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[aprssig] APRS IS question

Keith VE7GDH ve7gdh at rac.ca
Sat Dec 5 05:36:02 UTC 2009


Andrew VK4TEC wrote...

> 1. When two people connect to two different APRS IS servers, how does
> the system know to let which packet through?

An IGate gates everything (at least all UI frames) that it hears to the
APRS-IS.

> assuming each station hear and gated RF ---> INET

Could you explain that?

> 2. What is the "recency" checking that goes on ? ie sending packets
> too fast of the same payload?

The APRS-IS throws out duplicate packets if a copy was heard in the
previous 30 seconds.

> 3. What happens when I gate RF ---> INET packets that have already
> gone INET --- > RF

When you "gate RF ---> INET" your IGate is doing what it is supposed to
do. If you gate something that the APRS-IS already had, and someone (you
or someone else) gates it to RF and it is heard by an IGate again, I
would think that the APRS-IS would throw it out as it would be a
duplicate of a packet heard in the previous 30 seconds... unless there
were any delays of more than 30 seconds.

> My questions are more about the inner workings of the APRS-IS and why
> it does not simply fall into a giant RF loop

It's a good question. However, I think the APRS-IS would take care of
that potential problem by discarding packets that it saw as dupes. Any
comments from server operators or the server software designer?

73 es cul - Keith VE7GDH
--
"I may be lost, but I know exactly where I am!"




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