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TCP-group 1994


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Re: NEW TOPIC! : What about an AX.25 Bridge?



> Date: Thu, 22 Dec 1994 02:12:09 -0800
> From: Phil Karn <karn@unix.ka9q.ampr.org>
> Subject: NEW TOPIC! : What about an AX.25 Bridge?
> 
 
> Now if you wanted to establish bridges between full duplex repeaters
> they would probably work well.  But I'm kind of down on repeaters
> because of their low spectral efficiency (in terms of bits/sec/hz/sq
> km).

Hmmm, I don't understand your qualms with repeaters. How else are we
going to get a working network at 56kbps with the GRAPES modem? We
don't have any 56kbps repeaters built yet here on the Wet Coast
(January, we hope), maybe the Ottawa guys can tell us about their
experience with 56kbps repeaters.

At the moment, our 56kbps modems are running point to point; which is
fine for two stations, but if you add more stations you run into all
kinds of problems, ie hidden transmitters; and the MSK modulation
suffers a lot if there is too much path loss; so perhaps you could get
away with it in a small area. We will *have* to use high gain antennas
pointed right at the repeater if people want to use them at any
distance (and with few collisions). The repeaters will also be the
tcp/ip routers (ie with ethernet or other links), and this is a big
resource to share.

Anyway, the WA4DSY/GRAPES modems are based on ancient technology (in a
way, that will make them fairly easy to clone them and make cheap,
no-tune versions based on DSPs or programmable logic. The analog stuff
has advanced a lot too, making that part cheaper as well.), so I
suppose there is a better way to do 56kbps and higher (I know some
people with visions of T1 and T3 speeds dancing in their heads :-)

So how could 56kbps be done better? C=56kbps, B=70kHz for MSK right
now; plug that into Shannon's formula and solve for S/N, and get -1.3
dB, which makes me chuckle, because the modem's don't perform like
that. I guess MSK is really crappy, huh? But even if it is, any
replacement would have to be within reach. The 56kbps modems are
already too expensive ($300 or so), but that can be fixed (down to
$100 I suppose); anything newer and better and faster should be
buildable with common parts: FPGAs, DSP chips etc.

As for >=T1 speeds, things (I am told) will have to be point to point
(unless you want to buy modems worth the price of a new car), so
repeaters won't be necessary. This raises other things about spectrum
use, but they can be set up to use the bare minimum power recquired
and use very directive antennas, so frequency reuse would be possible.

> 
> Phil

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