edVirtus Reference

What Is the Public Switched Telephone Network?

How Does the Public Switched Telephone Network Connect Telephone Calls Around the World?

The Public Switched Telephone Network (PSTN) is the worldwide system of interconnected telephone exchanges, transmission links, and signalling systems that provides voice communication between fixed-line telephones. For more than a century, the PSTN formed the foundation of global voice communications, enabling any subscriber to call another subscriber almost anywhere in the world.

The basic principle is straightforward. When a telephone call is initiated, the caller's local exchange analyses the dialled number and establishes a communication path through a series of interconnected telephone exchanges. In traditional PSTN networks, this path is created using circuit switching, which reserves a dedicated communication circuit for the duration of the call. Once the conversation ends, the circuit is released and becomes available for other users.

A useful analogy is reserving a private railway track between two stations. While the train is travelling, no other train uses that track, ensuring uninterrupted travel. Similarly, the traditional PSTN reserves a dedicated communication path for each telephone call until the conversation is complete.

The PSTN has evolved significantly since its origins. Early telephone systems used manual switchboards operated by human operators, while later systems employed electromechanical switching and eventually fully digital electronic exchanges. Today, much of the transmission infrastructure consists of optical fibre, digital microwave links, and Internet Protocol (IP)-based backbone networks, even though users continue to experience a conventional telephone service.

One of the defining features of the PSTN is its use of internationally standardised signalling and numbering systems. Technologies such as Signalling System No. 7 (SS7) enable telephone exchanges to establish, manage, and terminate calls efficiently while supporting services such as caller identification, call forwarding, roaming, and emergency calling.

It is important to distinguish the Public Switched Telephone Network from Voice over Internet Protocol (VoIP). The traditional PSTN uses dedicated circuit switching to carry voice traffic, whereas VoIP converts speech into digital packets that are transmitted across packet-switched IP networks. Although many modern telephone providers now carry calls over IP infrastructure, they often continue to present a standard PSTN interface to customers, resulting in a gradual migration from circuit-switched to packet-switched technology.

Today, the traditional circuit-switched PSTN is being progressively replaced by all-IP communication networks in many countries. Nevertheless, the PSTN remains one of the most influential communication systems ever developed. Its principles of switching, numbering, signalling, and universal connectivity continue to underpin modern voice communication services, whether delivered over legacy telephone networks or contemporary broadband and mobile infrastructures.