System X (telephony)
Encyclopedia
System X was the name of the UK's first national digital
telephone exchange
system.
, and Standard Telephones and Cables
(STC) and first shown in public in 1979 at the Telecom 79 exhibition in Geneva Switzerland. In 1982, STC withdrew from System X and, in 1988, the telecommunications divisions of GEC & Plessey merged to form GPT
, with Plessey subsequently being bought out by GEC & Siemens
. In the late 1990s, GEC acquired Siemens' 40% stake in GPT and, in 1999, the parent company of GPT, GEC, renamed itself Marconi
.
When Marconi was sold to Ericsson
in January 2006, telent plc retained System X and continues to support and develop it as part of its UK services business.
, London and was a tandem junction unit which switched telephone calls amongst ~40 local exchanges. The first local digital exchange started operation in 1981 in Woodbridge, Suffolk
(near BT's Research HQ
at Martlesham Heath
). The last electromechanical trunk exchange (in Thurso
, Scotland
) was closed in July 1990—completing the UK's trunk network transition to purely digital operation and becoming the first national telephone system to achieve this. The last electromechanical local exchanges, Crawford, Crawfordjohn
and Elvanfoot, all in Scotland, were changed over to digital on 23 June 1995 and the last electronic analogue exchanges, Selby
, Yorkshire and Leigh on Sea, Essex were changed to digital on 11 March 1998.
. Concentrators are usually kept in local telephone exchanges but can be housed remotely in less populated areas. DLEs and DMSUs operate in major towns and cities and provide call routing functions.
Each Line Module unit converts analogue signals from a maximum of 64 subscriber lines in the access network
to the 64 kilobit digital binary signals used in the core network. This is done by sampling the incoming signal at a rate of 8 kS/s and coding each sample into an 8 bit word using Pulse Code Modulation (PCM) techniques. The Line Module also strips out any signalling information from the subscriber's line (e.g. dialled digits) and passes this to the Control. Up to 32 line modules are connected to a Digital Concentrator Switch unit using 2 Mbit/s highways giving each Concentrator Unit a capacity of up to 2048 subscriber lines. The Digital Concentrator Switch multiplexes
the signals from the Line Modules using time-division multiplexing
and concentrates the signals onto 30 time slots on up to 32 channel high speed highways for connection to the Digital Local Switching Unit via the Digital Line Termination units. The other two time slots on each highway are used for synchronisation and signalling.
Concentrator Units can either stand alone as remote concentrator
s or be co-located with the Digital Local Switching Unit.
known as PO CORAL (for Post Office CORAL).
System X is in the process of being replaced by BT's 21 Century Network (21CN)
. Many of the other main users of System X - in particular Jersey Telecom
and Kingston Communications
- have already replaced the bulk of their circuit switched networks with Marconi softswitch
es and Access Hub multiservice access nodes
.
System X is currently undergoing something of a renaissance; although the Falklands & Jersey have moved onto Softswitch
technology, operators such as BT
, Virgin Media
, Thus/Your Comms/Cable & Wireless have renewed their interest in keeping this "legacy network" switch operational for the foreseable future.
Digital telephony
Digital telephony is the use of digital electronics in the provision of digital telephone services and systems. Since the 1960s a digital core network has almost entirely replaced the old analog system, and much of the access network has also been digitized...
telephone exchange
Telephone exchange
In the field of telecommunications, a telephone exchange or telephone switch is a system of electronic components that connects telephone calls...
system.
Development
System X was developed by the UK Post Office (later to become British Telecom), GEC, PlesseyPlessey
The Plessey Company plc was a British-based international electronics, defence and telecommunications company. It originated in 1917, growing and diversifying into electronics. It expanded after the second world war by acquisition of companies and formed overseas companies...
, and Standard Telephones and Cables
Standard Telephones and Cables
Standard Telephones and Cables Ltd was a British telephone, telegraph, radio, telecommunications and related equipment R&D manufacturer. During its history STC invented and developed several groundbreaking new technologies including PCM and optical fibres.The company began life in London as...
(STC) and first shown in public in 1979 at the Telecom 79 exhibition in Geneva Switzerland. In 1982, STC withdrew from System X and, in 1988, the telecommunications divisions of GEC & Plessey merged to form GPT
Marconi Communications
Marconi Communications was a principal subsidiary of the Marconi Corporation plc and was formerly known as GEC Plessey Telecommunications . The company was a world leading telecommunications manufacturer...
, with Plessey subsequently being bought out by GEC & Siemens
Siemens AG
Siemens AG is a German multinational conglomerate company headquartered in Munich, Germany. It is the largest Europe-based electronics and electrical engineering company....
. In the late 1990s, GEC acquired Siemens' 40% stake in GPT and, in 1999, the parent company of GPT, GEC, renamed itself Marconi
Marconi Electronic Systems
Marconi Electronic Systems , or GEC-Marconi as it was until 1998, was the defence arm of The General Electric Company . It was demerged from GEC and acquired by British Aerospace on November 30, 1999 to form BAE Systems...
.
