OSI Reference Model

At the heart of any internetworked system is the OSI Reference Model. The OSI Reference Model is a conceptual model made up of seven layers, each specifying particular network functions. The OSI Reference Model was developed by the International Organization for Standardization (ISO) in 1984, and it is considered the architectural model for network-based communications. The OSI Reference Model describes how information from one computer application is transported across a network to the same (or similar) application on another computer.

The OSI Reference Model divides the processes involved with internetwork communication into smaller, more manageable pieces, or layers. Each layer is self-contained, although dependant upon the layer below for services, so that the processes assigned to each layer can be independently implemented. This layering provides for the solutions offered by one layer to be updated without affecting the other layers.

The following list details the seven layers of the OSI Reference Model:

• Layer-7: Application

• Layer-6: Presentation

• Layer-5: Session

• Layer-4: Transport

• Layer-3: Network

• Layer-2: Data-Link

• Layer-1: Physical

In order for network devices to communicate with each other, they must be able to speak the same "language," or protocol. Vendors, vendor consortiums, and technology working groups develop these protocols with the OSI Reference Model as a common reference point.

Chapter 2, "OSI Reference Model," discusses the model in further detail.