System X (telephony)

System X is the digital switching system installed in almost all telephone exchanges throughout the United Kingdom from 1980 until the 2020s.

System X was developed by Post Office Telecommunications (later to become British Telecom), GEC, Plessey, and Standard Telephones and Cables (STC), and was first shown in public in 1979 at the Telecom 79 exhibition in Geneva, Switzerland. STC withdrew from the project in 1982. In 1988, the telecommunications divisions of GEC and Plessey merged to form GPT, with Plessey subsequently being bought out by GEC and Siemens. In the late 1990s, GEC acquired Siemens' 40% stake in GPT. GEC renamed itself Marconi in 1999.

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, in September 1980, was installed in Baynard House, London and was a 'tandem junction unit' which switched telephone calls amongst some 40 local exchanges. The first local digital exchange started operation in 1981 in Woodbridge, Suffolk (near BT's Research HQ at Martlesham Heath). BT's 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.

In addition to the UK, System X was installed in the Channel Islands, and several systems were installed in other countries, although it never achieved significant export sales.

Separately from System X, BT developed the UXD5 ("unit exchange digital"), a small digital exchange which was cost-effective for small and remote communities. Developed by BT at Martlesham Heath and based on the Monarch PABX, the first example was put into service at Glenkindie, Scotland, in 1979, the year before the first System X. Several hundred of these exchanges were manufactured by Plessey and installed in rural areas, largely in Scotland and Wales. The UXD5 was included as part of the portfolio when System X was marketed to other countries.

System X covers three main types of telephone switching equipment. 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. The BT network architecture designated exchanges as DLEs / DMSUs / DJSUs etc. but other operators configured their exchanges differently depending on their network architecture.

With the focus of the design being on reliability, the general architectural principle of System X hardware is that all core functionality is duplicated across two 'sides' (side 0 and side 1). Either side of a functional resource can be the 'worker' with the other being an in-service 'standby'. Resources continually monitor themselves and should a fault be detected the associated resource will mark itself as 'faulty' and the other side will take the load instantaneously. This resilient configuration allows for hardware changes to fix faults or perform upgrades without interruption to service. Some critical hardware such as switchplanes and waveform generators are triplicated and work on an 'any 2 out of 3' basis. The CPUs in an R2PU processing cluster are quadruplicated to retain 75% performance capability with one out of service, instead of 50% if they were simply duplicated. Line cards providing customer line ports or the 2 Mbit/s E1 terminations on the switch have no 'second side' redundancy, although a customer can have multiple lines or an interconnect have multiple E1s to provide resilience.