tailieunhanh - Mạng và viễn thông P7

Setting Up and Clearing Connections The establishment of a physical connection across a connection-oriented network relies not only on the availability of an appropriate topology of exchanges and transmission links between the twoend-pointsbutalso on thecorrectfunctioning of alogical call set-upand ‘cleardown’ procedure. This is the logical sequence of events for establishing calls | 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 __7 Setting Up and Clearing Connections The establishment of a physical connection across a connection-oriented network relies not only on the availability of an appropriate topology of exchanges and transmission links between the two end-points but also on the correct functioning of a logical call set-up and cleardown procedure. This is the logical sequence of events for establishing calls. It includes the means by which the caller may indicate the desired destination the means for establishment of the path and the procedure for subsequent cleardown . In this chapter we discuss these call control capabilities of connection-oriented networks and we shall describe the related principles of interexchange signalling and review various standard signalling systems. We commence by considering telephone networks. We move to data networks and finally to connectionless networks. ALERTING THE CALLED CUSTOMER Figure shows the very simple kind of communication system which we have considered earlier in this book two telephones are directly connected by a single pair of wires without any intervening exchange. The users of the system in Figure A and B are able to talk at will to one another without fear of interruption. The problem with the equipment illustrated is the difficulty of alerting the other party in the first place to bring him or her to the phone. One easy solution would be to connect a bell at both ends in parallel with each telephone set. If the bell is designed to respond to a relatively high alternating current whenever such a current is applied from the calling end the bell at the called end rings. This was the earliest form of signalling used on telephone networks. The alternating current properly called ringing current was applied at the