Public switched telephone network

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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 telephone lines, fiber-optic cables, microwave transmission links, cellular networks, communications satellites, and undersea telephone cables interconnected by switching centers, such as central offices, network tandems, and international gateways, which allow telephone users to communicate with each other.

Contents

Originally a network of fixed-line analog telephone systems, the PSTN is almost entirely digital in its core network and includes mobile and wireless networks, all of which are currently[ when? ] transitioning to use the Internet Protocol to carry their PSTN traffic. [1]

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 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 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.

As of 2023, 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 modems 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

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 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.

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 SIP telephony, with an original completion date of December 2025, although this has now been put back to January 2027. See United Kingdom PSTN switch-off. Voice telephony will continue to follow the E.163 and E.164 standards, as with current mobile telephony, with the interface to end-users remaining the same.

Several other European countries, including Estonia, Germany, Iceland, the Netherlands, Spain and Portugal, have also retired, or are planning to retire, their PSTN networks. [7] [8] [9]

Countries in other continents are also performing similar transitions. [8]

See also

Related Research Articles

<span class="mw-page-title-main">ISDN</span> Set of digital telephony standards

Integrated Services Digital Network (ISDN) is a set of communication standards for simultaneous digital transmission of voice, video, data, and other network services over the digitalised circuits of the public switched telephone network. Work on the standard began in 1980 at Bell Labs and was formally standardized in 1988 in the CCITT "Red Book". By the time the standard was released, newer networking systems with much greater speeds were available, and ISDN saw relatively little uptake in the wider market. One estimate suggests ISDN use peaked at a worldwide total of 25 million subscribers at a time when 1.3 billion analog lines were in use. ISDN has largely been replaced with digital subscriber line (DSL) systems of much higher performance.

<span class="mw-page-title-main">5ESS Switching System</span>

The 5ESS Switching System is a Class 5 telephone electronic switching system developed by Western Electric for the American Telephone and Telegraph Company (AT&T) and the Bell System in the United States. It came into service in 1982 and the last unit was produced in 2003.

Circuit switching is a method of implementing a telecommunications network in which two network nodes establish a dedicated communications channel (circuit) through the network before the nodes may communicate. The circuit guarantees the full bandwidth of the channel and remains connected for the duration of the communication session. The circuit functions as if the nodes were physically connected as with an electrical circuit.

Digital subscriber line is a family of technologies that are used to transmit digital data over telephone lines. In telecommunications marketing, the term DSL is widely understood to mean asymmetric digital subscriber line (ADSL), the most commonly installed DSL technology, for Internet access.

In telecommunication, signaling is the use of signals for controlling communications. This may constitute an information exchange concerning the establishment and control of a telecommunication circuit and the management of the network.

<span class="mw-page-title-main">Time-division multiplexing</span> Multiplexing technique for digital signals

Time-division multiplexing (TDM) is a method of transmitting and receiving independent signals over a common signal path by means of synchronized switches at each end of the transmission line so that each signal appears on the line only a fraction of time according to agreed rules, e.g. with each transmitter working in turn. It can be used when the bit rate of the transmission medium exceeds that of the signal to be transmitted. This form of signal multiplexing was developed in telecommunications for telegraphy systems in the late 19th century but found its most common application in digital telephony in the second half of the 20th century.

Telephony is the field of technology involving the development, application, and deployment of telecommunication services for the purpose of electronic transmission of voice, fax, or data, between distant parties. The history of telephony is intimately linked to the invention and development of the telephone.

<span class="mw-page-title-main">Remote concentrator</span>

In modern telephony a remote concentrator, remote concentrator unit (RCU), or remote line concentrator (RLC) is a concentrator at the lowest level in the telephone switch hierarchy.

V5 is a family of telephone network protocols defined by ETSI which allow communications between the telephone exchange, also known in the specifications as the local exchange (LE), and the local loop. With potentially thousands of subscribers connected to the LE there is the problem of physically managing thousands of wires out to the local subscribers. Prior to the specification of V5 the manufacturers of exchange equipment had proprietary solutions to the problem. These solutions did not inter-operate and meant being tied into a single manufacturer's method at each exchange.

Voice over Internet Protocol (VoIP), also called IP telephony, is a method and group of technologies for voice calls for the delivery of voice communication sessions over Internet Protocol (IP) networks, such as the Internet.

Signalling System No. 7 (SS7) is a set of telephony signaling protocols developed in the 1970s, which is used to set up and tear down telephone calls in most parts of the world-wide public switched telephone network (PSTN). The protocol also performs number translation, local number portability, prepaid billing, Short Message Service (SMS), and other services.

Plain Old Telephone Service (POTS), or Plain Ordinary Telephone System, is a retronym for voice-grade telephone service employing analog signal transmission over copper loops. Originally POTS stood for Post Office Telephone Service as early phone lines in most parts of the world were operated directly by the local Post Office.

<span class="mw-page-title-main">DSLAM</span> Network device that connects DSL interfaces to a digital communications channel

A digital subscriber line access multiplexer is a network device, often located in telephone exchanges, that connects multiple customer digital subscriber line (DSL) interfaces to a high-speed digital communications channel using multiplexing techniques. Its cable internet (DOCSIS) counterpart is the cable modem termination system.

<span class="mw-page-title-main">Telephone hybrid</span> Telephone circuit element

In analog telephony, a telephone hybrid is the component at the ends of a subscriber line of the public switched telephone network (PSTN) that converts between two-wire and four-wire forms of bidirectional audio paths. When used in broadcast facilities to enable the airing of telephone callers, the broadcast-quality telephone hybrid is known as a broadcast telephone hybrid or telephone balance unit.

<span class="mw-page-title-main">Business telephone system</span> Telephone system typically used in business environments

A business telephone system is a telephone system typically used in business environments, encompassing the range of technology from the key telephone system (KTS) to the private branch exchange (PBX).

Direct inward dialing (DID), also called direct dial-in (DDI) in Europe and Oceania, is a telecommunication service offered by telephone companies to subscribers who operate private branch exchange (PBX) systems. The feature provides service for multiple telephone numbers over one or more analog or digital physical circuits to the PBX, and transmits the dialed telephone number to the PBX so that a PBX extension is directly accessible for an outside caller, possibly by-passing an auto-attendant.

In telecommunication, supervision is the monitoring of a telecommunication circuit for telephony to convey to an operator, user, or a switching system, information about the operational state of the circuit. The typical operational states of trunks and lines are the idle and busy states, seizure, and disconnect. The states are indicated by various electrical signals and electrical conditions depending on the type of circuit, the type of terminating equipment, and the type of intended service.

A managed facilities-based voice network (MFVN) is a communications network managed, operated, and maintained by a voice service provider that delivers traditional telephone service via a loop start analog telephone interface. MFVNs are interconnected with the public switched telephone network (PSTN) or other MFVNs and provide dialtone to end users. Historically, this was provided by equipment at Bell company central offices, however today's MFVNs can include a combination of access network, battery-backed customer premises equipment (CPE), network switches and routers, network management systems, voice call servers, and gateways to the broader PSTN.

<span class="mw-page-title-main">Telephone exchange</span> Interconnects telephones for calls

A telephone exchange, also known as a telephone switch or central office, is a crucial component in the public switched telephone network (PSTN) or large enterprise telecommunications systems. It facilitates the interconnection of telephone subscriber lines or digital system virtual circuits, enabling telephone calls between subscribers.

References

  1. ↑ 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.
  2. ↑ Werbach, Kevin D. (2013). "No Dialtone: The End of the Public Switched Telephone Network". SSRN Electronic Journal. doi:10.2139/ssrn.2241658. ISSN   1556-5068.
  3. ↑ Information sheets and timelines. Privatisation
  4. ↑ British Telecom Privatisation
  5. ↑ Moon, Jeremy; Richardson, J.J.; Smart, Paul (1986). "The privatisation of British Telecom: A case study of the extended process of legislation". European Journal of Political Research. 14 (3): 339–355. doi:10.1111/j.1475-6765.1986.tb00839.x.
  6. ↑ "TIA TSB-116". IHS Markit . Telecommunications Industry Association. Archived from the original on 8 July 2017. Retrieved 20 November 2011.
  7. ↑ "Global Learnings For The PSTN Switch Off | Yay". www.yay.com. Retrieved 4 October 2023.
  8. 1 2 "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.
  9. ↑ "Niðurlagning koparheimtaugakerfis Mílu". Míla ehf (in Icelandic). Retrieved 29 April 2024.
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