[aprssig] APRS Message workarounds
Robert Bruninga
bruninga at usna.edu
Thu Jan 17 11:00:05 EST 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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