Transferring Information Between Computers

Before delving into the mechanics of how information is transferred between computers, you must grow familiar with the terminology used to describe the transmitted data. Many of the layers of the OSI Reference Model use their own specific terms to describe data transferred back and forth. As this information is passed from higher to lower layers, each layer adds information to the original data—typically a header and possibly a trailer. This process is called encapsulation. Generically speaking, the term protocol data unit (PDU) is
used to describe data and its overhead.

Table 2-13 describes the terms used at the various layers of the OSI Reference Model. For instance, as data is passed from the session layer to the transport layer, the transport layer encapsulates the data PDU in a transport layer segment. For TCP and UDP in the TCP/IP protocol stack, the transport layer only adds a header. As the PDU information is passed down, each layer adds its own header and, possibly,
trailer.

Once the physical layer is reached, the bits of the data link layer frame are converted into a physical layer signal—a voltage, light source, radio wave, or other source according to the type of physical medium that is employed. When the destination receives the information, it goes through a reverse process of de-encapsulating information—basically stripping off the headers of the PDU information at each layer as the information is passed up from layer to layer of the OSI Reference Model.

Figure 2-10 shows an example of the process used for encapsulating and deencapsulating PDUs as data is passed down and back up the OSI Reference Model. In this example, you can see how the application, presentation, and session layers create the data PDU. As this information is passed down from layer to layer, each layer adds its own header.

The next few sections are to help you better understand the process that devices go through as information is transmitted between computers. The next section covers the details as to how information is encapsulated and sent down the protocol stack and then placed on the wire to the destination. The section following that covers
the reverse process: how the information is de-encapsulated at the destination and delivered to the application at the application layer. The third section looks at a more complex environment, where bridges, routers, and hubs are involved in the communication process to get information from the source to the destination.