Public switched telephone network
This article needs additional citations for verification. (December 2021) |
The public switched telephone network (PSTN) is the aggregate of the world's telephone networks that are operated by national, regional, or local
Originally a network of fixed-line
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 telephone numbers. The combination of the interconnected networks and a global telephone numbering plan allows telephones around the world to connect with each other.[2]
History
Commercialization of the telephone began shortly after its invention, with instruments operated in pairs for private use between two locations. Users who wanted to communicate with persons at multiple locations had as many telephones as necessary for the purpose. Alerting another user of the desire to establish a telephone call was accomplished by whistling loudly into the transmitter until the other party heard the alert. Bells were soon added to stations for signaling.
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, trunks were installed between exchanges. Networks were designed in a hierarchical manner until they spanned cities, states, and international distances.
Automation introduced
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
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
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
As of 2023[update], operators worldwide are in the process of retiring support for both last-mile analog telephony and ISDN, and transitioning voice service to
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
Operators
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.[3][4][5]
Technology
Network topology
The architecture of the PSTN evolved over time to support an increasing number of subscribers, call volume, destinations, features, and technologies. The principles developed in North America and in Europe were adopted by other nations, with adaptations for local markets.
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
Most automated telephone exchanges use digital switching rather than mechanical or analog switching. The
The call is carried over the PSTN using a 64
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
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
Voice quality in PSTN networks was used as a 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 latency and echo.[6]
Regulation
In most countries, the government has a regulatory agency dedicated to provisioning of PSTN services. The agency regulate technical standards, legal requirements, and set service 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.
Service retirement
In the United Kingdom, the copper POTS and ISDN-based PSTN is being retired in favour of
Several other European countries, including Estonia, Germany, the Netherlands, Spain and Portugal, have also retired, or are planning to retire, their PSTN networks.[7][8]
Countries in other continents are also performing similar transitions.[8]
See also
- List of country calling codes
- Managed facilities-based voice network
- Phreaking
- Plain old telephone service (POTS)
- Via Net Loss
References
- ^ Kushnick, Bruce (7 January 2013). "What Are the Public Switched Telephone Networks, 'PSTN' and Why You Should Care?". HuffPost. Archived from the original on 13 December 2019. Retrieved 4 July 2020.
- ISSN 1556-5068.
- ^ Information sheets and timelines. Privatisation
- ^ British Telecom Privatisation
- .
- ^ "TIA TSB-116". IHS Markit. Telecommunications Industry Association. Archived from the original on 8 July 2017. Retrieved 20 November 2011.
- ^ "Global Learnings For The PSTN Switch Off | Yay". www.yay.com. Retrieved 4 October 2023.
- ^ a b "What the UK can learn from the rest of the world when it comes to the shift to IP". BT Business. Retrieved 4 October 2023.