Path: utzoo!attcan!uunet!know!zaphod.mps.ohio-state.edu!usc!ucselx!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) Message-ID: <1990Dec12.135439.11186@ims.alaska.edu> Date: 12 Dec 90 13:54:39 GMT References: <1990Dec10.162413.13959@sparrms.ists.ca> <4986@optilink.UUCP> Organization: University of Alaska, Institute of Marine Science Lines: 47 In article <4986@optilink.UUCP> witkowsk@optilink.UUCP (Dave Witkowski) writes: >From article <1990Dec10.162413.13959@sparrms.ists.ca>, by mb@sparrms.ists.ca (Mike Bell): >> 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 > C message weighting matches your ear. But 3Khz flat will include very low frequency power line noise that won't affect your modem any more than it does your ear. Neither is a very good measure for modems. Also note that Dbrn is *not* a signal to noise power ratio. It is a measure of noise power. The bigger the number the more noise. I don't have Bellcore TRY-0064, but 20 DbrnC0 is a very quiet channel. My guess is that figure applies only to the local loop. End-to-End limits depend on how long the circuit is or how many carrier sections it goes through. For example a circuit greater than 100 miles can have up to 33 DbrnC0 ICN (Idle Channel Noise). Also satellite circuits are allowed higher noise and may have to be measured using a 1004Khz tone at 13 Db below test tone level to hold the channel up (vox). The tone is notched out, but the depth of the notch determines the *best* reading possible. ICN is not a good measure of real life performance of a modem on a circuit. Impulse noise, phase shifts, and gain shifts are the limiting factors on dial up lines. 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.