[Xastir] Weird plotting

[James Ewen]

On 3/1/07, Tom Russo <russo at bogodyn.org> wrote:
> This looks suspiciously like a digi that's holding packets too long before
> digipeating them.
> My memory's kinda dim on the subject, but as I recall
> it is stale packets being re-transmitted long after later posits are created.
>
> If you're seeing the behavior in a specific location, it's likely that the
> nearby digi is malfunctioning somehow.  You might try searching the APRSSIG
> archives for details, as I don't remember exactly what the mechanism is or
> what the comment traits were.

Mountaintop digipeaters that can hear far too much will exhibit this
phenomenon. Let's pretend for sake of simplicity that all APRS users
set their path for 1 hop, making use of the digipeaters to simply
digipeat once and once only. The i-gates grab this information and
forward it on to the APRS-IS.

So, let's also look at the situation where Dave KJ5KG is driving along
near BIGDIGI. BIGDIGI can hear 5 other digipeaters. As Dave drives
along, he can be heard by the digipeater, and also a local i-gate. As
Dave drives along, his packets get digipeated and gated to the
APRS-IS. However, in one location Dave only gets heard by BIGDIGI...
he is shadowed from the i-gate. BIGDIGI attempts to gain access to the
channel, however the neighboring digipeaters are busy making noise.
BIGDIGI has to wait for a chance to get on the air. Meanwhile Dave's
tracker pumps out another position report which is heard by the
i-gate. Finally BIGDIGI finds a spot to digipeat the packet it has
been holding, and does so. The i-gate dutifully sends the packet along
to the APRS-IS.

This easily shows how packets can be sent 1 - 2 - 3, and get heard on
the APRS-IS in 1 - 3 - 2 order.

In a quiet area, it is rare to see packets get out of order, but in
higher density areas, it can happen on a regular basis. One thing that
quite often happens though is that people think that their local
digipeater is not heavily used when in fact the digipeater is hearing
a very active channel.

Remember the assumption above about single hop paths? I did that
specifically to get you thinking about channel usage with used up
paths. BIGDIGI would never act upon any packets except those that it
heard locally. It would never need to digipeat packets already handled
by another digipeater. However, those digipeaters within earshot still
use up local airtime according to the local digipeater. It may sound
like a really quiet channel on the ground, but that digipeater up on
the mountain, or on that huge tower doesn't hear as quiet a channel.

Digipeaters with incorrect DWAIT and SLOTTIME or TXDELAY settings can
wait for quite a while in a noisy area. The noise level is determined
by adding the activity on your local digipeater PLUS all the activity
on ALL digipeaters that the local digipeater can hear.

When you use a single hop path, you affect the channel load for the
local digipeater, and also those it can hear. When you use a 2 hop
path you affect your local digipeater, all digipeaters that can hear
the local digipeater, and also all digipeaters that can hear the
digipeaters that heard your local digipeater. You affect people one
hop further than the number of hops you are using.

James
VE6SRV