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From: floyd@ims.alaska.edu (Floyd Davidson)
Newsgroups: comp.dcom.modems
Subject: Re: Capacity of a Channel (Was: Inexpensive 9600 baud modems)
Message-ID: <1990Dec13.061040.6753@ims.alaska.edu>
Date: 13 Dec 90 06:10:40 GMT
References: <1990Dec10.162413.13959@sparrms.ists.ca> <4986@optilink.UUCP> <1990Dec12.154139.20496@sparrms.ists.ca>
Organization: University of Alaska, Institute of Marine Science
Lines: 39

In article <1990Dec12.154139.20496@sparrms.ists.ca> mb@sparrms.ists.ca (Mike Bell) writes:
>In <4986@optilink.UUCP> witkowsk@optilink.UUCP (Dave Witkowski) writes:
>
>>> My book on information theory gives Shannon's result as:
>>> 
>>>     Channel Capacity = Bandwidth * log2( 1 + SignalPower/NoisePower )
>>> 
>>> 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.
>>>
>
>>Idle channel noise per BellCore TRY-0064 is:
>
>>20dBrnC for C-message weighting
>>39dBrn for 3-KHz flat response
>
>For those without calculators to hand, these suggest:
>	20db with 3k4Hz bandwidth -> 22640 bps (not sure what C-message means!)
>	39db with 3khz  bandwidth -> 38870 bps

Scratch that, totally.  Dbrn measures noise power, not signal to noise
ratio.  For example 20 DbrnC is less noise than 39 Dbrn.

C message weighting is a filter that has approximately the same 
roll off as your hearing.  Hence a 120 Hz power line harmonic can
be quite strong and not read very high with C message weighting or
bother you very much on a voice call.

If anyone really cares (doubtful) send me email and I'll look it
up and mail you some exact definitions.
 
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.