tailieunhanh - Mạng và viễn thông P30

Teletrafic Theory Telecommunication networks, like roads. are saidcarry ‘traffic’, consisting not of vehicles but of to telephone callsor data messages. The more traffic there is, the more circuits and exchanges must be provided. On a road network the more cars and lorries, the more roads and roundabouts are needed. In any kind of network, if traffic exceeds the design capacity then there will be pockets of congestion. | 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 30 Teletraffic Theory Telecommunication networks like roads are said to carry traffic consisting not of vehicles but of telephone calls or data messages. The more traffic there is the more circuits and exchanges must be provided. On a road network the more cars and lorries the more roads and roundabouts are needed. In any kind of network if traffic exceeds the design capacity then there will be pockets of congestion. On the road this means traffic jams on the telephone the frustrated caller receives frequent busy tones in a data network unacceptably long response times are experienced. Short of providing an infinite number of lines it is impossible to know in advance precisely how much equipment to build into a telecommunications network to meet demand without congestion. However there is a tool for dimensioning network links and exchanges. It is the rather complex statistical science of teletraffic theory sometimes called teletraffic engineering . This is the subject of this chapter. We begin with the teletraffic dimensioning method used for circuit-switched networks first published in 1917 by a Danish scientist A. K. Erlang. Erlang defined a number of parameters and developed a set of formulae which together give a framework of rules for planners to design and monitor the performamce of telephone telex and circuit-switched data networks. The latter part of the chapter deals with the dimensioning of data networks reviewing the techniques in such a way as to offer the reader a practical method of network design. TELECOMMUNICATIONS TRAFFIC Traffic is the term to describe the amount of telephone calls or data messages conveyed over a telecommunications network but this could cover any number of different scientific definitions. Possible definitions of traffic include the total number of