<communications> (PSTN, T.70) The collection of interconnected systems operated by the various telephone companies and administrations (PTTs) around the world. Also known as the Plain Old Telephone System (POTS) in contrast to Integrated Services Digital Network (and PANS).
The PSTN started as human-operated analogue circuit switching systems (plugboards), progressed through electromechanical switches and are now (1994) almost completely digital except for the final connection to the subscriber. Other things that make the PSTN less than bit-transparent include A-law to mu-law conversion or 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.
(16 Oct 1996)