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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

Protocols and Interfaces

These words have special meaning within the context of the OSI model: a protocol represents communication between logical or physical devices at the same layer of the model while an interface represents information moving between adjacent layers within the same device.

Thus, in “N” notation, protocols represent communication between layer N on one device and layer N on another device, while interfaces deal with communication between layer N and N+1 or Layer N and N-1 on the same device. The topics on OSI protocols and OSI interfaces describe these issues much better.

What do you get when you take a bunch of layers and put them all one on top of each other? Well, you get a stack. This term is used to refer to either the entire set of layers in a model or suite of technologies, or a partial set. Since each layer has protocols associated with it, this is also sometimes called the protocol stack.

These often-interchanged, somewhat vague terms refer to specific tasks or jobs performed at various layers in the model. An N-entity is a term that refers to a specific operation or job done at “layer N”. A function is basically the same thing. Facilities and services are what a layer provides to the layers above it. This is often expressed in N-notation as well: the “N+1” layer often uses a set of “N services” or “N facilities” provided by the “N” layer.

You may have just read all of that and said to yourself “why do they bother making this so complicated anyway?” Well, I can't really argue with you. Remember, I did say there was bad news here. Now that you know what all of this stuff is about, should you run into it you won't be… uh… too badly confused. J

Fortunately, the use of the buzzwords above is somewhat limited. Most references are to specific layer names or numbers, and in particular, the “N-1” and “N+1” stuff is rarely used in discussions of real-world technologies. However, it can be very useful in explaining the model itself, so you will see some of these terms as you read the rest of this chapter on the OSI model.

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