[aprssig] Premptive Digipeating

Robert Bruninga bruninga at usna.edu
Fri Nov 30 09:28:12 EST 2012

I cannot find previous threads on Premptive Digipeating.  So here goes.

PREMPTIVE DIGIPEATING DEFINITION:  When ON, the digi will search, starting
at the first unused field to the end for a match to any of its potential
matching aliases (MYCALL, MYALIAS, UIDIGI).  If a match, it will mark that
and all prior fields as USED and will digipeat it once.

PACKETS TO HOME SCENARIO:  In this scenario your goal is to get your
packets targeted to a particular digi or area.  The path WIDE2-2,HOMEX
With the HOMEX digipeater recognizing pre-emptive digipeating would make
sure that all your packets got to HOMEX in from 1 to 3 hops.  To get them
say 4 hops from City A to City B without flooding everywhere, the path
WIDE1-1,CITYA,WIDE2-1,CITYB would get to CITYB no matter where you are in
A or B or inbetween, but not involve ANY OTHER DIGI's than the shortest
path between them.  ETC...

Or CITYD,CITYC,CITYB,CITYA would let you go on a trip to CItyD, but your
packets would get back home to A no matter where you were inbetween.  When
close, then they would only go 1 hop.  Or if you wanted those at CityD to
anticipate your arrival, you might use the path CITYA,CITYB,CITYC,CITYD
and then your packets would all preceed you to CITYD until you got there,
but with fewer and fewer hops as you got close.

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

would preclude any use of any kind of WIDEn-N AFTER going through a gate,
and that is one of the main reasons for doing it.

Remember, these 4 hop Premptive SPECIFIC paths are MUCH less load on the
network than a WIDE4-4 which can potentially cause 34 DUPES, compared to
only a maximum of 4 hops with the explicit preemptive hop approach.

Have I missed anything?


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