In the late 1970s the International Standards Organization (ISO) began to develop a conceptual model for network communications called the Open Systems Interconnection Reference Model. It is commonly known as the OSI model .
In 1984 the model became the international standard for network communications, providing a conceptual framework that describes network communication as a series of seven layers. In the model, each layer is responsible for a different part of the process that takes place when two computers on a network establish a connection and move data between them. Figure 5.1 shows the layers of the OSI model. Table 5.1 provides a brief description of the role of each layer.
Figure 5.1. The OSI model is used to describe how a protocol stack initiates and controls network communication.
The OSI model is numbered from the bottom up. You will find, depending on what book or resource you are consulting, that the model layers are not always referred to by name . For example, the Network layer, which is the third from the bottom, is often referred to as layer 3 .
Table 5.1. The OSI Layers