Public switched telephone network

The public switched telephone network (PSTN) is the aggregate of the world's circuit-switched telephone networks that are operated by national, regional, or local telephony operators, providing infrastructure and services for public telecommunication. The PSTN consists of telephone lines, fiber optic cables, microwave transmission links, cellular networks, communications satellites, and undersea telephone cables, all interconnected by switching centers, thus allowing most telephones to communicate with each other. Originally a network of fixed-line analog telephone systems, the PSTN is now almost entirely digital in its core network and includes mobile[1] and other networks, as well as fixed telephones.[1]

The technical operation of the PSTN adheres to the standards created by the ITU-T. These standards allow different networks in different countries to interconnect seamlessly. The E.163 and E.164 standards provide a single global address space for telephone numbers. The combination of the interconnected networks and the single numbering plan allow telephones around the world to dial each other.


Commercialization of the telephone began in 1876, 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, so an attendant no longer needed to wait for the whistle.

Later telephones 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 communications outside this exchange area, trunks were installed between exchanges. Networks were designed in a hierarchical manner until they spanned cities, countries, continents and oceans.

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).


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.


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.


Network topology

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

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

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


  1. ^ a b Kushnick, Bruce (7 January 2013). "What Are the Public Switched Telephone Networks, 'PSTN' and Why You Should Care?". Huffington Post Blog. Retrieved 11 April 2014.
  2. ^ "TIA TSB-116". Retrieved 2011-11-20.
Area code 218

Area code 218 is part of the North American Numbering Plan (NANP) of the public switched telephone network for the northern part of the US state of Minnesota. It is one of Minnesota's original two codes, although its geographical area has been modified since inception. By area, the region is the largest area code in Minnesota, covering roughly the northern half of the state. It includes the cities of Duluth, Hibbing, Brainerd, Bemidji, Fergus Falls, and Moorhead.

According to a 1947 map of the NANP, the 218 region was originally r-shaped and covered about two-thirds of Minnesota. Area code 612 covered the remaining southeastern portion. In 1954, the shape of 218 was modified to coincide approximately with its current shape when the original southwestern portion of 218 was combined with the southern portion of 612 to form area code 507, which stretched across the southern fifth of Minnesota. A small change in the 1990s brought the Northwest Angle into the 218 area after being part of Bell Canada's Clearwater Bay exchange in Area code 807.

Because of the low population density in northern Minnesota, the region was unaffected when the 612 area was subdivided in 1996. The resulting area code 320, the former western portion of 612, runs the length of the southern border with 218, and the 612 area code has been reduced in size so much that it now just covers the city of Minneapolis and a few nearby suburbs.

The western portion of 218—generally everything from Brainerd westward—shares a LATA with the eastern half of North Dakota, including Fargo and Grand Forks. This means that several numbers in North Dakota's 701 aren't available for use.

Under present projections, northern Minnesota will not need another area code until mid-2028 at the earliest. Despite the proliferation of cell phones and pagers, particularly in Duluth and Fargo-Moorhead, 218 is nowhere near exhaustion.

Asterisk (PBX)

Asterisk is a software implementation of a private branch exchange (PBX). In conjunction with suitable telephony hardware interfaces and network applications, Asterisk is used to establish and control telephone calls between telecommunication endpoints, such as customary telephone sets, destinations on the public switched telephone network (PSTN), and devices or services on voice over Internet Protocol (VoIP) networks. Its name comes from the asterisk (*) symbol for a signal used in dual-tone multi-frequency (DTMF) dialing.

Asterisk was created in 1999 by Mark Spencer of Digium, today a division of Sangoma Technologies Corporation. Originally designed for Linux, Asterisk runs on a variety of operating systems, including NetBSD, OpenBSD, FreeBSD, macOS, and Solaris, and can be installed in embedded systems based on OpenWrt.

Bijoy Phone

Bijoy Phone is a Bangladeshi fixed line operator.It is a private public switched telephone network (PSTN) operator in Bangladesh. As of May 2008, total number of subscriber of this operator is 3.948 thousand.

Cable telephony

Cable telephony is a form of digital telephony over cable TV networks. A telephone interface installed at the customer's premises converts analog signals from the customer's in-home wiring to a digital signal, which is then sent over the cable connection to the company's switching center. The signal is then sent on to the public switched telephone network (PSTN). Cable telephone provides another revenue stream for cable television system operators and gives the consumer the convenience of a single bill for combined television, internet and telephone services.

Class-5 telephone switch

A Class-5 telephone switch is a telephone switch or telephone exchange in the public switched telephone network located at the local telephone company's central office, directly serving subscribers. Class-5 switch services include basic dial-tone, calling features, and additional digital and data services to subscribers connected to a local loop.

