[aprssig] Impossible to be digi+something else?

wb2osz at comcast.net wb2osz at comcast.net
Wed May 11 20:39:34 CDT 2016

KA2DDO asks:

> So, do any of the "soft" TNCs (AGWPE, DireWolf, etc.) have the ability to specify priority (DWAIT=0) transmission on a per-packet basis? ...
> I'm curious because I'm doing some research on an idea for improved digipeating, and it requires using an external program with a KISS TNC. As such, I'd like to be able to selectively decide whether my station is "polite" or "impolite" on a per-packet basis (assuming impoliteness is still preferred digipeater behavior).

This is a fundamental limitation of the KISS protocol.  An application doesn't have control over the transmit timing or any knowledge of when the frame is transmitted.

As others have pointed out, an APRS digipeater should transmit frames as soon as the channel is clear, intentionally causing collisions.

When transmitting original frames, there should be a random wait after the channel is clear, to reduce the chance of a collision.

Direwolf uses a heuristic when transmitting frames that arrive over the KISS protocol.   If any of the via digi fields have the "has been used" bit set, we assume the application is acting as a digipeater and there is no wait after the channel is clear.

If there are no digi addresses, or none of them have the "has been used" bit set, there is the usual random wait time after the channel is clear.

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