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[aprssig] APRS low-power-local ALT input channel

Robert Bruninga bruninga at usna.edu
Mon Sep 27 02:19:28 UTC 2004


I'm sorry that I may have confused everyone on Pete's concerns about
blind transmissions between the two separate issues of low power 
trackers with the issue of transmitting on a listen-only digi alternate
input.

Pete is "correct"  that users transmitting on 144.99 but listening on
144.39
are not going to hear the few other direct neighbors transmitting on
144.99.
But then as I have repeatedly pointed out, the number of other stations
one may typically hear direct is only a few, and each of them only 
transmits a few packets per hour yielding a vanishingly small
probability
of a collision.

Rather than arguing how many angels are on the head of this pin,
be it 1% or 2% or even 3% of  a chance of a collisions, everyone should
think for himself.   I assume that everyone's APRS can distinguish
between 
other users heard DIRECT and the vast majority heard via digis.   To
figure your own probability of a collision on the alt-input channel,
simply 
count the number of stations you hear DIRECT and multiply it by 4 
packets per hour per station.  The result is the total number of packets
per hour
that you would hear if everyone moved over to the alt input channel 
(where there are no digis).  Now divide that by 3600 which is about the
total number of packets the channel could handle per hour and the
result is the probability that you will experience a collision that you
could
have avoided.

In my case it is 0.004 (less than half a percent).   I'd say my station
is about
average for the Eastern states, 6 dB gain vertical at 80' HAAT and I can
hear 
only 4 other users direct (not counting 4 digis and 2 mobiles that drove
by).

Now the inverse of that number (99.6%) is not, however, my probabiliyt
of success.
My probability of success doesnt matter who I hear or not, it depends on
how many other users are also on the same alt-input channel and within
range
of MY local digi.  In my area, this might be a dozen other stations.  In
that
case, my probabiliy of successfully getting into the digi falls to about
98.8%.
A darn bit better than my probabiliy of success on 144.39 which is about
50% or less (or practically zero if I am using very low power).

If you dont like the 1.2% probability of a collision on the alt-input
channel
then dont operate there, or add a second receiver and you can reduce it
by about 1% to about 0.4%.  But that is what that channel is designed
for and
it is part of the network.  It is good for users, it is good for locals,
it is good
for the network and it is good for the others who stay on 144.39 too...
because they dont have you to collide with anymore.

Of course I assume everyone knows that this wont work on a mountain top
digi in LA that can hear 300 users direct!  But nothing can fix that. 
Any digi that
can hear more than the ALOHA number of stations (about 60 for average
APRS)
are completely overloaded and obsolete anyway.  Those digis must be
replaced 
by multiple low sites that dont have to hear so much QRM. no matter what

frequency they listen on.

de Wb4APR, Bob




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