Public Switched Telephone Network

The Public Switched Telephone Network (PSTN) is, as its most essential function, the service that can establish connections among the world's telephones that permit generally open access to one another. As a practical matter, it also encompasses the technical standards and operating procedures that both the public telephone network, and specialized nonpublic (e.g., military) telephone networks operate.

When telephones first were invented, they were interconnected by individual proprietary networks. A well-connected office, at the turn of the twentieth century, might have several telephones, each operated by a different company. As an early U.S. example, two of the larger telephone companies were Bell and Home; a Bell subscriber could not call a Home-only subscriber, and vice versa.

As telephone use became widespread in large companies, and then international, it became obvious that no single organization would control all the telephones. It was a practical necessity that telephone organizations, many of which were initially government-operated or regulated monopolies, would have to agree on a common set of technical standards, so different operating agencies could interconnect with one another.

While the first telephone operating networks used human operators to make connections even within the same company, it was apparent this could not increase indefinitely, as there would not be enough people to manage the connections. The telephone industry became the first to use a significant degree of automation.