Re: 56 kbps modems

[Bill Stewart <stewarts@ix.netcom.com>]

>> >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.

Or it can be worse.  Almost all voice traffic in the US these days,
either once it gets to your local telephone wire center or maybe before,
is carried on T1 digital connections, which use 64kbps digital voice -
it's sampled at 8000 samples/second, A/D converted using a non-linear
8-bit scale called mu-law (or A-law for Europe), and (for the most common
framing format) has a signalling channel stego'd onto the LSB of every 6th byte.
If you knew which the "robbed bit" was, you could get 63 kbps of digital data,
but since you don't, digital signals are limited to 56kbps since they
can't trust any of the low bits (analog doesn't lose much from this.)

Unless they're _really_ talking about ISDN "modems", I'm surprised
to hear somebody saying they can take 56 kbps, turn it into analog,
let the phone company quantize and mu-law the analog into 64kbps,
and still get the original 56kbps back out.  But if they can, well,
yee-hah, ISDN is nearly dead :-)  (Not totally dead; the signalling is
still useful for some applications, the convenience of two channels on
one wire pair is nice, and the fact that people can get 56kbps without
the phone company's help will pressure them into offering ISDN for
a lower price in areas where the Phone Company's idea of "all the market 
will bear" is substantially higher than voice pricing.)

(For Norm Hardy's comment on PBXs, the main transparent approach to that
is to use ISDN as the interface.  PBXs often use ISDN to reach either
local or long-distance phone companies, since they generally want more
than vanilla signalling anyway.)

#			Thanks;  Bill
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