Mạng và viễn thông P28

Network Routing, Znterconnection and ZnterworkingThe control of the routing of calls and connections (so-called ‘traffic’) across telecommunications networks is the most difficult but most important responsibility of a network operator. Only by careful planning and management of appropriate call and traffic routing plans can the network operator ensure successful connection of calls and the efficient use of network resources.
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Mạng và viễn thông P28 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) 28 Network Routing, Znterconnection and Znterworking The control of the routing of calls and connections (so-called ‘traffic’) across telecommunications networks is the most difficult but most important responsibility of a network operator. Only by careful planning and management of appropriate call and traffic routing plans can the network operator ensure successful connection of calls and the efficient use of network resources. In this chapter we discuss the techniques used network operators in establishing efficient call routing by patterns and the special problems caused by network interconnection and interworking, when calls or connections originated in one network have to be passed to another operator’s network for completion.27.1 THENEED FOR A NETWORK ROUTING PLAN We might choose, laudable as it may seem, to attempt to run our telecommunications network by completing as many calls as possibledelivering the greatestproportion of or data messages. The problem is that if we attempt to do so, we are bound adversely to affect the intelligibility of messages and the time it takes to deliver them. To attempt the ‘complete if you can’ routing philosophy, we simply programme the exchanges to route all messages ‘any way possible’, rather thanever fail anything when a circuit is free. Unfortunately, the networkwill perish of congestion and suffer appalling is signal quality. Studying the scenario, however, highly instructive andwe shall look at an example or two shortly. The rational and rewarding alternative to the ‘any way possible’ regime is to have network routing plan, togetherwithasupporting numbering plan. Theappropriate routing algorithms laid out by the routing and numbering plans are selected to control network traffic and to comply with the overall constraints which the transmission plan and engineering guidelines impose on end-to-end connections made across the network. To work within these various constraints, and still to achieve a network that is reason- ably cheap as well as highly efficient is an arduous test of planning and administration; 491492 INTERCONNECTION ROUTING, NETWORK AND INTERWORKING 1 A C I I I L - 1-q --l I - - - Busy direct circuits Connection established Figure 28.1 Uncontrolledcircularrouting but it is well worth while when we consider the alternative of uncontrolled network routing and its disastrous effect on network congestion and the quality of connections, which the examples in Figure 28.1 and 28.2 will illustrate. Intheexampleshown inFigure 28.1, a circuit-switched connection(suchasa telephonecall) is desired from exchange A to exchange B. Exchange A hasdirect circuits to B, but these are currently busy, so exchange A has made a connection to exchange C and passed the call on, intending to transit this exchange and connect via the direct circuits from to B. Unfortunately these circuits are also busy, and taking C by no cognisance of the call’s previous history, exchangeC extends the connection back to exchange A using a similar logic.(‘My direct circuits are busy, but I know thereis also a route via A’) The process continues in a circular fashion until either all the circuits between A and C also become busy, so that the call eventually fails, or finally a circuit becomes available on either of the routesA-B or C-B, in which casethe call eventually completes. In either eventuality the circular routing between A and C ties up network resources,restricting communication betweencustomers exchanges on A and C . Furthermore, even if the call does eventually complete, the transmission quality likelyis to be appalling or the delay in packet data delivery may be intolerably long, due to the large number of links in the connection. A second effect of uncontrolledrouting is showninFigure 28.2, whereacom- munication path hasfinally been completedover 9 individuallinks,transmitting8 intermediate exchanges. As in the circular routing example Figure 28.1, even though ...