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[aprssig] APRS low-power-local ALT input channel

Robert Bruninga bruninga at usna.edu
Sat Sep 25 17:15:26 UTC 2004


Pete is arguing that all APRS digipeaters should be full
duplex repeaters with cavitites so that everyone on the
output can hear everyone on the input and thus do a 
better job of avoiding collisions.  But unfortunately
1) A full dux repeater takes maybe 5 times the cost (cavities)
2) 30 times the volume of space
3) An impossible to obtain frequency coordination
4) and is NOT the network that APRS was designed to do

APRS grew because it uses simplex digis that anyone could
put up easily.  And the shortcomings of its lack of true
collision avoidance has not detered its popularity nor its
growth.   Im tired of hearing the sour grapes of:

"It is bad data network design to use a shared channel 
"with no attempt to have a collision avoidance method.

I suggest that those purists that  dont like the APRS simplex
ALOHA  should go design their full duplex network and we
will happily follow once you have it in place...  In the mean
time try to understand that network design is always a tradeoff
between what is "best" versus what can be done in limited
space, cost and spectrum.

>>> HamLists at ametx.com 09/25/04 7:30 AM >>>
>No.  Bob is again trying to justify the elimination of any type of
>collision avoidance based on the faulty premise that if it is not 100%
>effective, then don't do it.  

But when it can be CLEARLY SHOWN that it is less than 1%
effective, then what kind of sentient being would ever conclude that
it should be the driving factor in a network design that is impossible
to accomplish?   

> I stand by my statement: it is bad operating practice to 
>transmit without listening first to prevent intentional 
>interference on a shared channel. 

Motherhood and apple pie.  Of course it is.  But this is not
that situation.  We are talking about listening for low power
devices that cannot possibly hear each other on a channel
DESIGNED for such application...

> It is bad data network design to use a shared channel 
>with no attempt to have a collision
>avoidance method.

But to anyone that understands how APRS works, they
realize that that is how 99% of the APRS network is..
its just how the network was designed for overall
efficiency of materials, cost, infrastructure, time and
space...

de WB4APR, Bob
> -----Original Message-----
> From: Bill Herrmann
> Posted At: Saturday, September 25, 2004 3:07 AM
> Subject: RE: [aprssig] APRS low-power-local ALT input channel
> 
> At 11:50 PM 9/24/2004 -0400, Robert Bruninga wrote:
> >Ah, that is a common missunderstanding.
> >When any system uses HIGH site repeaters to serve many ground
> 
> While it's true that the ground level users are in an aloha 
> network it isn't true that the high site repeaters are. They 
> are truly in a CSMA network. I think what Pete was getting at 
> is that we need to be building those high site cross-channel 
> digipeaters so that they check for busy before they transfer 
> the packets from the alternate network to the main
> 144.39 network.
> 
> Pete - Is that where you were going?

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