<html><head><meta http-equiv="content-type" content="text/html; charset=utf-8"></head><body dir="auto">Packet fracticide is an essential part of digipeater function. The whole point of it is to reduce the time a packet occupies on the air, all while also eliminating duplicates that cause subsequent airtime to retransmit because of miss configured digis.<div><br></div><div>This has long been a problem for people to understand. Bob Bruninga has gone over this detail again and again. Stations can wait to transmit when there is a break, but in a noisy location, with lots of traffic, they also need to randomly move around in their interval to keep from occupying the same moment within a minute cycle.</div><div><br></div><div>It’s vital for fracticide to be understood and for regional digipeater systems to be far enough away from each other and main thoroughfares to get the 12db separation yo outlying areas. </div><div><br></div><div>This is actually science. It’s not impossible to get mostly right, and for areas without 12db of separation, low power fill in digis with TX delay can provide the packet again if absolutely required. But those fill in digis should never be 12db above mobiles at the digis!</div><div><br></div><div>Gregg WONDERLY<br><br><div dir="ltr">Sent from my iPhone</div><div dir="ltr"><br><blockquote type="cite">On Nov 22, 2021, at 9:55 PM, Scott Miller <scott@opentrac.org> wrote:<br><br></blockquote></div><blockquote type="cite"><div dir="ltr">
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On Randy's statement that "If the digipeater is correctly
configured, it will TX *at the instant it sees the end of packet
flag*" - a TNC should never be knowingly transmitting over another
signal. How do you decide when a signal is present? Typically that's
done one of three ways:<br>
<br>
1. SQL/COR signal from the receiver indicating a good carrier<br>
2. Receiver squelch with energy detect (VOX)<br>
3. Modem is locked and receiving valid data<br>
<br>
In none of those cases can the channel be assumed clear immediately
after a flag. There's some variation in how #3 is implemented but
for the OpenTracker series they all go by clock recovery status -
each bit transition is expected to fall within a certain time
window, and if a set number of transitions occur outside of the
defined window, lock is considered lost and this is passed on to the
carrier detect system which handles any configured delays.<br>
<br>
If you really think super-fast turnaround after the end of a packet
is important, I think that's the way to do it. It's sufficient to
wait just one or two byte times after the last flag. If you're still
getting bits at the expected times, the sender is probably still
sending. If it's all noise, it's probably safe to assume it's done.<br>
<br>
I'm pretty sure there are some implementations that don't do a
contiguous Bell 202 signal between packets - they might have some
dead time, or restart the modulator at a default phase value with
each packet - but I think it's fair to impose some requirements on
transmitters. If you want your back-to-back packets to go through,
you make sure they're seamless. Anything else is just wasting half
the benefit of the back-to-back packets anyway, because the
receiving side has to go through clock sync again.<br>
<br>
APRS is terribly inefficient already. Let's not make inefficiency a
requirement.<br>
<br>
Scott<br>
N1VG<br>
<br>
<div class="moz-cite-prefix">On 11/21/2021 11:35 AM, Kenneth
Finnegan wrote:<br>
</div>
<blockquote type="cite" cite="mid:CAFS5k-h+3iWH79x_Bt-Y4K=pjXo=iW8RaAwNJVV07iVpDsQJsA@mail.gmail.com">
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<div dir="ltr" class="gmail_attr">On Thu, Sep 23, 2021 at 7:14
AM Randy Love <<a href="mailto:rlove31@gmail.com" moz-do-not-send="true" class="moz-txt-link-freetext">rlove31@gmail.com</a>>
wrote:<br>
</div>
<blockquote class="gmail_quote" style="margin:0px 0px 0px
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<div dir="ltr">
<div>Sadly tho, most digipeaters are not configured
properly.</div>
</div>
</blockquote>
<div><br>
</div>
<div>A slightly different take on this is that there's a large
contingent of operators that think this fratricide theory is
nonsense. I haven't seen any robust justification that this
actually works other than "you know, FM capture effect is a
thing so this should work in theory"</div>
<div><br>
</div>
<div>There's just so many different scenarios where being able
to transmit more than one packet at a time is desirable that
regardless of fratricide actually working, I don't subscribe
to it actually being helpful:</div>
<div>1. Sharing the same TX delay for position and telemetry</div>
<div>2. Sharing the same TX delay for ACK and reply message</div>
<div>3. Sharing the same TX delay for multi-line messages or
bulletins because the message length limit is so ungodly
short because of the IBM PC screen width limits</div>
<div>4. Sharing the same TX delay for several digipeated
