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Restoration & preservation project

System X pre production concentrator

The single rack concentrator is a pre production prototype

System X pre production concentrator

The single rack concentrator on display at Rio de Janeiro. The exhibition featured many manufacturers of communications equipment, the British stand showing, amongst other things, System X and Prestel. British Telecommunication Systems Ltd was a short lived company set up with joint ownership by Plessey, GEC & STC to promote System X overseas.

What are we trying to do?

The Communications Museum Trust has a prototype System X concentrator rack and a selection of cards dating from 1979-80 which were used at the launch exhibition in Geneva in 1979 and a follow up exhibition in Rio in 1980. The rack and its cards have been unused since returning from Rio 35 years ago but we hope to restore the concentrator back to working order in isolation mode and hopefully eventually acquire the rest of a System X exchange to show a full working System X digital exchange.

As described in our eMuseum System X was the digital telephone switching system developed by the British Post Office and its 3 equipment suppliers (GEC, Plessey & STC) during the 1970s.

The first exchange was brought into service in 1980 and during the 1980s and 90s System X was used to replace the majority of the old analogue exchanges in the BT network (approximately 70%) with the remaining 30% using Ericssons AXE digital system.

A very small number of exchanges serving only a handful of customers used a third digital system called UXD5 (Unit eXchange Digital No 5). Restoration of a UXD5 (currently in storage) is a future project.

System X was also used by Kingston (upon Hull) Communications, Sky, Cable & Wireless, Jersey Telecom, Hong Kong, Barbados and some African countries but export orders were not as numerous as had been hoped.

System X was designed as a collection of subsystems such that each subsystem could evolve independently in design / internal implementation without affecting any other subsystem by having standardised interfaces but treating the internals as a 'black box'. This philosophy also suited the way Systen X was being developed - the design work could easily be divided up between the 3 companies by giving each company certain subsystems to work on.


In very simple terms digital exchanges are made up of 3 parts:

A core switching fabric called a 'route switch' or 'group switch' which connects 2MBit/sec digital lines together. Each of the 2Mbit/sec lines carry 30 calls simultaneously using time division multiplexing and lead either to other exchanges or to subscriber concentrators.

A subscriber concentrator terminates the customers phone lines, carries out analogue-digital conversion and concentrates them down onto 2Mbit/sec digital lines leading to the group switch. Concentration is needed for cost effectiveness - telephone traffic studies since telephony began have shown that each customers line is only in use for a very small percentage of the time therefore to provide enough exchange capacity for everyone to make a call at once is wasteful. The subscriber concentrator allows the core part of the exchange (the group switch) to be efficiently utilised.

The third part of a digital exchange is a central control computer often referred to as a 'processor'. The software running on the processor makes routing decisions and then instructs the group switch and concentrator what to do to make and break connections as required.

With System X the subscriber concentrator is called the Digital Subscriber Switching Subsystem (DSSS or DS3). This is what we currently have. Without a group switch or processor it can operate in isolation mode. This means it is capable of providing very basic telephone service between its own subscribers lines but not further afield. Calls are unbilled and more advanced facilities like Call Waiting, Ring back when free etc are not available.

The concentrator can be remote from the rest of the exchange - i.e in another exchange building many miles away. This allows a cost effective way of serving customers on smaller exchanges in villages or small towns without having to install a full System X exchange. This is called a remote concentrator unit (RCU).

Over time we would like to acquire a group switch (called the Digital Switching Subsystem - DSS) and central processor (called Processor Utility Subsystem - PUS)

System X also has various other hardware based subsystems as well, necessary for a fully functioning exchange, and we are keen to acquire whatever surplus / scrap equipment we can, especially equipment from the Analogue Line Termination Subsystem (ALTS) & Signalling Interworking Subssytem (SIS) as this provides interworking with old analogue exchanges which we also have in our collection.

Subscribers line unit (line card) for 4 lines

Subscribers line unit (line card) for 4 lines. You can see the components repeated for each of the 4 lines. These cards provide the interface between a phone line and the exchange and convert analogue voice to digital using Pulse Code Modulation

Line controller controls 8 line cards (32 lines). The line cards have an LED for each line to show if it is off-hook. The Line controller &  its 8 line cards form a line module of which there are 3 per shelf

A Line controller card controls 8 line cards (32 lines). The line cards have an LED for each line to show if it is off-hook. The Line controller & its 8 line cards form a line module of which there are 3 per shelf

MF4 Receiver cards detect tone dialling sounds and instruct the control system of the digit dialled. Pulse dialling is detected by the line controller card

MF4 Receiver cards detect tone dialling sounds and instruct the concentrator control system of the digit dialled. Pulse dialling is detected by the line controller card