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CCNA Certification All-In-One For Dummies by Silviu Angelescu

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Chapter 2: The OSI Reference Model

Exam Objectives

Describing the OSI reference model

Describing the purpose and basic operation of each layer in the OSI reference model

Describing the benefits of the OSI reference model

Describing the purpose and basic operation of the protocols in the OSI and TCP/IP models

Associating network devices to each layer in the OSI reference model

This chapter covers the Open Systems Interconnection (OSI) networking reference model. You discover the seven layers, their purpose, and how they relate to each other. The data encapsulation concept is also introduced.

Introduction to the OSI Reference Model

The International Organization for Standardization (ISO) defined the Open Systems Interconnection (OSI) reference model to standardize networking of devices from different vendors. The OSI reference model is mostly an architecture blueprint that networking and computer device manufacturers implement. The OSI model has never been implemented exactly as defined. The TCP/IP protocol stack is the closest implementation available today.

Seven Layers

The OSI reference model is designed in seven functional layers. Each layer has a precise mission, and each layer works fairly independently of the upper and lower layers. Upper layers use the services provided by lower layers, but the internal workings of each layer are not visible to other layers.

This independence is achieved through encapsulation and very clearly defined interfaces between layers. Here are ...

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