Q. Common control switching system?

Common Control Switching System: A functional block diagram of a common control switching system is displayed in Figure. Control functions in a switching system may be placed under four wide categories:

(a) Event monitoring.

(b) Call processing.

(c) Charging.

(d) Operation and maintenance

Events occurring outside exchange at the line units, trunk junctions and inter exchange signaling sender/receiver units are all monitored by control subsystem. Typical events include all requests and call release signals at the line units. Occurrences of events are signalled by operating relays that initiate control action. Control subsystem may operate relays in the junctions, receivers/senders and line units and hence command these units to perform certain functions. Events monitoring can be distributed. For illustration, line units themselves may initiate control actions on occurrence of certain line events. Whenever a subscriber goes off-hook, event is sensed, calling location is determined and market for dial tone and register finder is activated to seize a free register. Identity of calling line is used to determine line category and class of service to which the subscriber belongs. A register appropriate to line category is chosen, which then sends out dial tone to the subscriber, in readiness to receive dialing information. As soon as initial digits (usually 2-5) which identify exchange are received in register, register continues to receive remaining digits. Initial translator determines the route for call through the network and decides whether a call should be put through or not. It also determines charging methods and rates applicable to the subscriber. Initial translation can also take into account instructions from operating personnel and information regarding status of the network.