From: Bob Nielsen (tfip.wjqq@apex.net.au)
Date: Thu Sep 18 2003 - 22:05:46 EEST
You should also take a look at the AX25 Version 2, Level 2
specification: ftp://tapr.org/general/ax25.doc
Although most implementations reflect this version of the
specification, there is an update:
http://www.tapr.org/tapr/pdf/ax25.2.2.pdf
You can find additional information which may be useful on the TAPR web
site: http://www.tapr.org/html/pktf.html
73,
Bob, N7XY
On Thu, Sep 18, 2003 at 06:48:00PM +0000, Mike Davies wrote:
>
> 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 ?
>
>
> Mike de G0WZY
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