[aprssig] Delayed packets (yet again)

Wes Johnston, AI4PX wes at kd4rdb.com
Fri Aug 25 22:03:50 CDT 2006

Recently kc4pl and I saw enormous delays on RF.  We've seen igate delays
from ww6jc in a nearby town running ui-view.  At anyrate, kc4pl is running
TNC-x and an ax.25 stack connected to javaaprs.  What we saw was /were
delays on the order of 60 to 90 seconds.  I was sending packets that were
uniquely numbered so we'd know which got thru and which didn't.

We suspected that perhaps TNCx was hearing too many packets on frequency to
TX, so he QSY'ed the radio.  I send a few more packets.  Then it dawned on
us that he wasn't seeing the packets I send for 60 to 90 seconds.  As soon
as he saw them appear in the ax.25 stack, they were digipeater.  In the
process of reaching under the desk, he may have reset or momentarly
interrupted the power to the TNCx.... so we're not sure if the tnc-x is the
problem or the ax.25 stack in linux.  Or maybe some part of the system
needed a break. At this time, we're waiting for the delays to happen again
so we can figure out if it's TNC-x or ax25 stack.

Possible theories include that TNCx may have a memory leak...
tncx may spend so much time servicing interrupts in a busy channel that it
can't deliver data to the serial port.
ubuntu ax25 may have a problem.

Really don't know anything difinitive at this point.

On 8/25/06, Tim Cunningham <tim_cunningham at mindspring.com> wrote:
>  There was a delayed packet discussion specifically concerning IGates
> recently. We did a through study into the problem in my area and isolated
> the problem to a UI-View station operating as an IGate. Since that
> discussion there were a few other folks across the country who wrote
> off-line that they experienced the same issue with UI-View in their areas.
> This issue was specifically limited to a UI-View IGate that was injecting
> packets greater than 1 minute later than another IGate using APRS+SA. Many
> of the packets were time stamped to verify the delay. It was not a
> digipeater delay because we could see two IGates hearing the same packet
> from the same digipeater being injected into the APRS-IS greater than one
> minute apart.
> IGates that are experiencing delays should consider using filtered port
> 14580 and limit their data to under 100 to 200km using the m/100 to m/200
> qualifier. When an IGate tries to process the full feed they are going to
> run slower depending on their computer configuration and whether they are
> using a dial-up feed or a high speed link.
> There are many sources of delays that also include digipeaters. The first
> step is to identify if the delay is the USB to serial converter which should
> be fairly easy by visual observation on how long it takes a decoded packet
> to appear on the screen. Then, it becomes only a measure of time from that
> point to when it appears on the APRS-IS.
> 73's,
> Tim - N8DEU
> Huntsville, Alabama
>  ----- Original Message -----
> *From:* J T <w0jrt at yahoo.com>
> *To:* TAPR APRS Mailing List <aprssig at lists.tapr.org>
>  *Sent:* Friday, August 25, 2006 1:26 PM
> *Subject:* [aprssig] Delayed packets (yet again)
> In one of the many "delayed packet" discussions there was talk about some
> serial-to-USB converters causing issues.  I can't seem to find this
> discussion in the archives.  Does anyone recall the make(s)/model(s) of the
> serial-to-USB converters that are known or suspected of being problematic?
> And perhaps suggest a model that is known to work well?
> I've been communicating with an I-gate owner whose station is injecting
> delayed packets into APRS-IS.  He's too busy to study and fix the problem
> (unfortunately for the rest of us) so I'm trying to help gather some
> information that may make it easier to debug the problem.  He swapped TNCs
> but that didn't fix it.  He mentioned that he uses a serial-to-USB converter
> so hopefully I can talk him into experimenting with a different one or
> coming up with another solution.
> -Jerome, W0JRT
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