Question

What is meant by common control? Explain all the categories that are served by Common Control switching.

Answer 1

In some switching systems, the control subsystem may be an integral part of the switching network itself. Such system is known as direct control switching systems. Those systems in which the control subsystem is outside the switching network are known as common control switching system. Strowger exchanges are usually direct control systems, whereas crossbar and electronic exchanges are common control system. All stored program control systems are common control systems. Common control is also known as indirect control or register control.

Common Control Switching System: A functional block diagram of a common control switching system is shown in Fig. The control functions in a switching system may be placed under four broad categories:

(a) Event monitoring.

(b) Call processing.

(c) Charging.

(d) Operation and maintenance 

Events occurring outside the exchange at the line units, trunk junctions and inter exchange signaling receiver/sender units are all monitored by control subsystem. Typical events include all request and call release signals at the line units. The occurrences of the events are signaled by operating relays which initiate control action. The control subsystem may operate relays in the junctions, receivers/senders and the line units, and thus command these units to perform certain functions. Events monitoring may be distributed. For examples, the line units themselves may initiate control actions on the occurrence of certain line events. When a subscriber goes off-hook, the event is sensed, the calling location is determined and market for dial tone, and the register finder is activated to seize a free register. Identity of the calling line is used to determine the line category and the class of service to which the subscriber belongs. A register appropriate to the line category is chosen, which then sends out the dial tone to the subscriber, in readiness to receive the dialing information. As soon as the initial digits (usually 2- 5) which identify the exchange are received in the register, the register continues to receive the remaining digits. 

The initial translator determines the route for the call through the network and decides whether a call should be put through or not. It also determines the charging methods and the rates applicable to the subscriber . Initial translation may also take into account instructions from the operating personnel and information regarding the status of the network. 

 

If a call is destined to a number in an exchange other than the present one processing the digits, the initial translator generates the require routing digits and passes them on to the register sender. Here the digits corresponding to the subscriber identification are concatenated and the combined digit pattern is transmitted over the trunks to the external exchange. Register sender uses appropriate signaling technique, depending on the requirements of the destination exchange. If the call is destined to a subscriber within the same exchange, the digits are processed by the final translator. The translation of directory number to equipment number takes place at this stage. The final translator determines the line unit to which a call must be connected and the category of the called line. The category information may influence charging and connection establishment. In some practical implementations, both initial and final code translator functions are performed by a single translator. Controlling the operation of the switching network is an important function of the common control subsystem. This is done by marking the switching elements at different stages in accordance with a set of binary data defining the path and then commanding the actual connection of the path. Path finding may be carried out at the level of the common control unit or the switching network.