[aprssig] Digital two-way Radio communication in emergency situations

Brian Webster bwebster at wirelessmapping.com
Tue Sep 9 01:04:58 CDT 2014


Would Flexnet be something that could be a starting point? I know that in a
connected packet environment it dynamically changes packet sizes and such
based on what it detects for channel activity and retries.

http://www.afthd.tu-darmstadt.de/~flexnet/

Thank You,
Brian N2KGC


-----Original Message-----
From: aprssig-bounces at tapr.org [mailto:aprssig-bounces at tapr.org] On Behalf
Of Scott Miller
Sent: Monday, September 08, 2014 4:43 PM
To: TAPR APRS Mailing List
Subject: Re: [aprssig] Digital two-way Radio communication in emergency
situations

> Uh, if the digi (as opposed to a human administrator) specifies the 
> timeslot for each tracker, then those trackers have to be two-way (not
> transmit-only) in order to be able to hear their assignments. Pretty 
> much eliminates all current dedicated trackers, since they can't receive.
>
> And, as you point out, you still have the hidden transmitter problem 
> if you have any central authority (such as your suggested digi) 
> handing out slot assignments, and it can't hear every tracker.

Let's say every even-numbered second is allocated to non-coordinated
trackers.  If you don't have receive capability or haven't heard from a
coordinating authority, you transmit only in a randomized even-numbered
slot.  For truly deaf trackers, assuming equal-sized packets and all that,
your channel capacity is doubled using slotted ALOHA vs unslotted so you can
still carry as many non-coordinated trackers as before.

If you have receive capability, maybe you set a bit in the non-coordinated
packet to indicate you're willing to accept coordination.  A coordinating
digi offers a slot and you can accept it or not.

> Split frequencies only help a little if there is no coordination among 
> all the trackers, and the digis will just copy the chaos to their 
> output frequency.

Digis can coordinate amongst themselves more reliably that mobile stations.
They're typically fixed, in good locations, and up for long periods.  They
could much more easily coordinate time slots, and could rely on carrier
detect more.  For example, digi A might always start transmissions in even
seconds, and digi B in odd seconds.  They'll never start transmitting at the
same time, and can hold off if the other digi has more than a second's worth
of traffic to disgorge.  Slotting the digis also reduces channel overhead by
giving them a chance to aggregate traffic on the input channel.  It
introduces a bit of latency, but I'll take a hit of a few seconds for the
sake of greater reliability and capacity.

> Assuming we _can_ get two-way trackers that can listen for 
> assignments, is the AIS protocol still patented somewhere in the world 
> (despite the US government overturning the patent in the US)? I 
> wouldn't want to implement something that would get some users in 
> trouble due to patent infringement.

What specific claims are made by the AIS patent(s)?

> Just wondering if it's worth trying to implement a dynamic 
> time-slotting (as opposed to the existing static time-slotting the 
> transmit-only trackers use), and whether it could successfully coexist 
> with existing legacy transmit-whenever-I-want-to units.

Again, I'd like to see a new channel only allow units with a certain level
of smarts, including GPS-synchronized time slotting.  You could add a flag
to the protocol to indicate compliance so no one's tempted to dump their
non-compliant tracker on the new frequency and stomp on the other traffic.

Scott
N1VG

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