🚀 go-pugleaf

In article <josef.659001155@peun11> josef@nixdorf.de (Moellers) writes:
   ><flame>
   Hey, I needed this! The weather is lousy over here!

Here too.  :-)  -9 (celcius) right now.

   I know that the network layer is responsible for routing.  But I
   have never seen a modem which has a NETWORK LAYER built in B-{) But
   as You say Yourself: The physical layer knows about BITS and lines
   that are ACTIVE or INACTIVE.

The Network layer doesn't have to be X.25 or IP.  It can be anything
which provides the necessary services specific to a Network layer.

   Now I ask You: when the physical layer ACTIVATES a line, in the case
   of V.25bis or X.21, activating a line means dialling.

Well, yes, in a very specific sense, it does.  In general, though, you
separate taking your phone off hook and punching all the buttons.  (My
phone has this fancy feature wherein I just listen to the dial tone
for 5 seconds, and then it automatically dials a predefined number.
Neat programmability, but probably not what you think of. :-)

In this case, I'd say the Network layer knows about which physical
path things are going to take, goes through a dialling sequence to get
there and change state from idle to connected.

The way I look at ACTIVATE is "raise DTR".  That is clearly a Physical
layer thing to do.  Then I talk to the modem for a while, and after
the modem has done _it's_ Network layer job, I talk to the remote
host.  (Hopefully.)  Then I DEACTIVATE the line by dropping DTR.
Veeeery simple.  You need to separate the tasks at hand according to
the Principles that made the invincible heroes of ISO and CCITT select
their layers.

   ></flame>
   ...

   I was thinking about doing the same to the physical layer, i.e.
   implement ISO DIS 10022 e.g. using STREAMS. The physical layer would be
   implemented as a driver, the interface to the driver would be message
   based. The messages between the next layer/module up would be based on
   the primitives given in said standard.

Remember that the modem is actually relaying your information in a
transparent way, and that you have to persuade the modem to do this.
Such persuasion does not belong in the Physical layer.  Don't you see
that even though you use the same ----ing wire you're talking to two
different machines?  You ACTIVATE the signal level on the Physical
layer, then get some junk back which indicates that the dialer or
switch is willing to accept your dialling command, then awaits the
answer to your command, and _then_ you talk to a different host,
_through_ the modem/switch/network/whatever.  On an X.25 network, you
do the same thing, only it's more automated and hidden from you.  The
Call Request packet is taken care of by your nearest switch, which
assigns you a VC number and then does whatever it pleases with the
request.  This is clearly at the Network layer, right?  (Or do you
consider addressing in X.25 to be part of the Physical layer, as
well?)

   Now, either the state of the physical interface doesn't matter and the
   physical layer just sends whatever it is told to send and it is up to
   the "network" to interpret the first x characters as a number, or there
   has to be a way to tell the physical layer that "what now follows is a
   number to dial" and "what now follows is the data to transfer".

My favorite Physical layer doesn't care a ---- what kind of bits my
Data link layer asks it to get out on that wire.  Yours shouldn't,
either.  If it does, it suffers from megalomania, and should see a
Layer Shrink.

   >I thought this was blindingly obvious.

   Sorry, no!

Sigh.  Did this help, then?

--
[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
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