Connection & Disconnection of a call

Connection & Disconnection of a Call    The main task of the local exchane is to set up a call i.e. to connect calls between subscribers
   connected within the same exchange and further on in the network, to disconnect the call
   when the subscribers terminates and to charge the subscribers for the call.

In order to execute this task, the telephone exchange should be able.......

   to detect the hook off signal and identify the calling party (A-party).
   to receive and store the subscriber number called (B-party).
   to analyse the B number and connect to suitable outgoing line .
   to detect the B-party's telephone line status and to send the engaged tone if busy.
   to send the ringing tone to A subscriber and the ringing signal to B subscriber.
   to detect when B subscriber answers and switch off the ringing signal and tone.
   to connect both A and B subscribers together so that conversation could begin.

   When both the subscribers are in speech position their subscriber lines are under constant
   scanning. When either party terminates the call by putting down the handset, the exchange could
   then carry out the necessary measures in disconnecting the call. The call can be terminated
   by either party i.e. A or B can hangs up first. Different procedure should be carried in this two
   different cases.

   One common method is to consider the conversation closed when A (the calling subscriber)
   puts his handset down. Disconnection is then carried out immediately. On the other hand, if it
   is B who puts his handset down first, it may mean that the call is ended. But it is not absolutely
   certain. The exchange therefore connects a time supervision of about 90 seconds before the
   call is disconnected immediately.

Another method used is to have the time supervision carried out no matter who put the handset
down first. The earlier method is more widely used. Time supervision is shown in Figure 30.

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