OSI reference model

Unlike the TCP/IP protocol model, the Open Systems Interconnection (OSI) reference model consists of seven layers, which are the physical, data link, network, transport, session, presentation, and application layers, and this model was developed by the International Organization for Standardization (ISO). The OSI reference model is known to be the best layered model, which provides a detailed description about the flow of data from sending to receiving devices involved in the communication process. That is a result of the strict approach that the OSI is using in network layers compared to the lossless approach that the TCP/IP protocol model is using.

To better understand how the OSI reference model operates, let's look at the assembly line in the automobile factory. Everything starts from the chassis (or frame), which is placed as the first element in the assembly line. At each step of an assembly, one or more elements (or parts) of the vehicle are placed. After a considerable number of steps, at the end of the assembly line, a brand new vehicle comes out which is then sent for testing.

Almost the same happens in the OSI reference model when it comes to describing a communication session between two computers. In each layer of OSI, a certain activity is assigned that prepares the data for the subsequent layer, as shown in Figure 2.11: