The public switched telephone network (PSTN) is the concentration of the world's public circuit-switched telephone networks, in much the same way that the Internet is the concentration of the world's public IP-based packet-switched networks. Originally a network of fixed-line analog telephone systems, the PSTN is now almost entirely digital, and now includes mobile as well as fixed telephones.
The PSTN is largely governed by technical standards created by the ITU-T, and uses E.163/E.164 addresses (known more commonly as telephone numbers) for addressing.
The PSTN was the earliest example of traffic engineering to deliver Quality of Service guarantees. A.K. Erlang (1878-1929) is credited with establishing the mathematical foundations of methods required to determine the amount and configuration of equipment and personnel required to deliver a specific level of service.
In the 1970s the telecommunications industry conceived that digital services would follow much the same pattern as voice services, and conceived a vision of end-to-end circuit switched services, known as the Broadband Integrated Services Digital Network (B-ISDN). The B-ISDN vision has been overtaken by the disruptive technology of the Internet.
Only the very oldest and most backward parts of the telephone network still use analogue technology for anything other than the last mile loop to the end user, and in recent years digital services have been increasingly rolled out to end users using services such as DSL, ISDN and Cable Systems.
Many observers believe that the long term future of the PSTN is to be just one application of the Internet - however, the Internet has some way to go before this transition can be made. The Quality of Service guarantee is one aspect that needs to be improved on in the Voice over IP technology.
There are a number of large private telephone networks which are not linked to the PSTN, usually for military purposes. There are also private networks run by large companies which are linked to the PSTN only through limited gateways, like a large private branch exchange (PBX) system.