The Layered Architecture of the OSI Model

Each layer performs a specific set of functions that are required to transfer data between hosts. In addition, to facilitate smooth data transfer, each layer provides services to the layer above and below it. Figure 1.4 shows a simple diagrammatic representation of the position and appearance of these layers in the OSI model.

Figure 1.4: Position of layers in the OSI model.

Each layer is connected to the layer above and below it through an interface, which enables the layers to interact and communicate with each other. For example, the interface between the Application and Presentation layers is called the application-presentation interface. The OSI model states that data flowing from source to destination must always pass through each layer of the model at both ends. Figure 1.5 displays the flow of data from source to destination.

Figure 1.5: The data movement from source to destination in the OSI model.

In Figure 1.5, consider A as the source and B as the destination. According to the OSI model, any data starting from the Application layer of source A will travel up to the Physical layer. After reaching the Physical layer of destination B, the data moves up to the Application layer of B. At each layer, the data or message is modified according to the functionality of each layer. During the entire process, each layer of source A will communicate only with the corresponding layer of destination B. This means that the Presentation layer of A will communicate with the Presentation layer of B, and the Session layer of A will communicate with the Session layer of B, and so on.

The benefits of a layered protocol model are:

• Easy to troubleshoot, because each layer performs a specific function

• Interoperability of products from different vendors

• Ease in modifying programming interfaces

• Simplified troubleshooting

• Enhanced compatibility between diverse systems

• Future upgrade of a single layer does not affect other layers