PSTN

The Public Switched Telephone Network (PSTN) is the worldwide collection of circuit-switched telephone networks used for voice communications. Historically based on copper lines and analog signaling, it provides basic telephone service (POTS) and related features. It remains the traditional backbone of national and international voice traffic, alongside newer packet-switched technologies.

A PSTN call travels from a subscriber's set to a local exchange, through a hierarchy of switches and trunk lines, to the destination network. Local loops connect to exchanges; calls are carried on dedicated circuits via time-division multiplexing across intercity and international trunks. Calls are routed and billed using standardized numbering like E.164.

Signaling and control in many networks use Signaling System 7 (SS7), handling call setup, routing, number translation, and services. The PSTN provides voice, fax, and data over circuit-switched paths, with features such as caller ID and voicemail.

Modern networks increasingly use IP technology, but the PSTN continues through gateways and legacy interconnections. Interworking with IP allows VoIP and traditional telephony to coexist. The PSTN offers broad coverage and reliability, but can require centralized power and generally offers lower data capacity than broadband.

Regulation aims for universal service, number portability, and lawful intercept. Standards from ITU-T and national regulators guide compatibility and interoperability. The PSTN remains a foundational voice network worldwide, even as mobile and broadband networks expand.