TXE - Introduction

Introduction

Just before World War II, Tommy Flowers MBE, employed at the GPO, had been working on VF (Voice Frequency) signalling, using valves, and this had led him to realise that valves could be very reliable if not switched on and off. This gave him the confidence during the War to build the world's first digital computer called Colossus at Bletchley Park. This computer used about a thousand valves and was a major factor in helping British Intelligence to break the German codes, though details of this were only released in 2000. After the War, the success of Colossus encouraged him to contemplate the possibility of telephone exchanges each using tens of thousands of valves. He was told that this was impossible and he could not say he had already done it with Colossus because he was bound by the Official Secrets Act. However a fully electronic prototype Time Division Multiplex Model Exchange was constructed at the Post Office Research Station at Dollis Hill and then an experimental TDM exchange system was built and tested at Highgate Wood, North London in 1962 but it was found to be beyond the technology of the time : the solid state switching worked well, but the analogue transmission (which had worked on the short cable runs of a laboratory model at Dollis Hill) was too noisy for public service on the long cable runs of a large exchange. However, the principles would be used later, as transmission became digital, in the development of digital exchanges the world over including System X.

Siemens Brothers Ltd (later taken over by Associated Electrical Industries Ltd who renamed each section accordingly e.g. AEI Telecoms) had set up an electronic switching lab at Blackheath in London. This lab was headed by John Flood, who had been a founder member of Tommy Flowers' electronic switching team at Dollis Hill. In the Siemens team was an engineer called Jim Warman, who was probably the most talented switching system design engineer of that generation. It was his trunking ideas (sectionalisation, serial trunking, line scanning, route choice, repeat attempt etc.), which were to be central to the development of the British TXE exchanges.

Following the failure to win major contracts in Australia in 1960 and the subsequent failure of Highgate Wood, it was necessary for the British manufacturers to come up with something different. L M Ericsson of Sweden had 20 years of experience of manufacturing the crossbar system and reducing its cost, so there was no point in trying to compete with them (Plessy Telecommunications, a subsidiary company of The Plessey Company, took a different view and continued to urge the GPO to adopt crossbar). At this time, in the USA, Bell Labs were developing a system based on electronically controlled reed relays and this looked promising. One of Ericsson's marketing points for crossbar was that it used precious-metal contacts, but reed-relays would be even better as their precious metal contacts were hermetically sealed. Also their very short operating and release times (<1ms) made them ideal for electronic control and these reed-electronic exchanges were considered to be the most practical switching system to proceed with at the time and electronic enough, until a truly electronic system could be developed, although Tommy Flowers did not approve as he advocated going straight to a digital system.

The manager at AEI (W G Patterson) decided that reed-electronic space-division switching was the way to go and it was then that the term 'TXE'(Telephone Exchange Electronic) was coined, even although the reed relays themselves were not regarded as electronic components.

A much bigger team was needed to undertake the detailed development and AEI persuaded AT&E and STC to join them in the work. The initial result of their work was a prototype system called TXE1.