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

Henk de Groot henk.de.groot at hetnet.nl
Mon Sep 27 19:48:20 UTC 2004


At 10:38 27-9-2004 -0500, Wes Johnston wrote:
>And finally, if CSMA works so well, why did they switch to DAMA in europe?

I can answer that! CSMA works well if you play by the rules.

Channel access pseudo code:

1) if CARRIER DETECTED then goto 2); else goto 3)
2) wait SLOTTIME time; goto 1)
3) draw a random number between 0 and 255
4) if random number > PERSIST then goto 2)
5) START TRANSMISSION; goto 1)

Now what values of PERSIST and SLOTTIME should you use. That's exactly the 
problem in Europe with people who do not understand this channel access 
method. SLOTTIME is most of the time set to 10, which is 100ms, in most 
stations this is set okay. The ideal PERSIST value however depends on the 
number of users on the channel. If you are alone for example a value of 255 
is okay, you never have to leave room for another station since you are 
alone. If there are however 4 users, PERSIST should be set to 64, giving 
you a 1/4 chance of transmission. The other stations should also use this, 
giving everyboy an equal share.

The problem is the morons that keep PERSIST set to 255; which indeed works 
better for them at the expense of other stations. Other users see this and 
wonder why the throughput of this station is so much better compared to 
anybody else. So this moron (which doesn't know he is a moron, he is just 
ignorant) shares his "good" settings with its neighbouring stations. And so 
the channel troughput goes down the drain...

Education over the years didn't help, so finally the control was taken away 
by DAMA. While in CSMA every station is equal (HDLC Balanced mode), with 
DAMA you have a MASTER that controls the traffic. In DAMA the client holds 
its transmission until the MASTER gives a sign that it is okay to transmit 
by polling the client. No user can mess this up and if the user transmits 
without the consent of the MASTER its link will be terminated.

In APRS DAMA cannot work since it requires a connected mode of operation. 
Without that the MASTER would not know which stations to poll.

In short DAMA has nothing to do with the CSMA technology but with the 
technical (in)competence and willingness to cooperate by the users. 
Systemized CSMA networks which are correctly tuned perform just as good, if 
not better, than a DAMA network.

Kind regards,

Henk.






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