[aprssig] AX25 / KISS questions

Greg Trasuk trasukg at trasuk.com
Fri Jun 9 14:57:31 CDT 2017

> On Jun 9, 2017, at 10:20 AM, Fred Hillhouse <fmhillhouse at comcast.net> wrote:
> Greetings!
> As I read through the specs I have a couple of mysteries.
> https://www.tapr.org/pdf/AX25.2.2.pdf
> http://www.ax25.net/kiss.aspx
> The first byte is ‘C0’.
> The second byte-high nibble is a port byte - useful if talking to a multi-port device I presume.
> The second byte-low nibble is to flag to define the rest of the frame. Should be 00 unless actually controlling the TNC in some fashion.
> The last byte of the Destination Address Subfield, A7, is CRRSSID0. The C is the mystery. If A1-A6 contains a TOCALL, should C be a ‘1’ or ‘0’. My first inclination is to believe it should be a ‘0’ (Response V.2.X) if the packet is not a TNC command. But then, a ‘1’ could be signaling a command for a distant TNC. Or does it really matter the packet is being applied to APRS?

The C/R is used in the “connected” usage of AX.25.  You can get the idea from Section 4.3 of the AX.25 spec.  For APRS, since we just use UI frames, I don’t think it has any meaning in the “TO” address field. The spec just says “Either” for C/R.  It appears to be ‘0’ on any  frames I’ve seen on the air (although tbh I’m not absolutely certain of this, so feel free to check for yourself).  The high bit is used as “has-been-repeated” in the repeater address field, which follows the TO field.


Greg, VA3TSK

> Regarding bit (un)stuffing, this appears to not apply to the KISS packet but is an action taken at the point of transmission/reception.
> Please comment.
> Best regards,
> Fred N7FMH
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