ALOHA.TXT   Understanding your ALOHA circle         5 Aug 2004
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APRSdos can now compute and display your ALOHA circle or area
of optimum APRS range.  But first an explaination of ALOHA.

APRS users share a single channel using time division multiple
access called TDMA.  A fundamental concept in this channel
sharing process is the concept pioneered in the late 60's at
Hawaii University called ALOHA which is the simplest form of
TDMA network where those with data, simply send it and hope
there is no collision.  It was shown that the theoretical
maximum throughput of such a system is about 18% of channel
capacity.

This capacity can almost be doubled to 36% if stations use
carrier- sense-multiple-access (CSMA) where they listen first to
see if the channel is in use before transmitting.  But, in APRS
unfortunately, this only works if everyone can hear everyone else.
This is the case for digipeaters, but is not the case for the
users.  Typically, a single user only hears about 5% or less of
what the digi's hear.  Since the important packets on the air are
from users with very little CSMA advantage, it is very useful to
use the ALOHA concept in assessing channel loading in APRS.  We
will use 20% optimum channel loading as our goal.

Statistically, about 60 APRS users produce an average number of
packets that is about equal to this 20% ALOHA rate.  We call this
the ALOHA circle of ussers that can RELIABLY exchange APRS data.
If there are any more stations using the same network, their
packets only add QRM and interfere with local communications.
Thus, we call this 60 user area, the ALOHA circle.

Two new commands have been added to APRSdos (in the APRSmax version)
that let the user easily see his effecive LOCAL (reliable) APRS area.

  OPS-SHOWaloha   - computes and displays your ALOHA circle

  MAPS-PLOTS-HOPS - shows the number of hops to surrounding areas

OPS-SHOWaloha:  This command will compute the distance to all
stations and then draw YOUR ALOHA circle for your station
showing the radius from your station that contains a workable
number of APRS stations (typically about 60 that APRS can support
on a 1200 baud channel).  If you are getting stations from
outside your ALOHA circle, then they are QRMing your area and
colliding most of the time with local more valuable traffic.
We must educate people to CUT BACK on their path to their
own ALOHA circle.  Anything else is QRM to others...

MAPS-PLOTS-HOPS:  This command will first SORT all stations
by their PHG Height and then search through the DIGIPATH page
and compute the number of hops to each DIGI.  Then plot
them according to color and # of hops.  This display is
VERY REVEALING:

   1) It shows you how far away and how many hops some DIGIS
      are sending their POSITIONS inorder to be seen in YOUR
      area.  Many of these you just don't need to see the
      QRM.

   2) Band openings really show up with some short hop counts
      to some far away places

   3) The # of hops shown is the number of hops that that digi
      took to get its POSIT onto your screen.  For you to send
      your packets back to be seen in its area, you need to add
      ONE to the #.  But PLEASE, do not use WIDEn-N to reach
      anything more than say 2 or 3 hops.  instead choose a
      "named path" to communicate with someone in that area,
      Not flooding 100 digis in all directions..

NOTES:  If you have not done a OPS-SHOWaloha, then the first time
you do a MAPS-PLOTS-HOPS an ALOHA will automatically be done.  But
remember, this is computed ONLY for the stations in your P-LIST.
If you have done something like deleted a lot of home stations
or mobiles to clean up the MAP, or loaded a different .BK file
then your new ALOHA will be erroneous.  So the ALOHA circle only
has meaning if your P-LIST truely represents EVERY STATION on the
air on your 144.39 channel.


OTHER COMMANDS useful for making the above displays more useful:

MAPS-FEATURES-CALLS-NONE:  This will toggle off callsigns to
      make the display cleaner

PLIST-SELECT-DEL:  I used this to delete most of the home stations
      and other clutter so that my APRStest could make sure that
      it did not overwrite any digis and so after a fw hours I
      was sure to have captured all digis.  During this period
      I made sure that the 300 size limit didnt get filled up
      and just kept deleting non-digi stations and then saved the
      file as DIGIS.BK.  That is the file that you will get in
      this distribution.  But it only applies to Baltimore...

If you have more than 300 stations QRMing your area, you need to
make sure to capture all the digis inorder to get a full plot of
the digis.  Here are some steps I use to make sure I get all the
digis:

1) Let your system run for hours.  Before it gets to a full 9 pages
   do a FILE-SORT to get older stations first
2) On the P-List starting on page 1, DELete all old mobiles and any
   other non-digi stations
3) THen let it run over night to see what DX digis you might capture.
4) Again delete non-digis and FILE-SAVE the file as your own DIGIS.BK
5) Now turn off calls
6) Now do the MAPS-PLOTS-HOPS to get a nice network summary!


CAVEATS!  READ WITH CAUTION:

1)  Remember, this stuff is all hosed up by one count if any
digi has "ID" set to ON.  Because this automatically inserts the
digis call in the path and increements the hop counter, event though
this count is already counted for in the n-N decrement.

2) Also, sometime a N hop packet gets collided, but a later N+1 or
larger hop path will get heard.  SO remember that this display is
an instantaneous plot of what the LAST packet heard from a digi
took, it may not represent the average conditions.  So watch the
display every half hour or two to see what a DIGI actually does
most of the time...

Enjoy
Bob Bruninga, WB4APR
