OSI Reference Model

To pass the INTRO exam, you must be conversant in a protocol specification with which you are very unlikely to ever have any hands-on experience—the OSI reference model. The difficulty these days when discussing the OSI protocol specifications is that you have no point of reference—you simply cannot typically walk down the hall and use a computer whose main, or even optional, networking protocols conform to OSI.

OSI is the Open System Interconnection reference model for communications. Some participants in OSI's creation and development wanted OSI to become the networking protocol used by all applications on all computers in the world. The U.S. government went so far as to require OSI support on every computer that it purchased, as of a certain date in the early 1990s, which certainly gave vendors some incentive to write OSI code. In fact, in my old IBM days, they even had charts showing how the TCP/IP-installed base would start declining by 1994, how OSI installations would increase, and how OSI would be the protocol from which the 21st-century Internet was built.

What is OSI today? Well, OSI never succeeded in the marketplace. Some of the original protocols that comprised OSI are still used. The U.S. government reversed its decision to require OSI support on computers that it bought, which was probably the final blow to the possibility of pervasive OSI implementations. So, why do you even need to think about OSI for the CCNA exam? Well, the OSI model now is mainly used as a point of reference for discussing other protocol specifications. And because being a CCNA requires you to understand some of the concepts and terms behind networking architecture and models, and because other protocols are almost always compared to OSI, you need to know some things about OSI.

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