System X (telephony)

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System X was the 2nd national digital telephone exchange system to be used in the United Kingdom. The first was a UXD5-Glenkindie Scotland 1979.[1]



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 and develop 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 around 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[edit]

System X covers three main types of telephone switching equipment. Many of these switches reside 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[edit]

The concentrator unit consists of four main sub-systems, line modules, digital concentrator switch, digital line termination (DLT) units and control unit. Its purpose is to convert speech from analogue signals to digital format 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 that the call can be routed to its destination. In normal circumstances, it does not switch signals between subscriber lines but has limited capacity to do this if the 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/s 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 line, e.g., dialled digits, and passes this to the control unit. Up to 32 line modules are connected to a digital concentrator switch unit using 2 Mbit/s paths, giving each concentrator 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 paths for connection to the digital local switching unit via the digital line termination units. The other two time slots on each channel 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[edit]

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 Subscriber Switching Subsystem (DSSS) 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). The nature of the Time Switch-Space Switch architecture is such that the system is very unlikely to be affected by a faulty time or space switch, unless many faults are present.

Digital main switching unit[edit]

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. In inner London, larger versions of the DMSU exist and are known as DJSU's. The DMSU network in London has been gradually phased out and moved onto the larger DJSU switches over the years as the demand for PSTN phone lines has decreased as BT has sought to reclaim some of its floor-space.

Processor utility subsystem[edit]

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.

The original processor that went into service at Baynard house was known as the MK2 BL processor. It was replaced (though not perhaps in Baynard House?) by the POPUS1. One of these was in Lancaster House in Liverpool. Later, these too were replaced with a much smaller system known as R2PU or release 2 processor utility. This was the 4 CPU per cluster and 8 cluster system described above. In more recent times, some of the clusters were replaced with more modern hardware, akin to late-1990's computer technology, while the original processing clusters 0 to 3 were upgraded in many ways. There were many very advanced features in the system that explain why these are still in use today like self fault detection and recovery, battery backed ram disks, mirrored disk storage, auto replacement of a failed memory unit, the ability to trial new software and roll back to the previous version and a bespoke instruction set.

Replacement system[edit]

Many of the switches installed during the 1980s are near to or over 30 years old and still in use within local exchanges, giving an idea of their reliability.

System X was scheduled for replacement with Next Generation softswitch equipment as part of the BT 21st Century Network (21CN) programme. Some other users of System X – in particular Jersey Telecom and Kingston Communications – replaced their circuit switched System X equipment with Marconi XCD5000 softswitches (which are the NGN replacement for System X) and Access Hub multiservice access nodes. However, the omission of Marconi from the BT 21CN supplier list, the lack of a suitable replacement softswitch to match System X reliability, and the shift in focus away from telephony onto broadband – all led to much of the System X estate being maintained.

See also[edit]