Path: utzoo!censor!geac!torsqnt!news-server.csri.toronto.edu!cs.utexas.edu!samsung!zaphod.mps.ohio-state.edu!rpi!uupsi!sunic!nuug!ifi!enag From: enag@ifi.uio.no (Erik Naggum) Newsgroups: comp.protocols.iso Subject: Re: DESPARATE! Question on ISO/DIS10022 Message-ID: Date: 22 Nov 90 01:02:41 GMT References: Sender: enag@ifi.uio.no (Erik Naggum) Organization: Naggum Software, Oslo, Norway Lines: 82 Nntp-Posting-Host: hild.ifi.uio.no In-Reply-To: josef@nixdorf.de's message of 19 Nov 90 07:52:35 GMT Originator: enag@hild In article josef@nixdorf.de (Moellers) writes: > 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. > ... 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: , My opinions. Wail: +47-2-836-863 --