[aprssig] APRS messaging issues

Robert Bruninga 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
than 8%!

That's why messages should be SHORT, use minimum number of hpops and follow
the other 4 keey understandings of APRS message operations.

Bob


> 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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