Path: utzoo!utgpu!news-server.csri.toronto.edu!cs.utexas.edu!usc!wuarchive!uunet!usaos!wsrcc.com!wolfgang
From: wolfgang@wsrcc.com (Wolfgang S. Rupprecht)
Newsgroups: comp.dcom.modems
Subject: Re: Capacity of a Channel (Was: Inexpensive 9600 baud modems)
Keywords: Shannon, 9600, V.32, V.42, bis
Message-ID: <1990Dec11.185205.13752@wsrcc.com>
Date: 11 Dec 90 18:52:05 GMT
References: <136548@pyramid.pyramid.com> <16309@cbmvax.commodore.com> <1990Dec10.162413.13959@sparrms.ists.ca>
Organization: Wolfgang S Rupprecht Computer Consulting, Washington DC.
Lines: 32

mb@sparrms.ists.ca (Mike Bell) writes:
>>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.

Also, remember that the 3.4 kHz is the MINIMUM bandwidth the phone
line will present to the modem.  The sky is the limit.  Don't folks
regularly send 10 Mhz ethernet connections over (admittedly shorter
sections of) these wires?  I believe 64k baud ISDN links actually go
from the CO to your jack over these same wires.  The trick here is
that it is fairly easy to get good freq responses out of short
sections of twisted pair.  Its just that long runs are killers.  The
specs are a worst case spec.  In practice bandwidth is (hopefully)
going to be much better.

I wouldn't be surprised to hear that the 18k tb+ spec assumed that you
connected the two modems with a 3ft section of phone patch cord.

-wolfgang

-- 
Wolfgang Rupprecht    wolfgang@wsrcc.com (or) uunet!wsrcc!wolfgang
Snail Mail Address:   Box 6524, Alexandria, VA 22306-0524