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The first exchange to switch Pulse Code Modulation (PCM) signals from one group of lines to another, in digital form, was demonstrated at the Post Office Research Station on 15th February 1968. |
History
The ideas for digital switching go back to the time of World War II and are largely attributed to Tommy Flowers who is famous for the Colossus code breaking computer. Learn more about Colossus.
Alec Reeves of Standard Telephones and Cables (STC) invented Pulse Code Modulation (PCM) and a patent was granted as early as 1939. PCM was a means to send multi-channel signals over a high gain pair of cables. Typically, coaxial cables were the transmission medium and PCM systems evolved to become the backbone of inter-exchange signalling, until fibre optic cables were developed. Learn more about PCM (later).
On 15th February 1968 an exchange, using Time Division Multiplexing (TDM) and PCM was demonstrated at the Post Office Research Station at Dollis Hill. This digital tandem exchange was moved into Empress, West Kensington, London in April 1968 to switch calls from Acorn, Ealing and Shepherd's Bush. It was the first exchange to switch PCM signals from one group of lines to another in digital form.
Empress PCM
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The first exchange to switch junction calls using TDM and PCM was installed at Empress telephone exchange in April 1968. |
July 1980 saw the first System X junction tandem exchange installed in Baynard House and this marked the beginning of the Digital era. In the 1982 Network Master Plan, a network of 60 or so fully interconnected DMSUs (Digital Main Switching Units) were planned to form the very backbone of the evolving digital network... Learn more about The Digital Network (later).
System X
System X was the original specification developed by the British Post Office (B.P.O), Standard Telephone & Cables (S.T.C), General Electric Company (G.E.C.) and Plessey. The first production System X exchange, a junction tandem exchange TXD14, began working in Baynard House in July 1980.
System X designations are:
TXD01 Multiplexor
TXD02 Very Small Local Exchange
TXD03 Remote Concentrator Unit (RCU) parented on a DCCE (Digital Cell Centre Exchange).
MXD03 mobile version.
TXD04 Small Local Exchange
TXD05 as per UXD5
TXD06 Medium Local Exchange (MLE)
TXD07 Large Local Exchange (LLE)
TXD08 Digital Principle Local Exchange based on MLE above.
TXD09 Digital Principle Local Exchange based on LLE above.
TXD10 Medium Combined Trunk/ Local Exchange
TXD12 Medium Trunk Exchange
TXD13 Large Trunk Exchange
TXD14 Junction Tandem Exchange
System Y
System Y was the AXE10 version bought ' off the shelf ' from Thorn Ericsson and produced at factories in Brighton and Scunthorpe.
TXD20 an AXE10 exchange opened in Keybridge House, International Switching Centre, Vauxhall, London in 1984. And the first local AXE10, in Sevenoaks, Kent was brought into service just two years later (October 1986).
Other System Y designations are: TXD53 RCU parented on a central processor. TXD57 Large Local Exchange.
Next Generation Switches (NGS)
Towards the end of the 20th century the demand for telephone calls was never greater; phone-ins, 'chataway weekends', BT Together and more mobile to fixed network calls as everyone was keeping in touch far more.
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