Demarcation point

In telephony, the demarcation point is the point at which the public switched telephone network ends and connects with the customer's on-premises wiring. It is the dividing line which determines who is responsible for installation and maintenance of wiring and equipment—customer/subscriber, or telephone company/provider. The demarcation point varies between countries and has changed over time.

Demarcation point is sometimes abbreviated as demarc, DMARC, or similar. The term MPOE (minimum or main point of entry) is synonymous, with the added implication that it occurs as soon as possible upon entering the customer premises. A network interface device often serves as the demarcation point.

Dial plan

A dialing plan establishes the permitted sequences of digits dialed on subscriber or station lines with subscriber premises equipment, such as telephones and private branch exchange (PBX) systems. Dialing plans in the public switch telephone network (PSTN) have traditionally been more commonly referred to as dialing procedures. The dialing plan of a private telephone system or a customer premise equipment, such as an analog telephone adapter (ATA) or an IP phone, is sometimes also called dial plan. The digit sequences (numbers) permissible in a dialing plan may be as short as a single digit, e.g. for reaching an operator, or as long as a complete international telephone number, including trunk prefixes and international prefixes.


E.164 is an ITU-T recommendation, titled The international public telecommunication numbering plan, that defines a numbering plan for the worldwide public switched telephone network (PSTN) and some other data networks.

E.164 defines a general format for international telephone numbers. Plan-conforming numbers are limited to a maximum of 15 digits, excluding the international call prefix. The presentation of a number at the B-party device is usually prefixed with the plus sign (+), indicating that the number includes the country calling code. This is done by the B-party subscribers network by usually looking at the NOA (Nature Of Address) field of the signaling messages. When dialing, the number must typically be prefixed with the appropriate international call prefix (in place of the plus sign), which is a trunk code to reach an international circuit from within the country of call origination.

As described in by the ITU, the E.164 general format must contain only digits split as follows :

Country code (max 3 digits)

Subscriber number (max 12 digits)Alternative formats (with area codes and country specific numbers) are available.

The title of the original version and first revision of the E.164 standard was Numbering Plan for the ISDN Era.


An IP PBX ("Internet Protocol private branch exchange") is a system that connects telephone extensions to the public switched telephone network (PSTN) and provides internal communication for a business. An IP PBX is a PBX system with IP connectivity and may provide additional audio, video, or instant messaging communication utilizing the TCP/IP protocol stack.

Voice over IP (VoIP) gateways can be combined with traditional PBX functionality to allow businesses to use their managed intranet to help reduce long distance expenses and take advantage of the benefits of a single network for voice and data (converged network). An IP PBX may also provide CTI features.

An IP PBX can exist as a physical hardware device or as a software platform.

Interconnect agreement

An interconnect agreement is a business contract between telecommunications organizations for the purpose of interconnecting their networks and exchanging telecommunications traffic. Interconnect agreements are found both in the public switched telephone network and the Internet.

In the public switched telephone network, an interconnect agreement invariably involves settlement fees based on call source and destination, connection times and duration, when these fees do not cancel out between operators.

On the Internet, where the concept of a "call" is generally hard to define, settlement-free peering and Internet transit are common forms of interconnection. A contract for interconnection within the Internet is usually called a peering agreement.

Interconnect agreements are typically complex contractual agreements involving payment schemes and schedules, coordination of routing policies, acceptable use policies, traffic balancing requirements, technical standards, coordination of network operations, dispute resolution, etc. Legal and regulatory requirements are often an issue. For example, network operators may be forced by law to interconnect with their competitors. In the United States, the Telecommunications Act of 1996 mandated methods of interconnection and the compensation models for doing so.

Onetel Communication Ltd.

Onetel is a Bangladeshi fixed line operator.It is a private public switched telephone network (PSTN) operator in Bangladesh. As of May 2008, total number of subscriber of this operator is 37.796 thousand.

Plain old telephone service

Plain old telephone service (POTS), or plain ordinary telephone service, is a retronym for voice-grade telephone service employing analog signal transmission over copper loops. POTS was the standard service offering from telephone companies from 1876 until 1988 in the United States when the Integrated Services Digital Network (ISDN) Basic Rate Interface (BRI) was introduced, followed by cellular telephone systems, and voice over IP (VoIP). POTS remains the basic form of residential and small business service connection to the telephone network in many parts of the world. The term reflects the technology that has been available since the introduction of the public telephone system in the late 19th century, in a form mostly unchanged despite the introduction of Touch-Tone dialing, electronic telephone exchanges and fiber-optic communication into the public switched telephone network (PSTN).

