Group switching centre

A group switching centre (GSC) was a United Kingdom telephone exchange used to provide local and trunk routing to local exchanges within its own and if required, adjacent group areas. It formed a component of the trunk mechanisation hierarchy that was commenced in 1939. The concept of group switching centres became important with the introduction in the United Kingdom of subscriber trunk dialling (STD) in 1958, initially from Bristol central telephone exchange. They were typically located towards the centre of their corresponding group area, usually in a large town or city, and serviced a group of local and dependent telephone exchanges within their home and adjacent group areas. From 1 January 1958, the British network was divided into 639 charging groups. These areas provided the basis for tariff calculations for telephone calls.^[1]

History

Group switching exchanges were initially based on the Strowger step-by-step system as adopted by the British Post Office for the United Kingdom telephone network. Subsequent developments permitted the use of Crossbar switches designated as TXK1 which offered enhancements such as 2nd attempt routing; a facility not available on Strowger systems. Each of the 639 charging areas was associated or coterminous with one of 230 Group Switching Centres which contained register-translators (Also referred to as GRACE - Group Routing And Charging Equipment) required for local subscribers to originate self dialled trunk calls. During the introduction of International Direct Dialling (IDD), initially referred to as International Subscriber Dialling (ISD) within the UK, Group Switching Centres were equipped with international call timers and international register-translators to route calls to the relevant international exchange for onward routing abroad.

Prior to the introduction of Subscriber Trunk Dialling, most Group Switching Centres housed operators in an Auto Manual Centre (AMC) who were able to place any calls for subscribers in the local area that could not be directly dialled. Operators continued to be used after the introduction of Subscriber Trunk Dialling albeit on a reducing basis as direct dialled routings became increasingly available until universal direct dialling between U.K subscribers became available in 1979 via the Trunk Transit Network.

Functions

• Provide routing and charging control of inland subscriber dialled calls via register/translators.
• Provide routing and charging control of international subscriber dialled calls via international register/translators.
• Provide trunk routing based on trunk mechanisation and subsequently in conjunction with the Trunk Transit Network.
• Provide operator Services via the Auto Manual Centre (AMC).
• Provide inbound trunk routing to its own and, if required, adjacent charge groups.
• Provide an Emergency Manual Switching System (EMSS) for use during civil emergencies or nuclear war.
• Provide tandem routing of local and trunk calls as required.
• Provide local subscriber switching services.

References

1. The Scheme for Full Automation of the Telephone Service in the United Kingdom - Barron, D. A. 1959. Subscriber Trunk Dialling. The Proceedings of The Institution of Electrical Engineers. Vol. 106. Part B. No. 28. pp. 341–360.