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As you can see in Figure 2.1, the OSI model consists of seven layers. Each layer performs a specific function and then passes on the result to another layer. When a sending station has data to send, it formats a network

request and then passes that request to the network protocol at the top layer, the Application layer. The protocol that runs at the Application layer performs an operation on the request and then passes it to the next, lower layer. Each protocol at each layer below the Application layer performs its own calculations and appends its own information to the data sent from the layer above it. At the receiving station, the process happens in reverse. Figure 2.2 illustrates this basic process.

FIGURE 2.2 How data travels through the layers of the OSI model

n Information Flow    n

Network Medium

The OSI model is only a model; it is not a protocol. Nobody is running the “OSI protocol” (at least no one has developed one at the time of this writing). Let’s take a brief look at the layers of the OSI model and the basic protocol functions they describe. We’ll start at the top with the Application layer and work our way down to the Physical layer.

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