[ax25-layer2] AX.25 over raw serial interface

[Timothy J. Salo]

Can Yuce wrote:
> I've been trying to use AX.25 protocol over raw RS-232 serial 
> interface without using any particular TNC, soundmodem etc. 
> In my basic configuration, AX.25 protocol is running on a single 
> PC and through a standalone HF modem, two PC's each running 
> AX.25 protocols are connected with a back2back interface
> in order to perform experimental issues.

I assume that your modem simply takes an eight-bit byte received
from an asynchronous serial port and transmits it transparently
over the air.  The receiving modem does the reverse.

If this isn't the case, then please say so.

In this case, you application should simply read to and write
bytes from the a serial port (e.g., /dev/ttyS0, I think).

Your application will be responsible for determining where
messages start and end.  I recommend that you look at the
PPP spec for one scheme.  In short, the scheme is
byte-oriented and uses an escape character.

Specifically, look at RFC 1662, "PPP in HDLC-Like Framing",
Section 4, "Octet-stuffed framing".

   This chapter summarizes the use of HDLC-like framing with
   8-bit asynchronous and octet-synchronous links.

This provides the functionality of (i.e. replaces) the
flags and bit-stuffing described in the HDLC/X.25/AX.25
specs.

Once you have done this, you can send messages.  In your
case, that message would include all of the AX.25 message
between the beginning flag and the terminating flag.
If I was doing this, I would include a CRC of some sort.

So no, you don't need to do bit-stuffing, and you don't
need an explicit message length.

Someone can forward this to the TAPR Linux list if it is
useful -- I am not a member.

Hope this helps,

-tjs