packets which have queued up while waiting for CSMA/CA to
grab the channel</div>
<div><br>
</div>
<div>The fact that Kenwood only shows the last posit on the
display is a bad reason to try and get stations to transmit
multiple packets with "slight breaks" between them. Fine, so
now the Kenwood updates the display five times at.... some
fast interval and you still can only read the last posit
sent.</div>
<div><br>
</div>
<div>The fact that we expect stations to transmit single
packets at a time, but then don't actually document some
lower limit on packet interval or required idle time between
transmitting packets in the spec means it isn't actually
going to happen. When a TNC receives five packets to send to
RF, what is it supposed to do? Dump all five of them in a
single data stream, or buffer up the additional packets and
introduce one or several seconds of delay per packet in the
transmit queue?</div>
<div><br>
</div>
<div>The fact that some stations don't have enough buffer
space to handle more than one packet at a time sounds like a
problem we shouldn't still be accommodating in APRS routing
behavior in 2021. Fine for deaf trackers, but if a
digipeater can't handle more than one packet at a time, then
it sounds like the digipeater needs better hardware. I
suspect that many modems also don't handle two packets
sharing the same flag byte between them because the receive
state machine isn't implemented correctly to handle that,
but that should also be something that is documented/tested
so we can start improving the situation or at least be aware
of it instead of just assuming multiple-packet transmissions
shouldn't be happening.</div>
<div> </div>
<div>
<div dir="ltr" class="gmail_signature" data-smartmail="gmail_signature">--<br>
Kenneth Finnegan, W6KWF<br>
<a href="http://blog.thelifeofkenneth.com/" target="_blank" moz-do-not-send="true" class="moz-txt-link-freetext">http://blog.thelifeofkenneth.com/</a></div>
</div>
<br>
</div>
<br>
<div class="gmail_quote">
<div dir="ltr" class="gmail_attr">On Thu, Sep 23, 2021 at 7:14
AM Randy Love <<a href="mailto:rlove31@gmail.com" moz-do-not-send="true" class="moz-txt-link-freetext">rlove31@gmail.com</a>>
wrote:<br>
</div>
<blockquote class="gmail_quote" style="margin:0px 0px 0px
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<div dir="ltr">Define 'slight breaks', please.
<div><br>
</div>
<div>If the digipeater is correctly configured, it will TX
*at the instant it sees the end of packet flag*. </div>
<div>There should be minimal tx delay, only enough to
handle the time it takes the transmitter to come up and
stabilize, before the digipeater sends the packet it is
to digipeat. It is up to other stations on the channel
to listen before transmitting, but the digipeaters are
meant to clobber each other on the caveat that although
they might hear each other, most uses will be close
enough to one or the other digipeaters to have FM
capture to cover the one farther away.</div>
<div><br>
</div>
<div>Sadly tho, most digipeaters are not configured
properly.</div>
<div><br>
</div>
<div>Randy</div>
<div>WF5X</div>
<div><br>
</div>
</div>
<br>
<div class="gmail_quote">
<div dir="ltr" class="gmail_attr">On Wed, Sep 22, 2021 at
7:01 PM Eric H. Christensen <<a href="mailto:eric@aehe.us" target="_blank" moz-do-not-send="true" class="moz-txt-link-freetext">eric@aehe.us</a>>
wrote:<br>
</div>
<blockquote class="gmail_quote" style="margin:0px 0px 0px
0.8ex;border-left:1px solid
rgb(204,204,204);padding-left:1ex">‐‐‐‐‐‐‐ Original
Message ‐‐‐‐‐‐‐<br>
On Monday, September 20th, 2021 at 1:07 PM, Randy Love
<<a href="mailto:rlove31@gmail.com" target="_blank" moz-do-not-send="true" class="moz-txt-link-freetext">rlove31@gmail.com</a>>
wrote:<br>
<br>
> The burst method may not be compatible with local
digipeaters... or some TNC's even.<br>
><br>
> Some digipeaters transmit the split second they see
an end-of-packet indicator under the assumption that the
tx is done since the packet is completed. That would
cause the digipeater to collide with the rest of your
long packet that you are sending as a burst.<br>
><br>
> This is done on purpose to induce the
packet fratricide that is necessary for peak performance
of APRS digipeating. (i.e. the digipeater will only be
colliding with other digipeaters transmitting the same
packet )<br>
<br>
So I understand this *but*...<br>
<br>
...here's what I'm seeing in practice. My i-gate sends
out weather alerts for my local area to a path of
WIDE2-1. I just watched a thunderstorm warning go out
that contained three packets. These three packets went
out with only slight breaks in between, hardly any time
for the digipeater to retransmit before the next packet
came out leading to a collision. I've seen similar
results when transmitting messages and telemetry
parameters, too, meaning that at least half of the
packets are being wasted and delayed.<br>
<br>
--Eric WG3K<br>
</blockquote>
</div>
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