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Containing Network Overload Network designers like think that their traffic models ideally stable, andthat their forecasts to are of future traffic will never be real life, however, such assumptions are unwise, because any one of a number of problems may arise, resulting in network overload, and so congestion. The forecast may underestimate demand; there may be a short period of extraordinarily high pressure (for example at New Year, Christmas, or any public holiday, or following a natural disaster); or there may be a network link switch (exchange) failure. . | 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 37 Containing Network Overload Network designers like to think that their traffic models are ideally stable and that their forecasts of future traffic will never be wrong. In real life however such assumptions are unwise because any one of a number of problems may arise resulting in network overload and so congestion. The forecast may underestimate demand there may be a short period of extraordinarily high pressure for example at New Year Christmas or any public holiday or following a natural disaster or there may be a network link or switch exchange failure. Monitoring and controlling the network to detect and avoid network congestion and the resulting service degradation is a difficult task but an important one. This chapter discusses these new network management methods in more detail. THE EFFECT OF CONGESTION Congestion in a telecommunications network manifests itself to customers who are attempting to make calls as a network busy tone or as a delay in computer or data network response. It annoys the customer even worse the congestion can rapidly increase as the result of customer or equipment repeat attempts. Repeat attempts are further call attempts made in vain by customers hoping to make a quick connection by immediate re-dialling. These further call attempts only exacerbate the problem because they greatly increase the loading on exchange equipment . switch processors number stores senders and receivers and they add to the overall traffic volume. The congestion can then spiral out of control. The networks most at risk from congestion are those which have been kept to minimum circuit numbers in order to keep costs down. Also very much at risk are those networks that employ a high proportion of multi-link overflow routings because congestion