Routing in the PSTN

Routing in the PSTN is the process used to route telephone calls across the public switched telephone network.

Telephone calls are routed across a network of potentially many switching systems, often owned by different telephone carriers. Switching systems are connected with trunks. Each switch may have many neighbors. Neighboring switches owned by different operators are connected at interconnect points. The PSTN is not a full mesh network with the nodes of every operator directly connected to those of every other, which would be impractical and inefficient. Therefore, calls may be routed through intermediate operator networks before they reach their final destination. Efficient least-cost routing is an important procedure in PSTN routing.


SATel is a Bangladeshi fixed line regional operator. It is a private public switched telephone network (PSTN) operator in the South-East region of Bangladesh. As of November 2009, total number of subscriber of this operator is 17.577 thousand.

Secure Terminal Equipment

Secure Terminal Equipment (STE) is the U.S. government's current (as of 2008), encrypted telephone communications system for wired or "landline" communications. STE is designed to use ISDN telephone lines which offer higher speeds of up to 128 kbit/s and are all digital. The greater bandwidth allows higher quality voice and can also be utilized for data and fax transmission through a built-in RS-232 port. STE is intended to replace the older STU-III office system and the KY-68 tactical system. STE sets are backwards compatible with STU-III phones, but not with KY-68 sets.STE sets look like ordinary high-end office desk telephones and can place unsecured calls to anywhere on the public switched telephone network (PSTN), as well as secured calls on it via the phone's backwards compatible STU-III mode. There is a PC Card slot in the STE that allows a Fortezza Plus (KOV-14) Crypto Card or KSV-21 Enhanced Crypto Card to be inserted. When an NSA configured Crypto Card is present, secure calls can be placed to other STE phones. STE phones are "releasable" (unlike STU-III sets). All cryptographic algorithms are in the crypto card.

Newer STE sets can communicate with systems that use the Secure Communications Interoperability Protocol (SCIP) (formerly Future Narrowband Digital Terminal (FNBDT)). There are upgrade kits available for older units.

Telephone line

A telephone line or telephone circuit (or just line or circuit within the industry) is a single-user circuit on a telephone communication system. This is the physical wire or other signaling medium connecting the user's telephone apparatus to the telecommunications network, and usually also implies a single telephone number for billing purposes reserved for that user. Telephone lines are used to deliver landline telephone service and Digital subscriber line (DSL) phone cable service to the premises. Telephone overhead lines are connected to the public switched telephone network.

Telephone network

A telephone network is a telecommunications network used for telephone calls between two or more parties.

There are a number of different types of telephone network:

A landline network where the telephones must be directly wired into a single telephone exchange. This is known as the public switched telephone network or PSTN.

A wireless network where the telephones are mobile and can move around anywhere within the coverage area.

A private network where a closed group of telephones are connected primarily to each other and use a gateway to reach the outside world. This is usually used inside companies and call centres and is called a private branch exchange (PBX).

Integrated Services Digital Network (ISDN)Public telephone operators (PTOs) own and build networks of the first two types and provide services to the public under license from the national government. Virtual Network Operators (VNOs) lease capacity wholesale from the PTOs and sell on telephony service to the public directly

Telephone numbers in Venezuela

The Venezuelan Numbering Plan is an open telephone numbering plan with three-digit area codes and seven-digit telephone numbers that directs telephone calls to particular regions on a public switched telephone network (PSTN) or to a mobile telephone network, where they are further routed by the local network. The last revision of the current numbering plan is September 21, 2000.

Example landline calls (using Caracas as reference):

5551212 (within metropolitan area)

0212 5551212 (within Venezuela, Caracas excluded)

011 58 212 5551212 (from the U.S./Canada to Venezuela)

Voice browser

A voice browser is a software application that presents an interactive voice user interface to the user in a manner analogous to the functioning of a web browser interpreting Hypertext Markup Language (HTML). Dialog documents interpreted by voice browser are often encoded in standards-based markup languages, such as Voice Dialog Extensible Markup Language (VoiceXML), a standard by the World Wide Web Consortium.

A voice browser presents information aurally, using pre-recorded audio file playback or text-to-speech synthesis software. A voice browser obtains information using speech recognition and keypad entry, such as DTMF detection.

As speech recognition and web technologies have matured, voice applications are deployed commercially in many industries and voice browsers are supplanting traditional proprietary interactive voice response (IVR) systems. Voice browser software is delivered in a variety of implementations models.

Systems that present a voice browser to a user, typically provide interfaces to the public switched telephone network or to a private branch exchange.

Network topology
and switching

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