[aprssig] Premptive Digipeating

[Steve Dimse]

On Nov 30, 2012, at 9:41 AM, Lynn W. Deffenbaugh (Mr) wrote:

> (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.

It is important everyone keep in mind this further invalidates the use of APRS for any sort of network analysis. Over the years many people have tried to use the APRS IS to study propagation and network usage (it is very tempting!). This has always been doomed because of the duplicate filtering designed into the APRS IS. That is why propnet had to move to their own frequency.

Despite this feature of the APRS IS, the data out of a TNC still could be used for such studies of RF networks. However, to give an extreme example, when

K4HG>APRS,WIDE,WIDE,WIDE,WIDE,WIDE,WB4APR*:. . . . 

can be a single hop all conclusions based on path analysis are invalid.

I do think this is a useful feature for APRS RF networks where the bottom line is to move the data, but everyone needs to be aware of what is lost.

Steve K4HG