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[aprssig] Local Event using RELAY?

AE5PL Lists HamLists at ametx.com
Wed Mar 30 22:08:48 UTC 2005

> -----Original Message-----
> From: Henk de Groot
> Posted At: Wednesday, March 30, 2005 1:45 PM
> Subject: Re: [aprssig] Local Event using RELAY?
> Yes, I know. But when you are unable to detect any carriers 
> of the other APRS stations because they are too weak or 

If they break the squelch, they are heard and CSMA is in effect.

> slottime setting but otherwise pure ALOHA behaviour).

All of the ALOHA statistics are based on a central computer issuing retry commands.  This does not exist in APRS and it greatly affects the bandwidth capability numbers that is used.  That is why those numbers do not apply to APRS and it is a great disservice to the U of H's project to call transmitting in the blind "ALOHA".

> > another station is without being able to hear (at all) the other 
> > station.  In fact, the numbers you quoted are based on invalid 
> > assumptions and are of little value other than to mislead 
> people about 
> > operating a "deaf" station.
> Maximum APRS channel througput is much closer to this 18.4% 

It depends on where you are.  If that were the case here in the DFW area, why are we RELIABLY seeing 120+ stations per hour?

> time-slotting) than to anything you would expect from even 1 
> persist CSMA. This suggests APRS behaviour is more like ALOHA 
> than like CSMA. For CSMA the maximum channel load before 
> collisions kill it should be much higher.

This is only your conjecture and may apply to your area under your operating conditions.

> Anyway. CSMA with all the hidden transmitters like in APRS 
> (and in combination with the TX delays that HAM equipment 
> needs) has only a little gain over having no carrier sense at 
> all. Whether you like is or not, the carrier sense stuff in 
> APRS saves very little and my feeling is that its more there 
> to keep your conscience quiet than for any sensible technical 
> reason. The odd packet this will save is not worth the effort.

This is your opinion.  Thank goodness it is not how most people operate.  If it was, your ability to get reliable communications through for APRS would be LESS than 18% because of the increased beacon rates, etc. that people would try to overcome their short falls.  As I pointed out, your statements promote that everyone should transmit blind because CSMA doesn't help at all.  In fact, this is just plain wrong.  The numbers you use are naïve and without context to the real world of radio.  Yes, there are "hidden" transmitters.  Are they the majority of packets?  Not in the DFW area, maybe in yours.  If everyone transmits in the blind, then everyone will be "hidden" transmitters and you will see throughput drop to almost nothing.

Keep promoting blind transmissions, Henk.  Just don't do it in our area PLEASE!


Pete Loveall AE5PL
mailto:pete at ae5pl.net 

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