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[aprssig] "Best" packet decoder solution

Wes Johnston wes at ai4px.com
Tue Dec 28 16:57:37 UTC 2010


Hmmm.... did not know about this.  Looks like they did a test, and depending 
on if you are a half full or half empty kind of person, the results can be 
interpreted very differently...

http://www.stensat.org/projects/FX-25/FX-25_performance.htm

In a nutshell, 61 packets send, 9 would have been decoded with normal ax25. 
19 additional packets could have been received using the fx.25 method. 
Still, 1/2 of the packets were not recoverable.

A half full person might say that we could cut our packet loss due to error 
by a factor of 3  (9 vs 9+19).

I still can't find any info that tells how much longer the packets are on 
air.  Why hasn't this been pursued since 2006?  Not enough success in this 
test?  This looks like something that could be relatively easy to include in 
sent packets.

Wes


----- Original Message ----- 
From: "Jason KG4WSV" <kg4wsv at gmail.com>
To: "TAPR APRS Mailing List" <aprssig at tapr.org>
Sent: Tuesday, December 28, 2010 11:27 AM
Subject: Re: [aprssig] "Best" packet decoder solution


On Tue, Dec 28, 2010 at 8:40 AM, Wes Johnston <wes at ai4px.com> wrote:
> Hmm.... dopplegänger packets anyone? Two IDENTICAL packets back to back.
> At first blush you get the benefit of a 2nd shot at decoding a packet.

With "hard" decoding, double packets is a waste of time/bandwidth if
there's an error in either one. With the confidence level decoding,
maybe there would be some benefit.

> It's *really* a shame that AX25 doesn't support FEC at level 2.

Google FX.25.  This was proposed for (and maybe flown on) on a
cubesat. IIRC, they wrapped an AX.25 with some FEC information, so
that the packet could be decoded using standard AX.25 hardware, but
FEC-enabled hardware could attempt error correction.

-Jason
kg4wsv

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