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[aprssig] Tracker Smart Pathing

Robert Bruninga bruninga at usna.edu
Tue Mar 21 20:47:58 UTC 2006

>The problem is defining channel load....
>...  The easy metric is packets-per-minute.

Which is also perfectlly the -wrong- method, because as
the channel gets loaded more and more above about
18% of capacity, the actual number of packets decoded
begins to go down.  On a 100% collisions channel,
the throughput is close to zero packets per minute,
similar to a totally clear channel.  And the TNC cannot
tell the difference.

>Or possibly percentage of time that the squelch is 
>open or tones are present - that'd count collisions 
>as well. 

Yes, that is the only way to measure channel load.
-but- you cannot even use the DCD light in a true
CD TNC, beacuse the "true CD" (designed for use
with open squelch) is also only looking for valid 
AX.25 structure and it also does not detect a
collision in most cases.

So the only way to measure channel"load" is with
an RF carrier squelch circuit.  Which by definition is
a big step backwards in adding additional time delays
to every packet and also making the network
vulnerable to any old computer birdie that can 
completely shut down the digipeater.

Of course you can run the TNC open squelch and
use a separate squelch detector, but by now the
digi is taking on such radio surgery and complexity
that few if any digi owners are going to take the
trouble.  And if this is not done right, then it will
not work at all and only make matters worse.

So again, we need to be very careful about the smart digi
bandwagon which looks good on paper but has so many
pitfalls in implementation and human maintenance, as
to become a nightmare... in many cases.

Remember the KISS principle...

Just my 2 cents...

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