At one point in the history of the OSI model, many people thought that OSI would win the battle of the networking models discussed earlier. If that had occurred, instead of running TCP/IP on every computer in the world, those computers would be running with OSI.
However, OSI did not win. In fact, OSI no longer exists as a networking model that could be used instead of TCP/IP, although some of the original protocols referenced by the OSI model still exist.
So, why is OSI even in this book? Terminology. During those years in which many people thought the OSI model would become commonplace in the world of networking (mostly in the late 1980s and early 1990s), many vendors and protocol documents started using terminology from the OSI model. That terminology remains today. So, while you will never need to work with a computer that uses OSI, to understand modern networking terminology, you need to understand something about OSI.
The OSI model has many similarities to the TCP/IP model from a basic conceptual perspective. It has layers, and each layer defines a set of typical networking functions. As with TCP/IP, the OSI layers each refer to multiple protocols and standards that implement the functions specified by each layer. Just as for TCP/IP, the OSI committees did not create new protocols or standards in some cases, instead referencing other protocols that were already defined. For example, 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.
Today, the OSI model can be used as a standard of comparison to other networking models. Figure 1-14 compares the seven-layer OSI model with the commonly used five-layer TCP/IP model and the old original four-layer TCP/IP model.
Figure 1-14 OSI Model Compared to the Two TCP/IP Models
Note
The CCNA exam topics no longer mention the OSI or TCP/IP models; however, you should know both and the related terminology for everyday network engineering discussions. While today you will see the five-layer model used throughout the industry, and in this book, the figure includes the original RFC 1122 four-layer model for perspective.
Note that the TCP/IP model in use today, in the middle of the figure, uses the exact same layer names as OSI at the lower layers. The functions generally match as well, so for the purpose of discussing networking, and reading networking documentation, think of the bottom four layers as equivalent, in name, in number, and in meaning.
Even though the world uses TCP/IP today rather than OSI, we tend to use the numbering from the OSI layer. For instance, when referring to an application layer protocol in a TCP/IP network, the world still refers to the protocol as a “Layer 7 protocol.” Also, while TCP/IP includes more functions at its application layer, OSI breaks those into session, presentation, and application layers. Most of the time, no one cares much about the distinction, so you will see references like “Layer 5–7 protocol,” again using OSI numbering.
For the purposes of this book, know the mapping between the five-layer TCP/IP model and the seven-layer OSI model shown in Figure 1-14, and know that layer number references to Layer 7 really do match the application layer of TCP/IP as well.
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