jeffr@sco.COM (Jeff Radick) (09/16/89)
In article <8909111235.AA24396@WLV.IMSD.CONTEL.COM> mcc@WLV.IMSD.CONTEL.COM (Merton Campbell Crockett) writes: > ... >Excuse me, I'm confused. I thought that OSI was a model which described >the procedures (steps) to establish a connection and transfer data between >two nodes on a network regardless of the protocol suite employed. The ISO >IP/TPn and Internet TCP/IP protocol suites can be described by the model >as can networks based on Digital's DCA protocol suite, IBM's BSC protocol, >and the ISO 1745 protocol. > ... I think that whether what you say is true or not depends largely upon how you think of the OSI model. Note first of all that regardless of the interpretation, OSI is indeed a model but does not "describe procedures (steps) to establish a connection ... [etc]". OSI is a model of computer communication that defines general principles of protocol layering, and defines a particular set of layers that are defined to profide a particular set of services that interact in a particular set of ways, all for the purpose of communicating between computer systems. The OSI model does NOT define procedures for establishing communication or for transferring data, except for the manner in which some layers provide services to layers above them. For the most part, procedures are defined by the particular protocol definitions themselves. OSI purists will generally flame about your interpretation since the "Reference Model of Open Systems Interconnection" was defined by ISO as a specific framework within which to develop future ISO protocols. Thus the OSI model per se is not, in its purest interpretation, a general framework to be applied to other protocols. It does work for recent CCITT-defined protocols since CCITT has adopted the ISO OSI model (International Standard 7498) almost verbatim as CCITT recommendation X.200. The non-purist will consider this quibbling, since the OSI reference model can be taken as a general conceptual model which reasonably describes the functions undertaken by other protocol suites, in a reasonably modular form. However, use of the OSI model in the non-purist sense should also be done with care since most other protocol suites were built upon a different model, even if only trivially different; and trying to fit non-ISO and non-CCITT protocols into this model can often be like trying to force a square peg into a round hole. For example, the TCP/IP protocol suite was, as I understand it, designed with a 4-layer model in mind (application, transport, internet, network; see e.g. "The DARPA Internet Protocol Suite" on p. 2-27 of volume 2 of the _DDN_Protocol_Handbook_). The ideas comprising the OSI network layer are spread out over both the internet and network interface layers of the TCP/IP model, so they do not make a neat fit with each other. As another example, SNA is designed with a 5-layer model, which completely fails to discuss the areas covered by the OSI Application and Physical layers, and the layers in between have their functionality sorted out in a way different from the OSI model, so again we do not have a neat fit. Anyway, my point is that (1) Only the ISO and CCITT protocols designed for the OSI model can be properly said to fit this model; (2) Use of this model to describe other protocol suites can be useful but should be done with care. My $.02 on the matter. Jeff Radick Networking & Communications The Santa Cruz Operation, Inc. Internet MX mailers: jeffr@sco.COM UUCP: uunet!sco!jeffr or ucscc!sco!jeffr or just sco!jeffr Also probably usable: sco.UUCP or sco.UU.NET (to route via uunet)