[aprssig] AX25 / KISS questions

Fred Hillhouse fmhillhouse at comcast.net
Sat Jun 10 11:49:01 CDT 2017


Hi John,

I read the Direwolf source and found your comments to be remarkable. You obviously spent some time. Because of that I went with using  C=1 in the Destination Field (C=0 in the Source field). I also captured packets from APRSIS32 as well and noticed they were the same.

Thank you!

Best regards,
Fred N7FMH


-----Original Message-----
From: aprssig [mailto:aprssig-bounces at tapr.org] On Behalf Of John Langner WB2OSZ
Sent: Friday, June 09, 2017 6:25 PM
To: aprssig at tapr.org
Subject: [aprssig] AX25 / KISS questions

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 literature.

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 otherwise.

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,
Xastir
   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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