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[aprssig] Appalachian Trail Golden Packet Event update

Robert Bruninga bruninga at usna.edu
Tue Apr 7 14:38:40 UTC 2009


[Regarding the AT trail Golden Packet event:]

> I'd like to know what changed -- a few years ago
> I got chewed out royally for reporting the following:
> 
>>> KG6JWH>APU25H,ABAJO,WIDE7,LAMOSC,CAPTAN,MELROZ*,TRACE7-4
[UI]:
>>> =3703.52NN12048.48W#Todd's Big Motgage {UIV32N}
>>>
>>> Station is 1002 miles from me and appears 
>>> to have made it all the way here on 2 meters.  
>>> Looks like it took 10 hops to do it, though...
> 
> I didn't even SEND the packet and yet I was 
> in trouble over it --  and now we're actually 
> ENCOURAGING such operation?  I'm confused, I guess.

Ah, that's a good question.  There are many differences.  And it
is understanding those differences within the context of the
conditions that makes all the difference.  These are the
differences:

1) This experiment is not on the national shared APRS channel
2) It is on a dedicated one-time frequency
3) It does not use a FLOODING algorithm (TRACE7-7)
4) Which can generate nearly 200 packets over half the USA
5) It uses LINK7-7 in a linear network which makes only 7
packets
6) The network is physically layed out as a linear network only

This ad-hoc one day event is set up from the get-go to be a
linear system and to demonstrate the long haul potential of
packet (in linear links on a QRM free channel).  Such a system
with 2 or 3 hops can be used to get an emergency comm link
re-established in an emergency for low rate Communications.  Of
course links beyond 3 or 4 hops have lower and lower
probabilities (proportioanl to N squared), and we know in the
1990's that such links are deminishingly poor for CONNECTED
packet.  But for APRS using message-pushing (without acks) along
a low QRM linear set of links the success rate only deminishes
acccording to N and can be effective.

Conversly, the APRS national channel is a local communications
and info distribution channel in all directions simultaneously,
so flooding beyond the local network is QRM to everyone else.
Hence the requirment to keep all packets local (WIDE2-2 or
less).  In such a network where most digi's can hear 4 other
digis, the deminshing of success is N to the 4th power!
Virtually impossible.


Hope that helps.
Bob, Wb4APR




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