Signalling System WB700

Background

The WB700 had nothing to do with Civil Defence, but carried call charging information to small exchanges. It utilised the WB400, 72KHz carrier for this purpose, which avoided the expense of a separate network of lines conveying the charging information.

Prior to the introduction of "Subscriber Trunk Dialling" (STD) in UK during the early 1960's, telephone customers could only dial local calls themselves, a charge of one to four units depending on radial distance was levied irrespective of the call duration. Calls to places further afield were connected via the operator. When STD was introduced, local calls were charged by duration but not radial distance. STD calls were charged by radial distance rings and duration. The time of day and day of the week affected the charging rates of all calls. Ordinary customers were charged at a diffent rate than coinbox customers and public callboxes in telephone kiosks.

To enable small exchanges to charge for local calls by duration, it was decided to generate the timing signals in the main exchange and distribute this out to smaller units, therby avoiding the expense of sophisticated timing equipment in every small exchange. The Signalling System WB700 was devised to distribute these 'ORDinary' and 'CoinBox' timing signals.

Detail

At the main exchange, timing signals from the multi phase pulse supply were fed to the WB700 oscillator and modulator contained within the early warning Carrier Control Exchange (CCE) equipment cabinet. Timing pulses for Ordinary customers were converted to a 44Hz sub audio signal and Coinbox timing pulse were converted to a 26Hz sub audio signal. The sub audio signals were used to modulate the 72KHz carrier generated at the police station (CCP) as it passed through the cabinet for distribution around the network. At the small exchange, a Receiver Carrier WB700 was connected into the WB400 distribution. It detected the sub audio tones and operated the appropriate relay to extend the timing signal to the local call timing devices.

Replacement

In the early 1980's, the planned replacement of the WB400 by the WB1400 meant a rethink in the generation of timing pulses. The fall in cost of electronic circuitry and the invention of the integrated circuit since the 1960's made a new solution possible. A custom designed duplicated electronic clock was installed in the small exchanges to provide the call timing information. This was known as an Equipment Pulse Generator 2A (EPG2A). These were used until 1994 when the last step by step exchanges were replaced with a modern digital network.