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[aprssig] Local Event using RELAY?

Henk de Groot henk.de.groot at hetnet.nl
Tue Mar 29 18:32:34 UTC 2005

Hello Pete,

AE5PL Lists schreef:
> Your extensive attempt at justifying blind transmissions just add
> support to Robbie's statement that blind transmission is just not right,
> under any circumstances.  Your idea of common sense is sometimes very
> contrary to proper operating practices.

Now why does this come up now? We had lengthy discussions on the SIG about 
ALHOA and the maximum 18% channel load. Transmitting blind *is* exactly 
what ALOHA is all about and nobody argued agaist it.

APRS behaves like ALOHA; not because we want it, but because most of the 
time the other stations cannot be heard since they are too weak and out of 
sight of the receiver. So 90% of the time APRS stations already tranmsit 
blind as they can't hear the other stations, dispite carrier detection.

We have a max 18% channel load because of this. Under ALOHA conditions it 
is perfectly okay to transmit blind. The odd percentage you save by 
listening first is neglegable and waste of technology for only a very 
little gain.

I know this goes against your feeling but think about the 18% channel 
load, this means the chance of collision to begin with is already 0.18!

You are thinking about a situation where you actually can hear your 
neighbours, but that's not the way APRS works. If everybody could hear all 
the others and could utilize CSMA the maximum channel capacity would double!

Bobs proposal is sound and perfectly acceptable in an ALOHA environment. 
If you don't like it then you have to change to another technology 
(master-save, timeslotted, time allocated, duplex with feedback channel 
etc.). ALOHA is the way APRS works in practice due to its huge ammount of 
the hidden transmitter problems.

To be realy on the save side, operate the "deaf" relay with the minimum 
amount of power needed to reach the digi. In that case when a packet 
happens to collide, the other packet will most likely overpower the packet 
from the relay.

Kind regards,


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