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

OSI Reference Model Networking Layers, Sublayers and Layer Groupings 1 2 Interfaces: Vertical (Adjacent Layer) Communication

As a theoretical model, the OSI Reference Model comes complete with a set of terminology that is used to describe it and its constituent parts. This is sort of both good news and bad. The good news is that if you understand this terminology, it can help you comprehend much better most OSI model discussions, and also how technologies relate to the model. The bad news is that the terminology can also increase confusion—especially since it isn't always used consistently.

Let’s discuss a few of the terminology concepts you will often see used to refer to the OSI Reference Model. Many of these are illustrated in Figure 12.

The various layers of the OSI Reference Model are referred to in a variety of ways. They may have their names spelled out in full, or they may be abbreviated. They are also often simply referenced by their layer number. So for example, all of these refer to the same thing: “data link layer”, “Data Link Layer”, “DLL”, “L2”, “layer two”, “layer 2”.

Similarly, you will often see layer names being used as adjectives to describe protocols and technologies. A “layer 3 technology” is one that operates primarily at the network layer.

The letter “N” is often used to generically refer to a number within the computer world. With respect to the OSI Reference Model, it's common to see this letter used in discussions that relate generically to individual layers without mentioning a specific layer. You will hear terms like “N-functions” and “N-services”, which just refer to the functions and services provided within a particular layer.

As another example, you might see someone say that a particular technology “provides a useful service to the N+1 layer”. This just means it provides a function to the layer above the one at which it operates. Conceptually, every layer but the first and seventh have an “N-1” layer and an “N+1” layer, and so on. If you are looking at the network layer (layer 3) then the “N+2 layer” is the session layer (layer 5). See the discussion of model layer interfaces for more information.

OSI Reference Model Networking Layers, Sublayers and Layer Groupings 1 2 Interfaces: Vertical (Adjacent Layer) Communication