SystemX

2nd national digital telephone exchange system to be used in the United Kingdom.

System X was developed by the UK Post Office (later to become British Telecom), GEC, Plessey (later to be Marconi), and Standard Telephones and Cables (STC).

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\ 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.

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 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.

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.

Replacement system\ 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.