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[aprssig] conflicting APRS intructions

Robert Bruninga bruninga at usna.edu
Sat Oct 24 00:29:01 UTC 2009


> I've noticed that a stated goal for APRS is 
> somewhat conflicting. ... It is a single 
> information resource channel ", ... all 
> mobiles would have simple displays...
> 
> This implies 2-way communication.  Yet, else-
> where... the solution for "dead zones" in 
> the APRS network is a "wide1-1" digi...
> 
> This is in direct conflict with... 2-way 
> communication... If the mobile can't talk 
> to the major WIDE2 digi's in the area, they 
> likely can't "hear" them either. 

Ah, there is the error in your logic.  There is a world of
difference between what you can hear on your mobile down in a
deadzone compared to what the digi can hear from way up high.
The digi is hearing almost constant energy from every
surrounding digipeater and those beyond (it might not decode
them, but its noise floor includes them and all the other
hundreds of APRS transmitters in the region.  Down in the black
hole, the Mobile hears almost none of this QRM and can hear the
big digi much better than it can hear him.

> The WIDE1 "fill in" digi, in this case, 
> only serves to support "dump-one-way-trackers" 
> and does nothing to facilitate 2-way 
> communication between the station in the 
> dead zone and the rest of the APRS network. 

That is not a direct conclusion.  It serves ALL Mobiles either
one-way or two way to get out.  And it does no NEED to do
anything to help the mobile in many cases.  Sure there are dead
zones that are so dead, that the mobiles cannot decode the big
digis, and in this case, then a full function APRS digi may be
needed.  

But by definition, a FILL-IN is only placed in those areas to
help the mobiles get out and compete with the hundreds of other
signals that are hitting the big digis much closer to them.

> To this end, I really like what aprx is doing with the 
> experiment of "viscous digitpeating."  This seems to be a 
> BETTER solution to black holes than the above mentioned "fill 
> in" digi's (which are nothing more than "dumb-tracker" 
> relays.)  It doesn't completely solve the problem, as the 
> aprx digi can't know if a local mobile can hear other digi's, 
> but I think its a step in the right direction.  

Again, I caution everyone to understand that there is no way to
monitor the channel with a TNC and have any clue what is
happening to collisions.  It is very dangerous to count decoded
packets as any measure of what is happening on the channel.
Because any collision causes _notihng- to be heard.  Thus, any
algorithm based on "statistics" of what it hears is just
plainwrong in its conclusions.

The way to do this is to also monitor the channel noise level
and energy density.  This way  a smart system can not only tell
valid packets, but also all the OTHER packets and collisions
that it is NOT decoding, but that arte EQUALLY contributing to
the congestino problem.  We have to be careful to not jump to
simple conclusions, when the APRS channel is a congestion
limited channel and much more complex than what any one receiver
"decodes"...

Bob, WB4APR

> 
> Take care
> Gary / K3WOW
> 
> 
> 
> 




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