Order Tray | Contact Us | Home | SIG Lists

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

AE5PL Lists HamLists at ametx.com
Sat Sep 25 20:05:08 UTC 2004

> -----Original Message-----
> From: Robert Bruninga
> Posted At: Saturday, September 25, 2004 12:17 PM
> Subject: RE: [aprssig] APRS low-power-local ALT input channel
> 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

NO I AM NOT!!!!  In no way did I say anything to advocate that!!!!  I am
saying that collision avoidance is a necessary part of ANY shared
channel data network.  I am also saying that good operating practice
says that you listen on a frequency before transmitting to avoid
creating intentional interference.  Collisions will and do happen.  The
key is whether you take any action to avoid collisions.  As proposed,
your ALT input channel specifically excludes any collision avoidance on
the alternate channel.

> 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.

This is not "sour grapes".  It is a statement of FACT!

> I suggest that those purists that  dont like the APRS simplex 
> ALOHA  should go design their full duplex network and we will 

As pointed out already, we are not running an ALOHA network, but a
combination ALOHA/CSMA network.  What you are advocating is something
that the University of Hawaii determined to be untenable: a pure ALOHA
network with multiple stations on the same channel.

> 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?   

Bob, I don't where you cook up your numbers, but CSMA in the Dallas area
(over 100 users) is closer to 80% effective.

> 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...

And I quote "and some fixed stations".  And what about everybody else
who says "I'll go over to that channel so I don't have to compete for
144.39".  The flaw in your design (among others) is the fact that you
have no way to police the frequency and prevent everyone from moving
over to that frequency.

> > 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..

Wrong!  I fully understand how UI AX.25 packet works.  And 99% of APRS
depends on CSMA, not transmitting in the blind.

More information about the aprssig mailing list