catflap.org Online Dictionary Query

  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)
       
       

  public switched telephone network /pˈʌblɪk swˈɪtʃt tˈɛlɪfˌəʊn nˈɛtwɜːk/
  javna komutirana telefonska mreža, nepokretna komutirana javna telefonska mreža