The public switched telephone network (PSTN) is the worldwide collection of interconnected public telephone networks that was designed primarily for voice traffic.
The PSTN is a circuit-switched network. That is, a dedicated circuit (also referred to as a channel) is established for the duration of a transmission, such as a telephone call. This contrasts with packet switching networks, in which messages are divided into small segments called packets and each packet is sent individually. The Internet is based on a packet-switching protocol, TCP/IP.
Originally only an analog system, the PSTN is now almost entirely digital, even though most subscribers are connected via analog circuits, and it now includes mobile phones in addition to fixed-line phones. Only the very oldest and most backward parts of the PSTN still use analog technology for anything other than the final mile connections to individual homes and other end users. In recent years digital connections have been increasingly been made available to end users through such services such as ISDN (integrated services digital network), DSL (digital subscriber line) and cable.
There are numerous private telephone networks that are not linked to the PSTN, typically for military use. There are also many corporate networks that are linked to the PSTN only through limited gateways.
The roots of the PSTN can be traced back to 1876, when Alexander Graham Bell was awarded the first patent for the telephone, and the start of telephone service soon thereafter. Before the rise of the Internet, it was well understood that the PSTN would become increasingly important for data transmission, but it was thought that such transmission would occur using circuit switching, just as it is used by voice communications.
In recent years, however, it has become increasingly apparent that the long-term future of the PSTN is to become just another application of the Internet. That is, the voice traffic that is currently carried by the PSTN will be shifted to VoIP (voice over Internet protocol), thus allowing the PSTN infrastructure to be converted from circuit switching to packet switching. However, it will be necessary to make additional progress on improving the quality of VoIP before this can become a reality.
The PSTN is sometimes referred to as the plain old telephone system (POTS). However, the latter implies the older, analog system and the former is more inclusive.
Created October 20, 2005.