[aprssig] APRS Message workarounds
Robert Bruninga bruninga at usna.eduThu Jan 17 16:00:05 UTC 2008
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>> 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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