Public switched telephone network

It has been suggested that Routing in the PSTN be merged into this article. (Discuss) Proposed since July 2020.

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The public switched telephone network (PSTN) provides infrastructure and services for public telecommunication. The PSTN is the aggregate of the world's circuit-switched telephone networks that are operated by national, regional, or local telephony operators. These consist of telephone lines, fiber optic cables, microwave transmission links, cellular networks, communications satellites, and undersea telephone cables, all interconnected by switching centers which allow most telephones to communicate with each other.

Первоначально сеть фиксированных аналоговых телефонных систем, PSTN теперь почти полностью цифровая в своей базовой сети и включает в себя мобильные ^[1] и другие сети, а также фиксированные телефоны. ^[1]

Техническая работа PSTN соответствует стандартам, установленным ITU-T . Эти стандарты позволяют различным сетям в разных странах беспрепятственно соединяться между собой. Стандарты E.163 и E.164 обеспечивают единое глобальное адресное пространство для телефонных номеров. Комбинация взаимосвязанных сетей и единого плана нумерации позволяет телефонам по всему миру набирать друг друга.

History[edit]

Коммерциализация телефона началась в 1876 году, когда приборы работали парами для частного использования между двумя местами. Пользователи, которые хотели общаться с людьми в разных местах, имели столько телефонов, сколько необходимо для этой цели. Оповещение другого пользователя о желании установить телефонный звонок осуществлялось громким свистом в передатчик, пока другой абонент не услышал сигнал тревоги. Вскоре на станции были добавлены колокола для сигнализации , так что дежурному больше не нужно было ждать свистка.

Позже телефоны использовали принцип обмена, уже применявшийся в телеграфных сетях. Каждый телефон был подключен к телефонной станции, установленной для города или района. Для связи за пределами этой зоны обмена между станциями были установлены соединительные линии . Сети были спроектированы иерархически, пока они не охватили города, страны, континенты и океаны.

Automation introduced pulse dialing 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 meant that teletraffic engineering techniques needed to be deployed to deliver quality of service (QoS) guarantees for the users. 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.

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 use analog technology for the last mile loop to the end user. However, digital technologies such as DSL, ISDN, FTTx, and cable modems have become more common in this portion of the network.

Several large private telephone networks 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, such as a large private branch exchange (PBX).

Operators[edit]

The task of building the networks and selling services to customers fell to the network operators. The first company to be incorporated to provide PSTN services was the Bell Telephone Company in the United States.

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 nationalized company. In more recent decades, these state monopolies were broken up or sold off through privatization.^{[clarification needed][citation needed]}

Regulation[edit]

In most countries, the central has a regulator dedicated to monitoring the provision of PSTN services in that country. Their tasks may be for example to ensure that end customers are not over-charged for services where monopolies may exist. These regulatory agencies may also regulate the prices charged between the operators to carry each other's traffic.

Technology[edit]

Network topology[edit]

The PSTN network architecture had to evolve over the years to support increasing numbers of subscribers, calls, connections to other countries, direct dialing and so on. The model developed by the United States and Canada was adopted by other nations, with adaptations for local markets.

The original 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.

However, in modern networks the cost of transmission and equipment is lower and, although hierarchies still exist, they are much flatter, with perhaps only two layers.

Digital channels[edit]

Most automated telephone exchanges use digital switching rather than mechanical or analog switching. The trunks connecting the exchanges are also digital, called circuits or channels. However analog two-wire circuits are still used to connect the last mile from the exchange to the telephone in the home (also called the local loop). To carry a typical phone call from a calling party to a called party, the analog audio signal is digitized at an 8 kHz sample rate with 8-bit resolution using a special type of nonlinear pulse-code modulation known as G.711. The call is then transmitted from one end to another via telephone exchanges. The call is switched using a call set up protocol (usually ISUP) between the telephone exchanges under an overall routing strategy.

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 synchronous optical transmission, for example, SONET and Synchronous Digital Hierarchy (SDH) technologies, 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 standardized in ETSI areas as the V5 interface.

Impact on IP standards[edit]

Voice quality over PSTN networks was used as the benchmark for the development of the Telecommunications Industry Association's TIA-TSB-116 standard on voice-quality recommendations for IP telephony, to determine acceptable levels of audio delay and echo.^[2]

See also[edit]

• Managed facilities-based voice network

• Plain old telephone service (POTS)
• Via Net Loss

References[edit]

External links[edit]