n with signaling information (dialing) that is transmitted over the subscriber line. In the network, signaling is needed to transmit the control information of a specific call and circuits from one exchange to another. 2.6.1 Telephone Exchange The main task of the telephone or ISDN exchange is to build up a physical connection between subscriber A, the one who initiates the call, and sub- scriber B according to signaling information dialed by subscriber A. The speech channel is connected from the time when the circuit was established to the time when the call is cleared. This principle is called the circuit switch- ing concept and is different from packet switching, which has been used in data networks. In the past, the switching matrix was electromechanical and controlled directly by pulses from a telephone. Later, the control functions were inte- grated into a common control unit. Currently, the common control unit is an efficient and reliable computer or a multiprocessor system, including large amounts of real-time software. This kind of exchange is called a stored pro- gram control (SPC) exchange (Figure 2.9). Introduction to Telecommunications Network Engineering Exchange Lines or channels to the other exchanges Subscriber Switching (speech channels and lines matrix signaling channels in the case of channel associated signaling) Common control Common channel computer signaling connections to other exchange Channel associated signaling: Common channel signaling: each active telephone call has its own One data channel between exchanges signaling channel between exchanges is used for signaling purposes of all in addition to the speech channel telephone calls. This principle is similar to computer communications where data packets are transmitted between computers. Figure 2.9 SPC exchange and signaling principles used between exchanges. Every exchange between subscribers A and B connects a speech circuit according to signaling information that is received from a subscriber or from the previous exchange. If the e