Path: utzoo!attcan!uunet!van-bc!ubc-cs!news-server.csri.toronto.edu!helios.physics.utoronto.ca!ists!sparrms!sparatd!mb
From: mb@sparrms.ists.ca (Mike Bell)
Newsgroups: comp.dcom.modems
Subject: Capacity of a Channel (Was: Inexpensive 9600 baud modems)
Keywords: Shannon, 9600, V.32, V.42, bis
Message-ID: <1990Dec10.162413.13959@sparrms.ists.ca>
Date: 10 Dec 90 16:24:13 GMT
References: <136548@pyramid.pyramid.com> <16309@cbmvax.commodore.com>
Organization: Spar Aerospace Ltd, Toronto, Canada
Lines: 23

>In article <136548@pyramid.pyramid.com> lstowell@pyrnova.pyramid.com (Lon Stowell) writes:
>> In article <5435@navy19.UUCP> benyukhi@motcid.UUCP (Ed Benyukhis) writes:
>> >57 Kbps on the voice grade line that is band limited to 3.4 Khz is contrary
>> >to both Shannon and Nyquist rules.

And so on to a discussion based on dubious assumptions...

My book on information theory gives Shannon's result as:

    Channel Capacity = Bandwidth * log2( 1 + SignalPower/NoisePower )

Bandwidth for telephony ~= 3.4KHz

Telebit, in their literature for the T2500 Modem, claim "up to" 18000bps
in PEP mode without compression. To achieve this, the S/N power ratio would 
have to be >15.8dB.

Anybody know what Signal/Noise ratios are likely to be found on
normal phone lines? This would give an indication of the practical
limit to modem speeds.

NB. If the information source contains significant redundancy, (ie. is
non-random) then far higher *apparent* rates are possible.