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Empress, System X and the digital network
Post Office scientists scored another world first in 1968 with the first digital exchange, installed at West Kensington exchange in London. It was described as a major step towards a telecommunications system that is fast, efficient and cheap, and can carry any form of traffic — voice, vision and data. It's easy to forget how rapidly technology was developing: early in the Empress design, a study had to be made of which integrated circuits (ICs) would be suitable, and diode-transistor logic (DTL) was chosen — but by the end of development, transistor-transistor logic (TTL) was available with significant speed improvements. Empress remained in use for several years. Reusing many ideas from earlier solid-state experiments like Highgate Wood, its earlier problems were all but eliminated by using digital signals rather than noisy analogue ones, together with the newer components now available. The Empress exchange, as it was dubbed, acted as a tandem switching point, relaying calls between several other exchanges in West London such as Ealing and Shepherd's Bush.
The process of converting analogue signals to digital had been invented back in 1937 by British scientist Alec Reeves. Known as Pulse Code Modulation (PCM), it involves sampling the strength (amplitude) of the analogue signal at intervals, then encoding the sampled amplitude as a binary number that can be stored, switched and transmitted. Today PCM sits at the heart of every digital phone network, radio, TV, camera and piece of recorded music. Back in the '60s, the Post Office had begun using PCM digital lines between exchanges as a way of more efficiently carrying multiple phone circuits on one cable over long distances, without the noise and signal degradation of older analogue circuits. These inter-exchange circuits — "junctions" and "trunks" — still had to convert back to analogue at the distant end, since they were simply point-to-point wires: any time routing or switching was needed, the signal had to become analogue again, pass through an analogue exchange (Strowger, Crossbar, or one of the newer reed relay electronic exchanges), then convert back to digital to continue on its way.
Empress, 1968. The world's first digital exchange.
The obvious next step was a fully digital exchange, so phone calls could remain digital throughout the network, only needing to return to analogue near the customer. Empress was fully digital and fully solid-state, and proved to be the first switching of live telephone traffic digitally anywhere in the world. Its success spurred the development of a production-ready digital system that could be installed nationwide to replace not only the electromechanical Strowger and Crossbar systems, but also the relatively new reed relay analogue electronic systems too.
This system came to be known as System X.
From its inception in 1968, contracts with the three principal suppliers — GEC, Plessey and STC — were signed in 1973, and System X made its world debut at the Geneva Telecom Exhibition in 1979, seen as a great success for British Telecommunications. System X brought digital switching right across the country, with software-controlled ("Stored Program Control") exchanges designed to cover everything from small local exchanges to large trunk and international gateway exchanges.
System X was designed from the outset as a modular architecture of self-contained subsystems with clear boundaries, letting each evolve somewhat independently without breaking backward compatibility. This led to a range of subsystems, the most important being the Digital Switching Subsystem (DSS), the heart of a System X exchange, and the Processor Utility Subsystem (PUS), its central control computer.
The PUS evolved greatly over the years as new computer hardware became available. In 1987 a System X exchange broke the world record for simultaneous call handling capacity, under the watch of independent judges. Its control software has also evolved — largely rewritten in C (it was originally written in CORAL), with the real-time operating system replaced by the more modern, off-the-shelf LynxOS, a "Unix-like" real-time operating system from LynuxWorks.
The other major subsystem is the Digital Subscriber Switching Subsystem (DSSS), the part customers' lines connect to directly. The DSSS concentrates up to 4,096 lines down into between 2 and 16 digital links, each carrying up to 30 calls simultaneously.
The first System X local exchange opened in Woodbridge, Suffolk, in 1981, with installation progressing nationwide from 1983. Now the phone line was about far more than a conversation between two people — System X introduced services like three-way calling, automatic alarm calls, call barring, caller display, and ISDN. ISDN (Integrated Services Digital Network) extended the digital network from the local exchange all the way to the customer's premises using a digital line. Conventional phone lines are still analogue between customer and exchange, even on digital exchanges — this weak link in the chain reduces data throughput and introduces noise. ISDN, using a digital link end to end, offers the full data rate available over the phone network and crystal-clear sound. Each line runs at 64kbit/sec; ISDN let you multiplex 2 or 30 digital lines together as one connection, giving a variable data rate from 64kbit/sec up to 2Mbit/sec — at the cost of tying up 30 simultaneous phone calls' worth of capacity. ISDN became popular in radio broadcasting for remote interviews with crystal-clear sound, but never gained wide popularity for computer data transfer, mainly due to its cost.
Growth of System X was rapid — 55 exchanges went in over just 4 years. Early System X roll-out plans from 1983 show Crossbar and TXE4 expected to remain in use until 2014! The target was to replace all the large Strowger exchanges by 1992, which was achieved. The last Crossbar exchanges were replaced by 1994, the very last at Droitwich, Worcestershire. TXE2 exchanges and the very last Strowger exchanges were gone by June 1995, leaving just the TXE4 large analogue reed relay system operating in some areas. These too were replaced by March 1998, when the whole BT network became completely digital.
"Communications in the 90s" — from The Communications Museum Trust, on Vimeo.
Commemorative paperweight from March '98, celebrating the replacement of the last two TXE4 reed-relay analogue exchanges with digital ones — the point the BT network became entirely digital.
A System X exchange.
First System X exchanges
First System X exchanges brought into service (BIS), in order. The first two local exchanges were at Woodbridge, Suffolk, and Arrington, Cambridge. The first trunk exchange was at Cambridge.
| No. | Type & Location | BIS Date |
|---|---|---|
| 1 | DJSU Baynard House | July 80 |
| 2 | DLSU Woodbridge | July 81 |
| 3 | DJSU Baynard House (replaced switch No. 1) | July 81 |
| 4 | DLSU Arrington | Dec 81 |
| 5 | DMSU Cambridge | Dec 81 |
| 6 | DLSU Hale | Jun 83 |
| 7 | DMSU SEP* Coventry Spires | Dec 83 |
*System Enhancement Programme — the first evolution of the System X family. This was the first exchange brought into service after SEP.
DLSU = Digital Local Switching Unit, i.e. a local exchange (customer lines directly connected). DJSU = Digital Junction Switching Unit, a tandem exchange routing traffic between several DLSUs. DMSU = Digital Main Switching Unit, a trunk exchange handling long-distance traffic between many DLSUs and DJSUs.
If anyone has information on other switch BIS dates, wherever they fit in the table, please get in touch.
