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[aprssig] Packet Node on 144.390 ?

Joel Maslak jmaslak-aprs at antelope.net
Mon Sep 11 03:51:40 UTC 2006


On Sep 10, 2006, at 9:30 PM, Daron J. Wilson wrote:

>> In an area with one or two or three active users total - it wouldn't
>> be cost effective to build two separate systems when one would
>> work just fine.  Any more users and activity would make sense
>> to build a separate system to carry the additional load.
>
> In that small of area, they could just run on the voice repeaters,  
> it's just
> sharing bandwidth right?

I'll add that the repeater may be the more efficient option for  
emergency communication if all the stations can hear it effectively,  
more then two stations are communicating, it has a reasonable TX  
Delay, and at least one station on channel can't hear at least one  
other station.

Repeaters solve many of the hidden transmitter issues.  The only  
major issues using them are to ensure the repeater is "flat" (doesn't  
emphasize certain tones) and that the users use software carrier  
detect.  The problem with software carrier detect is if a voice user  
does try to use the repeater the packets won't detect that as an  
active station, and also the repeater's ID will clobber a few packets.

Hidden transmitter issues are ignored far more often than they should  
be.  Channel utilizations of 60% or higher are perfectly reasonable  
even with a reasonably large number of active stations - IF everyone  
can hear everyone.  If someone can't hear someone else (but maybe  
some node or central station can), you might be lucky to get a tenth  
of that.  That's because neither stations realizes it's clobbering  
another's transmissions.  (with APRS, this is even more likely since  
most stations are bad packet stations - they are low power, often  
mobile, with sometimes less than ideal geographies, and large area  
digis that can hear other large area digis that their users cannot  
hear; Adding connected mode packet to the mix will make things  
miserable for both APRS and connected-mode communities, as neither  
will work right)

It's also a lot less important to have good emergency packet stations  
in this configuration.

Surely there is a nearly unused voice repeater in nearly every area  
of the US (and possibly elsewhere in the world).

But of course no one likes to hear packet on their voice repeater,  
but it's fine to jam APRS!  ;)  So it probably won't even be  
considered a reasonable option.  But neither should using 144.39.

Besides, if you have a digi site for 144.39 that doesn't have any  
70cm equipment already, all you'll really need to add a second digi  
is a radio (cheap for 1200, more for 9600) and TNC (cheap for 1200,  
more money to do 9600).  And you can go 9600 instead of 1200 - so  
that you can increase the number of users and amount of information  
transmitted.  Heck, you can even build real packet nodes fairly  
easily rather than just relying on "dumb" digipeaters.

If the organization isn't interested in using voice repeaters or  
building new permanent digis, why not build a temporary digi?  One  
that fits in, say, an ammo can and can be carried by hand to a tall  
building or left in a car parked on a mountain top, in an area  
sufficient to provide the coverage needed during the disaster?  (I  
know where I live there are plenty of areas without good coverage by  
digis on 144.39, anyhow, and Murphy says that those are the most  
likely areas to need communication)  Build 2 or 3 of these portable  
boxes as true nodes and you could have a mobile network that you can  
use not only in your local area, but also in another area if another  
Katrina or something happens.  Heck, it might be a fun project!





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