public switched telephone network

telephone system based upon copper wires that carry analog voice signals, PSTN

The public switched telephone network (PSTN) is the network of the world's public circuit-switched telephone networks, in much the same way that the Internet is the network 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. It is sometimes referred to as the Plain Old Telephone Service (POTS).
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A domestic telecommunications network usually accessed by telephones, key telephone systems, private branch exchange trunks, and data arrangements. Note: Completion of the circuit between the call originator and call receiver in a PSTN requires network signaling in the form of dial pulses or multifrequency tones.

<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)