sconnection) instructions have to pass between the terminations and the exchange and also between exchanges for interexchange (junction) calls. These supervisory messages are conveyed by the exchange signalling systems which may transmit the relevant data over the same paths as the calls they control or they may use separate links for the purpose. Also needed is a control system for interpreting these instructions to enable appropriate paths to be set through the switches. The operation and design of exchange switching systems can be Control conveniently separated into these three separate, but interdepend- ent, functions: switching, signalling and control. In practical terms, the choice of switching technology is very dependent upon the transmission methods used in the network concerned and this in turn Figure 31.10 Generalised architecture of a complete crossbar or electronic exchange influences the choice of signalling and control methods. 31/8 Principles of switching systems B. Strowger, an undertaker in Kansas City, after whom the system vidual speech channel even though no signalling information is is named. Since then several systems have been developed. The transmitted during the greater part of the call. The use of stored most successful of these employ mechanical devices called crossbar program control has given rise to a more efficient signalling means, switches or arrays of reed relays. Modern versions of these systems known as common channel signalling. In this, one signalling chan- use stored program control (spc) in which the operations to set up nel serves a large number of speech channels and consists of a direct each call are determined by software in the exchange processors data link between the control processors (computers) in the ex- (computers). changes concerned. Each new item of information relating to a call The new generation (digital) systems employ totally solid-state is sent as a digital message containing an address label identifying switching as well as stored program control. the 