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Note from a satisfied customer

  W. Curtiss Priest wrote in a message to Mike Bilow:
   MB> they are perfectlyindistinguishable from the point of view of 
   MB> an outside observer.
   WCP> Agreed.  And indeed the 28.8 Kbps modem is digital as much
   WCP> as the ISDN "modem" is digital.
  No.  A telephone circuit is composed of a number of separate physical links,
  each of which may itself be analog or digital.  The end-to-end connection
  between two analog modems may be digital, but the individual physical links in
  the middle are unaware that the circuit is in use for a digital purpose, and so
  they carry the information as carefully as possible, carrying both data and
  noise with equal faithfulness because they cannot tell the difference.
  Where the individual links in the middle know that the end-to-end circuit is
  digital, as they do with ISDN, analog noise introduced along the way can be
  distinguished easily from real data and stripped out on each link.
  -- Mike