[aprssig] APRS and the I-95 Corridor

[Cap Pennell]

Ian N8IK wrote:
> For mobiles check that your path is WIDE1-1,WIDE2-1 and if really
> remote try
> WIDE1-1,WIDE2-2.  That way the home RELAYs can respond.  See
> http://www.ew.usna.edu/~bruninga/aprs/fix14439.html
>
> 73, Ian N8IK


Yes, but very few places are _so_ remote that WIDE1-1,WIDE2-2 (three digi
hops in all directions) is required or helpful.

WIDE1-1,WIDE2-1 from a mobile or portable station is far better, with plain
WIDE2-2 (or less) enough from a home or Base.
http://web.ew.usna.edu/~bruninga/aprs/APRSpaths.gif

If it's so bad that 3 hops seem needed, adding a (WIDE1-1 ONLY) fill-in
digi, perched on a local high point, can help out those wayward mobile
stations.  Using a fill-in digi on Point Sur last weekend, we operated the
(very remote) Big Sur Marathon trackers with good results, and the _mobiles
did not need any settings changed_ from their daily routines.  For local map
display using APRS, we re-labeled the map symbols on the marathon course
with tactical calls. http://bsim.aprsca.net  Worked swell!

For good reason, RELAY and WIDE and TRACE are obsolete, at least in North
America.  Please don't become part of (or cooperate with) that old problem.
73, Cap KE6AFE

"Boilerplate text" follows:
To explain the only problem (and my only concern) for VHF APRS:
Our only big problem in APRS is coping with the "reduced throughput" that
adversely affects all the APRS users on our single 144.390 MHz national VHF
frequency.  Because of the limited amount of available airtime ("bandwidth")
at 1200bps on the shared frequency, only so many packets can be decoded at a
given receiver, only one at a time.  Any more than that simply "collide"
(share airtime) at the receiver.  Usually none of those colliding packets
are successfully decoded by the receiving station (though _occasionally_ the
"FM capture effect" allows the strongest signal to get through).  Most of
these collisions occur at high elevations between all the mountain-top
digipeaters which all hear each other over great distances up there.  We
rarely notice many collisions because we can only hear one or two digis from
our individual low-elevation Valley or coastal QTHs.  By themselves, these
collisions (reduced throughput) are not a big problem because (with APRS's
Unconnected protocol) the transmitting APRS station will try again in a
short while.  But with so many users sharing the nationwide frequency, the
real problem crops up when we hams try so hard to compensate for the reduced
throughput we notice.  Naturally, we are tempted to try to compensate by
setting broader digipaths, more frequent transmissions, higher power, and
putting up new digipeaters for "better coverage."  These things usually only
make matters worse and tend toward a slow "death spiral" of the VHF network.

Tragedy of the Commons:
APRS suffers the classic fate of all limited resources as well documented
since the 1830's.
http://en.wikipedia.org/wiki/Tragedy_of_the_commons
Whenever there is a balance between individual interests and the common
good, human nature guarantees the overloading and ultimate demise of the
common resource.  In this case, APRS throughput.  This is easy to understand
since the benefit of adding one more packet to the network always
immediately benefits the SENDER, but the negative "cost" is spread over
everyone else.  There is no natural solution, other than the establishment
of "Golden Rules" to live by for all concerned.