OSI Seven Layer Model
As shown in Figure 1, the OSI reference model is built, bottom to top, in the following order: physical, data-link, network, transport, session, presentation, and application. The physical layer is classified as layer 1 and the top layer of the model, the application layer, as layer 7.
Figure 1 The OSI seven layer model.
Each layer of the OSI model has a specific function. The following sections describe the function of each layer, starting with the physical layer and working up the model.