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[aprssig] Beacon rate/distance

Robert Bruninga bruninga at usna.edu
Fri Oct 9 01:13:17 UTC 2009


> Hey Bob, -- what is the recommended beacon 
> rate these days?  I remember it used to be 
> once a mile, but perhaps that's a bit much now. 

Depends...

> I've [tried] anywhere from once per mile to 
> once per TEN miles plus corner pegging via 
> SmartBeaconing. Not sure what to do any more. 

I'll probably get flack for this.. But...

As most people know, I consider smart beaconing to be a good
tool for vehicle tracking applications for special events or
special needs where a good vehicle track must be maintained...
Usually in most areas, there is not enough bandwidth to let more
than a few users do this at a high enough rate for good tracks,
or the channel can get overloaded.  Fine for events, but not so
good for 50 commuters on the national channel.

Conversly, if users set the smart beaconing to such a low and
sparse rate to share the channel equitably, then in many cases,
the inconsistancy in position and lack of regular timing might
actually detract somewhat from the objective of maintaining
continuity of communications contact.  Also, the loss of a
single packet due to collision under smart beaconing is the loss
of a lot more information than the loss of a single packet at a
regular rate.

For APRS as a communications network with the expectation to
maintain regular contact with all its mobile stations, then, I
prefer -regular- rates, so that the recepients can know what to
expect as to the next position and/or can know how many
positions per hour to expect.  Also then the network sees
generally constant load over general periods.

In that context, we came up with "proportional pathing" which we
got Kenwood and some trackers to implement.  If set to a one
minute rate, then once a minute, the packet is transmitted
DIRECT, LOCAL (no digipeat) and then every other minute it goes
ONE HOP and every 4th minute it goes 2 hops.  The result is that
you get frequent (and REGULAR) one minute updates on your
nearest mobiles, but only a 2 minute rate in the local area (via
the local digi) and only a 4 minute rate in the region.  The
result (compared to the usual 2 hop mobile) is only 37% channel
loading.  Yet you are getting a high rate when near someone. 

This high 1 minute rate nearby is valuable.  Two cars going at
70 MPH in opposite directions with a 3 minute rate can have up
to 6 miles ambiguity and probablly wont hear each other direct.
(12 miles or more if there is a packet lost due to collision via
the local digi).  But with proportional pathing, you always get
a 1 minute rate direct between mobiles close by with very little
chance of a collision (the digi holds off)..

So I recommend a 1 minute rate if you have proportinal pathing.
A 2 minute rate for travelers who do not have proportional
pathing, (or a 1 minute rate and one hop) and a 3 or 5 minute
rate for routine commuters who simply want to show that they are
on the air.  These settings give everyone about the same share
of the network.

I'm just not sure what to suggest for smart beaconing, since it
is so dependent on application, and the vehicle's intent in any
situation.  Some settings can generate abusively high rates when
a car is in a parking lot, or spotty rates on the open road at
different speeds if not set up properly.  Which is part of the
challenge with smart beaconing, which depends on the users to
make proper settings based on what they are doing.

Hope that helps.
Bob, Wb4APR




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