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[aprssig] Too many digi's ?

Robert Bruninga bruninga at usna.edu
Fri Oct 20 19:13:59 UTC 2006

> I've been speculating to myself that maybe we don't have
> digis.  If we got the existing digis down to a reasonable
> thereby reducing the cell size, and added more digis we might
> both better coverage and better control of the traffic. 3000'
> may be great for a voice repeater, but I suspect it isn't all
> great for a digital network node.

One thing to consider with the multiple lower digis, is to keep
the big digi up there but put its output on another frequency so
that it does not QRM everything else from its high location.  If
you want to hear the big picture, tune in to its output.  And
having one permanent Igate over there will make sure what it
hears gets into the APRS-IS, but does not add QRM everywhere.
Think of this as instantly doubling channel capactiy because now
every packet heard by that big digi does not get digipeated back
onto the same channel.

But then not having the output on 144.39 kinda defeats the
purpose of APRS.  So here is the recommended way to do it.
Shift the HIGH site digi to input on 144.99 and the output
remain on 144.39.  This way low power and local stations or HT's
can get into the high-site digi using +600 on transmit but
without any contention or collisions with other traffic on
144.39.  These local users get priority over all out-of-area
packets on 144.39.

Throughput for all local users to that digi goes up TREMENDOUSLY
because they now have no competition with any other traffic but
themselves.  Out of area packets are not hear by this digi with
it's 144.99 input.

> Maybe the high-site digis should be set so that they don't 
> repeat a  packet unless it isn't repeated by someone else 
> in a reasonable time, thereby leaving the high-site digis 
> to do fill-in and backup work.

That is another way to do it, but the alt-input digi takes only
the setting of the -600 offset at the digi and it's done.

De Wb4APR, Bob
> I don't have any data to support this, but I wouldn't mind
> some numbers or simulations or tests to convince me one way or
> other.
> Random thought before my caffeine takes effect...
> -Jason
> kg4wsv
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