oop to the local exchange. For switched services, such as those provided for telephony, telex and packet switching of data, the transmission links provide circuits which are temporarily connected together as required by switching equipment at the nodes. Private circuits leased from the PTO, either analogue or digital, are provided by circuits in the same transmission bearer network. However, these circuits are connected together in a semi-permanent fashion at the nodes in order to provide fixed routings. In a PSTN, in order to route the call to the required address, the Figure 29.2 Simple network configuration: (a) fully calling subscriber must signal to the exchange that a call is required interconnected network; (b) star network; (c) partially interconnected network and then send it to a signal indicating the number of the called subscriber. At the end of the call, the subscriber sends a 'clear' signal causing the switches to release the circuits used in the connection. information to be transmitted between the exchanges at all these In the case of the simplest telephone call, the call and clear signals nodes (Welch, 1981). are respectively a loop and a disconnection of the subscriber's line, It can therefore be seen that a telecommunication network may be produced by the 'off hook' and 'on hook' states of the telephone considered to consist of the following interacting sub-systems: handset. The called number is sent by a succession of disconnection- S produced by the telephone dial. A more complex call may be Transmission systems. routed through several nodes. It is then necessary for this signalling 2. Switching systems. Transmission Bearer Network Customer Figure 29.1 Provision of different services over common transmission bearer networks 29/4 Telephone networks In a multi-exchange area, subscribers connected to each exchange will wish to communicate with subscribers connected to other exchanges. Thus, there is a need for circuits between the exchanges. These circuits are called junctions. The number of jun