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[aprssig] PPersistance and Slottime settings.

Robert Bruninga bruninga at usna.edu
Tue Nov 1 02:43:12 UTC 2005

>>> bruce.coates at sasktel.net 10/28/05 8:24 AM >>>
>I'm just preparing to replace a failed UIDIGI 
>digipeater and am trying to figure out what 
>would be a suitable setting for Slotime and PPersistance. 

Normally APRS has always been recommended to
have ALL digipeaters suppress all delays and to
immediately seize the channel and digipeat immediately.
The reason is that typically, eacch digi hears 4 or so 
other digis.  If they respectuflly hold off to make
sure that other digis have had a chance to digi first,
then each packet guarnatees 5 time slots as each
digi repeates in turn.  And then 5x5 more time
slots as each additional digi radiates outward.

This is extremely inefficient.

In APRS the next 4 digis are all supposed to key up
at the same time., and then the next tier of 12
more digis will all key up at the same time.  The
result being only 2 total digipeating time slots are
used up and the channel clears up very quickly.

So I guess I think there should be no delays.
it just slows the network down drastically.


Saskatoon, the home of the VE5BNC-3 IGate, has half a dozen highways that 
radiate out from it.  The idea is to set digipeater timing so that it packet 
tend to travel towards the central digi.

Here's how I think it works if two of my digipeaters hear the same signal 
from a station half way between them.  If this station uses a path of 
WIDE2-2, my hope is to have the outlying digipeat it first, followed by the 
central digi where it would stop.  What I'm trying to avoid is having the 
central one digipeat first and then every outlying digipeat the second hop.

Here are the timing parameters that I'm looking at.

Digi will use aggressive timing and will be more likely to be the first to 
digipeat.  This would be used on outlying digipeaters.
PPersistance = 255
SlotTime = 1

Digi will use more relaxed timing and will be less likely to be the first to 
digipeat.  This would be used by the digi in the middle of town.
PPersistance = 64
SlotTime = 10

First, am I right?
Second, should I care?
Third, is it worth trying to fiddle with the timing in the hope of 
optimizing things?

73, Bruce - VE5BNC 

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