Control functions in telephony

Phonic information can be sent from a particular point to a distant point by using a telephone. Transmission bridges and cradle switches are two important components of a telephone. Transmission bridges which are nothing but mutual inductance circuits are kept at a local switching office. Cradle switches are employed to provide a connection between handset and exchange. It is very important to carry out some control functions before and during the speech signal is transmitted. The major control functions are signaling, hunting, preparing, holding, releasing, supervising and metering. By ‘signaling’ function, progress of the call is informed to caller and the receiver. For this, different signals used are dial tone, ringing tone, busy tone and available tone. ‘Hunting’ is a search operation for finding a free switch at a particular switching stage. In the process ‘preparing’, switches and links are prepared for its function when they are free. ‘Holding’ helps to avoid external interference during a call. ‘Releasing’ function helps to release from one call and thus enabling other calls if was in waiting. ‘Supervising’ function supervises the devices involved during a call to maintain the connection without disruption. The registry and billing process is done by ‘metering’.