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[aprssig] APRS needs...

Scott Miller scott at opentrac.org
Tue May 27 15:00:02 UTC 2008


I was thinking more along the lines of having the digis broadcast a map 
of slot utilization as heard at each site for a 5-minute net cycle. 
It'd be based on DCD activity and not decoded packets, and would 
distinguish between no activity, non-receipt (weak or colliding signals) 
and good copy.  Each slot map would be transmitted direct only.

Listening for your own packets is still a good idea - that'd tell you if 
your packets are the ones making it through in your timeslot.  If you're 
not in a good slot, you'd make a list of slots that are open in every 
digi you can hear, and then pick one in a pseudo-random sequence.

A certain set of slots would still be reserved for 'deaf' trackers and 
wouldn't be subject to coordination.

Operating on the network would require meeting some basic technical 
standards for TXD, maximum packet length, timeslot support, and so on. 
Most of that burden would fall on the equipment developers.

Keeping an unregulated input channel would still be a good idea. 
Transmit-only weather and telemetry stations with no GPS wouldn't be 
able to participate in the timeslotting, with no GPS time sync and no 
ability to slave to another station.

Anyway, I don't expect any of this to just happen in APRS.  It's stuff I 
plan to experiment with, though, and hopefully make available for 
self-contained systems.  Something like Baker to Vegas would be easier 
to set up if it didn't require tedious manual assignment of timeslots.

Scott
N1VG

Robert Bruninga wrote:
>> I *really* don't like the idea of just moving traffic 
>> to 9600 baud without implementing some kind of 
>> intelligent channel access scheme. 
> 
> I heard of one recently something like this:
> 
> If your packet is decoded (and you see the copy), then that is
> considered a reservation for that slot one minute later.  Anyone
> else who transmitted and did not receive a copy, can assume they
> collided and so that slot is taken.
> 
> Simplistically, this would work for systems where everyone wants
> 1 minute beacons.  It has to get more complex to have variable
> rates (which APRS requrires, becase everyone is doing tomething
> different)...  THen each beacon has to include its rate, so
> everyone knows what slot it is reserving.
> 
> But in practical terms, this would never work on the variable
> topography and anarchy of APRS..
> 
> But it was a nice concept on closed nets...
> 
> Bob, Wb4aPR
> 
> 
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