Modelo Osi

CHAPTER

3

Understanding the OSI Reference Model
In years past, the need to understand the Open Systems Interconnection (OSI) reference model for networking grew rapidly. The U.S. government passed laws requiring vendors to support OSI software on their systems or they would no longer buy the systems. Several vendors even predicted that the global Internet would evolve towards using OSIas the protocol instead of TCP/IP. As we near the turn of the century, however, OSI has been implemented on a much smaller scale than predicted. Few vendors push their OSI software solutions, if they have them. Several components of the OSI model are popularly implemented today. For example, OSI NSAP network layer addresses are often used for signaling in Asynchronous Transfer Mode (ATM)networks. However, full seven-layer OSI implementations are relatively rare today. So why have a whole chapter on OSI? The biggest reason is that the OSI seven-layer reference model is an excellent point of reference for describing the concepts and functions behind other network protocol implementations. References to Layer 2 switching and Layer 3 switching, which are popular topics today, refer tothe comparison to Layers 2 and 3 of the OSI model. Cisco courses make generous use of the OSI model as reference for comparison of other network protocol implementations. So, this chapter will not actually help you understand OSI fully, but rather it will discuss OSI functions in comparison with popularly implemented protocols.

How to Best Use This Chapter
By taking the following steps, youcan make better use of your study time:

• • •

Keep your notes and answers for all your work with this book in one place for easy reference. Take the quiz and write down your answers. Studies show retention is significantly increased through writing facts and concepts down, even if you never look at the information again! Use the diagram in Figure 3-1 to guide you to the next step inpreparation for this topic area on the CCNA exam.

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Chapter 3: Understanding the OSI Reference Model

Figure 3-1 How To Best Use This Chapter
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Review Chapter using charts and tables

End-of-chapter exercises

“Do I Know This Already?” Quiz
1. Name the sevenlayers of the OSI model.

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2. What is the main purpose of Layer 3?

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NA260801

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“Do I Know This Already?” Quiz

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3. What is the main purpose of Layer 2?

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4. Describe the process of data encapsulation as data is processed from creation until it exits

a physical interface to a network. Use the OSI model as an example. _______________________________________________________________________ ______________________________________________________________________________________________________________________________________________
5. Describe the services provided in most connection-oriented protocol services.

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6. Name three terms popularly used as a synonym for MAC addresses....