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[aprssig] Fundamental APRS capacity

Robert Bruninga bruninga at usna.edu
Mon Oct 4 04:45:44 UTC 2004


Fundamental APRS channel capacity:

There is a limit to APRS throughput on 144.39 or any frequency.
That limit has been said to be about 60 stations.  No matter 
where you live or how dense or sparse the activity, the 1200
baud channel can only handle a certain maximum number of
packets before reliability to others falls off.

Draw a circle around your nearest 60 stations, and if you are
sending packets beyond that range, you are being inconsiderate
and adding QRM to others.  In LA, the range of that circle may
only be 15 miles.  In Wyoming, it might be 100 miles.

Doesnt matter the size, only the number of stations.

We MUST educate users to this fundamental 1200 baud
limitation.  To that end, I am asking ALL authors to draw this
ALOHA circle on all maps so that no user can claim ignorance
of his realistic APRS communications range.  Being good
stewards of the channel must be the responsibilily of the
software to SHOW the user his limits.

Here is the algorithm to exactly (in real time) determine what
the ALOHA circle range is for any user:

* 1200 baud can handle about 60 packets per minute or 1800/30mins
* Sort all stations heard on RF by range
* Starting with closest, add these number of packets per 30 mins
  - Mobile in motion = 15 per 30 mins
  - Other Mobile      =  7 per 30 mins
  - WX station         = 6 per 30 mins
  - DIGIs                =  3 per 30 mins
  - Home stations  =   2 per 30 mins
* Multiply each of these by the number of copies (C).
* C begins as 1, but as you encounter each digi, add 1 to C
* When the total gets to 1800, STOP
* The range of that last station is the limit of the local
  network.  That is the ALOHA circle and should be plotted
  on the map.

Users should be encouraged to NOT routinely send any packets
beyond their ALOHA circle because doing so is JAMMING other
users with un-invited QRM. 

Thats the facts folks.  We can argue and fine tune the assumptions
above, but once we nail down that algorithm, I want to see it
in all APRS programs.  We have GOT to get users to understand
how limiting the 1200 baud channel is and to CUT BACK their
expectations, and use APRS locally, or via the Internet.  But
sending packets routinely outside the ALOHA circle is 
inconsiderate and everyone suffers.

I have implemented this in APRSmax and it works beautifully.
Interstingly enough, it does work out to be about 50 to 60 stations
as we estimated before.  But now it is much more dynamic to 
match actual conditions.  I was shocked when I ran it the first
time...

My QTH is half way between Baltimore and Washingon DC.
My ALOHA circle is 50 miles and it contains 43 stations of
which 13 are digis, 12 are mobiles, and 6 are WX stations.!
It was shocking to me to see the number of digis.  Oh
and I can hit EVERYONE of those digis with a 2 hop path.
Fortunately 3 of those digis are RELAY only ones and another
one is the 144.99 INPUT digi, so it is not quite as bad as it
looked..

But seriiously folks.  Lets get this ALOHA circle on all
MAPS so that people know what they are dealing with.
Its real, it can be calculated, and it IS the limit of a
fully saturated channel.  We must get people to live
within it. 

de Wb4APR, Bob





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