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Understanding the OSI Reference Model

   

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Understanding the OSI Reference Model

  

 

How to Best Use This Chapter

  

 

“Do I Know This Already?” Quiz

  

 

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CCNA Exam Certification Guide

From: CCNA Exam Certification Guide
Author: Wendell Odom
Publisher: Cisco Press (Trade/52)
More Information

3. Understanding the OSI Reference Model

The following table outlines the CCNA exam objectives that are reviewed in this chapter. The numbers shown correspond to the master list of objectives found in Chapter 1, “What Is CCNA?”

Objective

Description

1

Identify and describe the functions of each of the seven layers of the OSI reference model.

2

Describe connection-oriented network service and connectionless network service, and identify the key differences between them.

3

Describe data link addresses and network addresses, and identify the key differences between them.

4

Identify at least 3 reasons why the industry uses a layered model.

5

Define and explain the 5 conversion steps of data encapsulation.

6

Define flow control and describe the three basic models used in networking.

7

List the key internetworking functions of the OSI Network layer and how they are performed in a router.

29

Describe the two parts of network addressing, then identify the parts in specific protocol address examples.

60

Define and describe the function of a MAC address.

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 OSI as 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 to the 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, you can 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 in preparation for this topic area on the CCNA exam.

Figure 3-1. How To Best Use This Chapter
   

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