What is a Pstn or a Public Switched Phone Telephone Network?

By: Gloria Moore

PSTN stands for public switched telephone network. It is the network of the public circuit-switched telephone networks around the world, just as the internet is the network of the various IP-based packet-switched networks. At first, PSTN was only a fixed-line analogy telephone system network, however, these days, it is almost exclusively digital and includes mobile phones as well as land lines.

The technical standards developed by the ITU-T are what generally govern the PSTN, as it uses E.163/E.164 addresses - that is, regular telephone numbers - in order to set its addresses.

At first, telephones didn't have any network at all, but were in private use, paired up in their wiring. When users wished to speak to other people, they had as many phones as were necessary in order to communicate. If the person wished to speak to someone else, they whistled into a transmitter until the other person heard it and picked up

Later, a bell was added in order to signal an incoming call. Soon, a switch-hook was also added so that the exchange principle in telegraph networks could be employed. Each user's home phone was hooked up to a local telephone exchange, and each of those telephone exchanges were then hooked up to trunks. These networks were all connected in a hierarchical layout until finally neighborhoods, cities, countries, and the world was all hooked up together. Thus, the PSTN was born.

The exchange system changed when automation entered into the picture through pulse dialing. This made exchanges much more sophisticated and required fewer human participants to connect a call. Later, multi-frequency dialing - which brought about the SS7 (signaling protocol) network - became the standard connection for the majority of exchanges by the end of last century.

Though the phone network was created using voice connections that were analog, and used manual switchboards, the automated telephone exchanges replaced those switchboards, and digital switch technologies were also introduced. Today, switches almost always use digital circuits among exchanges, while two-wire analog circuits are still utilized for connecting the majority of telephones. In the PSTN, the basic digital circuit is a 64-kilobits-per-second channel.

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