Path: utzoo!attcan!uunet!husc6!uwvax!umn-d-ub!umn-cs!ns!logajan
From: logajan@ns.UUCP (John Logajan x3118)
Newsgroups: sci.electronics
Subject: Re: 9600 baud modems
Summary: bandwidth and information rate
Message-ID: <478@ns.UUCP>
Date: 18 Jul 88 19:11:49 GMT
References: <1127@nunki.usc.edu>
Organization: Network Systems Corp. Mpls MN
Lines: 21

In article <1127@nunki.usc.edu>, grking@nunki.usc.edu (Greg King) writes:
> 9600 baud modems can operate over normal telephone lines.
> the bandwidth of normal telephones is about 3 kHz.
> how do these modems work?

With a bandwidth of 0-3000 hertz, you can simultaneously send tones at any
arbitrarily spaced frequency intervals, say every 10 hertz.  The presence or
absence of a tone can have some information meaning.  Thus you could, in
the 10hz case, send some 300 bits of information simultaneously.  The more
tones you send, they lower in amplitude they have to be, because the aggragate
volume could exceed that allowed.  But by proper phase choice, the rate of
volume increase to number of tones added, is much less than a linear relation!

Anyhow, the basic idea is parallel information flow, rather than the serial,
but much simpler, single frequency jobs such as the 110/300 baud modems of old.

The 1200 and 2400 use a form of phase encodeing, which from another point of
view is the same as multiple simultaeous frequencies.  Just like the 9600 guy.

- John M. Logajan @ Network Systems; 7600 Boone Ave; Brooklyn Park, MN 55428 -
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