Public switched telephone network or PSTN

Overview of Public switched telephone network or PSTN

The public switched telephone network (PSTN) offers public telecommunications infrastructure and services. The PSTN is the global network of circuit-switched telephone networks, which is run by national, regional, or local telephony providers. Telephone lines, fiber optic cables, microwave transmission connections, cellular networks, communications satellites, and underwater telephone cables are all joined via switching centers, allowing most phones to interact with one another. 

The PSTN, which was once a network of fixed-line analog telephone systems, is now nearly fully digital in its core network and incorporates mobile and other networks in addition to fixed lines. 

The PSTN’s technical functioning is per the ITU-T standards. These standards enable various networks in different countries to easily interact. The E.163 and E.164 standards establish a unified worldwide address space for telephone numbers. Telephones all around the world may dial each other because of linked networks and a unified numbering plan. 

History of Public Switched Telephone Network

The telephone was first commercialized in 1876, using equipment operated in pairs for private use between two sites. Users who needed to speak with people in various places might do so by using as many telephones as they needed. To notify another user of the wish to make a phone call, whistled loudly into the transmitter until the other party received the alarm. Bells were quickly installed to stations for signaling, eliminating the need for an attendant to wait for the whistle. 

Later telephones used the exchange idea that was already in use in telegraph networks. Each telephone was linked to a telephone exchange set up for a certain town or area. Trunks were built between exchanges to provide communication outside of this exchange region. Networks were built hierarchically until they stretched over towns, countries, continents, and seas. 

Automation enabled pulse dialing between the telephone and the exchange, allowing one subscriber to contact another subscriber on the same exchange directly, but long-distance calling across several exchanges needed manual switching by operators. Later, more advanced address signaling systems, such as multi-frequency signaling methods, enabled customers to make direct-dialed long-distance calls, culminating in the Signalling System 7 (SS7) network, which regulated calls between most exchanges by the end of the twentieth century. 

With the expansion of the PSTN, teletraffic engineering techniques were required to be employed to provide quality of service (QoS) assurances to consumers. A. K. Erlang’s work provided the mathematical underpinnings for methodologies used to calculate the capacity needs, equipment configuration, and the number of employees needed to provide a given level of service. 

In the 1970s, the telecommunications sector began deploying packet-switched network data services using the X.25 protocol, which was already being used in the PSTN. 

In the 1980s, the industry began planning for digital services with the assumption that they would follow a similar pattern to voice services and envisioned end-to-end circuit-switched services known as the Broadband Integrated Services Digital Network (B-ISDN). The disruptive technology of the Internet surpassed the B-ISDN concept. 

The oldest portions of the telephone network still utilize analog technology for the last mile loop to the end-user at the turn of the twenty-first century. Digital technologies like DSL, ISDN, FTTx, and cable modems, on the other hand, have grown more widespread in this segment of the network. 

Several major private telephone networks, mainly for military purposes, are not linked to the PSTN. Private networks are maintained by major corporations that are only connected to the PSTN via restricted gateways, such as a large private branch exchange (PBX). 


The network operators were in charge of developing networks and selling services to clients. In the United States, the Bell Telephone Company was the first to incorporate to provide PSTN services. 

However, in other nations, the responsibility of establishing telephone networks passed to the government since the investment required was very high, and telephone service was quickly becoming an important public utility. For example, in the United Kingdom, the General Post Office joined together a variety of private firms to establish a single nationalized corporation. These governmental monopolies have been broken up or sold off through privatization in recent decades. 


In most nations, the central government has a regulator who is solely responsible for overseeing the supply of PSTN services in that country. Their responsibilities may include, for example, ensuring that end users are not overcharged for services when monopolies exist. These regulatory authorities may also set the fees that operators charge to transport one other’s traffic. 

PSTN Technology 

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, etc. The model developed by the United States and Canada was adopted by other nations, with adaptations for local markets. 

The initial idea was that telephone exchanges would be organized into hierarchies, such that if a call could not be processed in a local cluster, it would be sent to one higher up for further processing. This decreased the number of connecting trunks necessary across large distances between operators while still keeping local traffic distinct. 

However, the cost of transmission and equipment is cheaper in current networks, and, while hierarchies still exist, they are considerably flatter, with maybe just two levels. 

Digital channels 

Instead of mechanical or analog switching, most automated telephone exchanges employ digital switching. The trunks that connect the exchanges are likewise digital and are referred to as circuits or channels. Analog two-wire circuits, on the other hand, are still utilized to link the last mile from the exchange to the telephone in the home (also called the local loop). The analog audio stream is digitized at an 8 kHz sample rate with the 8-bit resolution to transmit a typical phone call from a calling party to a called party using G.711 nonlinear pulse-code modulation. The call is then routed through telephone exchanges from one end to the other. The call is moved across telephone exchanges using a call setup protocol (typically ISUP) as part of an overall routing plan. 

The call is routed via the PSTN using a 64 kbit/s channel built by Bell Labs. Digital Signal 0 is the name given to this channel (DS0). The fundamental granularity of circuit switching in a telephone exchange is the DS0 circuit. Because DS0s are aggregated in time-division multiplexing (TDM) equipment to produce greater capacity communication lines, they are also known as timeslots. 

A Digital Signal 1 (DS1) circuit transports 24 DS0s on a T-carrier (T1) line in North America or Japan, or 32 DS0s (30 for calls plus two for framing and signaling) on an E-carrier (E1) line in the majority of other countries. The multiplexing function in contemporary networks is relocated as close to the end-user as feasible, generally into cabinets along the roadside in residential areas or into big commercial facilities. 

These aggregated circuits are routed from the original multiplexer to the exchange through the access network, a collection of equipment. Although certain sections still utilize the older PDH technology, the access network and inter-exchange transit employ synchronous optical transmissions, such as SONET and Synchronous Digital Hierarchy (SDH). 

There are several reference points specified inside the access network. Most of these are primarily of relevance to ISDN, but one – the V reference point – is of more general significance. This serves as the link between a primary multiplexer and an exchange. The protocols at this reference point were standardized as the V5 interface in ETSI regions. 

Impact on IP standards 

To define acceptable levels of audio delay and echo, speech quality over PSTN networks was used as the baseline for the creation of the Telecommunications Industry Association’s TIA-TSB-116 standard on voice-quality guidelines for IP telephony. 


Roosho is a Telecommunication engineer with more than 10 years of experience in VoIP and Unified Communications. His expertise has helped him complete more than 100 projects for Feds, Public Universities, Large Group of Companies in his 10 years of experience, and he is still growing with the industry. He loves to share his ideas about his experience and expertise with the world. That’s why VoIP Bible has made him the lead technical content writer of VoIP Bible.


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