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Public Switched Telephone Network (PSTN)  

Public switched telephone network (PSTN)

The public switched telephone network (PSTN) is the network of the world's public circuit-switched telephone networks, in much the same way that the Internet is the network of the world's public IP-based packet-switched networks. Originally a network of fixed-line analog telephone systems, the PSTN is now almost entirely digital, and now includes mobile as well as fixed telephones.

The PSTN is largely governed by technical standards created by the ITU-T, and uses E.163/E.164 addresses (more commonly known as telephone numbers) for addressing.

Architecture and context

The PSTN was the earliest example of traffic engineering to deliver Quality of Service (QoS) guarantees. A.K. Erlang (1878–1929) is credited with establishing the mathematical foundations of methods required to determine the amount and configuration of equipment and the number of personnel required to deliver a specific level of service.

In the 1970s the telecommunications industry conceived that digital services would follow much the same pattern as voice services, and conceived a vision of end-to-end circuit switched services, known as the Broadband Integrated Services Digital Network (B-ISDN). The B-ISDN vision has been overtaken by the disruptive technology of the Internet. Only the oldest parts of the telephone network still use analog technology for anything other than the last mile loop to the end user, and in recent years digital services have been increasingly rolled out to end users using services such as DSL, ISDN, FTTX and cable modem systems.

Many observers believe that the long term future of the PSTN is to be just one application of the Internet - however, the Internet has some way to go before this transition can be made. The QoS guarantee is one aspect that needs to be improved in the Voice over IP (VoIP) technology.

There are a number of large private telephone networks which are not linked to the PSTN, usually for military purposes. There are also private networks run by large companies which are linked to the PSTN only through limited gateways, like a large private branch exchange (PBX).

Early history

The first telephones had no network but were in private use, wired together in pairs. Users who wanted to talk to different people had as many telephones as necessary for the purpose. A user who wished to speak, whistled into the transmitter until the other party heard. Soon, however, a bell was added for signalling, and then a switchhook, and telephones took advantage of the exchange principle already employed in telegraph networks. Each telephone was wired to a local telephone exchange, and the exchanges were wired together with trunks. Networks were connected together in a hierarchical manner until they spanned cities, countries, continents and oceans. This was the beginning of the PSTN, though the term was unknown for many decades.

Automation introduced pulse dialing between the phone and the exchange, and then among exchanges, followed by more sophisticated address signaling including multi-frequency, culminating in the SS7 network that connected most exchanges by the end of the 20th century.

Digital Channel

Although the network was created using analog voice connections through manual switchboards, automated telephone exchanges replaced most switchboards, and later digital switch technologies were used. Most switches now use digital circuits between exchanges, with analog two-wire circuits still used to connect to most telephones.

The basic digital circuit in the PSTN is a 64-kilobits-per-second channel, originally designed by Bell Labs, called Digital Signal 0 (DS0). To carry a typical phone call from a calling party to a called party, the audio sound is digitized at an 8 kHz sample rate using 8-bit pulse code modulation (PCM). The call is then transmitted from one end to another via telephone exchanges. The call is switched using a signaling protocol (Signaling_System_7) between the telephone exchanges under an overall routing strategy.

The DS0s are the basic granularity at which switching takes place in a telephone exchange. DS0s are also known as timeslots because they are multiplexed together using time-division multiplexing (TDM). Multiple DS0s are multiplexed together on higher capacity circuits into a DS1 signal, carrying 24 DS0s on a North American or Japanese T1 line, or 32 DS0s (30 for calls plus two for framing and signalling) on an E1 line used in most other countries. In modern networks, this multiplexing is moved as close to the end user as possible, usually into cabinets at the roadside in residential areas, or into large business premises.

The timeslots are conveyed from the initial multiplexer to the exchange over a set of equipment collectively known as the access network. The access network and inter-exchange transport of the PSTN use synchronous optical transmission (SONET and SDH) technology, although some parts still use the older PDH technology.

Within the access network, there are a number of reference points defined. Most of these are of interest mainly to ISDN but one – the V reference point – is of more general interest. This is the reference point between a primary multiplexer and an exchange. The protocols at this reference point were standardised in ETSI areas as the V5 interface.

UK Telephone Switch Hierarchy

The forerunner of British Telecom, the General Post Office, also organized its intercity trunk network along similar hierarchical lines to that of North America. However, because of the significantly smaller geographic area involved, fewer levels of connection were required, and no formal numbering of class offices was made.

There were a few special exceptions to the following description, notably those involving Northern Ireland, some of the Channel Dependencies, and the few locations in England which were served by non-GPO companies, such as Hull (Kcom) and Portsmouth.

In the early days of manual exchanges, outlying areas (eventually called dependent exchanges) were connected through progressively larger locations (eventually called group switching centres) into one of the main cities - Birmingham, Edinburgh, Glasgow, Liverpool, London, and Manchester. As automation began to be established in the network, this was refined into a system of approximately fifty tandem locations for Group Switching Centres, with an additional layer of perhaps a dozen Wide Area Tandems to provide for busy periods, emergency routing, etc. There were also some additional Local Tandems to handle traffic in the London Metropolitan Area without involving the GSCs, although this was a later development, as it required common control signalling for identification.

Subscriber Trunk Dialing

The dialing codes used by trunk operators to connect calls were originally assigned and established to ensure speed with pulse dialing equipment. With the advent of subscriber dialed calls, numbering patterns were reassigned to provide for mnemonic methods of improving customer performance. STD codes all began with 0. The largest cities, which had seven digit local numbers, were allocated special codes - London, 01; Birmingham, 021; etc. Smaller towns were typically allocated a code based on the first letters of their name, translated into digits on the telephone dial. For example, OXford translated into 09 on the British phone dial, so the original STD code for Oxford was 0096. However, because of subscriber dialing errors, there was an early decision to eliminate codes which began with "00" and Oxford soon became 0865, the 86 standing for UNiversity.

Some of the smallest towns connected to the trunk network only through nearby switches. In those cases, STD codes were composed of combination of the code for the nearby switch, plus some additional digits that were unused in that nearby switch, but which served two purposes (1) to identify the end location, and allow the nearby switch to complete the call (2) to "pad out" the overall length of the dialing string, since a small town might only have a three-digit telephone number, and allow the network to move to a more-standard number length.

As step offices became rarer, Subscriber Trunk Dialing Codes no longer followed the original rules, and were significantly revised in the mid-1990s, with further changes as wider use of mobile phones and non-BT competition came into the UK market. There are now some 70000 local exchange codes in use in the UK. The largest trunk carrier, British Telecom, connects the local network through some 60 transit (tandem) switches.

Technological development

In common with most countries, the development of technology allowed for different networking, and the maintenance of a formal hierarchy disappeared into a distributed network. By the mid-1990s, a revised structure had appeared, reflected by the replacement of the old departmental area codes by the assignment of regional codes and a major renumbering scheme for strategic planning, privatization, and deregulation under the auspices of ART, the Autorité de régulation des télécommunications (Regulatory Authority for Telecommunications - since 2005, ARCEP, as responsibility for postal services was added). After 1996, the country prepared for complete deregulation of the telephone network.

Thus, the local exchanges (zones à autonomie d'acheminement) are connected somewhat differently by various carriers. However, the largest of these, based upon the (partially) privatised former government network, is a two-level long distance hierarchy, based on 80 CTS (centre de transit secondaire) and 8 CTP (centre de transit primaire) locations. In addition, there are 12 CTI (centre de transit internationaux) for connections to areas which are not integrated into the French telephone network [note that some overseas locations are considered "domestic" for telecommunications purposes].

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