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TCP-group 1995

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KISS and experimental AX.25

kf5mg wrote in a message to Mike Bilow:

 k>     I read an article ( 11/15/93 by Tom McDermott - n5eg )
 k> in the TPRS  Quarterly Report this morning that talked about
 k> Error-Correction codes for  packet radio.

How can I get a copy of this?  Glenn Elmore and I have been having an
interesting exchange of mail on error correcting code recently.

 k> It got me to
 k> thinking...  does a KISS TNC pass all received  packets
 k> (including invalid ax.25 packets ) to NOS and NOS determines
 k> if the  packet is valid and send a REJ (if needed) or does
 k> the KISS TNC determine  that a packet is invalid and send a
 k> REJ frame?  If the KISS code sends all  packets to NOS.... 

REJ is sent when a frame passes valid FCS but is rejected for some reason,
usually because it has been received out of sequence.  AX.25 requires that
frames be received in strict sequence, and that frames received out of sequence
be discarded.

If a frame fails FCS, then the KISS TNC does not pass it to the computer.

 k> NOS could be modified to run the error correction code  on
 k> invalid packets and attempt to correct them before sending
 k> an REJ frame.

No, that isn't going to happen.

 k>        Also....  if you added data (changed the
 k> data structure) for an ax.25  SABM frame...  ( Bits for
 k> compression, encoding, etc ) will the KISS TNC  pass the
 k> SABM frame to NOS or will it decide it's an invalid packet
 k> and  REJ the SABM frame itself?  

In KISS mode, all transmitted frames, including SABM, REJ, and so on, are
generated by the software.  The TNC wraps frames in the FCS and flags, and
sends them out.  On receive, the TNC strips flags and FCS, and passes the
otherwise raw frames to the computer.  There is no protocol implementation in
the TNC when in KISS mode.

 k>     I'm assuming that even if the KISS code handles some of
 k> the REJ frames itself... the KISS code on the TNC is still
 k> responsible for validating received packets and sending the
 k> REJ. The other alternative would be an even lower level TNC
 k> program validating the packet and sending valid packets to
 k> the KISS code and sending the REJ frame itself. 
 k>      As always... any info would be appreciated. Thanks.

Nice idea, but frames which fail FCS are discarded.
-- Mike

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