Chapter 2. The Open Systems Interconnect Model (OSI)

Terms you'll need to understand:

• Protocol Data Unit (PDU)

• Encapsulation

• De-encapsulation

• Media Access Control address

• Network address

• Connection-oriented communication

• Connectionless communications

• Logical Link Control sub-layer (LLC)

• Media Access Control sub-layer (MAC)

• The seven OSI layers

Techniques you'll need to master:

• Understand the rationale behind the development of the OSI model

• Know the differences between connection-oriented and connectionless communications

• Learn the functions of each layer and sub-layer in the OSI model

• Understand how the OSI model passes between network nodes

• Describe the five steps of encapsulation and de-encapsulation

The International Organization for Standardization ( ISO ) developed the Open Systems Interconnection ( OSI ) reference model in 1984 for a host of reasons, including

• Increased interoperability between vendors

• Reduction of complex network operation to more easily understood pieces

• Simplification of future enhancements by keeping changes in one layer from affecting other layers

• Providing a method or model to understand network and internetwork communications

The good folks at Cisco can barely get out three sentences without referencing the OSI model. They use it to define their products, troubleshoot communications problems, and describe network operation, to name just a few. If you already know the OSI model, feel free to skim this chapter, but do not skip it. This is one of the areas where you definitely need to think " the Cisco way. "

As we go through the layers of the OSI model, we will italicize terms that Cisco uses in describing a particular layer. Pay attention to these italicized terms because you will run across them on the test and it is important you use them the way Cisco uses them. In most cases, we will give you a definition of the term the first time we use it, but in some cases we will defer the definition to a following section where it is better defined in context. (Yes, we will tell you when that occurs.)