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The TCP/IP Guide  9  The Open System Interconnection (OSI) Reference Model       9  Key OSI Reference Model Concepts

Key OSI Reference Model Concepts 1 2 3 "N" Notation and Other OSI Model Layer Terminology

The most important OSI Reference Model concept is that of networking layers. It’s not an exaggeration to say that layers are really the heart of the OSI model—the entire point of the model is to separate networking into distinct functions that operate at different levels. Each layer is responsible for performing a specific task or set of tasks, and dealing with the layers above and below it. The rest of this section will deal with many of the different nuances of this layer orientation.

The OSI Reference Model is comprised of seven conceptual layers, each assigned a “ranking” number from one to seven. The layer number represents the position of the layer in the model as a whole, and indicates how “close” the layer is to the actual hardware used to implement a network. The first and lowest layer is the physical layer, which is where low-level signaling and hardware are implemented. The seventh and highest layer is the application layer, which deals with high-level applications employed by users: both end users and the operating system software.

You can see that as we proceed from the first layer to the seventh, we move up the layer stack and in so doing, increase our level of abstraction. This means that the higher a layer is in the stack, the more it deals with logical concepts and software, and the less it deals with the hardware of a network and the “nuts and bolts” of making it work.

The first layer is the most concrete, as it deals with the actual hardware of networks, and the specific methods of sending bits from one device to another. It is the domain of hardware engineers and signaling experts. The second layer is a bit more abstract but still deals with signaling and hardware. As you proceed through the third, fourth and subsequent layers, the technologies at those layers become increasingly abstract. By the time you reach the seventh layer, you are no longer dealing with hardware or even operating system concepts very much; you are in the realm of the user and high-level programs that rely on lower levels to do the “heavy lifting” for them.

Key OSI Reference Model Concepts 1 2 3 "N" Notation and Other OSI Model Layer Terminology