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

J. Gary Bender, WS5N WS5N at mindspring.com
Mon Sep 27 17:41:17 UTC 2004

Except for the issue of desensing of ALT input at the digipeater, it appears to 
me that virtually all other issues being raised against ALT input also apply to 
the .39 primary channel.  Taking it that way -- we should not be running APRS.  
It either can't work, or is bad operating practice.

Here in the sticks, relative to the east coast, more than 90% of the packets I 
see are digipeated.  My weather station and 10 watt mobile can compete, but an 
ALT input for deaf low power pocket trackers with crappy antennas would benefit 
from the reduced contention from high power digis and fixed stations.  Chances 
of success for a packet would be up to 10 times better.  With desensing ... less 
improved, but still several notches better.  My guess is that dense urban areas 
would benefit even more.

J. Gary Bender, WS5N
Tijeras, New Mexico USA

On Mon, 27 Sep 2004 09:19:04 -0500, AE5PL Lists wrote:
> 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
> directly.
> 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.
> 73,
> Pete Loveall AE5PL
> mailto:pete at ae5pl.net

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