[aprssig] AX25 / KISS questions

John Langner WB2OSZ wb2osz at comcast.net
Fri Jun 9 17:24:32 CDT 2017

The AX.25 spec says this about the "C" bits in the destination and source
addresses.  Section 6.1.2, figure 6.1.

Frame Type        Dest.SSID C-Bit   Source SSID C-Bit
---------------   ---------------   -----------------
Previous versions        0                0
Command (V.2.X)          1                0
Response (V.2.X)         0                1
Previous versions        1                1

The two "C" bits must be the opposite of each other.  This is very important
for the traditional "connected" mode.

I could find no mention of these two bits in the APRS protocol spec or other

After spending a lot of time studying the AX.25 spec, I think "command"
would be correct.  The state diagrams, in the back, indicate that a UI frame
that is not a "command" is an error.  APRS is built on top of AX.25 so we
would expect it to follow the same rules unless explicitly mentioned

When this question came up before, I hacked together something to observe
what is actually used in practice.

Source C   Dest C   meaning, examples.
--------   ------   ------------------
   0      0       (previous versions)   WXtrac, APRS+SA, KPC-3, Tiny Track,
   0      1       (command)   Uidigi, UIview, APRSISCE, APRSdroid
   1      0       (response)  Kenwood, Yaesu, APRSISCE
   1      1       (previous versions)   OpenTrack

We can have philosophical discussions about what they "should" be but the
reality is that we find all 4 combinations being used.

Any implementation should simply ignore them for incoming frames.

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