tailieunhanh - Mạng và viễn thông P6

In this chapter we deal with the mechanics of the switching process, describing the sequence of functionsnecessary to establishconnectionsacross a cover the principles of circuit switching (as would be used in voice or circuit data networks) as well as the statisticalmultiplexing switching techniques of packet and cell switching. In addition, we describe in outline some of the better known types of switch technology. | 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 6 The Principles of Switching In this chapter we deal with the mechanics of the switching process describing the sequence of functions necessary to establish connections across a telecommunications network. We shall cover the principles of circuit switching as would be used in voice or circuit data networks as well as the statistical multiplexing switching techniques of packet and cell switching. In addition we describe in outline some of the better known types of switch technology. CIRCUIT-SWITCHED EXCHANGES In circuit-switched networks a physical path or circuit must exist for the duration of a call between its point of origin and its destination and three particular attributes are needed in all circuit-switched exchanges. First the ability not only to establish and maintain or hold a physical connection between the caller and the called party for the duration of the call but also to disconnect clear it afterwards. Second the ability to connect any circuit carrying an incoming call incoming circuit to one of a multitude of other outgoing circuits. Particularly important is the ability to select different outgoing circuits when subsequent calls are made from the same incoming circuit. During the set-up period of each call the exchange must determine which outgoing circuit is required usually by extracting it from the dialled number. This makes it possible to put through calls to a number of other network users. Third the ability to prevent new calls intruding into circuits which are already in use. To avoid this the new call must either be diverted to an alternative circuit or it must temporarily be denied access in which case the caller will hear a busy or engaged tone or the data user will receive an equivalent message or signal. Exchanges are usually designed as an .