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[aprssig] Too many digi's ?

Robert Bruninga bruninga at usna.edu
Sun Oct 22 16:01:31 UTC 2006

> > ...the alt-input digi takes only the setting of 
> > the -600 offset at the digi and it's done.
> But if this is a single transceiver listening on 
> 144.990 and transmitting on 144.390, it would be 
> transmitting blind without listening on 144.390.
> It's a nice idea, but wouldn't it really take two 
> TNCs & two radios to do it properly... 

It depends on what you are tyring to do.  If it is a High
mountain digi that covers hundreds of APRS users, then one
reason for doing this is to give LOCAL priority to LOCAL users.
If the -local- packet steps on 144.39 QRM from surrounding digis
that are a hundred miles away and are of less value than local
traffic, then I see no problem with it.  It is simply a way of
giving local packets local priority.

Or another way to look at it, is a mountain digi that only
serves a few tens of users in a large area, but who have nothing
in common with the users in the next state who are colbbering
this digi with out-of-area packets that are generally of no
interest to this area.  Many digis are hearing 95% or more of
the channel busy without out of area packets and these all
collide with locals making their local LAN have poor
reliability.  By making this an alt-input digi for locals, now
then these local packets have priority over all external
packets.  Since the dozens of local users are typically only 5%
of the overall packets this digi  is hearing, this gives these
locals 100% priority but only at the expense of about a 5%
impact on still seeing out of area packets.

In otherwords, the alt-input digi gives absolute priority to
local packets and then "allows" out of area packets to be heard
too if the local channel is momentarily quiet.

But if the local area does want to preserve out-of-area packets
as well as locals ones, then you are correct that dual receivers
will be needed to avoid colisions of the local and distant
packets at the transmitter.

> or at least one TNC & a transceiver on 144.990 / 144.390
> plus a receiver on listening on 144.390 to at least listen to 
> see if the frequency was clear? Of course, in busy areas, 
> it may never be clear.  Perhaps it would be no worse than 
> mobiles transmitting from an RF black hole and not knowing 
> that someone the next valley over was already beaconing.

Yes, that is about what it is like.  It gives local packets
Brick-bat priority over out of area packets...


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