Britain is saddle with System X developed by the UK Post Office (later to become British Telecom), GEC, Plessey, and Standard Telephones and Cables (STC), and first shown in public in 1979.
System X was the name of the UK's first national digital telephone exchange system.
It was developed by the UK Post Office (later to become British Telecom), GEC, Plessey, and Standard Telephones and Cables (STC), and first shown in public in 1979.
The first System X digital exchange started operation in 1979 in Woodbridge (near BT's Research HQ at Martlesham Heath). The last electromechanical trunk exchange (in Thurso, Scotland) was closed in July 1990 - completing the UK's transition to purely digital operation, and becoming the first national telephone system to achieve this.
System X covers 3 main types of telephone 'switching' equipment. Firstly, the concentrator, which converts analog signals used in the access network to digital binary signals used in the core network. It also multiplexes customer lines using time-division multiplexing so that less cabling can be used further into the network. Around 4000 lines can be multiplexed to run over 16 cables.
Secondly, the Digital Local Switching Unit (DLSU), which connects to the concentrator and routes calls to different DLSUs or DMSUs depending on the destination of the call. Thirdly, The Digital Main Switching Unit (DMSU) deals with calls that have been routed by DLSUs or another DMSU and is a 'trunk switch', i.e. it is not connected to any concentrators. Each switch is controlled by a Processor Utility Subsystem, aka Processor.
Many of these switches reside all over the United Kingdom. Concentrators are usually kept in local telephone exchanges, with DLSUs and DMSUs operating in major towns and cities. Most of the switches are now near to or over 20 years old, giving an idea of their reliability.
System X is to be replaced by BT's 21 Century Network (21CN). |
