The ALOHA Circle (or local reliable APRS network size)     7 Nov 2011
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                                                               WB4APR

Original 13 Jan 2003

There are three things that are very important to the network and
user education and which some software does does *not*  show to the
user to help him be a good steward of the shared channel.  These are
the proper display of PHG circles, the ALOHA range and number of
hops between users.  Software should insure that users are aware of 
the RF network they are in.  See these crude APRSdos examples of what 
every user needs to know about his network:

Simple PHG:    http://aprs.org/aloha/mapPHGs.gif 
Aloha Circle:  http://aprs.org/aloha/alohamap.gif 
Digi Hops:     http://aprs.org/digis/digihopsX.gif 

Users that are not aware of the network limitations and the number
of hops it takes to hit their local area are killing the network, 
because they have no PHG displays and no way of knowing what paths 
to use, nor how busy the channel is, nor anything about the local RF 
network.

1) Software should display the PHG circles from all digis to make 
users aware of their relative magnitude of converage.

2) All digis should be displayed in green and their PHG circles for 
DIGIS also in GREEN to distinguish the circles from the hundreds of 
user and other station circles. 

3) The ALOHA circle showing the limitations of the local network
around own station should be displayed on all maps by default.


THE ALOHA CIRCLE:
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Many users these days have no appreciation of the limits of the 144.39 
or other national APRS network even though it is trivial to calculate 
and display since it is just the definition of a 100% full channel 
based on the TX rates of surrounding stations.  The ALOHA circle is 
that circle that contains the number of users that equals a 100% full 
channel. 

I have called on all software to permanently display the calculated 
ALOHA circle for *own-station* and always display it on the users map 
so that the user is always aware of his effective range and SHOULD NOT 
send his packets beyond that 100% saturated channel range...

Here is the spec for the ALOHA circle:
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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 N stations.  (We used to say
that N was about 60.  Now we want all software to compute it live
based on the immediate live activity and mixture of 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 drastically.

Draw a circle around your nearest N (was 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 150 miles.

This is *independent* of topology.  The size is only limited by
the number of stations and their statistical transmit rates.

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 determine the ALOHA circle range 
for own-station.  Do this once  every 30 minutes or so.  (You have
to wait at least 30 minutes from power up to make the first
computation anyway or use old data)...

* Start with the "digi-copies" vairable C equal to 1.
* 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
  for each station.
  - 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 digi-copies (C) when you
  add it to the accumulating total...
* C begins as 1, but as you encounter each digi, add 1 to C
* When the total accumulated gets to 1800, STOP
* The range of the last stations is the limit of the local
  network of 100% channel saturation.  That is the ALOHA 
  circle and should be plotted on the map (every time the
  map is rendered).

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 APRSdos (the MAX version) and it works 
beautifully.  Interstingly enough, it does work out to be about 50 
to 60 stations as we had estimated before.  But now it is dynamic to 
match actual conditions and the mixture of mobiles or non-mobiles
on the air at any one time..  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. 

USING FINDU TO FIGURE YOUR ALOHA CIRCLE:
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Use FINDU to "measure" your network saturation limit and to help you
DESIGN the optimum DIGI coverage for YOUR area anywhere from Maine to
LA!

Simply do the CLICK FOR NEARBY STATIONS, and lets say the reliable 
ALOHA channel capacity for APRS for one digi is on the order of 40 home
stations and 10 mobiles and 5 digis and 5 WX stations.  That adds up to 
about 60 stations.  Since FINDU displays the nearest 50 stations and 
this information is SORTED by distance, just go down to the bottom of 
the list, and that is the reliable ALOHA network coverage for your area!  

From now on, think in terms of your ALOHA circle.  If your LOCAL digi
covers lots more than your local ALOHA circle, then your channel is 
beyond saturation and reliability suffers... and your local digi is NOT 
a local digi!

de WB4APR, Bob