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System X was the name of the UK's first national digital telephone exchange system.

System X 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 at the Telecom 79 exhibition in Geneva Switzerland. In 1982 STC withdrew from System X and in 1988 the telecommunications divisions of GEC & Plessey merged to form GPT, with Plessey subsequently being bought-out by GEC & Siemens. In the late 1990s GEC acquired Siemens' 40% stake in GPT and in 1999 the parent company of GPT, GEC, renamed itself Marconi.

When Marconi was sold to Ericsson in January 2006, telent plc retained System X and continues to support it as part of its UK services business.

The first System X unit to enter public service was in September 1980 and was installed in Baynard House, London and was a tandem junction unit which switched telephone calls amongst ~40 local exchanges. The first local digital exchange started operation in 1981 in Woodbridge, Suffolk (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 trunk network transition to purely digital operation, and becoming the first national telephone system to achieve this. The last electromechanical local exchanges, Crawford, Crawfordjohn and Elvanfoot, all in Scotland, were changed over to digital on 23 June 1995 and the last electronic analogue exchanges, Selby, Yorkshire and Leigh on Sea, Essex were changed to digital on 11 March 1998.

System X covers 3 main types of telephone 'switching' equipment. 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.

The Concentrator Unit consists of four main sub-systems, Line Modules, Digital Concentrator Switch, Digital Line Termination (DLT) units and Control. Its purpose is to convert speech from analogue to digital and concentrate the traffic for onward transmission to the Digital Local Switching Unit (DLSU). It also receives dialled information from the subscriber and passes this to the DLSU so the call can be routed to its destination. In normal circumstances it does not switch signals between subscribers' lines but has limited capacity to do this if connection to the DLSU is lost.

The Line Module units convert analogue signals from a maximum of 64 subscriber lines in the access network to the 64 kilobit digital binary signals used in the core network. This is done by sampling the incoming signal at a rate of 8 kHz and coding each sample into an 8 bit word using Pulse Code Modulation (PCM) techniques. The Line Module also strips out any signalling information from the subscriber's line (e.g. dialled digits) and passes this to the Control. Up to 32 line modules are connected to a Digital Concentrator Switch unit using 2 Mbit highways giving each Concentrator Unit a capacity of up to 2048 subscriber lines. The Digital Concentrator Switch multiplexes the signals from the Line Modules using time-division multiplexing and concentrates the signals onto 30 time slots on 32 channel high speed highways for connection to the Digital Local Switching Unit via the Digital Line Termination units. The other two time slots on each highway are used for control and signalling.

Concentrator Units can be either stand alone or co-located with the Digital Local Switching Unit.

The Digital Local Switching Unit (DLSU) connects to the concentrator and routes calls to different DLSUs or DMSUs depending on the destination of the call. The heart of the DLSU is the Digital Switch which consists of Time Switches and a Space Switch. Incoming traffic on the 30 channel highways from the Concentrator Units is connected to Time Switches. The purpose of these is to take any incoming individual Time Slot and connect it to an outgoing Time Slot and so perform a switching and routing function. To allow access to a large range of outgoing routes, individual Time Switches are connected to each other by a Space Switch. A Processor Utility Subsystem (PUS) controls the switching operations and is the brain of the switch. The PUS is divided into up to 8 'clusters' depending on the amount of traffic dealt with by the switch. Each cluster contains 4 Central Processing Units (CPUs) and also the memory stores of the processor (STRs) and also backing stores of this memory. The PUS was coded with a special real-time version of the CORAL66 programming language.

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. As with DLSUs, DMSUs are made up of, amongst other things, a digital switch 'block' and a processor. In the British PSTN network, each DMSU is connected to every other DMSU in the country, enabling almost congestion-proof connectivity for calls through the network.

Many of the switches are now near to or over 20 years old, giving an idea of their reliability.

System X is in the process of being replaced by BT's 21 Century Network (21CN). Many of the other main users of System X - in particular Jersey Telecom and Kingston Communications - have already replaced the bulk of their circuit switched networks with Marconi softswitches and Access Hub multiservice access nodes

• System Y
• AXE telephone exchange
• TXE