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

Henk de Groot henk.de.groot at hetnet.nl
Wed Sep 29 16:09:12 CDT 2004

At 22:39 29-9-2004 +1000, Andrew Rich wrote:
>I have just picked up on the conversation
>are u talking about split freqs ?
>And the sharing rates ?

No, about the alt channel idea (separate RX only channel to enter APRS) and 
if there is a need/benefit for DCD checking on such a channel. The 
discussion wandered off a little to a ALOHA/CSMA discussion.

To wrap it up:

APRS on the main channel is half ALOHA and half CSMA. Stations do hardly 
see eachother but they do see the digipeater; the digipeater sees a lot of 
stations. So on the main channel DCD checking pays off enough. You can only 
defend not doing any DCD checking while using this channel when very low 
power transmitters are used.

On the alt channel stations hardly see eachother and there is no digi 
transmitting there, only listening. So DCD checking is near useless and may 
just as well be skipped without any noticable effect. Having mobile 
stations with no DCD checking at all can be very well defended in this 
case, even when using more powerfull mobile stations. Some people seem to 
disagree with this assessment, even if it doesn't make any sense.

Someone wondered why Europe switched to DAMA, if that was not a proof CSMA 
didn't work. I tried to explain the way the channel is seased in CSMA and 
the problem of misconfiguration by the end-users. That's how we got to talk 
about PERSIST and SLOTTIME. User education didn't work, the DAMA solution 
takes the control away from the users, bypassing this education problem. 
CSMA can outperform DAMA if tuned properly. The need for DAMA does not 
proof CSMA doesn't work, it only proves that it doesn't work on HAM radio 
AX.25 connected networks because of its users.

That's about it, I think you caught up now, the rest was mud-throwing and 
repeats of what has been said and explained before.

Kind regards,


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