When Marconi was sold to Ericsson
Ericsson
Ericsson , one of Sweden's largest companies, is a provider of telecommunication and data communication systems, and related services, covering a range of technologies, including especially mobile networks...
in January 2006, telent plc retained System X and continues to support and develop it as part of its UK services business.
Implementation
The first System X unit to enter public service was in September 1980 and was installed in Baynard HouseBaynard House
Baynard House is a Brutalist office block in Queen Victoria Street in the Blackfriars area of London, occupied by BT Group. It was built on the former site of Baynard's Castle. Most of the site under Baynard House is a Scheduled Ancient Monument....
, London and was a tandem junction unit which switched telephone calls amongst ~40 local exchanges. The first local digital exchange started operation in 1981 in Woodbridge, Suffolk
Woodbridge, Suffolk
Woodbridge is a town in Suffolk, East Anglia, England. It is in the East of England, not far from the coast. It lies along the River Deben, with a population of about 7,480. The town is served by Woodbridge railway station on the Ipswich-Lowestoft East Suffolk Line. Woodbridge is twinned with...
(near BT's Research HQ
Adastral Park
Adastral Park is a science campus based at Martlesham Heath near Ipswich in the English county of Suffolk.When the site opened it was known as the Post Office Research Station, but it was subsequently renamed BT Research Laboratories or BT Labs and later Adastral Park to reflect an expansion in the...
at Martlesham Heath
Martlesham Heath
Martlesham Heath village is situated 6 miles east of Ipswich, in Suffolk, England. This was an ancient area of heathland and latterly the site of Martlesham Heath Airfield...
). The last electromechanical trunk exchange (in Thurso
Thurso
-Facilities:Offices of the Highland Council are located in the town, as is the main campus of North Highland College, formerly Thurso College. This is one of several partner colleges which constitute the UHI Millennium Institute, and offers several certificate, diploma and degree courses from...
, Scotland
Scotland
Scotland is a country that is part of the United Kingdom. Occupying the northern third of the island of Great Britain, it shares a border with England to the south and is bounded by the North Sea to the east, the Atlantic Ocean to the north and west, and the North Channel and Irish Sea to the...
) was closed in July 1990—completing the UK's trunk network transition to purely digital operation and becoming the first national telephone system to achieve this. The last electromechanical local exchanges, Crawford, Crawfordjohn
Crawfordjohn
Crawfordjohn is a small village and civil parish located in South Lanarkshire, Scotland.It is west of Abington and north east of Leadhills, near junction 13 of the M74. It lies to the north of the Duneaton Water, a tributary of the River Clyde. It is known for the manufacture of curling stones....
and Elvanfoot, all in Scotland, were changed over to digital on 23 June 1995 and the last electronic analogue exchanges, Selby
Selby
Selby is a town and civil parish in North Yorkshire, England. Situated south of the city of York, along the course of the River Ouse, Selby is the largest and, with a population of 13,012, most populous settlement of the wider Selby local government district.Historically a part of the West Riding...
, Yorkshire and Leigh on Sea, Essex were changed to digital on 11 March 1998.
System X Units
System X covers 3 main types of telephone 'switching' equipment. There are many of these switches residing all over the United KingdomUnited Kingdom
The United Kingdom of Great Britain and Northern IrelandIn the United Kingdom and Dependencies, other languages have been officially recognised as legitimate autochthonous languages under the European Charter for Regional or Minority Languages...
. Concentrators are usually kept in local telephone exchanges but can be housed remotely in less populated areas. DLEs and DMSUs operate in major towns and cities and provide call routing functions.
Concentrator Unit
The Concentrator Unit consists of four main sub-systems, Line Modules, Digital Concentrator Switch, Digital Line Termination (DLT) units and Control. Its purpose is to convert speech from analogue to digital and concentrate the traffic for onward transmission to the Digital Local Exchange (DLE). It also receives dialled information from the subscriber and passes this to the DLE so the call can be routed to its destination. In normal circumstances, it does not switch signals between subscribers' lines but has limited capacity to do this if connection to the DLE is lost.Each Line Module unit converts analogue signals from a maximum of 64 subscriber lines in the access network
Access network
An access network is that part of a telecommunications network which connects subscribers to their immediate service provider. It is contrasted with the core network, which connects local providers to each other...
to the 64 kilobit digital binary signals used in the core network. This is done by sampling the incoming signal at a rate of 8 kS/s and coding each sample into an 8 bit word using Pulse Code Modulation (PCM) techniques. The Line Module also strips out any signalling information from the subscriber's line (e.g. dialled digits) and passes this to the Control. Up to 32 line modules are connected to a Digital Concentrator Switch unit using 2 Mbit/s highways giving each Concentrator Unit a capacity of up to 2048 subscriber lines. The Digital Concentrator Switch multiplexes
Multiplexing
The multiplexed signal is transmitted over a communication channel, which may be a physical transmission medium. The multiplexing divides the capacity of the low-level communication channel into several higher-level logical channels, one for each message signal or data stream to be transferred...
the signals from the Line Modules using time-division multiplexing
Time-division multiplexing
Time-division multiplexing is a type of digital multiplexing in which two or more bit streams or signals are transferred apparently simultaneously as sub-channels in one communication channel, but are physically taking turns on the channel. The time domain is divided into several recurrent...
and concentrates the signals onto 30 time slots on up to 32 channel high speed highways for connection to the Digital Local Switching Unit via the Digital Line Termination units. The other two time slots on each highway are used for synchronisation and signalling.
