<communications> (PSTN, T.70) The collection of interconnected systems operated by the various telephone companies and administrations (telcos and PTTs) around the world. Also known as the Plain Old Telephone System (POTS) in contrast to xDSL and ISDN (not to mention other forms of PANS). The PSTN started as human-operated analogue circuit switching systems (plugboards), progressed through electromechanical switches. By now this has almost completely been made digital, except for the final connection to the subscriber (the "last mile"): The signal coming out of the phone set is analogue. It is usually transmitted over a {twisted pair cable} still as an analogue signal. At the telco office this analogue signal is usually digitised, using 8000 samples per second and 8 bits per sample, yielding a 64 kb/s data stream (DS0). Several such data streams are usually combined into a fatter stream: in the US 24 channels are combined into a T1, in Europe 31 DS0 channels are combined into an E1 line. This can later be further combined into larger chunks for transmission over high-bandwidth core trunks. At the receiving end the channels are separated, the digital signals are converted back to analogue and delivered to the received phone. While all these conversions are inaudible when voice is transmitted over the phone lines it can make digital communication difficult. Items of interest include A-law to mu-law conversion (and vice versa) on international calls; robbed bit signalling in North America (56 kbps <--> 64 kbps); data compression to save bandwidth on long-haul trunks; signal processing such as echo suppression and voice signal enhancement such as AT&T TrueVoice. (2000-07-09)

The public switched telephone network (PSTN) provides infrastructure and services for public telecommunication. The PSTN is the aggregate of the world's circuit-switched telephone networks that are operated by national, regional, or local telephony operators.

A telephone network is a telecommunications network that connects telephones, which allows telephone calls between two or more parties, as well as newer features such as fax and internet. The idea was revolutionized in the 1920s, as more and more people purchased telephones and used them to communicate news, ideas, and personal information.