Mạng và viễn thông P32

Network Trafic ControlTraffic control is as much art as it is science, and the profits are gratifying. The right control mechanisms and routing algorithms in the right places will determine the overall performance and efficiency of our network. What is more, they will giveus simpler administration, better management and lower costs. This chapter describes a number of these admirable devices: first the simple methodscommonlyusedforoptimizingnetworkroutingundertypical‘normal-dayloading’ conditions; then we look at recent more powerful and complex techniques, together with some of the practical complications facing telecommunications today. ...
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Mạng và viễn thông P32 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) 32 Network Trafic Control Traffic control is as much art as it is science, and the profits are gratifying. The right control mechanisms and routing algorithms in the right places will determine the overall performance and efficiency of our network. What is more, they will giveus simpler administration, better manage- ment and lower costs. This chapter describes a number of these admirable devices: first the simple methodscommonlyusedforoptimizingnetworkroutingundertypical‘normal-dayloading’ conditions; then we look at recent more powerful and complex techniques, together with some of the practical complications facing telecommunications today. Finally we note how several net- work operators have found ways interconnecting with the multiple carriers that have emerged of lately as a result of market deregulation.3 . NETWORKS 21 Anymetropolitan,international,trunk,transitorcorporatenetwork, by virtue of its nature andsheer size has to include a considerable number of switches (exchanges). If there are many inter-connections between switches, individual these any call crossing the network will have plenty of alternative paths open to it. It is in fact the number of pathpermutations whichdeterminestherobustness of thenetwork to individual link failure and sets the level or grade of service provided (i.e. the prob- ability of successful call connection). It offers the customer what he most wants, an acceptablechance of makingasuccessfulcall or informationtransfereachtime. Particular attention must be paid to choosing the call-control mechanisms which are going to determine the routing of our customers’ individual calls, connections or mes- sages, bearing in mind that in an ideal network we aim overall to achieve a controlled flow of traffic throughout the network at reasonable cost and without the penalty of complicated administration. Various methods and preliminary calculations can now be considered in turn. 571572 NETWORK TRAFFIC CONTROL32.2 SIZING CIRCUIT-SWITCHED NETWORKS First of all, to find how many circuits a telecommunications network will need to meet traffic demand, a mathematical model is required to predict network performance, and it will comprise at least two parts 0 a statistical distribution to represent the number calls in progressat anygiven time of 0 a forecast of the overall volume of traffic Erlang’s model provides a statistical methodfor approximating telephone traffic, which is based on his measurements of practical telephone networks. A short reminder follows. Erlang devised a method of calculating the probability of any given number of calls being in progress at any instant in time. The probability density function used for the calculation is termed the Erlang distribution. The Erlang formula, a related and complex iterative mathematical formula, can be manipulated into a number of different forms. The most common form is used to calculate the required number of circuits (or circuit group size) to carry a given traffic volume between two exchanges, under theconstraint of having to meet a given grade of service. The traffic valueinput into this formula is the measurement of trafic intensity. The intensity of traffic on a route between any two exchanges in a network is equal to the average number of calls in progress, and is measured in Erlangs. In Chapter30 we discussed how, for planning purposes, it was normal measure the to route traffic intensity during the busiest hour of activity, or so-called route busy hour. Using this value and the Erlang formula, the number of circuits required for the route can be calculated according to a given target grade of service. The grade of service ( G O S ) of a telephone route between any two exchanges in a circuit-switched network is the fractional quantity of calls which cannot be completed due to network congestion. The lower the numerical value of GOS, the better the per- formance. A typical target value used in many trunk and international networks 1%, is or 0.01 (in other words 1% of calls cannot be completed due to network congestion, the other 99% can). The calling customer, however, probably only perceives end-to-end an grade of service, on a call-by-callbasis, of around 5 % (i.e. 5% lostcalls).This is because most connections comprise a number of links and exchanges, each of which is likely to be designed to inflict a 1% loss. The common method is to dimension circuit groups within a network usin ...