Highgate Wood telephone exchange

It was built in Grand Avenue, in the London suburb of Muswell Hill,[1] by members of the Joint Electronic Research Council (JERC).

Although different contractors had been responsible for the system design, installation and manufacturing of various sections of the exchange, the commissioning had been the work of teams drawn from each of the parties to the agreement.

Two of them were based on slightly different applications of the time-division pulse-amplitude modulation (TDM) principle, and the third, at Leighton Buzzard, on space-division switching.

The model of Highgate Wood had worked "reasonably satisfactorily" [see Harris 2001] in the laboratory but the analogue transmission was too noisy on the long cable runs in an actual exchange.

TDM exchange operation in the UK did not become possible until pulse-code modulation (PCM) was developed, providing a digital transmission solution which led to System X.

Roy Harris states that it carried mainly artificial traffic and was looked after on a care-and-maintenance basis until it was taken out of service in 1965.

This led to the implementation of reed relay and crossbar technologies alongside Strowger extensions until the arrival of digital SPC exchanges in the mid-80s.

It was also decided that the experimental equipment should provide the full service and maintenance facilities of a comparable electro-mechanical exchange.

For these reasons the Highgate Wood installation was bigger than the small number of lines would have seemed to warrant and about 400,000 electronic components were fitted.

The installation also used time-sharing in the setting-up of the calls and in their control, which was something that all subsequent digital exchange design would follow.

At the time in Strowger exchanges ranks of selectors were interconnected by means of a complex trunking system dictated by the method of setting up the calls and by the need to economise in the total number of switching points.

For this purpose each subscriber's line termination was examined for a period of 280 microseconds every 224 milliseconds, a process known as scanning.

This capacity was not fully exploited at Highgate Wood although the register and most of the control equipment was potentially capable of dealing with the traffic from a large metropolitan exchange.

It monitored the lines by scanning the "highways" successively, examining each pulse channel for a period of one microsecond.

Both the register and the supervisory equipment had "persistence timers" which, in effect, substituted the B and C relays and the S and Z pulses of the Strowger system.