[aprssig] AX.25 + FEC = FX.25

Rob Riggs mobilinkd at gmail.com
Mon Jan 13 22:49:57 EST 2020

Hi John,

Thanks for publishing this.

All of the FX.25 specs seem to ignore the distinction between the HDLC
layer (framing, bit-stuffing, FCS) and the AX.25 frame.

This looks like it might be a bit of a challenge to implement in memory-
and cpu-constrained devices. All of the HDLC framing, including
bit-stuffing, must be done and stored in memory, prior to applying the
FEC.  And the raw HDLC frame needs to be saved, including stuffed bits,
while demodulating in order to do the error recovery.

I still may give implementing this a shot at some point.

Kind Regards,

Rob Riggs WX9O
Mobilinkd LLC

On Mon, Jan 13, 2020 at 8:09 PM John Langner WB2OSZ <wb2osz at comcast.net>

> What can you do if your radio signal isn't quite strong enough to get
> through reliably?  Move to higher ground?  Get a better antenna?  More
> power?   Send data very very slowly with narrow bandwidth?
> Sometimes those are not options.  Another way to improve communication
> reliability is to add redundant information so the message will still get
> through even if small parts are missing.
> FX.25 is a clever scheme to add forward error correction (FEC) to AX.25
> while maintaining total compatibility with older equipment.  FX.25 has been
> used for more than a decade with some Amateur Radio satellites.
> Implementations were oriented toward that very specialized application and
> not suitable for general purpose use.
> FX.25 has recently been integrated into a general purpose TNC making it
> very
> easy to use.  Many other AX.25 based applications can now gain the benefits
> while retaining complete compatibility with older equipment.
> For more details, see
> https://github.com/wb2osz/direwolf/blob/dev/doc/AX25_plus_FEC_equals_FX25.pd
> f
> <https://github.com/wb2osz/direwolf/blob/dev/doc/AX25_plus_FEC_equals_FX25.pdf>
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