Re: Note from a satisfied customer

["Andrew C. Esh" <andrewes@cnt.com>]

  W. Curtiss Priest wrote:
  > 
  > One fine point.  It may sound picky, but all modem/ISDN transmissions
  > are analog.  This phrase of using analog versus digital is simply
  > incorrect.
  
  No, you're wrong. If you define analog that way, then there is no such
  thing as "digital" in either transmission, or inside computers. Every
  digital signal, at some physical level, has to be represented by an
  analog signal. All T1, T3, LAN, Fiber, Microwave, ... it's all analog.
  Even RS-232 is analog: When the voltage on the line reaches a certain
  level, it's counted as a 1 or a 0 bit (depending on the polarity),
  thereby being translated into a digit. The same polarity and analog
  limit system is used throughout all "digital" computers. The only truly
  digital mediums I can think of are punched cards, paper tape, and
  CD-ROMS, since they actually contain discrete digits. Even CD-ROMs are
  read by measuring the reaction to the reflectivity of a suface pit, by
  using an analog circuit. The analog level of laser energy returned
  determines the value of the bit. 
  
  What makes ISDN digital, is that at no level available to the customer 
  is analog. With POTS, the changes in the frequency of your voice are
  put directly onto the physical media, as an analog signal. With ISDN,
  your voice is digitized, and sent at one level as digital information.
  What happens below that level to actually transport the bits is
  irrelevant. The service is digital, and is measured in bits per second,
  no matter what is being transported across it.
  
  The reason this is a significant improvement over analog phone service
  is because you are guaranteed that a certain number of bits can be
  transmitted through the connection, and it's the responsibility of the
  service provider to make sure all the bits get there intact. With analog
  service, a certain amount of distortion and static can be allowed, and it won't
  noticably degrade the service. This is not acceptable for digital service.
  For each customer, the service provider (SP) must supply a much larger "envelope"
  of analog bandwidth in order the pare it down to an agreeable digital signal.
  The whole system is defined by its digital characteristics, so whatever
  the SP has to provide in order to obtain that digital service is what they
  are required to do. With analog service, all they had to provide is 3000Hz
  of bandwidth, and not even cleanly.
  
  ISDN may not be purely digital, but neither is anything else, and what we
  negotiate for when we buy ISDN is digital bits per second, not analog
  frequency bandwidth.
  
  -- 
  Andrew C. Esh			mailto:andrew_esh@cnt.com
  Computer Network Technology	andrewes@mtn.org (finger for PGP key)
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