The OSI Model

The ISO standards committee created a list of all the network functions required for sending data (such as an e-mail) and divided them into seven categories. This model is known as the OSI seven-layer model. The OSI seven-layer model was released in 1984; it is illustrated in Figure 1-1.

Figure 1-1 Each of the Seven Layers of the OSI Model Represents Functions Required for Communication

Application

Presentation

Session

Transport

Network

Data Link

Physical

Upper Layers

Lower Layers

NOTE You might also have heard people talk about OSI Layers 8 and 9. Although they are not official, Layer 8 is commonly known as the political layer, and Layer 9 is the religious layer. These lightheartedly represent all the other issues you might encounter in an IT project.

KEY POINT

The OSI model represents everything that must happen to send data. The important thing to remember is that the OSI model does not specify how these things are to be done, just what needs to be done. Different protocols can implement these functions differently. For example, the open-standard Internet Protocol (IP) and Novell's Internetwork Packet Exchange (IPX) protocol are different implementations of the network layer.

As also shown in Figure 1-1, the seven layers can be thought of in two groups: the upper layers and the lower layers. The term upper layers often refers to Layers 5 through 7, and the term lower layers often refers to Layers 1 through 4, although this terminology is relative. The term upper layer also refers to any layer above another layer.

The upper layers are concerned with application issues—for example, the interface to the user and the format of the data. The lower layers are concerned with transport issues—for example, how the data traverses the network and the physical characteristics of that network.

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