The OSI Model

There are two models that we are going to look at. One is the TCP/IP model, the other is the OSI Model (Open System Interconnect). The OSI model has a total of seven layers while we have only 5 layers in the TCP/IP model. Why do we discuss the OSI model now rather than before TCP/IP- The TCP/IP model was being used in practice well before the OSI model was devised. Thus, while there are similarities in the models, TCP/IP is the model that is used.

These layers (in both the OSI and the TCP/IP models) are identical up to the Network layer. In OSI the MAC layer is the called the Link Control Layer, also called the Data Link Layer.

From layer three onwards the two models are not quite the same, primarily because the TCP/IP model was created long before the OSI model.

We are going to look at the OSI model, and then see how it relates to the TCP/IP model.

The Transport Layer is responsible for how datagrams or packets are transported from one PC to another.

Above that we have the Session Layer. What is the Session Layer used for?

If we needed to record session information, what happens to the information between key-strokes. For example, if you started off a Secure shell session between two PC's the session layer is responsible for keeping that session alive between key-strokes.

On top of the Session Layer we have the Presentation Layer, which provides the standard look and feel that is presented to the final layer, the Application Layer.

So if you are writing an application like telnet or SSH (Secure Shell) or ftp (file transfer protocol), the presentation layer is used to present the application with a standard look and feel, what they call a API, Application Program Interface.

Figure 1.1. The OSI Model