[aprssig] AT Test Results

Bob Bruninga bruninga at usna.edu
Mon Jul 27 20:46:54 CDT 2009

Here is my quick summary for everyone...  It will take me a while to put all the detail reports coming all together.  

Katahdin     - no station
Maine Relay  - qsy of existing KQ1L-6 didnt happen?
MtWashington - OK But isolated!  Heard some from Glock
NewHempshire - no station. Radio crashed night before... 
Greylock     - OK Heard to MD mtns and Mt Wash
Sams-pointNY - no station
Camelback PA - OK Heard WV to Greylock, tho Glk vy rare
G.D. Hill    - OK Heard WV to Greylock, tho Glk vy rare
MD Mountains - OK Heard WV to Greylock, tho Glk vy rare
WV Mountains - OK but late setup
AppleOrchard - OK, but isolated
Comers Rock  - Pouring rain and did not get to top site
Roan Mountn  - OK heard to Huntsville
Clingmans    - OK.Heard Roan to Huntsville
Springer Mn  - OK Heard Roan to Huntsville (no digi)
Lookout Mtn  - OK Heard Roan to Huntsville direct to Clingmans
Huntsville   - OK Heard Roan to Huntsville via Lookout
Great fun with MURPHY to be sure.  Just wait till next year.  This event is to get VHF and PACKET people out and operating.  We get no "respect" at FD, so we are creating our own national annual event VHF field packet event.

Start thinking about next year.  What additional LINEAR trails can be activated?  Mexico to Canada across the Rockies?
San Diego to Seattle?

But we did find a bug in the D700 digipeater.  It does not recognize its OWN originated packet as a dupe.  Fortunately, that dupe then does not further propogate because all the surrounding digis do recognize it as a dupe.  And due to DWAIT=0, it occurs at the same slot as the second hop, so no damage is done to propogation.  The only unmitigated issue is that the 1st digi will hear the originators dupe, and the 2nd digi's packet at the same time, and may not "recognize" these digipeates due to the collision.

This does not happen on any through packets, so it does not impact anything but its own immediate area.

We still want more proof, but here is what we think happens.  In this linear string of digipeaters below, digi 0 is the originator of a packet...

5 4 3 2 1 0 1 2 3 4 5

0 originates a packet.
both 1's take the first hop
both 2's take the 2nd hop
both 3's take the 3rd hop and so on...

BUT at the same time as "both 2's are taking the 2nd hop, the originator "0" is making its mistaken dupe packet.  SO... both 1's actually hear the same packet from both sides (0 and 2) at the same time and there is a collision.

There is no impact on the outware bound packet since it is far away and still moving outbound.  But because both 1's hear a collision there will be two effects:

a) if a perfect collision, the 1's will never decode the outward bound next hop (even though it was successful) of packets originated at 0.

b) if one of the two is 10 dB stronger than the other, then he will always hear either 0 or 2.  If 0 is stronger, he never hears the 2 (when 0 originates) he might think that 2 is broken (even though it is working just fine and the packet did go that way).  If 2 is stronger, then he won't even hear the dupe and there is no problem.

Again, this originators dupe does not impact throughput, nor success of any link packets, though it does make for confusion for those watching the detail.  ANd it only impacts packets originated within the digi itself.

For the purists, the workaround is to just let the digi's only be digis, and then use a separate HT or other APRS station at that site for all messaging.

More as we learn it.


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