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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 (known more commonly as telephone numbers) for addressing.
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 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 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) .
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 (SS7) 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.
In order to organize automated operator dialing, and later Direct Distance Dialing (DDD), AT&T divided the various switches in its network in to a hierarchy containing five levels (or classes). This was a formal expansion of the network structure that had developed within AT&T Long Lines as local telephone exchanges had been connected together. As long distance calling was originally established, it could take up to seven minutes to complete a connection to another major city, and small points would need to have "call back" appointments made with long lead times for circuits to be reserved. 333333
By the mid-1920s, a revised manual system where "local" toll operators connected tandem routes (a process formally called Combined Line and Recording) as needed to complete telephone calls, reduced the process to an average of two minutes, but still meant that some complex routings might interconnect as many as sixteen points! As long distance services grew in the Contigous Continental US (48 states) and Canada, the amount of overhead equipment and people required to determine and establish Rates and Routes became excessive. As technology improved, network design included consideration of more automated and defined procedures. Thus, beginning with a switch installed in Philadelphia PA in 1943, AT&T began to automate the system, and establish a new switch hierarchy, which lasted until the breakup of AT&T in the 1980s.
The underlying principle of the five-level hierarchy was to provide economies of scale by establishing direct connections between centralized call "collection points" (essentially the Class 4 offices) where economically feasible, and to provide additional concentration points (Class 1 through 3) to handle overflow traffic that could not be handled directly, or to handle traffic to locations which were less likely to be dialed from a given point - usually longer distances and/or smaller locations in other parts of the North American dialing plan. The North American plan differed from those of other continents in the existence of three concentration levels of hierarchy for domestic (here defined as including all those points "within" the dialing plan) calls, a need not required where the larger geographic area was broken into several national plan jurisdictions. However, it is important to note that this was not a strict hierarchy of absolute levels. If enough call traffic existed between geographic areas, for example, a Class 4 office could have direct trunk connections not only to a Class 3 office, but to a Class 2 or Class 1 office, and vice versa. For example, the Class 2 switch in Toronto (OTORON0101T2) had connections not only to the Class 1 switch in Montréal (MTRLPQ0201T1), but to the Class 1 switch in White Plains (WHPLNY0201T1), one of the Class 2 switches in New York City (NYCMNYAA02T2) and a Class 3 switch in Buffalo (BFLONYFR04T3). Network engineers re-worked the system as necessary to balance off call completion percentages with budgetary limitations. In fact, minor changes were made almost every month.
Initially excluded from the development of the North American network were locations that eventually would become part of the North American Numbering Plan Area - Alaska, Hawaii, some other United States possessions, various outlying Northern and rural portions of Canada, and much of the Caribbean. These areas were handled as International Calls until more advanced computer hardware and software allowed them to be included in the automated, integrated systems in later decades. After the spread of stored program control switching, many services of Class 1 through 3 could be delegated to newer switches in the class 4 and 5 offices, and that portion of the network became obsolete, although it was partially replaced by the establishment of multiple long distance carrier networks, connected to the local networks through their points of presence.
The class 1 office was the Regional Center (RC). Regional centers served three purposes in the North American toll network (a) their connections were the "last resort" for final setup of calls when routes between centers lower in the hierarchy were not available (b) they were staffed by engineers who had the authority to block portions of the network within the region in case of emergencies or network congestion (c) they provided collection points (until the development of more advanced computer hardware and software for toll operators) for circuits that would be passed along to one of the international overseas gateways (which operated as special centers outside the formal North American hierarchy). The regional centers updated each other on the status of every circuit in the network. These centers would then reroute traffic around the trouble spots and keep each informed at all times. There were twelve Regional Centers in North America, ten in the United States, nine of which were operated by AT&T (White Plains NY, Wayne PA, Pittsburgh PA, Norway IL [which wasn't a real place, but a rural crossroads a distance away from Chicago - an underground office built in a cornfield with hardened construction to withstand nuclear attack], Rockdale GA, St Louis MO, Dallas TX, Denver CO, and Sacramento CA), one by GTE (San Bernardino CA). Two centres in Canada were operated on behalf of the Trans-Canada Telephone System, one by Bell Canada (Montréal PQ), and one by Saskatchewan Telephone, (Regina SK).
The class 2 office was the Sectional Center (SC). The sectional center typically connected major toll centers within one or two states or provinces, or a significant portion of a large state or province, to provide interstate or interprovincial connections for long-distance calls. At various times, there were between 50 and 75 active class two offices in the network.
The class 3 office was the Primary Center (PC). Calls being made beyond the limits of a small geographical area where circuits connected directly between class 4 toll offices would be passed from the toll center to the primary center. These locations use high usage trunks to complete connection between toll centers. The primary center never served dial tone to the user. The number of primary centers in the network fluctuated from time to time, ranging between 150 and 230.
The class 4 office is the Toll Center (TC), Toll Point (TP), or Intermediate Point (IP). A call going between two end offices not directly connected together, or whose direct trunks are busy, is routed thru the toll center. The toll center is also used to connect to the long-distance network for calls where added costs are incurred, such as operator handled services. This toll center may also be called the tandem office because calls have to pass through this location to get to another part of the network. Toll centers might have been operated either as interstate facilities, under the operation of AT&T Long Lines (GTE in a few cases), or by local telephone companies, handling long distance traffic to points within a particular operating company territory. Class 4 offices continue to exist, although with considerable changes, as they handle local exchange company interconnections, locally charged or long distance rated, or provide facilities for connection to long distance company points of presence.
The class 5 office is the local exchange or end office. It delivers dial tone to the customer. The end office, also called a branch exchange, is the closest connection to the end customer. Over 19,000 end offices in the United States alone provide basic dial tone services.
In modern times only the terms Class 4 and Class 5 are much used, as any tandem office is referred to as a Class 4. This change was prompted in great part by changes in the power of switches and the relative cost of transmission, both of which tended to flatten the switch hierarchy. The breakup of the Bell System, and the need for each of the surviving regional operating companies to handle long distance interconnections, also promoted the inclusion of inter-regional and international processing through larger Class 4 offices.
The special requirements of placing calls to locations outside main Canadian/United States points meant that these calls were handled by special operators in locations where connections could be monitored to other countries. The technology to automate these connections began to develop in the 1960s (see Bell Laboratories Record 42:7, July-August 1964), and as the decade of the 1970s progressed, North American customers who were served by electronic offices began to be able to directly dial to an increasing number of international points (service between ESS offices in New York and London began on March 1, 1970). However, since points could not be connected until equipment in both countries was converted to electronic switching, implementation to many locations took some time, and while the majority of calls began to be connected via automated systems by the 1990s - after the termination of the five-level hierarchy - the majority of countries were still connected via manual intervention until the beginning of the 21st century.
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 small 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 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.
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.