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Digital switching

Geneva Telecom '79 Exhibition

System X is launched (1979) : enter System X

The System X family of digital exchanges marked a leap forward into the era of microprocessors.

System X was conceived by the Advisory Group on System Definitions, an alliance involving Post Office and industry groups set up in 1968 to define the shape of the future telephone network and how to get there.

Collaborative development between the Post Office and its three principal equipment suppliers - GEC, Plessey and STC - culminated in the first of the British-designed digital switching systems designated 'System X', first shown to the world at the international Telecom '79 exhibition in Geneva.

Launch of System X at Baynard House, 1980

System X goes live (1980) : the switch from analogue to digital

The first System X digital exchange was brought into service on July 1, 1980, at London's Baynard House and formally inaugurated in September of that year. This was a tandem junction unit which switched PCM traffic between around 40 of London's telephone exchanges. The following year a second System X exchange was brought into service at Woodbridge in Suffolk. This was a 'local' exchange connected directly to customer lines.

The development of 'System X' exchanges was the cornerstone of modernisation of the existing network, by replacing analogue exchanges with digital switching centres interconnected with digital transmission links. It enabled an increased variety of facilities and services to be made available to the telecommunications user, resulting in ISDN (Integrated Services Digital Network) and ISDN 2.

AXE10 telephone exchange

Ericsson digital system is adopted (1985) : Ericsson's System Y

In 1985 British Telecom placed an order worth around £100 million for an AXE 10 digital switching system manufactured by Thorn Ericsson. The idea was to adopt a competitive alternative to System X and, perhaps inevitably, it was designated 'System Y'.

The first AXE 10 exchange was opened the following year at Sevenoaks. The Ericsson system did more than just provide a commercial alternative to System X. It also allowed the modernisation of the network to be speeded up. The new exchanges provided the same range of extra facilities known as Star Services (later known as Select Services) as System X, including code calling, repeat last call, three-way calling, call diversion, call waiting, call barring, reminder call and charge advice.

'Value-added' services helped telecoms companies to increase network usage and stay profitable - greater capacity on networks was driving down the amount they could charge for the calls. 

Modern telephone exchange

The all-digital national network (1990) : the 20-year switch over

The first digital exchange appeared in 1968. But it was to be more than two decades before the last analogue exchange based on the old electromechanical Strowger system was finally closed down.

Modernisation of the trunk network began with the opening of System X exchanges in Birmingham, Coventry, Leeds and the City of London. The initial phase of modernisation was completed in November 1988 with the opening of the 53rd digital trunk exchange in Norwich.

When the last traditional electromechanical trunk exchange at Thurso, Scotland, was closed in July 1990, the BT long-distance telephone network became totally digital - the first national telephone system in the world to be so. 

Circuit Board from 'Pathfinder' digital telephone exchange - a Connected Earth artefact, now in the Milton Keynes Museum collection

'Pathfinder' digital exchange (c. 1975) : dipping a toe into digital switching

The change from the old-style Strowger technology to modern digital switching didn't happen overnight but involved a long process of research, development and testing. Many ideas were explored before the improved technology was firmly decided and Pathfinder represented part of this process of evaluation.

The Pathfinder switchboard was an advanced test model that was reliable enough to carry public telephone traffic. It was installed as a mini-branch exchange on the internal system at the Post Office's Martlesham research centre.

Pathfinder was never installed as a stand-alone system in its own right. Being a sub-section of a bigger network meant that it was never fully responsible for delivering all customer needs, but it served as a useful step on the road to the digital upgrade.

BT Laboratories, Martlesham Heath, after opening in 1975

BT Laboratories, Martlesham (1975) : new ideas, new location

The British telephone industry has a long tradition of innovation and, in 1975, Queen Elizabeth II opened a new research and development centre at Martlesham Heath in Suffolk dedicated to research and development.

The Martlesham facility replaced the old Post Office research station at Dollis Hill in north London and is now the home of BT Laboratories.

The location was perfect because the surrounding countryside is relatively flat and therefore ideal for testing the radio-based communication systems in vogue at the time. Before the Post Office moved in, it had been an RAF base, used for aviation research during the First World War and later as a base for fighter planes in the Second World War.

Today it is one of the most advanced centres for telecommunications research in Europe. It has supported many 'leading edge' developments, including BT's pioneering work on optical fibre technology and submarine cable transmission systems. Now called Adastral Park, it is host to a science park and is one of BT's five satellite earth stations.

fun and games

Can you beat our games? Explode equipment to see what's inside, hear the changing sounds of telecommunications, see how telecommunications designs have changed over time or send an e-postacard.

what's on

The UK's first permanent gallery dedicated to the history of information and communication technologies opens in the new Information Age gallery at London’s Science Museum.

audio history

Take a trip down memory lane with extracts of the interviews which have been recorded as part of the Connected Earth oral history programme.

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