Source : Free On-Line Dictionary of Computing

Public Switched Telephone Network
     
         (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)