Public Switched Telephone Network (PSTN)

The Public Switched Telephone Network (PSTN) is the historical global collection of interconnected, circuit-switched telephone networks operated by various national, regional, and local carriers. The PSTN provides the foundational infrastructure for traditional voice call services. It is also known as the Plain Old Telephone Service (POTS) or the traditional landline network.


The origins of the PSTN date back to the late 19th century with the invention of the telephone. Early telephone networks were localized and required manual connection of calls by operators. The development of automatic switching led to the gradual formation of a globally interconnected network allowing users to seamlessly place calls to each other anywhere in the world.

Technical Components and Operation

The PSTN is a complex system based on the following key components:

  • Telephones: The devices used to make and receive calls. Traditional landline phones convert sound into electrical signals and vice versa.
  • Local Loop: The copper wires that connect a telephone to the local telephone exchange (also known as a central office).
  • Telephone Exchanges: Switching centers that establish connections between phones within a local area and route long-distance calls.
  • Trunk Lines: High-capacity lines (both copper and fiber-optic) that interconnect telephone exchanges over long distances.
  • Signaling protocols: Standards governing call setup, connection, and termination.

How PSTN Works

  1. Call Initiation: When a user lifts the handset and dials a number, the telephone sends a signal to the local telephone exchange.
  2. Circuit Switching: The local exchange establishes a temporary, dedicated circuit between the caller's phone and the destination phone.
  3. Call Routing: Long-distance calls are routed through multiple telephone exchanges to reach the correct destination.
  4. Signal Conversion: Voice is transmitted as analog electrical signals across most of the PSTN infrastructure.
  5. Call Termination: When either party hangs up, the telephone exchange disconnects the circuit.

PSTN Features

  • Universal Addressability: Through the E.164 standards, every phone on the PSTN has a unique, globally recognized phone number.
  • High Reliability: PSTN's design prioritizes uptime, even during power outages in localized areas.
  • Basic Functions: PSTN traditionally supports core calling features like caller ID, call waiting, and voicemail.

Transition to VoIP

With the advent of the internet and Voice over Internet Protocol (VoIP), the PSTN is increasingly being replaced by newer digital communications technologies. VoIP converts voice into data packets and transmits them over the internet, offering cost savings, flexibility, and enhanced features. Many countries are gradually phasing out the PSTN in favor of all-IP networks.

Challenges and Limitations

  • Aging Infrastructure: PSTN relies on legacy copper-wire technology, which is expensive to maintain and can be limited in capacity.
  • Limited Features: The PSTN primarily supports voice calls and a few basic features compared to modern VoIP systems.
  • Geographic Restrictions: International calls over PSTN can sometimes be expensive.