Introduction to the OSI Model

   The OSI Stack

The OSI reference model describes a theoretical protocol stack, shown on the OSI Model Diagram. The model consists of seven layers of services and protocols, from the concrete Physical Layer, which contains protocols that transmit bits over physical media, to the abstract Application Layer, where programs such as electronic mail (e-mail) reside.

The OSI model, consisting of seven layers, falls logically into two parts. Layers 1 through 4, the "lower" layers, are concerned with transmitting raw data between computers. Layers 5 through 7, the "higher" layers, are concerned with communication between applications.

Each layer of the OSI model describes the services that a protocol provides, but it does not specify exactly how a protocol must do that. Thus, several different protocols provide the functions of OSI Layer 3 (the Network Layer), and a developer can create a new one at any time.

We will now study each of the layers, from the lowest to the highest, to learn about the services and protocols at each level.