Date: Thu, 12 Apr 2001 
From: Bob Bruninga, WB4APR 
To: TAPR APRS Special Interest Group 
Cc: amsat-bb@AMSAT.Org,
Subject: TX-Cycle Recommendation?

ANSWER: One packet randomly evey 2 mins.

Some numbers.  Lets say a good pass lasts 10 mins.  Thats 600 secs, but
its half duplex so thats 300 secs of 100% channel capacity.  An ALOHA
channel falls apart above 18% loading (what we are seeing now), so only
about 60 could be successful.  With one second packets, then these 60
people should only attempt one packet randomly once every 2 minutes. 

This gives them 5 shots at the prize with an excellent probability of
success... *IF* everyone else but them would be quiet...<WINK>

It's actually twice this (120) because as each one gets in, he should
STOP, and then his shot gets turned over to someone else.

NOW THE GOOD PART.  In the long run, after the initial excitement, I would
doubt that there will be 120 people actively operating in each footprint
on every pass. (PRIME-TIME and FIELD DAY excluded!)  Thus, you can count
on a good chance at pinging the ISS to show off HAM radio anytime,
anyhwere, when it comes into view (if everyone else follows the ISS
UtIquette)...

This is not to discourage people with more important DEMOS to increase
their rate until successful, because the value to HAM radio of someone
else's demo is more important to us all than our individual 1 seconds...

One more thing.  Your signal will vary relative to everyone else by at
least 10 dB thoroughout the pass, so even if you randomly collide with
someone every time, sooner or later you are going to win.

Just a thought. I'd love to see a rigorous analysis.

de WB4APR, Bob