[aprssig] Careless AX.25 encoding in APRS messages

Matti Aarnio oh2mqk at sral.fi
Fri Oct 2 15:56:06 EDT 2009

On Fri, Oct 02, 2009 at 08:18:02AM -0500, Pete Loveall AE5PL Lists wrote:
> Be careful, the currently accepted spec is v2.0 on the TAPR site and close
> attention should be paid to section " Command/Response Procedure"
> (TAPR web site).  Specifically, if both C bits are set to zero or both are
> set to one, it is assumed to be a pre-v2.0 version of AX.25 and acceptable.
> Therefore, all of the listings with either both C bits set to zero or both
> set to one are legitimate and within spec.

So it really is a matter of "ignore these bits"..  And reserved bits can also
be sent as zeros and corrent system infra ignores them, even though they are
specified to be sent as one.

For myself, and other stumbling on these issues, I collected most details and
useful hints I could come up with into following web page:


Can you name any additional details ?  Like recommendation of what deviation
an 2200 Hz tone, and 1200 Hz tone should make on NBFM ?   That would be an
"gold standard" to tune our modems and transmitters/receivers to.

> 73,
> Pete Loveall AE5PL
> pete at ae5pl dot net

73 de Matti, OH2MQK

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