This article is about the TXK telephone exchange system. For other uses, see TXK (disambiguation).
TXK (Telephone eXchange Crossbar) was a range of Crossbar exchanges used by the British Post Office telephone network, subsequently BT, between 1964 and 1994. TXC was used as the designation at first, but this was later changed as TXC sounded too much like TXE the code used for later electronic exchanges. Prior to this the GPO had standardised on Strowger for automatic switching and had resisted the adoption of Crossbar, preferring to wait for its electronic switching research to bear fruit. The development of electronic systems however took longer than anticipated and the British equipment manufacturers, particularly Automatic Telephone & Electric (ATE), which later became part of the Plessey group feared that continuing to focus the bulk of their production on Strowger equipment would harm their export sales as Crossbar had already become popular throughout the world.
In response to this, ATE, and later Plessey, developed their own crossbar system, the 5005, and pushed for the GPO to adopt it as an interim measure. Normally the GPO preferred to develop systems in co-operation with the manufacturing companies, from whom they could then purchase competitively rather than allowing one manufacturer to sell it a proprietary system. The situation however was becoming critical, waiting lists for telephone service in the UK were growing embarrassingly long and the manufacturers were becoming more and more reluctant to supply Strowger in the quantities needed by the GPO. Eventually the GPO relented and decided to accept Crossbar equipment into its network.
TXK1
This code was given to Plessey's 5005A switch, which was used for local exchanges in non-director areas or group switching centres / sector switching centres (tandem exchanges). The 5005A was a two wire version of the 5005, meaning the transmit and receive speech was routed through the switch over one pair of wires. The first one was installed at Broughton, Preston, in 1964 as a field trial replacing Broughton's manual exchange. The village was chosen due to its relative proximity to the Plessey factory and research centre at Edge Lane, Liverpool.
Mainstream installation of TXK1s commenced in 1968, and many were installed throughout the UK until the late 1970s when the more modern TXE4 electronic system became available. TXK1 remained in use in the BT network until March 1994, when the last one at Droitwich, Worcestershire was replaced with a digital exchange.
The Plessey 5005 was made up of routers and distributors. A distributor performed subscriber concentration and could serve up to 500 lines. A router made up the core of the exchange for switching traffic between distributors or between a distributor and an external junction/trunk. The exchange at Broughton was initially dimensioned to serve 2,000 lines and therefore had four distributors and one router. A 5005 could serve up to 100,000 lines, but no inland exchanges of this size were used in the UK.
TXK was also manufactured by GEC Telecommunications of Coventry based on the Plessey 5005 design. GEC provided Sector Switching Centres in the London director area at Ilford, Wood Green, Colindale, Ealing, Kingston, Croydon and Eltham. These exchanges were controlled by the GEC designed Mk 1c processor.
TXK2
Plessey 5005T (TXK2) at Paddington, Sydney, Australia
This code was used for Plessey 5005T exchanges which was a 4 wire version of the 5005 (separate pair for transmit & receive). In the UK TXK2s were only used as an international gateway, initially at Wood Street (WS) exchange in the City of London. This switch had been previously sold by Plessey to the Overseas Telecommunications Commission Australia (OTC) for use as an international gateway at Sydney. The British Post Office was planning with Plessey Ltd for a switch at Wood Street but with advanced facilities. Plessey was in difficulty with this development and could not make Wood Street work on time and in-budget. As OTC had replaced their 5005T with an Ericsson switch, so the 5005T was air-freighted back to the UK in desperation to be put into service at Wood Street. The 5005T was identical to the 5005A except for the 4 wire switching, the lack of subscriber line circuits and concentration stage (i.e. no distributors).
The switch which Plessey struggled with was also 4-wire but controlled by manual boards – the tallest boards ever made at the Beeston factory. The cords, of course, did not carry the speech. They controlled the routing from end to end via the crossbar 4-wire switches. The operators selected the route and the crossbar used mass-marking to find a path. One of the problems was the design of the high capacity relay. It was developed from the 3000 type, but with wire-wrap connections and more springs. The lifting comb proved to be difficult to make, and much Araldite was used to rectify the faults.
Plessey further supplied the British Post Office with TXK2 switches at DeHavilland ISC (Burnt Oak) and Mondial ISC in Central London. The first switch was similar technically to the Wood St Relief Unit, but the latter was larger and came with the developments of Codesender Router and Line Terminations for CCITT R2 (MF(C) with E&M line signalling).
TXK3
TXK3 was the code allocated to STC's BXB 1100, a 2 wire version of the French Pentaconta system. It was used by the GPO for local exchanges in director areas (London, Manchester, Birmingham, Liverpool, Edinburgh and Glasgow) and for some non-director exchanges in Northern Ireland. The first one opened at Edinburgh Liberton in 1971.
TXK4
This code was used for STC's BXB1121 which was a four-wire version of the French Pentaconta system, used by the British Post Office for main trunk exchanges. They were all replaced in the 1980s by System X.
TXK5 & 6
These codes were allocated to two Ericsson Crossbar systems used as international gateways in London. TXK5 was LM Ericsson ARM200 and TXK6 was the same manufacturer's AKE13 system.
