Path: utzoo!utgpu!news-server.csri.toronto.edu!rutgers!uwm.edu!bionet!hayes.ims.alaska.edu!floyd
From: floyd@ims.alaska.edu (Floyd Davidson)
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: <1990Dec13.105202.11328@ims.alaska.edu>
Date: 13 Dec 90 10:52:02 GMT
References: <136548@pyramid.pyramid.com> <16309@cbmvax.commodore.com> <1990Dec10.162413.13959@sparrms.ists.ca>
Organization: University of Alaska, Institute of Marine Science
Lines: 48

In article <1990Dec10.162413.13959@sparrms.ists.ca> 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.
>
>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.


I did a little research and looked it up.  The old Ma Bell thought
a reasonable S/N power ratio was 24 dB.  However  ( ain't there
always a catch?), that is a static measurement.  The numbers for
impulse noise are 1 per minute over a 15 minute period with a
threshold of 6 dB below data level.  So calculate the "Channel
Capacity" for a 24 dB S/N, and then figure on one hit per minute.
(Numbers are from "Notes on the Network", ATT, 1980.  They may
have changed slightly since 1980, but I doubt it enough that I
didn't bother comparing with the 1986 revision.)

Also in another article 20 dBrnC was mentioned.  That is maximum
idle channel noise on a subscriber loop, as measured at the
customer end.  It does not include any connection past the
line switcher.  It is also the quietest portion of any connection
you will ever dial up!

Floyd
 
-- 
Floyd L. Davidson                             floyd@hayes.ims.alaska.edu
Salcha, AK 99714                    paycheck connection to Alascom, Inc.
 When I speak for them, one of us will be *out* of business in a hurry.