System X (telephony)
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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.
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 Units 
System X covers 3 main types of telephone 'switching' equipment. There are many of these switches residing all over the United Kingdom. Concentrators are usually kept in local telephone exchanges but can be housed remotely in less populated areas. DLEs and DMSUs operate in major towns and cities and provide call routing functions.
Concentrator Unit 
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 Exchange (DLE). It also receives dialled information from the subscriber and passes this to the DLE 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 DLE is lost.
Each Line Module unit converts 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 kS/s 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/s 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 up to 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 synchronisation and signalling.
Concentrator Units can either stand alone as remote concentrators or be co-located with the Digital Local Switching Unit.
Digital Local Exchange 
The Digital Local Exchange (DLE) connects to the concentrator and routes calls to different DLEs or DMSUs depending on the destination of the call. The heart of the DLE is the Digital Switching Subsystem (DSS) which consists of Time Switches and a Space Switch. Incoming traffic on the 30 channel PCM 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. The Time Slot inter-connections are held in Switch Maps which are updated by Software running on the Processor Utility Subsystem (PUS).
Digital Main Switching Unit 
The Digital Main Switching Unit (DMSU) deals with calls that have been routed by DLEs or another DMSU and is a 'trunk switch', i.e. it is not connected to any concentrators. As with DLEs, DMSUs are made up of a Digital Switching Subsystem and a Processor Utility Subsystem, amongst other things. 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.
Processor Utility Subsystem 
The Processor Utility Subsystem (PUS) controls the switching operations and is the brain of the DLE or DMSU. It hosts the Call Processing, Billing, Switching and Maintenance applications Software amongst others. The PUS is divided into up to 8 'clusters' depending on the amount of telephony traffic dealt with by the DLE/DMSU. Each cluster of processors contains 4 Central Processing Units (CPUs), the main memory stores (STRs) and the backing store memory. The PUS was coded with a version of the CORAL66 programming language known as PO CORAL (Post Office CORAL) later known as BTCORAL.
Replacement System 
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 21st 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 X is currently undergoing something of a renaissance; although the Falklands & Jersey have moved onto Softswitch technology, operators such as BT, Virgin Media, Thus/Your Comms/Cable & Wireless have renewed their interest in keeping this "legacy network" switch operational for the foreseable future.
See also