[OZAPRS] Help with digi paths

[David and Ann Fraser]

Hi all,

I can quite understand the responses to my posting, as it flies in the
face of conventional logic for connected mode packet.  But APRS is a
one-to-many broadcast scheme.

Appended is a relevant posting made by WB4APR, and extracts from
UIDIGI and DIGI_NED configuration files (used worldwide, not just the
USA).

73 David ZL3AI.

TAPR APRS Special Interest Group Digest for Monday, February 09, 2004
---------------------------------------------------------------------
Subject: UIDWAIT must be OFF on APRS
From: "Robert Bruninga" <bruninga at usna.edu>
Date: Mon, 09 Feb 2004 09:01:57 -0500
X-Message-Number: 1

It has come to my attention that some digi owners are
mistakenly setting UIDWAIT to ON.  This is not correct.
The default is OFF and should be OFF on 144.39
everywhere.

With UIDWAIT OFF (the default, and way the system
was deisgned), a DIGI will seize the channel first and
all other stations will wait.  The DIGI has priority.
Packets will propagate rapidly.

With it ON, then the digis are just like all other users
and have to wait for the channel to completely clear 
befoer they can send on a packet.  From their HIGH
locations, the channel rarely clears and this just 
drastically slows down the entire network.

Example of a 2 hop packet that hits first one digi, and 
and then its 3 next tier neighbors:

With UIDWAIT OFF (as designed), after the first digi 
digipeats a packet, we want its 3 neighbor digis to all 
digipeat the result at the SAME time (since they are 
all going in threee different directions, there is no
collisions in the directions of each of these digis.)

With UIDWAIT ON (as is being wrongly done in some
areas), the channel throughput will be drastically  cut
by 3 to 4 times because each of these 3 digis will wait
just like all other users for the channel to be completely
clear before each one of them (in turn) digipeats.  This
is disasterous to APRS throughtput!

APRS was designed with FRATRICIDE as a fundamental
part of the system.  That is, ALL digis at the same tier
outward from the source are supposed to all digi at the
same time for maximum channel efficiency and for also
killing off most of the dupes.

Please note, that the ONLY place that this fratricide
kills packets is in the area where the packet originated
in the first place!  Yet, the digi in that area has ALREADY
digipeated the packet once BEFORE.  SO again
UIDWAIT must be OFF for maximum channel efficiency,
and for eliminating some dupes.  Having it ON will triple
or quadruple channel overload.

de WB4APR, Bob


----------------------------------------------------------------------


Extract from N8DEU's website
----------------------------
http://www.qsl.net/n8deu/uidigi_eprom_settings.htm

PPersistence

//PPersistence (default 64)

PPersistance = 64

This is an AX.25 link layer parameter. There should be no need to change
the default unless you have a specific application.

However, a discussion on the APRS SIG in September 2003 dictated some
adjustment should be made to reduce dupes in adjacent digipeater nodes. It
was highly recommended that DWAIT be set to 0. Since UIDIGI does not
implement a DWAIT parameter, setting PPERSISTENCE=255 will in effect make
the digipeater very aggressive in seizing the channel. This parameter is
used in conjunction with SlotTime to configure a setting close to DWAIT=0.

We changed PPersistence to the following setting for our network:

PPersistence=255

==========================================================================
==
SlotTime

//Slottime (default 10)

SlotTime = 10

This is an AX.25 link layer parameter. There should be no need to change
the default unless you have a specific application.

However, a discussion on the APRS SIG in September 2003 dictated some
adjustment should be made to reduce dupes in adjacent digipeater nodes. It
was highly recommended that DWAIT be set to 0. Since UIDIGI does not
implement a DWAIT parameter, setting SLOTTIME=1 which is the lowest
setting will in effect make the digipeater very aggressive in seizing the
channel. This parameter is used in conjunction with PPersistence to
configure a setting close to DWAIT=0.

We changed SlotTime to the following setting for our network:

SlotTime=1


Extract from AX25_MAC.INI file in DIGI_NED version 0.3.5
--------------------------------------------------------
;
; Port 1
 P   1:255      ; (8-255) Persistent value for transmission (=P/255)
 W   1:0        ; (10 ms) Slottime
 T   1:25       ; (10 ms) TX-Delay: PTT on -> transmission of data
 @TA 1:4        ; (10 ms) TX-Tail: end of frame -> PTT off
 @C  1:10       ; (0-63)  DCD (0=Hard-DCD, >0 Adjustment speed for
Soft-DCD)
 @D  1:0        ; (0-1)   Duplex (0=off, 1=on)
 X   1:1        ; (0-1)   PTT (0=off, 1=on)
;


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