[aprssig] KISS Protocol Question
brett.friermood at gmail.com
Wed Oct 17 19:47:46 CDT 2012
Thanks for the replies. And the off-list one from Andrew.
That's what I was missing. For some reason I forgot all about it being AX.25.
Thanks for the links. I should be good now for a while.
On Wed, Oct 17, 2012 at 3:41 PM, Heikki Hannikainen <hessu at hes.iki.fi> wrote:
> It can also be useful to browse existing open source implementations of the
> protocols. For example, here are the ax25 and kiss implementations of aprx,
> in C:
> There are lots more of those around the internets.
> On Wed, 17 Oct 2012, Lynn W. Deffenbaugh (Mr) wrote:
>> The AX.25 header portion is bit-shifted ASCII along with some bit flags in
>> 7 octet "chunks". Once you get through those (as flagged by one of the
>> bits), the packet payload for APRS packets is straight ASCII, hence what you
>> are seeing.
>> KISS is just a wrapper (and escaper if the boundary character appears
>> inside) for a raw AX.25 packet. You'll need to grok the AX.25 spec to be
>> able to interpret the actual packet headers. And then you'll need to grok
>> aprs101.pdf to interpret the APRS packets in the payloads.
>> http://www.tapr.org/pub_ax25.html links to the PDF for AX.25. Because
>> you're doing KISS, you can ignore the descriptions of "bit-stuffing" and the
>> FCS (checksum) as those are handled by the TNC. If you're only interested
>> in APRS packets, you can ignore everything about connected mode packets and
>> focus solely on UI packets, but the interpretation of the header portion is
>> the same.
>> Just remember that every packet starts with a destination address followed
>> by the source address optionally followed by path components and finally
>> followed by some sort of payload. The first two addresses are opposite of
>> what we're used to seeing in humanly-readable packet interpretations.
>> Lynn (D) - KJ4ERJ - Author of APRSISCE for Windows Mobile and Win32
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