[aprssig] Local Event using RELAY?

John Kraus jfkraus3 at cox.net
Thu Mar 31 08:25:43 CST 2005

At 07:19 AM 3/31/2005, you wrote:
>         From: John Kraus
>         Posted At: Wednesday, March 30, 2005 10:21 PM
>         Subject: RE: [aprssig] Local Event using RELAY?
>         At 09:00 PM 3/30/2005, you wrote:
>                 Ah ha!!!  The light came on for me!  Henk and I are
>talking apples to Pete's
>                 oranges...   Apples and Oranges...
>         This discussion is silly.
>         The key is to understand that the primary factor that influences
>whether a station is CSMA or ALOHA is, at its most basic, simply a
>matter of antenna height or more succinctly HAAT.
>         If a station is running near a unity gain antenna at less than
>10 ft above ground it is almost always functionally ALOHA.  The
>exception being when it is on a high site or is located in a station
>dense area.
>         Conversely if it is located at 300ft HAAT or on a mountain it is
>mostly CSMA.
>I did not receive Wes's email via TAPR so I will respond to his email as
>a SIG post in my next post.  As to your statements above, they are
>incorrect.  Reread my original post.  That station with the low HAAT is
>most definitely running CSMA to over 90% of the packets being generated.
>Actually, it is running closer to CSMA for 99% of the packets on the
>frequency when you take the time to do the math and take into
>consideration what a station can _hear_ versus what a station can
>_decode_.  This is a very important distinction which your statements do
>not take into account.
>Pete Loveall AE5PL
>mailto:pete at ae5pl.net

You are located in a "station dense area".  The station in my example is 
not. Neither are stations located throughout much of the geographic area of 
the US.  I stated in my example that the low station could only hear the 
digi.  It is ALOHA to everyone else on the frequency.

The key concept that continues to be missed is that APRS is a mixed CSMA / 
ALOHA network.   Stations that are CSMA should not transmit 
blind.  Stations that are almost always ALOHA can be granted the ability to 
transmit blind under special circumstances.

The single easiest way to differentiate is by looking at RF range and that 
is largely a function of antenna height and topology.

============here is my text again.===============

Ask this question:

If the transmitting station can not hear ANY STATION but the digi who will 
be harmed by its transmission in the blind? If the digi and the ALOHA 
collide who loses?

The answer in both cases is nobody.

Nobody is harmed by the blind transmission.

The ALOHA station must accept the loss of this packet as part of "its" 
network design it is expected and unavoidable therefore not harmful.
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