[aprssig] Preemption Digipeating

Jason KG4WSV kg4wsv at gmail.com
Thu Oct 14 12:15:53 EDT 2010

On Thu, Oct 14, 2010 at 10:32 AM, Lynn W. Deffenbaugh (Mr)
<ldeffenb at homeside.to> wrote:
> A question about Preemptive Digipeating:

I've got one, too:  Is preemptive digipeating defined anywhere?  Or
maybe I should say other than the T2 source code, which as far as I
know is t he only device to implement it.

> If a station transmits a packet with a path of WIDE1-1,WIDE2-1,SAR and the
> SAR-1digipeater is configured to preemptively digipeat on SAR, what
> will/should the digipeated packet path be A, B, or C?
> A) - WIDE1-1,WIDE2-1,SAR-1,SAR*
> B) - SAR-1*,WIDE1-1,WIDE2-1
> C) - SAR-1,SAR*,WIDE1-1,WIDE2-1
> I tried this with a T2-135/OT2m and got D) WIDE1-1,WIDE2-1,SAR-1* which is
> basically A with callsign substitution instead of insertion.

This is actual preemption.  Preemption as used here is defined as "a
taking possession before others".  The SAR preemptive digi takes
possession of the PATH and the packet out of turn, i.e. before the
WIDE digis use it, which is the normal progression.

> I personally find B or C more useful as it preserves the original desired
> path rather than preemptively removing the path by jumping the Used flag to
> the end of the path.

B and C _reorder_ the path, not just the _processing_ of the path.
That's not preemption, it's reordering, mangling, munging, etc.


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