Concentrator Units can either stand alone as remote concentrator
Remote concentrator
In modern telephony a remote concentrator, Remote Concentrator Unit , or Remote Line Concentrator is the lowest level in the telephone switch hierarchy.Subscribers' analogue telephone/PSTN lines are terminated on concentrators...
s or be co-located with the Digital Local Switching Unit.
Digital Local Exchange
The Digital Local Exchange (DLE) connects to the concentrator and routes calls to different DLEs or DMSUs depending on the destination of the call. The heart of the DLE is the Digital Switching Subsystem (DSS) which consists of Time Switches and a Space Switch. Incoming traffic on the 30 channel PCM highways from the Concentrator Units is connected to Time Switches. The purpose of these is to take any incoming individual Time Slot and connect it to an outgoing Time Slot and so perform a switching and routing function. To allow access to a large range of outgoing routes, individual Time Switches are connected to each other by a Space Switch. The Time Slot inter-connections are held in Switch Maps which are updated by Software running on the Processor Utility Subsystem (PUS).Digital Main Switching Unit
The Digital Main Switching Unit (DMSU) deals with calls that have been routed by DLEs or another DMSU and is a 'trunk switch', i.e. it is not connected to any concentrators. As with DLEs, DMSUs are made up of a Digital Switching Subsystem and a Processor Utility Subsystem, amongst other things. In the British PSTN network, each DMSU is connected to every other DMSU in the country, enabling almost congestion-proof connectivity for calls through the network.Processor Utility Subsystem
The Processor Utility Subsystem (PUS) controls the switching operations and is the brain of the DLE or DMSU. It hosts the Call Processing, Billing, Switching and Maintenance applications Software amongst others. The PUS is divided into up to 8 'clusters' depending on the amount of telephony traffic dealt with by the DLE/DMSU. Each cluster of processors contains 4 Central Processing Units (CPUs), the main memory stores (STRs) and the backing store memory. The PUS was coded with a version of the CORAL66 programming languageCORAL66 programming language
CORAL is a programming language originally developed in 1964 at the Royal Radar Establishment , Malvern, UK, as a subset of JOVIAL. Coral 66 was subsequently developed by I. F. Currie and M. Griffiths...
known as PO CORAL (for Post Office CORAL).
Replacement System
Many of the switches are now near to or over 20 years old, giving an idea of their reliability.System X is in the process of being replaced by BT's 21 Century Network (21CN)
BT 21CN
The 21st Century Network programme is the network transformation project of the UK telecommunications company BT Group plc for data and voice. It had been intended to move BT's telephone network from the present AXE/System X Public Switched Telephone Network to an Internet Protocol system...
. Many of the other main users of System X - in particular Jersey Telecom
Jersey Telecom
JT Global is the former monopoly incumbent operator in the Bailiwick of Jersey. Jersey is incorporated into the UK National Telephone Numbering Plan area codes of +44 1534 for landlines and +44 7797 for Jersey Telecom mobiles, +44 7700 for Cable and Wireless mobiles and +44 7829 for Jersey Airtel...
and Kingston Communications
Kingston Communications
KCOM Group , formerly known as Kingston Communications, is a UK communications and IT services provider. Its headquarters is in Kingston upon Hull, East Riding of Yorkshire, where subsidiary business unit KC serves local residents and businesses with Internet and telephony services...
- have already replaced the bulk of their circuit switched networks with Marconi softswitch
Softswitch
A softswitch is a central device in a telecommunications network which connects telephone calls from one phone line to another, typically via the internet, entirely by means of software running on a general-purpose computer system...
es and Access Hub multiservice access nodes
MSAN
A Multi-service access Node also known as a Multi-service access gateway is a device typically installed in a telephone exchange which connects customers' telephone lines to the core network, to provide telephone, ISDN, and broadband such as DSL all from a single platform.Prior to the deployment...
.
System X is currently undergoing something of a renaissance; although the Falklands & Jersey have moved onto Softswitch
Softswitch
A softswitch is a central device in a telecommunications network which connects telephone calls from one phone line to another, typically via the internet, entirely by means of software running on a general-purpose computer system...
technology, operators such as BT
BT Group
BT Group plc is a global telecommunications services company headquartered in London, United Kingdom. It is one of the largest telecommunications services companies in the world and has operations in more than 170 countries. Through its BT Global Services division it is a major supplier of...
, Virgin Media
Virgin Media
Virgin Media Inc. is a company which provides fixed and mobile telephone, television and broadband internet services to businesses and consumers in the United Kingdom...
, Thus/Your Comms/Cable & Wireless have renewed their interest in keeping this "legacy network" switch operational for the foreseable future.