[aprssig] Premptive Digipeating

Lynn W. Deffenbaugh (Mr) ldeffenb at homeside.to
Fri Nov 30 09:41:44 EST 2012

(Added missing * on my example)

On 11/30/2012 9:28 AM, Robert Bruninga wrote:
> GATEWAY SCENARIO:  IN this scenario there is a special event on FREQB with
> lots of FREQB digis that desires to get at least one (and only 1) copy of
> each packet into the 144.39 system via a cross-channel gateway with the
> callsign GATE.  The path of FREQB7-7,GATE,WIDE2-1 would make sure that a
> packet got everywhere in the special event network but only one copy
> always gets through the gateway at least one hop into the APRS network.
> (such as the linear appalachan trail event or an underground multihop cave
> event...
> <snip>
> Have I missed anything?

I believe it is worth clarifying that the preemptive digipeater marks 
the component that it pre-empted as used when digipeating and only the 
path AFTER that component is still available.  This is implied by your 
FREQB7-7,GATE,WIDE2-1 example, but I believe it should be an explicit 
comment something like:

Note that in this case, when GATE hears the packet, regardless of how 
many FREQB hops it has used, the digipeated packet will have the GATE 
marked used.  For instance, if it took two hops on the FREQB network to 
get to GATE (arriving as FIRST,SECOND*,FREQB7-5,GATE,WIDE2-1), the 
digipeated packet would be FIRST,SECOND,FREQB7-5,GATE*,WIDE2-1 providing 
for one remaining WIDE2 digipeater hop on the gated frequency and 
completely hiding the remaining 5 hops on FREQB.

Lynn (D) - KJ4ERJ - Author of APRSISCE for Windows Mobile and Win32

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