[aprssig] UIDIGI Preemptive Digipeating (was: Why RELAY, WIDE... is so bad....)

[Greg Noneman]

The renewed discussion relating to RELAY and WIDE got me thinking about 
the local situation in Southern California.

As I stated in another thread, the 4 metro LA digipeaters made a change 
in February to support single hop digipeating.  They do not support the 
normal flooding technique, but rather generate a single digipeat when 
WIDE1-1, WIDE2-2, WIDE2-2 OR WIDE3-3 is received.  Two of the four 
digipeaters (those running UIDIGI firmware) also support RELAY.  This 
change was made to help reduce the congestion in the area, and I 
believe has been relatively successful in doing so.

One of the remaining issues is that many stations unnecessarily use 
RELAY as their first hop in the digipath.  While this can help in some 
situations, it usually just creates additional traffic on the channel 
by adding an extra hop to the path.  This defeats the single-hop 
concept.  Much of the greater LA area is within reach of one of the 
four high-level digipeaters.

Now I've been successfully using UIDIGI firmware since 2000. It has 
been used on one of the SoCal metro digis since 2000 (v 1.8b6) and was 
recently installed on another within the last few weeks (v 1.9b3).  For 
some reason, it just dawned on me yesterday that the preemptive 
digipeating feature of UIDIGI solves our local problem!

Here's our particular situation:

   The N6EX-4 runs UIDIGI firmware.

   UIFLOOD: disabled
   UITRACE: disabled
   UIDIGI: RELAY, WIDE1-1, WIDE2-2, WIDE2-2, WIDE3-3
   Peemptive Digipeating: ON
   Preempted Calls: WIDE1-1,WIDE2-2,WIDE2-1,WIDE3-3

   Assume the following packet is launched:  
W6XYZ>APRS,RELAY,WIDE2-2.....

  Without preemptive digipeating, if the digipeater heard the packet 
directly, it would respond as follows:  W6XYZ>APRS,N6EX-4*,WIDE2-2.....

This packet would then most likely be heard by the remaining three 
metro digis and be transmitted three more times.  Additionally, any of 
the outlying digipeaters that heard the packet directly from N6EX-4 
would act on the WIDE2-2 and propagate it outward from LA.

   Now with preemptive digipeating enabled, the digi responds as 
follows: W6XYZ>APRS, N6EX-4*.....

One hop, into an IGate and it's done!   This is just what is wanted.  
Should the digi not hear it directly but it is heard by a low level 
(i.e home) digi, it will respond on the second half of the path.

I don't know why it took me so long to see the value in the preemptive 
digipeating feature, but now I'm a believer!   Many of you running 
UIDIGI may already be aware of the benefits of this function, but I 
wanted to pass along my experience with it.

I know this only applies to those running TNC2s with UIDIGI, but for 
situations such as ours, I think is a significant enhancement.


73,
Greg
WB6ZSU