transformation that will divide it into smaller pieces that can be sent across the transmission medium.
Layer 7 takes a manageable piece of the data and adds its own information (called the header) to it. The piece of the original data is now referred to as the payload. The entire unit (the payload plus the header) is referred to as a protocol data unit (PDU) and it is passed down to Layer 6.
Layer 6 then treats the Layer 7 PDU as a payload, and adds its own header. The resulting
PDU is then passed down to Layer 5. The process continues through all the layers of the
OSI. This process of encapsulation enables each layer to communicate with the others, and also allows the receiving computer to process the message.
Each layer considers what has been passed down to it from an upper layer to be "data." It treats the entire higher-layer message as a data payload. It does not concern itself with what was added by the upper layers. At the end of the encapsulation process, a frame is formed. Figure 5-3 illustrates how each layer appends a header to the PDU it receives from the layer above. Notice that Layer 2 also adds a footer (or trailer). Notice also that no header is added at Layer 1.
Figure 5-At the application, presentation and session layers
An item of information is considered data as it is generated and passed down through the upper three layers of the OSI, which are often collectively known as the application layer
At the transport layer
Data is passed down to the transport layer (Layer 4) where it is encapsulated to include source and destination port numbers. A port number is used to identify the specific application (such as FTP or e-mail) that produced the packet, and the corresponding application on the destination machine that should receive the packet. At this point, the data is considered a segment.
Confirmation