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Table Of Contents  The TCP/IP Guide
 9  The Open System Interconnection (OSI) Reference Model

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Comparing the OSI Reference Model to Network Architectures and Protocol Stacks
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OSI Reference Model Networking Layers, Sublayers and Layer Groupings
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Key OSI Reference Model Concepts

The OSI Reference Model is valuable as a tool for explaining how networks function, and for describing the relationships between different networking technologies and protocols. To accomplish this, the model relies on a number of important concepts and terms. To understand the model, then, I need to explain what these are and how they relate to the model as a whole.

In this section, I describe the key concepts that define how the OSI Reference Model works. I begin with a discussion of how the model uses layers, perhaps the single most important model concepts. I then talk about some of the notation and jargon you are likely to see used in general discussions of the model. I define in more detail what “interfaces” and “protocols” are in the context of the model. I then explain the important concept of data encapsulation and the terminology used to refer to messages in the OSI Reference Model: PDUs and SDUs. Finally, I put together most of the preceding issues by describing how the various layers work to handle the routing of messages on a theoretical basis.

Note: Many of the concepts discussed in this section are also used by many people in describing the details of various individual technologies. The best example is the word “protocol”, which is used just about everywhere to refer to just about anything. My focus here is on the conceptual definitions of such terms. However, the conceptual discussions here often also apply to real-world technologies. For example, data encapsulation is performed as data is sent between protocols and technologies at various OSI layers, pretty much as I describe it here.


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Comparing the OSI Reference Model to Network Architectures and Protocol Stacks
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OSI Reference Model Networking Layers, Sublayers and Layer Groupings
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