The Free On-line Dictionary of Computing (30 December 2018):

Public Switched Telephone Network
Plain Old Telephone System
PSTN

    (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)