PSTN (Public Switched Telephone Network)

Man talking on telephone
Getty Images/John Lund/Marc Romanelli

The Public Switched Telephone Network (PSTN) is the global collection of interconnects originally designed to support circuit-switched voice communication. The PSTN provides traditional Plain Old Telephone Service (POTS) - also known as landline phone service - to residences and many other establishments. Parts of the PSTN are also utilized for Internet connectivity services including Digital Subscriber Line (DSL) and Voice over Internet Protocol (VoIP).

PSTN is one of the foundation technologies of telephony - electronic voice communications. While the original forms of telephony including PSTN all relied on analog signaling, modern telephony technologies employ digital signaling, work with digital data, and also support Internet connectivity. The rollout of Internet telephony allows both voice and data to share the same networks, a convergence that the worldwide telecommunications industry is moving toward (for largely financial reasons). A key challenge in Internet telephony is to achieve the same extremely high reliability and quality levels that traditional telephone systems achieved.

History of PSTN Technology

Telephone networks were expanded worldwide during the 1900s as telephones became a routine fixture in homes. Older telephone networks used analog signaling but were gradually upgraded to use digital infrastructure. Most people associate the PSTN with the copper wiring found in many homes although modern PSTN infrastructure also uses fiber optic cables and leaves copper only for the so-called "last mile" of wiring between home and the telecommunication provider's facilitates.The PSTN utilizes the SS7 signaling protocol.

Household PSTN telephones are plugged into wall jacks installed in homes using telephone cords with RJ11 connectors. Residences don't always have jacks in all the right locations, but homeowners can install their own telephone jacks with some basic knowledge of electrical wiring.

One PSTN link supports 64 kilobits per second (Kbps) of bandwidth for data.

The PSTN phone line can be used with traditional dial-up network modems for connecting a computer to the Internet. In the early days of the World Wide Web (WWW), this was the primary form of home Internet access but was made obsolete by broadband Internet services. Dial-up Internet conections supported up 56 Kbps.


Integrated Services Digital Network (ISDN) was developed as an alternative to PSTN that provides both telephone service and also digital data support. ISDN gained popularity in larger businesses due to its ability to support large numbers of phones with low installation costs. It was also offered to consumers as an alternative form of Internet access supporting 128 Kbps.


Voice over Internet Protocol (VoIP), sometimes also called IP telephony, was designed to replace the circuit switched phone services of both PSTN and ISDN with a packet switched system based on Internet Protocol (IP). The first generations of VoIP services suffered from reliability and sound quality issues but have gradually improved over time.

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