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

[John Langner WB2OSZ]

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
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