[Date Prev][Date Next][Thread Prev][Thread Next][Date Index][Thread Index]

Note from a satisfied customer

  W. Curtiss Priest wrote in a message to Mike Bilow:
   WCP> ISDN is simply another form of analog transmission.
  * * *
   WCP> At each "step" from 0 to 1 we have a transition of a voltage
   WCP> or phase angle or whatever that makes a transition in zero
   WCP> time.  This is a true digital step function.  But if you
   WCP> check your communications handbook you find it takes
   WCP> infinite bandwidth to send a step function in zero time.
  This is a pointless argument.  Let me remove the "technical" issue and try to
  explain digital communications by example.
  Consider two people who need to communicate, one named Sender and the other
  Receiver.  They are within earshot of each other, so they agree on the
  following "analog" communications protocol:
     Protocol A:
     Sender shouts floating point numbers to Receiver in sequence.  Each number
     is between 0 and 1.  Receiver writes them down.
  It is possible to send a lot of information in each data packet, because each
  data packet can have one of an infinite number of states.  Suppose, however,
  that this high bandwidth channel proves unnecessary or unsuited to the kind of
  data being sent, and they decide to use instead the following "digital"
     Protocol D:
     Exactly the same as Protocol A, except that all numbers less than 0.5 are
     written down as "0" and all numbers greater than or equal to 0.5 are 
     written down as "1".
  Obviously, our new digital Protocol D uses exactly the same transmission method
  as our analog Protocol A, and the interesting point is that they are perfectly
  indistinguishable from the point of view of an outside observer.  It is only
  the meaning attached to the data which makes the protocol digital instead of
  -- Mike