From: Dennis Boone (wbak@queensu.ca)
Date: Thu Sep 18 2003 - 22:17:09 EEST
> You should also take a look at the AX25 Version 2, Level 2
> specification: ftp://tapr.org/general/ax25.doc
This addresses the original poster's link layer questions (#2, #3).
To address his physical layer questions:
For most amateur VHF packet, I believe the physical layer is
equivalent to Bell 202. I've not yet managed to find a copy of that
spec online, but there are a number of documents about chips which
implement it, for example:
http://laspace.lsu.edu/aces/Docs/Component%20Docs/mx614.pdf
which might prove helpful. A tour of the soundmodem code for Linux
might also be enlightening.
Dennis Boone
KB8ZQZ
> > I'm trying to find out what happens at the physical level with
> > Ax25 packet. I've tried looking at the source code but either I'm looking
> > in the wrong place, or I don't understand.
> >
> > I'm assuming tones of 1200Hz and 2200Hz for 1200bps
> >
> > I have several questions concerning transmission.
> >
> > 1) Am I right in thinking that the tone changes every time a zero
> > bit is sent, and not changed when a 1 bit is sent? If so, what tone
> > frequency do I start with (for the first zero bit in the $7E frame start
> > octet)
> >
> > 2) Which order are the bits sent ? LSB or MSB first ?
> >
> > 3) What is the polynomial for the frame check CRC. I have seen two
> > quoted, x^16 + x^12 + x^5 + 1 and also x^16 + X^15 + x^2 + 1 which is
> > right, or is it something else ?
> >
> > 4) Given that one of the tones is the same as the baud rate, i.e.
> > one complete cycle for one bit, does it matter where in the cycle I start
> > the tone ?
> >
> > 5) I'm assuming there is no phase shift between bits. Is this
> > correct ?
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