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

AE5PL Lists HamLists at ametx.com
Mon Sep 27 09:19:04 CDT 2004

Ok, time to debunk your numbers, Bob.  I have refrained from doing so to
this point out of deference to your contributions to APRS, but you have
left out some very important considerations which make your numbers just
plain incorrect and misleading.

> -----Original Message-----
> From: Robert Bruninga
> Posted At: Sunday, September 26, 2004 9:21 PM
> Subject: RE: [aprssig] APRS low-power-local ALT input channel
> 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 

Average beacon rate of 15 minutes?  For trackers (your primary thrust
with this thread), maybe times 60 would be more accurate because that is
what they will be set at.  Weather stations beacon at a maximum of every
15 minutes and the move is to every 5 minutes.  That leaves fixed
stations which might help pull up your average number, but minimally
according to your statements from before regarding number of fixed
stations vs. mobile/portable stations.  Add on to that all of the
objects generated by many fixed stations, and the number far exceeds 4
packets per hour per station.

Second, you are using a number of how many stations you can DECODE via
direct.  My station (including my mobile) sees many more stations than
it can decode due to white noise, flutter, etc.  Those stations must be
factored in, as well, since all of those stations will supposedly switch
to your local frequency.

And all of those other stations (300 per hour according to your count at
your QTH)?  You are not competing with them for transmission on RF, only
with their packets that are being digipeated by your near-by WIDEn-n
which you do see.

So, you are incorrect trying to state that CSMA only prevents about 1%
of collisions at your location.  It prevents in excess of 80% collisions
because you are directly seeing more stations than you are decoding AND
you are only competing with the other stations in your area for the
WIDEn-n digipeater.

Let's look at your numbers further.  If you have only 9 stations on the
frequency within sight of the digipeater you want to use, beaconing at
an average rate of 3 minutes (most are mobile, according to your
comments), you end up with 3 beacons per minute.  Seems like that leave
57 seconds wide open per minute.  Well, yes and no.  The odds of a
collision are actually much higher than 3/60 or 5%.  The reason they are
higher is because that over time, it is likely that the beacon intervals
will slide in such a way as to cause a collision to occur.  Also,
statistics show that packet transmission will not occur on an average,
but in a bursty manner.  Unfortunately, once a collision does occur, it
is also likely that the collision will continue to occur for an extended
time as there is no ability in APRS to recognize the loss of a packet
and therefore recycle to transmit again at a random interval (a key
requirement within the original ALOHA tests, by the way).

Now, those 9 stations (this is assuming that nobody else in range of
your digi sets up the same system on the same frequency) would see each
other in one form or another.  True, one station might not see all 8
other stations, but one station will see (being conservative) 3 other
stations which each see 3 stations which covers the entire area.
All-in-all, there is a large opportunity for carrier detect to work.

Finally, let's look at your digipeater design as implemented at a
WIDEn-n site.  Every time the WIDEn-n side transmits, your local
receiver is desensed and cannot copy anything on the "local" side.  Talk
about needing cavities.  Also, as pointed out before, the PocketTracker
(sorry Scott for misspelling, if it is) only supports 144.39 and 144.34.
You have excluded them from your configuration with promoting 144.99 for
a frequency.

When we have run events on 144.39 with no problems, even though a quick
survey of the frequency shows very little open space.  Why?  Because we
have well positioned WIDEn-n digipeaters which are not running high
power that allow for CSMA to work.  Yes, there are collisions.  But much
fewer than if CSMA was not used.  We have also run events on 144.34 (and
a couple of other frequencies) with a digipeater covering the event area
and an IGate providing full network interoperability.  Yes, we have ATV
in our area, but we have found that we can work with that group on an
event-by-event basis.

Do as you please, Bob.  And the same for others around the world.  But
understand that the basis for the need for a blind ALT input channel is
flawed.  You ignore actual operating procedures that occur (beacon rates
are much higher than you state, people will use excessively long paths
"because it doesn't directly affect me" nailing the 144.39 network even
worse than now, among other things).  You try to say that you are in
ALOHA competition for 144.39 with 300 other stations when you are only
in competition for 144.39 with the WIDEn-n digipeater and what it sees

I am done with this thread.  Unless you totally misrepresent what I have
said (as you did earlier in this thread), I am out of here :-)

Have fun.


Pete Loveall AE5PL
mailto:pete at ae5pl.net 

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