[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,
   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.

aprssig mailing list
aprssig at tapr.org

This email has been checked for viruses by Avast antivirus software.

More information about the aprssig mailing list