[aprssig] Impossible to be digi+something else? (DWAIT=0)

spam8mybrain spam8mybrain at yahoo.com
Wed May 11 12:29:06 CDT 2016

So, does this mean that fill-in digis (WIDE1-1) should not use DWAIT=0 (since they do not have the best local view)?
What would the rule-of-thumb be to define a digi as qualifying for DWAIT=0?
Andrew, KA2DDO 

-------- Original message --------
From: Robert Bruninga via aprssig <aprssig at tapr.org> 
Date: 05/11/2016  11:13 AM  (GMT-05:00) 
To: TAPR APRS Mailing List <aprssig at tapr.org> 
Subject: Re: [aprssig] Impossible to be digi+something else? (DWAIT=0) 

Lynn has it right.  Here is my summary: The DWAIT=0 intentional priority for all APRS digiepaters for maximum network throughput is based on the fundamental assumption that all such digipeaters are at HIGH SITES.  That is, their TNC’s are in the best position in the area to hear anything that is going on and to ONLY transmit when the channel in the entire area is clear. This does not matter if the transmission contains the DIGI’s own beacon, or weather, or frequency objects or user packets.  It does not matter.  The DIGI which has the best ears in the area is in the best place to make the decision to use the channel to maximize throughput, avoid collisions and only transmit when the channel is clear. There are two related corollaries: 1)      ALL digis that have the same pending packet will ALL transmit at the same time (intentional fratricide) so that particular APRS data will take up only one time slot, no matter how many digis heard it.  Yet surrounding next-hop digis WILL hear it. 2)      ALL user systems and any system that is NOT at the digi site or other such highest-in-the land site will set DWAIT to 1 or other value so that the DIGI gets first chance at the clear channel and the user will not get stepped on. Notice that #2 really does nothing to help the network, it only helps that user avoid getting clobbered every time by the digi!   We have not really stressed that so much, since the network does not care.  But it is useful to users who want to maximize their throughput to make sure to NOT have DWAIT=0 and choose some other value.  I have never studied what that optimum value is.  It only needs to be milliseconds since once the DIGI does transmit, then the user TNC is going to hold off until the DIGI packet is finished anyway. Hope that helps. Bob, WB4APR From: aprssig [mailto:aprssig-bounces at tapr.org] On Behalf Of Lynn W. Deffenbaugh (Mr) via aprssig
Sent: Wednesday, May 11, 2016 10:23 AM
To: TAPR APRS Mailing List
Subject: Re: [aprssig] Impossible to be digi+something else? I have asked this question several times and the answer is always the same.  Digipeaters are not supposed to delay their digipeated packet, but ARE supposed to wait for a clear channel.  So they're "semi-polite" as it were.  From http://www.choisser.com/packet/part03.html (a definition of DWAIT):

DWAIT: Used to avoid collisions, DWAIT is the number of time units the TNC will wait after last hearing data on the channel before it transmits.
So a DWAIT of zero will not stomp on other packets, but will jump in as soon as they "last heard data on the channel".  Not really monopolizing in my book, just being aggressive with getting their data on the air.  As long as the digi's other packets are not too long, too frequent, or too numerous, I wouldn't see a need to modify the DWAIT by packet type.

Lynn (D) - KJ4ERJ - Author of APRSISCE for Windows Mobile and Win32

On 5/11/2016 9:44 AM, spam8mybrain via aprssig wrote:I was re-reading some of the old postings about digipeating, where they state that digipeaters (only) should be "impolite" and use a DWAIT of zero to avoid channel clutter. This would seem to imply that no station can be a digipeater and something else (weather station, etc.) unless the full functionality is built into the TNC, because there is no way for an external application to tell a KISS TNC on a packet-by-packet basis which packets are DWAIT 0 and which are to have normal competition for airtime. Either the digi would be "polite" and cause additional airwaves clutter, or the non-digi functionality would be "impolite" and dominate the channel.  So, do any of the "soft" TNCs (AGWPE, DireWolf, etc.) have the ability to specify priority (DWAIT=0) transmission on a per-packet basis? How does this jive with the proposed sat-gate mode? 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). Any feedback would be welcome. Andrew Pavlin, KA2DDO 

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