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.
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).
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.
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.
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.
It should be noted that this hierarchy has been obsolete since the early 1980s, but it lives on in the terms "Class 4" and "Class 5", referring to tandem and end-office switches respectively. The PSTN in the United States was essentially restructured with the 1984 Divestiture of AT&T. The old Long Lines network remained with AT&T, but its internal routing became non-hierarchical with the introduction of more advanced computer-controlled switching. Each major long distance carrier can have its own internal routing policies, though they generally start with the same principles and even components.
With Divestiture, the network in the US was divided into Local Access and Transport Areas (LATAs), calls within which were carried by Local Exchange Carriers (LECs), while calls between them were carried by Interexchange carriers (IXCs). LATAs generally have one or more tandem switches which interconnect end office switches.
While the following discussion refers to AT&T and (principally) to the United States, it is important to remember that until 1956, AT&T controlled Bell Canada and thus influenced corporate decisions north of the border. Bell Canada provided local operations in most of Ontario and Quebec, and both in its capacity as the largest telecommunications carrier in Canada and because of its historic operations in the Atlantic and Prairie provinces, dominated decisions over long distance practices. Canadian authorities agreed that integration of Canadian long distance services into a trans-national network was valuable to both countries, so that U.S. and Canadian services were integrated for networking capabilities at an early stage into what eventually became the foundation for the North American Numbering Plan Area.
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 Contiguous 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.
For control and oversight of the entire network hierarchy, AT&T established a Network Control Center in New York City in 1962, renamed the Network Operations Center and relocated to Bedminster, NJ in 1977. Engineering supervision was also centralized in eight regional Network Management Centers. The realignment and dispersion of functions were done, in part, to ensure maximum network integrity in the event of a national emergency, a major concern in that era. The basic structure of this unit, although significantly altered since the AT&T divestiture in the 1980s, still exists as the Global Operations Center, with domestic regional centers in Colorado and Georgia.
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.
Please note that the currently attached diagram of switch hierarchy is incorrect, as it identifies Class 1 points with International switching. International connections were located in places generally close to cable, later satellite, termination locations, and were not directly related to Class 1 switches. Major international connection points were located in Oakland, California; Miami, Florida; and New York, New York, with a number of secondary international operator toll points. Only after the rapid expansion of ESS terminal offices did operator handling of international calls begin to be off-loaded into the domestic network structure, as international calling services began to be customer dialable, ca. the mid-1980s. This in part paralleled the demise of the five-level hierarchy, so identification of international switches and class one offices is incorrect.
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.
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.
Under these conditions, early network development revolved around two major distinctions, "Paris" and "not Paris." Within metropolitan Paris, automated step switches appeared, with a level of tandem switching, before World War II. In the rest of the country, automation was confined to major cities, with a high level of manual intervention.
World War II heavily damaged the French telephone system, so that by the end of the War, only about 140 automatic exchanges (mostly in Paris and its banlieues) and 228 manual exchanges were fully operational. Repair of much of the network had been deferred during the war due to lack of parts, as well as co-opting of technical personnel for German military needs.
The network had a minimal hierarchy, with most connections routed into a central tandem in each department, and from there to Paris. As greater installation of private telephones, for both small business and private residences, increased in the 1970s, direct connections among the tandems in adjacent regions were installed, and a three-level tier of switches, local, tandem, and regional interconnection was implemented, with final routing through Paris. Also, during the 1970s and 1980s, the smaller rural switches were replaced and combined with nearby automated offices, and a closed numbering scheme was adopted for dialing consistency.
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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