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[aprssig] Aprx v2 beta, now with Viscous Digipeater (can't work)

Robert Bruninga bruninga at usna.edu
Thu Oct 22 17:55:18 UTC 2009


> To implement this on the T2 shouldn't 
> require more than one extra bit of 
> storage per entry....
> I'm tempted to go try it out now...

Please don't until this "visious" digipeating has been fully
characterized.

I can see NO easy SOLUTION to the problem of the
digi-not-hearing some if not most of all packets on the air from
two or more adjacent nearly equal digi's due to collisions, and
therefore, its delayed repeating will not solve the problem, but
simply MULTIPLY the number of dupes and QRM.

I admit that the problem IS solvable by one technique.  That is
designing AND placement of the ANTENNA such that *EVERY*
surrounding wide-area-digipeater is guaranteed to arrive with at
least 13 dB or more of DIFFERENCE in signal level to every other
digipeater in range.  This DOES solve the problem by making sure
each packet can be heard, but ONLY if done correctly and with
critical attention to RF detail.

My experience is that this WILL NEVER BE DONE satisfactorally in
practice in the field and updated EVERY time ANY surrounding
digi changes ANYTHIGN in its antenna, feed-line, power, or
nearby structures..  When this critical alignment is NOT done or
not done perfectly and not maintained with EVERY surrounding
change in anything, then the result of the "visious" digipeating
is a MULTIPLICATION of QRM and dupes which is bad for the
efficiency of the network.

I simply cannot see it work in practice with so many systems
being installed willy-nilly plug-n-play, install-and-forget that
we see today.

Another set-and-forget way to make it work is to have a separate
BEAM and separate packet  RECEIVER pointed at each surrounding
wide-area digi, so that every packet is received without FM
collision.  But this is a huge expense and effort for a small
area FILL-IN..

The FINAL and ULTIMATE solution has always been for APRS to go
to a DUAL-FREQUENCY system which separates the USER UPLINK from
the DIGI OUTPUTS and the local traffic frome the regional
traffic.  But this too is an extrordinary jump... And requires
all new dual-band-dual-receive mobiles and users..

Bob, WB4APR




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