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From: koch@motcid.UUCP (Clifton Koch)
Newsgroups: comp.dcom.modems
Subject: Re: Capacity of a Channel (Was: Inexpensive 9600 baud modems)
Message-ID: <5753@navy22.UUCP>
Date: 12 Dec 90 16:57:37 GMT
References: <1990Dec11.185205.13752@wsrcc.com>
Organization: Motorola Inc., Cellular Infrastructure Div., Arlington Heights, IL
Lines: 31

From article <1990Dec11.185205.13752@wsrcc.com>, by wolfgang@wsrcc.com (Wolfgang S. Rupprecht):
> mb@sparrms.ists.ca (Mike Bell) writes:
> 
> 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.

  We're talking about modems which have to used normal voice telephone systems
to transfer data.  ISDN is not the same, though it does use the same twisted
pair to transmit the data, it is done in an entirely different manner.
Ethernet is an entirely different matter, and though it can still use twisted
pair, it is subject to a lot more constraints.

  The bandwidth is *not* limited to 3.4KHz by the wires.  The bandwidth is
limited due to the way the audio is trasmitted at the central office (phone
company).  The audio signal is sent by 12 bit samples compressed to 8 bits (by
either Mu-law or A-law compansion, depending on the country) sent at 8,000
samples per second.  Each audio channel is then multiplexed into a serial
stream of 24 or 30 channels (again depending on the country).  Nyquists theorm
limits the bandwidth to 4Khz, though in practice it is limited to about 3.4KHz
to avoid aliasing.
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