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[aprssig] APRS Message workarounds

Robert Bruninga bruninga at usna.edu
Thu Jan 17 16:00:05 UTC 2008


>> 2) Recognize that your message had 2 to 4 times 
>> higher chance of getting through then you will 
>> ever see in getting an ACK back.
> 
> This (and many of your other statements about 
> how messaging works) makes the assumption that 
> there is a 50% packet loss per hop. 

I also gave an example for a 70% reliable digipeater.

> Has any testing been done to see if this assumption 
> is correct?

Everyone who has tried to use messages experiences it every
time.

> Show your work, please.  (:

Sure.  Choose any probability you want.  Let that be P.  P is
from 0 (nothing works) to P=1 means every packet is 100%
successful.  Let N be the number of hops.  Then the probabilly
of success is P^N.  Here are the probabilities:

One hop    = P
Two hops   = P^2
Three hops = P^3
ACK one hop = P * P     (or P^2)
ACK 2 hops  = P^2 * P^2 (or P^4)

So for 80% reliable hops, then each of the above values would be
80%, 64%, 50%, 64%, and 41%.

Of course someone with 50W close to a digi, can probably get
P=.99 whereas with 5 W it will be much lower.  Also, after that
first hop, then the next hop is usually one digi competing with
other digis and so P is lower.

*** BUT *** the point was not the actual value, but that the
probability of an ACK is always p^2 worse than the message
getting through.  And so fixed rate clients that did not
implement any of the ack-enhancement features are doomed to poor
message throughput.

Bob, Wb4APR







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