> Date: Thu, 30 Sep 1999 17:28:20 +0300 (EEST)
> From: Tomi Manninen <[EMAIL PROTECTED]>
> Subject: Re: AX.25 Layer 1
>
> That's I believe because there is no such definitive source available. As
> I said very little if anything is specified in the AX.25 specs when it
> comes to layer 1.
o I believe that it is appropriate for the AX.25 spec to focus on
link layer issues and 1) remain silent on physical layer issues,
and 2) operate independent of what ever media people choose to use.
o I _think_ the 1200 bps "standard" is the Bell 103 spec (or one of
the Bell modem specs). I would have to go look stuff up to be
sure...
> Tim said HDLC bit stuffing etc is a layer 2 issue, where does one actually
> draw the line between physical layer and link layer?
A reasonable question. A few thoughts follow, without the benefit of
a lot of thought or looking anything up.
o One answer is that bit stuffing is motivated by the need for AX.25
to delimit frames, (i.e., to be assured of unambiguously finding
flags), not by the need of the underlying transmission techniques
for a certain density of ones or zeros.
o Another answer is that if something is required for use with all
media, it is probably a link layer issue. Again, according to the
spec, bit stuffing is required by AX.25 for all media.
o Of course, the division between the physical layer and the link
layer changes over time. Generally, the physical layer is
responsible for finding the bits (encoding and decoding the bits)
and the link layer is responsible for finding the frames in the
stream of bits (flags and bit stuffing). (I suppose the
responsibility for finding the characters is sometimes the
responsibility of the physical layer, [e.g., async, and I think
SONET] and sometimes the responsibility of the link layer, [e.g.,
HDLC]). But, if the physical layer had some mechanism to delimit
frames, then one could argue that finding the frames is a
physical layer issue and therefore the link layer doesn't need to,
for example, have flags and bit stuffing. Ethernet is an
example of a physical layer that delimits frames. I wouldn't
use flags and bit stuffing if I were shipping AX.25 frames over
Ethernet. Which goes to show how hard it is to write good
protocol specs and why we have zillions of sub-layers rather than
the standard seven layers.
-tjs
