🚀 go-pugleaf

In article <josef.658659224@peun11> josef@nixdorf.de (Moellers) writes:

   To re-phrase it and make it easier to answer, here are a couple of
   questions I am interest in:

   - When using X.21 or V.25bis, is it the responsibility of the physical
     Layer to do the actual dialing?

   - If it is, how do I specify which number to dial when using ISO/DIS
     10022?

<flame>

Hey you, get the idea that we have _layering_, here.  It's the NETWORK
LAYER which takes care of such things as routing and all that,
including dialling the stupid telephone number, or instructing the
DATA LINK LAYER to do it, which sends the necessary instructions to
the PHYSICAL LAYER, to be more precise.  The PHYSICAL LAYER, however,
doesn't have a clue whether it's transferring Physical Layer Service
Data Units (i.e. _bits_ :-) meaningful to a higher layer as dialling
(such as your modem's NETWORK LAYER), or bits meaningful to a higher
layer as this message.  The PHYSICAL LAYER knows about BITS and lines
that are ACTIVE or INACTIVE.  That's it.  End.  Period.  No more.
Dialling doesn't _exist_ at the PHYSICAL LAYER.  Aaarrgghh!

If you regard the Seven Holy Layers as the Ten Holy Commandments, you
lose.  The ISO RM for OSI is intended to describe the interrelation-
ship of functions, not to cast things in stone.  Telephone wires
(PHYSICAL LAYER) require a whole set of OSI layers to be useful to
you, qua PHYSICAL LAYER.  Considering dialling, PCM encoding, routing,
charging stuff, the whole song and dance, do you think they crammed
all that into the PHYSICAL LAYER?  "How do I specify reverse charging
or authenticate the caller using ISO/DIS 10022?"  You DON'T.

WHY is it that layering and hierarchical structures are so inordi-
nately difficult to understand?  Could it be that people have the
wrong philosophy of concepts?  That would explain why Germans seem to
have the most problems all over the ISO framework.

The answer to your "desperate" question goes like this: CCITT chose to
publish a set of principles to guide choosing the layers, in Appendix
A to CCITT X.200.  Go _read_ it.  _Don't_ come back until you have
_grasped_ the ideas behind each layer, the choices, etc.  Then you
will _understand_ that your question makes NIL sense.

Now get this: Eventually, the physical layer does _everything_.  That
does not mean it's meaningful to implement X.400 or X.500 in hardware.

</flame>

The considerate answer to your question is: I don't think you have
understood the ISO reference model.  Even though some physical layer
somewhere is going to output DTMF codes after having detected the dial
tone (V.25bis) or send bits corresponding to the subscriber number
(X.21), you have a _different_ physical layer for DTMF codes than for
QAM signals, which is yet a different physical layer from that which
eventually churns your bits out on the proverbial wire (fiber, copper,
radio, whatever) in the appropriate form.  Just as there are many
different application layers, there are many different physical
layers.  Now, luckily (!), we have this fantastic reference model to
guide us in eliminating the specific features of each instance of each
layer, and instead talk about more abstract functions and services
provided up and down the hierarchical model.

I thought this was blindingly obvious.

--
[Erik Naggum]	Snail: Naggum Software / BOX 1570 VIKA / 0118 OSLO / NORWAY
		Mail: <erik@naggum.uu.no>, <enag@ifi.uio.no>
My opinions.	Wail: +47-2-836-863
--