Re: 56 kbps modems

[tcmay@got.net (Timothy C. May)]

At 3:24 PM 9/13/96, Enzo Michelangeli wrote:
>On Fri, 13 Sep 1996, Asgaard wrote:
>
>> >U.S. Robotics and Rockwell International are planning new modems with
>> >speeds up to 56 kbps a second, almost double the speed of the fastest
>> >rate now available.  The new devices should be available by the end of
>> >the year, although their top speed initially may be less than 56 kbps.
>> >(Wall Street Journal 12 Sep 96 B11)
>>
>> People who seemed to know used to say that 'the Shannon limit'
>> set an absolute upper limit around 40 kbps. Has Shannon been
>> proven wrong, or what?
>
>Well, it all depends on the signal-to-noise ratio. Also, if the noise is
>not white gaussian the situation can be even better.

No, this is not what's important in this case.

(Besides, the "upper limit" is really more about the Nyquist Limit.
Shannon's Theorem says that even in the presence of noise, this limit can
be approached if proper coding schemes and whatnot are used.)

While it is true that a noisy channel can reduce the effective channel
capacity to something less than its capacity (for some particular coding
scheme), the upper limit on channel capacity is whatever it is.

As to the original question about modems and 40 kbps vs. 56 kpbs rates,
this depends on "tricks" involving definitions of a "symbol" (a la the
familiar argument about baud vs. bits per second). "Trellis coding," for
example.

I'm not a modem designer, but I'm not surprised to see these incremental
improvements...none involve huge gains, and  none involve getting, say, a
megabit per second through a 6 kHz (or whatever) audio line--now that
_would_ violate Shannon's Theorem, the Nyquist Limit, etc.

--Tim May

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