Order Tray | Contact Us | Home | SIG Lists

[aprssig] Re: APRS SSn-N Spec.

Cap Pennell cap at cruzio.com
Fri Nov 30 09:32:00 UTC 2007


Thanks, Chuck.  I'm pleased someone is still reading along as the packet
paths keep scrolling by.  hi hi

The solution in rural Wyoming or places like that is to support more fill-in
digis and/or more IGates, build and maintain more network infrastructure
where there is none, especially if the objective is reaching a IGate.  But
there won't always be seamless network coverage where there are no people to
provide and support digis and IGates.  <grin>  However, even in those remote
unpopulated areas, most often the problem is reaching _the first_ digipeater
in the path, not the second (or third).  WIDE1-1,WIDE2-1 can help a mobile
station find that first hop into the network, (by allowing use of any nearby
fill-in digis) better than can plain WIDE2-2.

But my experience differs from yours regarding I-5 between Sacramento and
Redding.  My experience (on that portion of I-5 too) is the main trick is
getting into _the first_ digi.  Surely more trips to and fro (experiments)
can help us learn more about our stations and what to do about any gaps in
the existing VHF network capabilities.

I believe _any_ path is okay _while_ the operator is paying attention to the
effect their own packets and the packets of other stations are having on the
shared frequency, watching the packets scroll by and interpreting each one.
And experimenting is good, and it helps.  But the "reduced throughput"
caused by broader paths and more frequent transmissions persists even when
the sender doesn't always notice the effect their station is having on
everybody else (even at great distances).

The best operators will realistically and frequently evaluate just _whom_ is
their intended and interested "audience", where those interested viewers are
located, _how often_ are those viewers eager to see that operator's new
position update on their map, and how are they viewing the packets (using RF
or internet).  Then those operators will courteously set a path no broader
and transmissions no more frequent than required to get that job done.  If
the makeup or location of their interested audience changes, the needed
digipath and transmission interval may also change.  A big ego is not the
guiding motivation of these operators.

But not many users want to _always_ work this hard or to learn this much to
be able to operate courteously, changing digipaths and transmission
intervals to most efficiently and courteously suit the actual current VHF
network situation.  Most would rather set a path and transmission interval
that suits most situations without causing excessive problems to the
network, and then just leave their setup alone and call it good.  It's those
stations (most of us) that find optimal a routine setting of WIDE1-1,WIDE2-1
mobile and plain WIDE2-2 or less from home.

Yes, those folks that have to do things a little different don't have to
feel like they are breaking rules just to live with their situation.  The
only Rules in APRS are those imposed by the FCC (in the USA).  But the FCC
does require good amateur practice and that each station must cooperate in
making the most effective use of the amateur service frequencies
<§97.101(a)&(b)>.
73, Cap KE6AFE
http://info.aprs.net/index.php?title=Paths

"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.

> -----Original Message-----
> From: aprssig-bounces at lists.tapr.org
> [mailto:aprssig-bounces at lists.tapr.org]On Behalf Of The Bland Ranch
> Sent: Thursday, November 29, 2007 20:36 PM
> To: TAPR APRS Mailing List
> Subject: RE: [aprssig] Re: APRS SSn-N Spec.
>
>
> Quoting Cap Pennell <cap at cruzio.com>:
>
> > With mobiles able to set WIDE1-1,WIDE2-1 and then travel
> anywhere _without
> > ever having to change_ their path......
>
>
> Cap,
>
> I beg to differ with you.
>
> Right off the bat, there are MANY places in Wyoming where the
> WIDE1-1,WIDE2-1
> path won't get you to an I-Gate. I was operating in southern and eastern
> Wyoming for six week mobile and I can assure you I had to vary from your
> recommendation or not be seen on the servers. Another place I just learned
> about is Interstate 5 north of Sacramento, toward Redding. Even
> the SLIGHTLY
> longer path I used often did not get my packets to the servers,
> but I got more
> there than with the shorter path.
>
> Yes, I realize when I get into the areas rich with I-Gates and
> digipeaters that
> I need to change my path or risk getting an automated note. But
> there are many
> areas that are not as blessed with hardware like the Bay area or other
> metropolitan areas.
>
> I believe a dis-service is done to the aprs community with such broad
> statements. Is it a great policy? Absolutely! Is it true enough to be
> universal? Not at all and I feel some slack is warranted to allow
> for places
> where it does not work. Those folks that have to do things a
> little different
> don't have to feel like they are breaking rules just to live with their
> situation.
>
> Chuck Bland
> NA6BR





More information about the aprssig mailing list