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British Pioneers — the birth of electronic & digital switching
The end of the Second World War was something of a blessing for technologists who had given all their time, intellectual capacity and working life to the war effort. One of the most outstanding was Tommy Flowers, a researcher at the GPO Telecommunications Research Station at Dollis Hill, north London, and creator of Colossus — the first electronic computer, which, working at Bletchley Park, helped break the German code system and shorten the war.
In 1947 Tommy Flowers returned to telecommunications and started work on the signalling and control developments needed for Subscriber Trunk Dialling (STD). At this stage the telephone system was electromechanical (using Strowger switching), and there was a recognised need for something more reliable and flexible.
A lot of early research happened at Dollis Hill in the 1950s. Early experiments used the first transistors, which weren't yet reliable, though Pulse Code Modulation (PCM) was showing promise as a practical form of transmission. By the mid-1950s, switching research was diverted to the design and development of ERNIE (Electronic Random Number Indicator Equipment) for the launch of Premium Bonds — a major political initiative to encourage National Savings.
Following ERNIE, work on electronic switching moved forward, and a field trial took place using a combination of transistor and valve circuits at the Highgate Wood Exchange, Grand Avenue, Muswell Hill, London. The scale is striking: 3,000 valves, 26,000 transistors and 350,000 additional components, in what was the world's first solid-state electronic telephone exchange. The first call was made in September 1962, and the system was accepted by the Post Office in December 1962.
By the end of the 1960s, electronic components were getting more reliable, and the decision was taken to move to electronic exchanges, bypassing advanced mechanical systems like Crossbar. Strowger's time was running out — or so it seemed. But much of the early work on electronic switching didn't progress as planned, and Crossbar had to be adopted as a stop-gap measure after all. One of the main problems at Highgate Wood was that the solid-state components of the time couldn't handle analogue signals very well. Solid-state switching would have to wait a few more years for digital systems; meanwhile, research turned to reed-relay-based switching with solid-state control — a halfway house between electromechanical and electronic systems. Tommy Flowers didn't agree with this direction, and resigned from the Post Office.
Reed relay exchanges — a cross between crossbar & electronic
The problems with solid-state electronic switching led to the development of two systems using reed relays, dubbed TXE (Telephone eXchange Electronic). The use of reed relays was somewhat controversial — some developers didn't consider them properly "electronic," while others considered them electromagnetic diodes. Still, they enabled more advanced electronic control systems to be used for routing, more cost-effectively than the alternative Crossbar electromechanical systems.
Crossbar and reed relay exchanges both use the matrix, or crosspoint, principle: lines are arranged in a matrix so that any two can be connected together at the point where they overlap — the crosspoint. Replacing the large Crossbar mechanisms with small reed relays let exchanges of considerable size be built taking up far less space, consuming less power, and being manufactured more quickly and cheaply.
A reed relay consists of two small contacts in a glass tube, surrounded by an electromagnet coil. When the coil is energised, the contacts close — imagine one of these at each crosspoint in a matrix, with the control system connecting two lines together by energising the coil at their crosspoint.
A field trial went successfully into service in 1960 as TXE1, at Leighton Buzzard, operating well and remaining in use until 1977. TXE1 was designed for large exchanges (over 2,000 lines); a system economic for smaller exchanges, dubbed TXE2, was also developed, trialled at Peterborough and Leamington Spa. This was successful, and full production was undertaken by Plessey, with the first TXE2 exchange in place at Ambergate in 1966. The Communications Museum Trust holds what we believe is the only remaining part of the Ambergate TXE2; the only preserved and working TXE2 is at The National Telephone Kiosk Collection @ Avoncroft.
The hundredth TXE2 was installed in 1970, and several hundred were in use by the mid-1970s, serving villages and small towns. It was also the first time electronic switching had been economical for small exchanges, and many systems were successfully exported abroad by UK manufacturers.
Following the widespread rollout of TXE2s for small exchanges, and the success of the prototype large reed exchange TXE1 at Leighton Buzzard, planning was in place for TXE3 — a refined version of a large reed exchange for nationwide installation. It never happened: TXE3 was deemed too expensive, and a revised design, TXE4, succeeded it as the production model, later developed further into TXE4A. The first TXE4 was installed at Sutton Coldfield in 1976, some ten years after the first TXE1 trial. The delays meant Crossbar had to keep meeting demand for telephone lines at large exchanges through the late '60s and early '70s. By the mid-1980s, some 8 million lines of reed electronic exchanges had been installed.
