The model actually is used to describe how data that's generated by a user, such as an email message, moves through a number of intermediary forms until it is converted into a stream of data that can actually be placed on the network media and sent out over the network. The model also describes how a communication session is established between two devices, such as two computers, on the network. Because other types of devices, such as printers and routers, can be involved in network communication, devices (including computers) on the network are actually referred to as nodes. Therefore, a client computer on the network or a server on the network would each be referred to as a node.

                           When data is sent by a network node (for example, a computer sending an email message, as already mentioned), that data moves down through the OSI stack and then is transmitted over the network media. When the data is received by a node, such as another computer on the network, it moves up through the OSI stack until it is again in a form that can be accessed by a user on that computer.

                           Each of the layers in the OSI model is responsible for certain aspects of getting user data into a format that can be transmitted on the network. Some layers are also for establishing and maintaining the connection between the communicating nodes, and other layers are responsible for the addressing of the data so that it can be determined where the data originated (on which node) and where the data's destination is.

                          An important aspect of the OSI model is that each layer in the stack provides services to the layer directly above it. Only the Application layer, which is at the top of the stack, would not provide services to a higher-level layer.

                     The process of moving user data down the OSI stack on a sending node (again, such as a computer) is called encapsulation. The process of moving raw data received by a node up the OSI stack is referred to as de-encapsulation.

                      To encapsulate means to enclose or surround, and this is what happens to data that is created at the Application layer and then moves down through the other layers of the OSI model. A header, which is a segment of information affixed to the beginning of the data, is generated at each layer of the OSI model, except for the Physical layer. This means that the data is encapsulated in a succession of headers—first the Application layer header, then the Presentation layer header, and so on. When the data reaches the Physical layer, it is like a candy bar that has been enclosed in several different wrappers.

                         When the data is transmitted to a receiving node, such as a computer, the data travels up the OSI stack and each header is stripped off of the data. First, the Data Link layer header is removed, then the Network layer header, and so on. Also, the headers are not just removed by the receiving computer; the header information is read and used to determine what the receiving computer should do with the received data at each layer of the OSI model.

                           It is by using these headers that the sending computer is able to communicate with the receiving computer and provide the receiving computer with either information or actual instructions related to the disposition of the data at the various levels of the OSI model. Using the candy bar analogy again, the situation on the receiving computer would be like opening a candy bar enclosed in many layers of wrapping, with each individual wrapper providing important instructions on how to unwrap the candy further and eventually eat the candy bar.

                           Figure 5.2 provides a diagram of the encapsulation and de-encapsulation processes. The header information supplied at each layer of the OSI model by the sending computer will be used by the receiving computer as it massages the data into a format that can actually be accessed by a user on that computer.