs (separate books could be sent simultaneously in Figure 1.10). The labelling allows packets to be sent 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 a second before transmission is possible. This possibility of slight delay leads to another description of packet-switching as a store-and-forward technique. Packet-switching is the technique behind X.25, frame relay and many other computer network techniques. Circuit-switched networksare generally necessary when very rapid or instantan- eous interaction is required (as is the case with speech). Conversely, packet-switched Received so far Page number page number r--------- l I I - Paae S Page L I’ I I Today’s i I post Tomorrow’s post ‘I L ---A Transportmechanism l the postman ) Figure 1.10 A simple ‘message switched’ communication system
  13. 14 INFORMATION AND ITS CONVEYANCE networks are more efficient when instantaneous reaction is not required, but when very low ‘corruption’ (either through distortion or loss of information) is paramount. Cell-switching is a specialized form of packet-switching in which the packet lengths are standardized at a fixed length. As we shall see in Chapters 25 and 26, cell switching or cell relay switching is the basis of multimedia networks, the broadband integrated services digital network (B-ZSDN) and ATM (asynchronous transfer mode). 1.8 CONSIDERATIONS FOR NETWORK PLANNERS We have established the economic value of switched networks (as opposed to direct wire systems), and briefly addressed the needs for basic communication (the physical, switching and protocol needs). What other factors need to be provided in order to enable networks to function in a manner fit for purpose? The following list is a brief summary of some of the factors which require consideration. Transmission and configuration plan A plan laying out guidelines according to which appropriate transmission media may be arranged so that adequate end-to-end conveyanceof information is achieved. The plan will include safeguards to ensure that thereceived signal is loud enough, clear enough and free of noise and interference. Numbering and routing plans The numbering plan is crucial to the ability of the network to deliver communications to the appropriate destination. Much like reading an address on an envelope, it is the inspection of the destination number that permits the determinationof destination and appropriate route within the network. Usage monitoring plan A usage monitoring plan is needed to ensure continuing and future suitability of the network. Adequate measurements of network performance are required so that early steps may be taken, as necessary, to adjust the network design, or expand its overall throughput capacity to meet demand. Charging and accounting plan Publictelecommunicationoperators (PTOs) needreimbursementfortheir services. Specific equipment may be required to monitor individuals’ use of the network, so that bills can be generated. Maintenance plan A 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 of these items may be appropriate to any type of telecommunications network, regardless of what type of information it is carrying (for example, telephone, data or multimedia network). Each is discussed more fully in later chapters.
  14. STANDARDS TECHNICAL FOR TELECOMMUNICATION SYSTEMS 15 1.9 TECHNICAL STANDARDS FOR TELECOMMUNICATIONS SYSTEMS As a way of promotinggreatercompatibility between varioustelecommunications systems 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 in more detail in Chapter 40. However,twoofthemostsignificant and influentialbodies are worth mentioning now. These are: 0 the International Organization for Standardization (ISO) 0 the International Telecommunications Union (ITU), and specifically its sub-entities, thestandardizationsector (ITU-T, formerly CCITT, consultativecommittee f o r international telephones and telegraphs) and the radiocommunication sector (ITU-R, formerly CCIR, consultative committee for international radiocommunication). The standards and recommendations of these bodies commonly two are used throughout telecommunications, and are often referred to by this book.
Đồng bộ tài khoản