Data Flow in the OSI Model

The OSI model presents a standard data flow architecture, with protocols specified in such a way that the receiving layer at the destination computer receives exactly the same object as sent by the matching layer at the source computer. Figure A.2 shows the OSI model data flow.


Figure A.2 OSI Model Data Flow

The sending process passes data to the application layer. The application layer attaches an application header and then passes the frame to the presentation layer.

The presentation layer can transform data in various ways, if necessary, such as by translating it and adding a header. It gives the result to the session layer. The presentation layer is not aware of which portion (if any) of the data received from the application layer is the application header and which portion is actually user data, because that information is irrelevant to the presentation layer's role.

The process of adding headers is repeated from layer to layer until the frame reaches the data link layer. There, in addition to a data-link header, a data-link trailer is added. The data-link trailer contains a checksum and padding if needed. This aids in frame synchronization. The frame is passed down to the physical layer, where it is transmitted to the receiving computer.

On the receiving computer, the various headers and the data trailer are stripped off one by one as the frame ascends the layers and finally reaches the receiving process.

Although the actual data transmission is vertical, each layer is programmed as if the transmission were horizontal. For example, when a sending transport layer gets a message from the session layer, it attaches a transport header and sends it to the receiving transport layer. The fact that the message actually passes through the network layer on its own computer is unimportant.