Internet Core Protocols: The Definitive Guide

An Introduction to the OSI Reference Model

The OSI Reference Model is a conceptual model that uses seven “layers” to identify the various functions provided by a network, and these seven layers can be used to compare different protocols using a common framework. Each layer within the OSI Reference Model has a very specific function, and each layer depends on the other layers in order for the entire model to function properly. Each layer only communicates with the layers immediately above or below it. If there is a problem at one layer, it is the responsibility of that specific layer to provide feedback to the layers surrounding it.

The OSI Reference Model is extremely useful as a tool for discussing various network services. For example, if we were to look at a simple network service such as printing a document to a locally attached printer, we could use the OSI Reference Model to determine how this simple task was being achieved. We could also use the model to determine how printing over a Novell network was done, or how printing over a TCP/IP network was accomplished. Because all three of these examples use the same model, they can all be compared to each other even though they all use extremely different technologies to achieve the same objective.

Not all networking technologies have seven layers, nor do they all match up to the seven layers in the OSI Reference Model exactly. Most of them do not match it except in small, specific ways, although all of them can be compared ...