[aprssig] Error checking within APRS packets

Bill V WA7NWP wa7nwp at gmail.com
Wed Jun 22 12:43:13 EDT 2011

>> The bit-stuffing process should mean that an bit field of 01111110 on the
>> ASCII side gets translated to 011111010 over-the-air and  so can never get
>> confused with a start/end flag.
> The point is that APRS is not taking advantage of that trick. Because APRS
> is explicitly forbidding that pattern to occur anywhere in the information
> field of its packets.

The APRS spec says to never use a tilde in the text field?   If it's
true, that's silly.   It's just another ASCI character.  The
bit-stuffing magic happens at the hardware packet level and is totally
independent of what we put in the packet data -- for APRS or any other
packet application.

>>  There
>. Even the same AX.25 could be improved. For example, I would be
> quite keen to see proper FEC in place of CRC.

>  But, in my opinion, most imporantly is the layer 1 that should be improved
> as it could bring most benefits. So, I suppose it would be even better if
> FEC is introduced there as more powerful schemes could be used and also the
> upper layers would be indipendent and it could optionally use different
> levels of FEC adaptively as needed.

Check out the presentations on FAX.25 (or something by a similar name...)

AX25 is 'good enough' for what we're doing now.  The real need is
something better to do with it...  :-)


Bill - WA7NWP

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