System X: the prototype concentrator

A piece of System X from 1979–80

The Communications Museum Trust holds a pre-production System X subscriber concentrator rack, and a selection of its cards, dating from 1979–80. They were used at the System X launch exhibition in Geneva in 1979, and at a follow-up exhibition in Rio de Janeiro in 1980. The rack has also featured in our Flowers to Fibre exhibition.

System X in brief

As described on our Digital Switching page, System X was the digital telephone switching system developed by the British Post Office and its three equipment suppliers (GEC, Plessey & STC) during the 1970s.

The first exchange entered service in 1980. During the 1980s and 90s, System X replaced approximately 70% of the old analogue exchanges in the BT network; the remaining 30% used Ericsson's 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). The Trust's collection includes a UXD5.

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. Each could evolve independently, without affecting any other, because the interfaces were standardised and the internals treated as a "black box". This philosophy also suited the way System X was developed: the design work could easily be divided between the three companies by giving each certain subsystems to work on.

How a digital exchange works

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

1. A core switching fabric, called a "route switch" or "group switch", which connects 2Mbit/sec digital lines together. Each 2Mbit/sec line carries 30 calls simultaneously using time division multiplexing, and leads either to other exchanges or to subscriber concentrators. In System X this is the Digital Switching Subsystem (DSS).
2. A subscriber concentrator, which terminates the customers' phone lines, carries out analogue-to-digital conversion, and concentrates them down onto 2Mbit/sec digital lines leading to the group switch. Concentration is needed for cost effectiveness: traffic studies since telephony began have shown that each customer's line is in use for only a very small percentage of the time, so providing enough exchange capacity for everyone to call at once would be wasteful. The concentrator lets the core of the exchange be used efficiently.
3. A central control computer, often called the "processor". The software running on it makes routing decisions, then instructs the group switch and concentrators what to do to make and break connections. In System X this is the Processor Utility Subsystem (PUS).

The concentrator

With System X the subscriber concentrator is called the Digital Subscriber Switching Subsystem (DSSS, or DS3). This is the part the Trust's rack represents.

Without a group switch or processor, a concentrator can operate in "isolation mode". This provides very basic telephone service between its own subscribers' lines, but not further afield. Calls are unbilled, and more advanced facilities such as Call Waiting and Ring Back When Free are not available.

A concentrator can be remote from the rest of the exchange, in another exchange building many miles away. This is a cost-effective way of serving customers on smaller exchanges in villages or small towns without installing a full System X exchange, and is called a remote concentrator unit (RCU).

System X also has other hardware subsystems needed for a fully functioning exchange. The Analogue Line Termination Subsystem (ALTS) and Signalling Interworking Subsystem (SIS), for example, provide interworking with older analogue exchanges.

Inside the concentrator

1979 launch brochures