I would restate that a little: it is a mistake to put error control at too high a
layer, or to put it in multiple layers (unless it is doing distinctly different jobs
in those layers). However, to make error control a function of the physical layer
would be contrary to any definition of the OSI layers that I've ever seen. The
physical layer deals with nitty-gritty stuff like voltages, connector standards,
modulation techniques, Medium Access Control and the like.
> is that more clear to me than in radio. The link layer should not have to know
> whether the link is on HF or VHF, narrowband or wideband, noisy or clear, and
> so on. When we put error control into AX.25, we made the protocol close to
> useless on HF. My point is that any error control mechanism has to be closely
> tailored to physical layer issues, and that is where we have fallen down with
> packet radio. Furthermore, it is my opinion that the half-baked error control
> in AX.25 is largely responsible for arresting the development of intelligent
> physical layer error control which would have been far more useful and
> effective.
On the contrary, I think that the link layer needs to be tailored to the particular
nature of the data link involved. It's just not practical to make all of the possible
physical layers look equivalent to the link layer. What is needed is a family of data
link protocols, each of which is tuned for a particular data link environment (bit error
rates, error patterns, propagation delays, etc). Obviously AX.25 is a poor choice for
HF data links, but that just illustrates the point that no single link layer protocol
can do it all. Hopefully, when we get up to the network layer, we can start to see a
dramatic reduction in the number of different protocols needed. :-)
I don't think we have any fundamental disagreement - it just boils down to semantics.
If you use the terminology of the OSI model, you have to stay within its terms of
reference.
Barry
-- Barry McLarnon VE3JF/VA3TCP | Internet: tpecxrsr.gtnu@tele.fi Ottawa Amateur Radio Club | AMPRnet: ppiulib.okjrjszh@roskapostia.tunk.net Packet Working Group | Web: http://hydra.carleton.ca