[aprssig] APRS messaging issues
bruninga at usna.edu
Tue Jan 6 14:01:59 CST 2015
>> APRS operators must understand that ACKS in packet radio
>> are much less reliable than outgoing messages.
>This makes no sense at all.
Yes it does and is the #1 reason for poor throughput in packet radio
systems and why APRS was invented in the first place.
Do the math. Say the channel has a 50% probablity of getting any single
packet digipeated. This give an outogoing message of two hops have a 50%
times 50% probablity of getting out 2 hops. That is a 25% chance. On
average it would take 4 transmissions to get through.
THe ACK on the other hand only gets sent ONCE and only after hearing the
incoming message. SO not only does the incoming message only have a 25%
chance of getting there then the ACK also only has a 25% chance of gettin
back. So the chance that the SENDER gets an ACK is 25% times 25% or less
That's why messages should be SHORT, use minimum number of hpops and follow
the other 4 keey understandings of APRS message operations.
> Bob, I don't disagree with your suggestions for making effective use of
> APRS messaging, but let's get the facts straight. Why do you claim ACKs are
> less reliable than messages?
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