PSTN network topology

A PSTN network topology is the switching network topology of a telephone network connected to the public switched telephone network (PSTN).

In the United States and Canada, the Bell System network topology was the switching system hierarchy implemented and operated from the late 1940s to the 1980s for the purpose of integrating the diverse array of local telephone companies and telephone numbering plans to achieve nationwide Direct Distance Dialing (DDD) by telephone subscribers. It was the precursor of the world-wide interconnected public switched telephone network (PSTN).

The ideas originated in the Bell System in the United States, but were soon adopted by other countries where telephone operators were facing similar issues, even when service smaller geographic areas. The system in the United Kingdom implemented by the General Post Office resulted in fewer switching levels than the Bell System.

The Bell System

In the late 1940s the Bell System devised plans to consolidate the various incompatible local telephone numbering plans of its constituent service areas into a unified network, that later became known as the North American Numbering Plan (NANP). This was a prerequisite to achieve Direct Distance Dialing (DDD) by customers, first implemented in New Jersey in 1951. In addition to devising a unified numbering plan, AT&T reorganized the nationwide system that was spearheaded by AT&T Long Lines of local telephone exchanges and central offices into a hierarchical network containing five levels, called classes, of switching systems.

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.

The newly deviced hierarchy was maintained into the early 1980s, when technological advances and business models rendered it increasingly obsolete, but the hierarchical features live on in terms, such as Class 4 and Class 5 telephone switch, 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 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 Bell System divestiture, the network in the US was divided into local access and transport areas (LATAs). Calls within LATAs 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 1975, 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 routing 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 (TOROON0101T2) 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.

Class 1 (Regional Center)

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 initially staffed by engineers who had the authority to block portions of the network within the region in case of emergencies or network congestion – although these functions were transferred after 1962 to the Network Control/Operations Center and the distributed Network Management Centers (see below) (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 [a rural crossroads west of Chicago at the intersection of US highway 52 and IL highway 71 – an underground office built with hardened construction to withstand nuclear attack], Conyers, GA in Rockdale County, St Louis, MO, Dallas, TX, Denver, CO, and Sacramento, CA), one by GTE (San Bernardino, CA). Two centers 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).

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.

Class 2 (sectional centers)

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.

Class 3 (primary centers)

The class 3 office was the Primary Center (PC). Calls being made beyond the limits of a small geographical area where circuits are not 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.

Class 4 (Toll Centers)

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, or whose direct trunks are busy, is routed through 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.

Class 5 (Local Exchange / Feature Group)

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.

Class 5 has also over time begun to include Centrex style solutions such as Hosted PBX, as well as services like VoIP SIP Trunking.

Updated on September 19, 2017

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