The OSI Reference Model

The OSI Reference Model

Having a model in mind will help you understand how the pieces of the networking puzzle fit together. The most commonly used model is the Open Systems Interconnection (OSI) reference model. The OSI model, first released in 1984 by the International Standards Organization (ISO), provides a useful structure for defining and describing the various processes underlying open systems networking.

The OSI model is a blueprint for vendors to follow when developing protocol implementations. The OSI model organizes communication protocols into seven layers. Each layer addresses a narrow portion of the communication process. Figure 2.1 illustrates the layers of the OSI model.

Although you will examine each layer in detail later in this chapter, a quick overview is in order. Layer 1, the Physical layer, consists of protocols that control communication on the network media. Layer 7, the Application layer, interfaces the network services with the applications in use on the computer. The five layers in between—Data Link, Network, Transport, Session, and Presentation—perform intermediate communication tasks.

You should learn the names and the order of the seven OSI layers for the Networking Essentials exam. The following two phrases help you remember the first letters of the layers:

All People Seem To Need Data Processing (top down)

Please Do Not Throw Sausage Pizza Away (bottom up)

Choose one, depending on whether you are most comfortable working from the top of the model down or from the bottom up.

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