[aprssig] Mobile Digi (TEMPn-N)

Lynn W. Deffenbaugh (Mr) ldeffenb at homeside.to
Mon Jun 15 11:55:00 CDT 2009

William McKeehan wrote:
> Why would it be a bad idea for the D700's to run around with WIDE1-1 enabled
> all of the time?

Consider this scenario:

Three operators with mobile D700's with WIDE1-1 enabled all show up at 
the local club.  For every beacon from each of them, the other two hear 
it directly and decide to digipeat it (after all, the WIDE1-1 is intact, 
they're on the same parking lot!).  Due to minor timing delays, they all 
listen for the other and take turns on the channel.   One beacon is now 
3 beacons.  All three hit the neighborhood WIDE1-1 local fill-in digi 
(the one that's SUPPOSED to be doing the work).  Only the original 
packet gets digi'd here (the other two have already marked the WIDE1-1 
used (WIDE1*)).  Now we've got four copies of the packet on the air 
headed over to the WIDE2-1 digi on the mountain top.  It hears all four 
copies and for the sake of argument also hears the original packet 
directly.  It digis ALL of them giving 5 more copies on top of the four 
copies already on the air giving NINE COPIES FOR EACH BEACON!

The idea behind digipeaters (as I understand it) is to provide reception 
and rebroadcast so a broader area can receive.  This is best done with 
fixed stations on reasonable antennas that can hear mobiles and digipeat 
the packets so other mobiles can hear it.  Mobile to tower to mobile 
communications is MUCH more reliable (and predictable) than mobile to 
mobile communications.  For the WIDEn-N APRS infrastructure to work, it 
needs to be predictable in any given area.  If a digi is needed, get one 
up on a tower in someone's back yard.  Driving around with a mobile is 
just going to provide spotty coverage sometimes, and massive amounts of 
channel overload at other times.

Now, the idea behind a TEMPn-N is that when the local infrastructure has 
gone down, or specific coverage for specific stations is needed in a 
specific area, then the stations that NEED the coverage add TEMP1-1 to 
their path and the pre-configured mobiles will pick up and digi just 
those packets, not saturate the needed channel all the time with every 
packet that they hear.

Lynn (D) - KJ4ERJ - Just providing food for thought and testing my 
understanding of the way things work.

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