Pulse dialing is a signaling technology in telecommunications in which a direct current local loop circuit is interrupted according to a defined coding system for each signal transmitted, usually a digit. This lends the method the often used name loop disconnect dialing. In the most common variant of pulse dialing, decadic dialing, each of the ten Arabic numerals are encoded in a sequence of up to ten pulses. The most common version decodes the digits 1 through 9, as one to nine pulses, respectively, and the digit 0 as ten pulses. Historically, the most common device to produce such pulse trains is the rotary dial of the telephone, lending the technology another name, rotary dialing.
In electronics and telecommunications, a crossbar switch is a collection of switches arranged in a matrix configuration. A crossbar switch has multiple input and output lines that form a crossed pattern of interconnecting lines between which a connection may be established by closing a switch located at each intersection, the elements of the matrix. Originally, a crossbar switch consisted literally of crossing metal bars that provided the input and output paths. Later implementations achieved the same switching topology in solid-state electronics. The crossbar switch is one of the principal telephone exchange architectures, together with a rotary switch, memory switch, and a crossover switch.
The General Electric Company, or GEC, was a major British industrial conglomerate involved in consumer and defence electronics, communications, and engineering. The company was founded in 1886, was Britain's largest private employer with over 250,000 employees in the 1980s, and at its peak in the 1990s, made profits of over £1 billion a year.
The Strowger switch is the first commercially successful electromechanical stepping switch telephone exchange system. It was developed by the Strowger Automatic Telephone Exchange Company founded in 1891 by Almon Brown Strowger. Because of its operational characteristics, it is also known as a step-by-step (SXS) switch.
The Plessey Company plc was a British electronics, defence and telecommunications company. It originated in 1917, growing and diversifying into electronics. It expanded after World War II by acquisition of companies and formed overseas companies.
System X is the digital switching system installed in telephone exchanges throughout the United Kingdom, from 1980 onwards.
GEC Plessey Telecommunications (GPT) was a British manufacturer of telecommunications equipment, notably the System X telephone exchange. The company was founded in 1988 as a joint venture between GEC and the British electronics, defence and telecommunications company Plessey. The next year, after a joint holding company of GEC and the German conglomerate Siemens AG acquired Plessey, GPT was converted into a 60/40 GEC/Siemens joint venture. The GPT name ceased to be used in the mid-1990s, and in 1998 the company was amalgamated into Siemens Communications.
SP-1 was the name of a computerized telephone exchange manufactured by Northern Electric in Canada. It was introduced in 1971
System Y is the terminology used by BT, the main operator of the telephone network in the United Kingdom, to refer to the Ericsson AXE digital switching system.
The British Approvals Board for Telecommunications (BABT) is a telecommunications certification body.
A marker is a type of special purpose control system that was used in electromechanical telephone central office switches. Switches employing markers belong to a class of switches known as "common control", as the purpose of a marker is to control the closure of contacts in the switching fabric that connect a circuit between the calling party and the called party. This is in contrast to "direct control" switches, where the switching elements were controlled directly by the customer's dial, such as the Step by Step switch. The term marker came from its use to mark a path of links through the switching fabric. A marker's comprehensive view of the switching fabric allowed it to find and assemble a path from one terminal to another, if the links were available, unlike the earlier graded progressive systems in which a path might not be found.
Stored program control (SPC) is a telecommunications technology used for telephone exchanges controlled by a computer program stored in the memory of the switching system. SPC was the enabling technology of electronic switching systems (ESS) developed in the Bell System in the 1950s, and may be considered the third generation of switching technology. Stored program control was invented in 1954 by Bell Labs scientist Erna Schneider Hoover, who reasoned that computer software could control the connection of telephone calls.
A reed relay is a type of relay that uses an electromagnet to control one or more reed switches. The contacts are of magnetic material and the electromagnet acts directly on them without requiring an armature to move them. Sealed in a long, narrow glass tube, the contacts are protected from corrosion. The glass envelope may contain multiple reed switches or multiple reed switches can be inserted into a single bobbin and actuate simultaneously. Reed switches have been manufactured since the 1930s.
TXE, was a family of telephone exchanges developed by the British General Post Office (GPO), designed to replace the ageing Strowger systems.
The Highgate Wood telephone exchange was the first all-electronic telephone exchange in Britain. It was built in the London suburb of Highgate by members from Joint Electronic Research Council (JERC).
The push-button telephone is a telephone that has buttons or keys for dialing a telephone number, in contrast to having a rotary dial as in earlier telephone instruments.
A telephone exchange, telephone switch, or central office is a telecommunications system used in the public switched telephone network (PSTN) or in large enterprises. It interconnects telephone subscriber lines or virtual circuits of digital systems to establish telephone calls between subscribers.
A group switching centre (GSC) was a United Kingdom telephone exchange used to provide local and trunk routing to local exchanges within its own and if required, adjacent group areas.
The Automatic Telephone and Electric Company was a British telephone exchange manufacturer established in 1911. After several name changes and acquisitions, the company was merged into Plessey in 1961.
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