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TCP-group 1995


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Re: Radios for higher speeds,



> From: Phil Karn <karn@qualcomm.com>
>
> >Why it is nessesary to have a 15KHz bandwidth for 9.6Kb, when in the wireline
> >case it is sufficient a 2.4KHz for 28.8Kb ??
>
> There's a fundamental tradeoff between bandwidth and S/N ratio. On a phone
> line where you have high S/N, you can use higher-order modulation schemes
> to get many bits/sec per Hz.
>
> You *could* apply these same schemes to radio channels but, as Richard
> Nixon once said, "that would be wrong". Modulation schemes that give many
> bits/sec/Hz are inherently more sensitive to noise and interference, which
> is much more of a problem on radio than on phone lines.

It is worth mentioning that telephone modems implement error detecting/
correcting protocols that really depend on the full-duplex nature of
the telephone line to achieve reasonable performance.  Also, the
really snazzy telephone encoding formats are very sensitive to
changes in the phase response of the link; on a telco line, this
don't change that often.  On a radio link, this can change several
times a second.

Maybe it is about time for a FAQ; this is not the first time I've
seen the "The phone modems get 28kbits/sec, why isn't ham radio like
that?" argument.

[I'm worried about you, Phil; when did you start quoting Nixon?]

Dana






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