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

A.J. Farmer ajfarmer at spenet.com
Sat Sep 25 09:42:25 CDT 2004

I agree that it is bad to transmit "blindly", but I don't think Bob is
advocating that.  As a matter of fact, he specifically said that the digi
should "check for busy" on 144.39 before transmitting.

The way I see impementation of an ALT-input digi is to *ADD* the ALT input
via an additional TNC and radio or a multi port TNC.  Therefore, the digi is
listening on both 144.39 and 144.99.  Therefore, it would *NOT* be
transmitting blindly on 144.39 and collision avoidance is maintained.

What about the "PocketTracker" type devices that have no receiver circuitry
at all?  They have no means of collision detection.  


A.J. Farmer, AJ3U

-----Original Message-----
From: aprssig-bounces at lists.tapr.org [mailto:aprssig-bounces at lists.tapr.org]
On Behalf Of AE5PL Lists
Sent: Saturday, September 25, 2004 7:31 AM
To: TAPR APRS Mailing List
Subject: RE: [aprssig] APRS low-power-local ALT input channel

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.  Ground level users are in a combination
aloha/CSMA network.  In fact, in a local operation (which is what we are
talking about), ground based devices do see a high percentage of other
transmitted packets (the need here is not to be able to decode those
packets, but to see the signals).  I stand by my statement: it is bad
operating practice to transmit without listening first to prevent
intentional interference on a shared channel.  It is bad data network
design to use a shared channel with no attempt to have a collision
avoidance method.


Pete Loveall AE5PL
mailto:pete at ae5pl.net  

> -----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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