The Open System Interconnection (OSI) Reference Model
Models are useful because they help us understand difficult concepts and complicated systems. When it comes to networking, there are several models that are used to explain the roles played by various technologies, and how they interact. Of these, the most popular and commonly used is the Open Systems Interconnection (OSI) Reference Model.
The idea behind the OSI Reference Model is to provide a framework for both designing networking systems and for explaining how they work. As you read about networking, you will frequently find references to the various levels, or layers, of the OSI Reference Model. The existence of the model makes it easier for networks to be analyzed, designed, built and rearranged, by allowing them to be considered as modular pieces that interact in predictable ways, rather than enormous, complex monoliths.
In fact, it's pretty much impossible to read a lot about networking without encountering discussions that presume at least some knowledge of how the OSI Reference Model works. This is why I strongly advise that if you are new to the OSI Reference Model, you read this chapter carefully. While it is all arguably background material, reading it will help form an important foundation in your understanding of networks, and will make the rest of the Guide make more sense at the same time.
If you are quite familiar with the OSI Reference Model, you may wish to skip this chapter of the Guide, or just skim through it. You can always return later to brush up on particular issues, as needed. There are also many links that come back to the descriptions of the individual layers from various parts of the Guide.
In the pages that follow, I describe the OSI Reference Model in detail. I begin with a history of the model, and a discussion of some general concepts related to the OSI model and networking models overall. I provide a useful analogy to help you understand how the reference model works to explain the interaction of networks on multiple levels. I then describe each of the seven layers of the OSI Reference Model, and conclude with a summary of the layers and their respective functions.
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