What is 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.

OSI Layers The OSI reference model consists of seven layers. Each layer defines a set of typical networking functions. When OSI was in active development in the 1980s and 1990s, the OSI committees created new protocols and specifications to implement the functions specified by each layer. In other cases, the OSI committees did not create new protocols or standards, but instead referenced other protocols that were already defined. For instance, the IEEE defines Ethernet standards, so the OSI committees did not waste time specifying a new type of Ethernet; it simply referred to the IEEE Ethernet standards.

Because OSI does have a very well-defined set of functions associated with each of its seven layers, you can examine any networking protocol or specification and make some determination of whether it most closely matches OSI Layer 1, 2, or 3, and so on. For instance, TCP/IP’s internetworking layer, as implemented by IP, equates most directly to the OSI network layer. So, most people say that IP is a network layer, or Layer 3, protocol, using OSI terminology and numbers for the layer. Of course, if you numbered the TCP/IP model, starting at the bottom, IP would be in Layer 2—but, by convention, everyone uses the OSI standard when describing other protocols. So, using this convention, IP is a network layer protocol.

Cisco requires that CCNAs demonstrate an understanding of the functions defined by OSI for each layer, as well as some example protocols that correspond to each OSI layer. The names of the OSI reference model layers, a few of the typical protocols at each layer, and the functions of each layer are simply good things to memorize for the INTRO exam. And frankly, if you want to pursue your Cisco certifications beyond CCNA, these names and functional areas will come up continually.