[aprssig] AX.25 Digipeating precise timing?

Scott Miller scott at opentrac.org
Mon Jul 22 14:04:26 EDT 2019

I made a special version of the Tracker3 a few years ago specifically 
for tracking ambulances and medical support vehicles at Burning Man.  
They were all 9600 baud and transmitted two copies of each position 
packet in quarter-second time slots, so the whole network of 24 trackers 
had a cycle time of 6 seconds.  The receive side was a D700, no 
digipeating involved.  It worked well, but required transmitters with a 
pretty fast TXD time.

Those were TX-only, though, and I never built in any slotted digipeater 
support.  I suppose I could add that to my to-do list for the Tracker4 - 
maybe give it the option to reserve every other second or every fourth 
second for digipeated packets.


On 7/22/2019 9:15 AM, Robert Bruninga wrote:
> AX.25 experts needed:
> We finished the Appalachian Golden Packet event with 15 hops to Maine and
> 14 hops back to Georgia.
> At 9600 baud each packet is 0.5 seconds and we think we can increase
> throughput significantly by totally changing our beacon plan, messaging
> plan and getting the 2X benefit of 9600 baud.
> This means we would like to better understand the exact AX.25 packet
> timing.
> I just timed 9600 baud packets between three kenwoods as digipeaters and
> conclude an end to end packet with no collisions woiuld take about 8
> seconds.  I want to analyze the probability of success with say a packet
> starting at each end and meeting in the middle, what is the chance of
> anialation of one, or both.  This depends on the delay that all
> digipeateers (with Dwait off) will wait before transmitting.  It is only
> during this window when a given digi, could hear two packets before it
> transmits.
> Anyone remember this stuff?, or am I starting from scratch..
> Radios are set to TXD of 200ms...  But that is after the radio is already
> keyed.
> Bob, Wb4APR
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