[aprssig] Consecutive Packet Send

Kenneth Finnegan kennethfinnegan2007 at gmail.com
Sun Nov 21 14:35:07 EST 2021

On Thu, Sep 23, 2021 at 7:14 AM Randy Love <rlove31 at gmail.com> wrote:

> Sadly tho, most digipeaters are not configured properly.

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"

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:
1. Sharing the same TX delay for position and telemetry
2. Sharing the same TX delay for ACK and reply message
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
4. Sharing the same TX delay for several digipeated packets which have
queued up while waiting for CSMA/CA to grab the channel

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

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?

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.

Kenneth Finnegan, W6KWF

On Thu, Sep 23, 2021 at 7:14 AM Randy Love <rlove31 at gmail.com> wrote:

> Define 'slight breaks', please.
> If the digipeater is correctly configured, it will TX *at the instant it
> sees the end of packet flag*.
> 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.
> Sadly tho, most digipeaters are not configured properly.
> Randy
> WF5X
> On Wed, Sep 22, 2021 at 7:01 PM Eric H. Christensen <eric at aehe.us> wrote:
>> ‐‐‐‐‐‐‐ Original Message ‐‐‐‐‐‐‐
>> On Monday, September 20th, 2021 at 1:07 PM, Randy Love <rlove31 at gmail.com>
>> wrote:
>> > The burst method may not be compatible with local digipeaters... or
>> some TNC's even.
>> >
>> > 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.
>> >
>> > 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 )
>> So I understand this *but*...
>> ...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.
>> --Eric WG3K
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