The special set of rules for peer level communication is called a protocol. You will notice that the application layer is at the top of the model and the physical layer is at the bottom. In order for the e-mail application on the Macintosh to request mail from the server on the PC, the Macintosh request must logically transverse its way down the seven layers on the local device and work back up the seven layers in the PC mail server. The design of the layers is modular, therefore, each layer has its own set of protocols. The upper layers Keep in mind that this is an abstract model.
Professionals often refer to the upper layers as the application layer.
Although, the upper three layers of the reference model include the application layer, presentation layer, and session layer. It is merely that, the application. The application layer provides the windowing or interface between the user applications and the lower layers of the model. The application layer is at the top of our reference model containing familiar programs such as telnet, e-mail, and browser. The presentation layer is concerned with data formatting.