The public switched telephone network ( PSTN) is the aggregate of the world's telephone networks that are operated by national, regional, or local telephony operators. It provides infrastructure and services for public telephony. The PSTN consists of , fiber-optic cables, microwave transmission links, cellular networks, communications satellites, and undersea telephone cables interconnected by , such as central offices, network tandems, and international gateways, which allow telephone users to communicate with each other.
Originally a network of Landline Analog signal telephone systems, the PSTN is now predominantly digital in its core network and includes Cellular network, Satellite phone, and landline systems. These interconnected networks enable global communication, allowing calls to be made to and from nearly any telephone worldwide. Many of these networks are progressively transitioning to Internet Protocol to carry their telephony traffic.
The technical operation of the PSTN adheres to the standards internationally promulgated by the ITU-T. These standards have their origins in the development of local telephone networks, primarily in the Bell System in the United States and in the networks of European ITU members. The E.164 standard provides a single global address space in the form of . The combination of the interconnected networks and a global telephone numbering plan allows telephones around the world to connect with each other.
History
Later telephone systems took advantage of the exchange principle already employed in telegraph networks. Each telephone was wired to a telephone exchange established for a town or area. For communication outside this exchange area, trunking were installed between exchanges. Networks were designed in a hierarchical manner until they spanned cities, states, and international distances.
Automation introduced between the telephone and the exchange so that each subscriber could directly dial another subscriber connected to the same exchange, but long-distance calling across multiple exchanges required manual switching by operators. Later, more sophisticated address signaling, including multi-frequency signaling methods, enabled direct-dialed long-distance calls by subscribers, culminating in the Signalling System 7 (SS7) network that controlled calls between most exchanges by the end of the 20th century.
The growth of the PSTN was enabled by teletraffic engineering techniques to deliver quality of service (QoS) in the network. The work of A. K. Erlang established the mathematical foundations of methods required to determine the capacity requirements and configuration of equipment and the number of personnel required to deliver a specific level of service.
In the 1970s, the telecommunications industry began implementing packet-switched network data services using the X.25 protocol transported over much of the end-to-end equipment as was already in use in the PSTN. These became known as public data networks, or public switched data networks.
In the 1980s, the industry began planning for digital services assuming they would follow much the same pattern as voice services and conceived end-to-end circuit-switched services, known as the Broadband Integrated Services Digital Network (B-ISDN). The B-ISDN vision was overtaken by the disruptive technology of the Internet.
At the turn of the 21st century, the oldest parts of the telephone network still used analog baseband technology to deliver audio-frequency connectivity over the last mile to the end-user. However, digital technologies such as DSL, ISDN, FTTx, and cable modems were progressively deployed in this portion of the network, primarily to provide high-speed Internet access.
, operators worldwide are in the process of retiring support for both last-mile analog telephony and ISDN, and transitioning voice service to Voice over IP via Internet access delivered either via DSL, Cable modem or fiber-to-the-premises, eliminating the expense and complexity of running two separate technology infrastructures for PSTN and Internet access.
Several large private telephone networks are not linked to the PSTN, usually for military purposes. There are also private networks run by large companies that are linked to the PSTN only through limited gateways, such as a large private branch exchange (PBX).
Operators
In some countries, however, the job of providing telephone networks fell to government as the investment required was very large and the provision of telephone service was increasingly becoming an essential public utility. For example, the General Post Office in the United Kingdom brought together a number of private companies to form a single state ownership. In more recent decades, these state monopolies were broken up or sold off through privatization. Information sheets and timelines. Privatisation British Telecom Privatisation
Technology
Network topology
A key concept was that the telephone exchanges are arranged into hierarchies, so that if a call cannot be handled in a local cluster, it is passed to one higher up for onward routing. This reduced the number of connecting trunks required between operators over long distances, and also kept local traffic separate. Modern technologies have brought simplifications
Digital channels
The call is carried over the PSTN using a 64 kbit/s channel, originally designed by Bell Labs. The name given to this channel is Digital Signal 0 (DS0). The DS0 circuit is the basic granularity of circuit switching in a telephone exchange. A DS0 is also known as a timeslot because DS0s are aggregated in time-division multiplexing (TDM) equipment to form higher capacity communication links.
A Digital Signal 1 (DS1) circuit carries 24 DS0s on a North American or Japanese T-carrier (T1) line, or 32 DS0s (30 for calls plus two for framing and signaling) on an E-carrier (E1) line used in most other countries. In modern networks, the multiplexing function is moved as close to the end user as possible, usually into cabinets at the roadside in residential areas, or into large business premises.
These aggregated circuits 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 use synchronization optical transmission, for example, SONET and Synchronous Digital Hierarchy (SDH) technologies, although some parts still use the older PDH technology.
The access network defines a number of reference points. 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 standardized in ETSI areas as the V5 interface.
Impact on IP standards
Regulation
Technology retirement
Several other European countries, including Estonia, Germany, Iceland, the Netherlands, Spain and Portugal, have also retired, or are planning to retire, their traditional networks.
Countries on other continents are also performing similar transitions.
See also
- List of country calling codes
- Managed facilities-based voice network
- Phreaking
- Plain old telephone service (POTS)
- Via Net Loss
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