Re: 33.6 USR Modem

Marc Lindahl (shark@dti.net)
Fri, 29 Mar 1996 01:06:17 -0400


Minor point: the baud rating of the modem (28.8, 33.6, etc.) is the
maximum under 'ideal' conditions (by which I mean a 'standard' line, which
is a Ma-Bell specified noise floor and frequency response). It's a raw
rate, but a real rate. Any compression algorithms (like V.42) are not
included in this number. The higher bauds are obtained by more efficient
coding schemes.

If you transmit a binary file thru one of these modems, you get the baud
rate. If you transmit a text file, with V.42 on, you can get a much higher
rate, like up to 115K for alot of them, in bursts. Obviously a pkzipped
file can't be compressed much more, so it'll go thru at 33.6 or 28.8 or
whatever (but the file's smaller anyway).

Minor point 2: usually the phone co keeps most lines close to ideal, since
traditionally they've just been wires. The bandwidth is about 300-3600Hz,
and the noise level averages about -30dBm, worst case. But more and more
(especially in Europe) because of digital tech, they use digital
speech-oriented compression algorithms to save on bit rates (for example,
one popular ADPCM algorithm gives them a 4:1 compression, so they can
switch 4 calls for the price of 1!). While these work OK for speech, the
high speed modems depend on clear channel characteristics, like high slew
rate, which are compromised by these algorithms.

> As far as phone line quality is concerned, I agree about "Ma Bell" having
>limited B/W. Where I disagree, is how the increased modem compression
>technology
>is disregarded! The 33.6 firmware chip is providing higher levels of
>compression
>for the limited power of the phone lines, albeit a relatively "small" increase!

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shark@dti.net (Marc Lindahl)
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