[aprssig] Alternate Freq Input Digi's

Robert Bruninga bruninga at usna.edu
Wed Oct 3 13:37:27 CDT 2007

>> The digi will... be a 144.99 in 144.39 out...
> ... But listening on 144.990 and transmitting 
> on 144.390 it wouldn't know when the 144.39
> frequency was clear. ... It would be better 
> with two TNCs and 2 radios so it could listen 
> before transmitting.

In many cases, that is not really needed.  Here is the value of
a 144.99 input into a 144.39 output WIDE1-1 digi:

1) Gives local priority to local low-power users.
2) They do not collide with any other uplink packets
3) Their reliability is excellent on 144.99
4) The 144.99 channel has NO OTHER DIGIS OR TRAFFIC
5) When they are digi'ed over to 144.39, they are no different
than any other hidden-transmitter-packet that cannot hear other
users direct either.

So in most cases, there is no difference in performance on

Looking at the big picture, what the local digi hears on 144.39
around here is usually 90 to 98% packets from out-of-area. Just
look in the immediate vicinity of any digi in our Mid-Atlantic
area, and there may be only 2 to 5 stations in the footprint
(hitting it direct) around any one digi out of the 300 packets
per 10 minutes that same digi digipeats. 

This is why the reliability is not that great for any local
user, because he has to take his collision chances with 90 to
98% of out-of-area packets of less interest in his community.
But in some local areas, they want to give priority to local
use.  Or to give a back-door input to low-power APRS such as
hikers in a mountain park... So they do not have to compete with
all the noise on .39 to be heard.  SO they use .99 instead.

A WIDE1-1 listening on .99 for these RARE inputs and digipeating
out on 144.39 gives these local users high reliability of being
heard, and by digipeating on 144.39 they get an equal chance to
evertything else there.  That is why, if one wants to give them
priority, then that cross freqeuency digi does not need to have
two TNC's and two receivers, it is like any other user suffering
the same hidden transmitter problem as all other users there,
and so it is no different than if the original user had used 50
watts.  He still will step on any out-of-area packets.

So I don't have any problem with somone setting up a local
WIDE1-1 priority digi listening on 144.99 (where there are
extremely few LOCAL users) digipeating to 144.39 at only enough
power to hit the nearest 144.39 digi.  Listening first would
make no difference in the overall performance of 144.39.  Yet it
would give those local users with low power on 144.99 local
priority.  And local performance is what APRS is all about. 

Again, 90 to 98% of all packets heard by many digis on the
relatively low terrain of the east coast are NOT first hop
packets.  This alternate input digi simply gievs first-hop-local
packets priority over out-of-area packets.

Just a perspective.

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