Group switching centre


A group switching centre 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.
Group switching centres formed a component of the trunk mechanisation hierarchy that was commenced in 1939. The concept of group switching centres became important with the introduction of subscriber trunk dialling 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.

History

Group switching centres were required because it was not cost-effective to have direct circuits between every exchange in the country. The GSC serving the calling party would act as a concentrator for trunk calls and an operator or, if an STD dialled routing was available, it would establish the call to the appropriate GSC serving the called party's exchange.
The area code for the called party would be translated by a register/translator into the routing code that set the route through the GSC's for the calling party to the called parties' exchange. a maximum of 6 routing digits was possible hence giving restrictions on the number of exchanges that could be directly dialled.
Different technologies were used for the unit, the Type 1 using cold cathode devices and the Type 4 magnetic drum, suitable for use in Director areas. Types 2, 3 and 5 were electromechanical. Type 5 registers used PO type-4 uniselectors for storage, directly interfacing with sender/receivers that used SSMF2 multi-frequency signalling that was used on the Transit Network. Type 6 was an all-electronic unit, Type 10 was an electromechanical incoming register using PO type-4 uniselectors, accepting signals from SSMF2 Sender/Receivers and pulsing-out the digits as LD. Types 12, 13 and 14 were electronic, using various technologies.
Until the Transit System was introduced in 1971, directly-dialled call routings could only involve one intermediate GSC due to transmission requirements, all other calls requiring an operator-assisted routing. GSC's were initially based on the Strowger step-by-step system as adopted by the Post Office for the 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 required for local subscribers to originate self dialled trunk calls. During the introduction of international direct dialling, initially referred to as international subscriber dialling 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 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 UK subscribers became available in 1979 via the trunk transit network.

Functions