UK telecoms history
General ideas
A telecommunications system does two things:
- Concentrates traffic (phone calls and internet access) into manageable ‘chunks’ that make economic use of resources.
- Switches the phone call or internet access, from its origin in the network to its destination. For economic reasons switching is concentrated in particular places in the network.
- Provides access to the network from homes, business premises and public spaces.
The concentration of traffic and its switching do not have to happen in the same physical location. The reason for concentration and switching is to make economic use of the equipment and the plant that connects sites together. If the equipment and plant is kept busy nearly all the time then it is being economically used.
On these pages you will find information on; Manual , Electromechanical (Strowger and Crossbar). Electronic, Digital and Future exchanges.
In the beginning
There were manual exchanges, which fall into two main types:
- Local Battery.
- Central Battery.
Electromechanical
There are two main types of electromechanical telephone switching used in public switching systems Stowger and Crossbar..
Strowger
Strowger is an electromechanical telephone switching system, known by that name in the UK and some English-speaking territories, and as ‘step’ or Step-by-Step (SxS) in North America. These systems used a ‘space’ switch that was under individual relay control; some exchanges had an element of common control.
Read more: Electromechanical switching systems
Crossbar
Peter Walker gave a talk on Crossbar switching at the THG’s AGM in 2000. In his article, Peter describes the principles of Crossbar switching and the history of the system. These systems used a ‘space’ switch made into a matrix of relay type contacts. Operation of the contacts was by joint activation of horizontal and vertical magnets under relay common control.
Read more: Crossbar Switching Systems
| Exchange | Dates | Notes |
| TXK1 | Plessey 5005 in service from 1968-1994. | Also made by GEC, used in London Sector switching centers with GEC control system. |
| TXK2 | Plessey 500T, used a s 4wire international gateway exchange. | Wood Street, Mondial and DeHaviland International Switching Centres in London. |
| TXK3 | STC BXB 1100, (British XBar). British version of French Pentaconta system. From 1971. | Local exchanges in Director areas. |
| TXK4 | STC BXB 1121 4 wire trunk exchange used in the ‘transit’ network up to 1980. | Replaced by System X by 1980 |
| TXK5 TXK6 | LM Ericsson ARM200 and AKE 13 international gateway exchange. | — |
| STC=Standard Telephones & Cables | GEC=General Electric Company |
Electronic
The history of TXE exchanges to follow in full later. These exchanges had a ‘space’ switch using small reed relays to make a crossbar like switching matrix. The common control of the exchange was electronic and ranged from discrete component architecture to systems using integrated circuits.
| Exchange | Dates | Notes |
| TXE1 | Development started 1963, Used at Leighton Buzzard 1968. Small Exchanges. Heavily based on the AEI REX (reed electronic exchange) design being introduced by Post Office Telecoms at the time of the GEC take over of AEI. | Prototype, developed by AEI, AT&E and STC. |
| TXE2 | Development 1963, Prototypes at Peterborough and Leamington Spa 1965. First production type at Ambergate (Derbyshire) 1966. | Developed by Plessey, commercially sold as PENTEX. |
| TXE3 | Cost reduced TXE1 development 1964 for exchanges greater than 2000 subs. Used a test bed at Amour House in Telecommunications HQ, trailed there 1969-1970. First exchange installed at ROYal in London, but scrapped before it came into service. | AEI designed and built but fell foul of GEC take over who withdrew from contract. |
| TXE4 | Cost reduced TXE3 developed by STC in 1974, also called TXE4 RD (Rectory Design) after its first installation at Rectory exchange in Birmingham 1976. Large exchange for up to 40,000 subscribers or 5,000 Erlangs of traffic. | Taken out of service 1998 |
| TXE4A | Improved design of TXE4 by STC in 1975, first installed in 1981 at Leicester. | Last example taken out of service in 1998 |
| TXE4E | Improved software to give same facilities as System X or Y. | — |
| TXE5 | Never produced. | — |
| TXE6 | Extension to Strowger exchanges. | Used between 1971 and 1976 |
| AEI=Associated Electrical Industries. AT&E=Automatic Telephone and Electric GEC=General Electric Company. STC=Standard Telephones & Cables |
Digital
Digital exchanges switch and control calls completely electronically. The switch is a time domain highway on which all the traffic sits sharing the highway with all other calls and supervisory signals. It has its routes in techniques developed in the early 1950’s in the ‘Highgate Wood’ experimental exchange of the GPO.
The future
The future of telecommunications for speech lies in the amalgamation of telecom methods and techniques from the World Wide Web.
Telephone boxes
Can’t tell your K2 from your K6? is it a Scottish phone box? All these and more questions about the UKs best loved or abused Public Telephone Kiosks.