Relay-Version: version B 2.10 5/3/83; site utzoo.UUCP Path: utzoo!mnetor!seismo!sundc!hqda-ai!cos!howard From: howard@COS.COM (Howard C. Berkowitz) Newsgroups: comp.dcom.lans,comp.protocols.misc Subject: Re: OSI-model software Message-ID: <338@cos.COM> Date: Mon, 22-Jun-87 19:35:11 EDT Article-I.D.: cos.338 Posted: Mon Jun 22 19:35:11 1987 Date-Received: Wed, 24-Jun-87 06:54:45 EDT References: <1204@botter.cs.vu.nl> <1680@munnari.oz> <192@ditmela.OZ> <933@bloom-beacon.MIT.EDU> Organization: Corporation for Open Systems, McLean, VA Lines: 90 Keywords: osi, iso, internetworking Summary: LLC != "perfect virtual wire" Xref: mnetor comp.dcom.lans:570 comp.protocols.misc:91 In article <933@bloom-beacon.MIT.EDU>, martillo@athena.mit.edu (Yakim Martillo) writes: > In article <8613@amdahl.amdahl.com> chuck@amdahl.UUCP (Charles Simmons) writes: > >In article <920@bloom-beacon.MIT.EDU> martillo@athena.mit.edu (Yakim Martillo) writes: > >>IEEE 802.2 and ISO level 2 puzzle me as well. These are protocols for > >>the communications subnet and I don't quite understand why IEEE and > >>ISO are trying to define communications subnet protocols for all time. > >>Shouldn't communications subnet protocols be medium-dependent? > > >Actually, if you look closely at 802.2, you'll notice that the IEEE > >defines an interface within the datalink layer. 802.2 defines > >the logical link control layer, and is, and should be, medium-independent. > >Underneath the logical link control layer there exists a medium > >access control layer which is, naturally, highly medium-dependent. > > No, this is incorrect because the supposedly medium-independent part > of the level 2 (logical link control) is basically a member of the > X.25 level 2 like family of protocols. These protocols make or > enforce some very specific assumptions about the medium and how you do > networking. They assume you really want to establish a perfect > virtual wire at level 2 on a relatively noisy telephone-line-like > medium and that you either never do internetting or will do so at the > cost of some very complicated flow control procedures. They also > assume you will not have transmit windows at higher protocol layers. > NO to the No! There are several classes of Level 2 LLC (Logical Link Control; IEEE 8802/2). Type I is connectionless and does not ^^^^^^^^^^^^^^ ^^^ try to provide a "perfect virtual wire;" it is significantly different from the LAP-B HDLC-style link protocols used for the X.25 family, LLC Type I has been adopted as the media-independent part of LAN-environment Layer 2 by the NBS OSI Implementors' Workshop (for 802.3 CSMA/CD and 802.4 Token Bus Media Access Control), by the MAP/TOP User Group for 802.3 and 802.4, and by the Corporation for Open Systems (for 802.3, 802.4, and 802.5 Token Ring). By contrast, COS adopted ISO 7776 as the link control layer for X.25 WAN applications, another example of how practical OSI protocol stacks do not require only one protocol at each layer. > > This family of protocols was developed for the PTTs which have a very > specific type of medium and have some very specific constraints on the > type of services they can provide and on the type of services which > they want to provide. The PTTs consider remote terminal sessions and > file transfer to constitute the full range of data communications for > computers. In the worldwide environment, the view of PTT's cannot be ignored. The challenge is interfacing the LAN and WAN environment in the real world, which has political, regulatory, and economic constraints as well as technical ones. PTT's doing packet work are apt to be more amenable to data concerns than "old-line" telephone operating companies, who believe real men do it in 4 kilohertz analog channels implemented on twisted pair. There is hope, however. A few months ago, I presented COS' current activities to ANSI Committee T1, which is the main telecommunications standards body for the U.S. I reminded them that about three years previously, I had been on the T1 task force concerned with developing the workplan for performance/quality of service standards. At that meeting, an old time telephony standards man took me to task about not really understanding how it all worked; that I was up in the theoretical stratosphere of X.25. At the T1 meeting recently, I found that same old-timer in the bar, complaining how his customers "didn't understand what their software told them; that they simply didn't understand retransmission algorithms in X.25." I pointed out that in three years, our telephony pioneer had moved from below Layer 0 of the OSI Reference Model into Layer 2. I predicted that in 1993, he would be a UNIX hacker developing OSI applications, and we OSI people would have moved into another dimension. (Further discussion on last point to rec.sf-lovers, please. Comments that OSI people already are in another dimension will be routed, using IP if you like, to /dev/null.). -- -- howard(Howard C. Berkowitz) @cos.com {seismo!sundc, hadron, hqda-ai}!cos!howard (703) 883-2812 [ofc] (703) 998-5017 [home] DISCLAIMER: I explicitly identify COS official positions.