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

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Why Understanding The OSI Reference Model Is Important To You
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Comparing the OSI Reference Model to Network Architectures and Protocol Stacks
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How To Use The OSI Reference Model

I have been trying to make the point in this section that the OSI Reference Model is a very important tool in understanding how networks function. However, while some people tend to play down the OSI model too much, suggesting that it really isn't very relevant today, there are others who go to the other extreme. They use it too much, overanalyzing and trying to use the model in a way that was never intended.

The most common mistake is attempting to try to “make everything fit” into the layered structure of the OSI model. (I must confess to falling into this trap myself on occasion.) When I first started laying out the structure of this Guide, I wanted to organize everything based on where it fell in terms of OSI model layers. I quickly discovered that this was like attempting to put pegs of various shapes into a board containing only round holes. I had to change my approach. I ended up organizing the Guide based on the OSI layers where it made sense, and using a different structure where it did not.

I recommend that you learn from my experience. A simple rule of thumb is this: refer the OSI Reference Model if it helps you make sense of technologies and understand how they work; don't use it if it makes things more complicated instead of more clear. In particular, bear the following in mind:

  • It can be very hard to even figure out where some technologies fall within the model. Many protocols were designed without the OSI model in mind, and they may not fall neatly into one layer or another. Some overlap two or more layers; other protocol suites may have two protocols that share a layer.

  • The boundaries between the upper layers (session, presentation and application) get particularly fuzzy. Some protocols are clearly designed to fit at one of these layers, while others may overlap all three. It is for this reason that I do not categorize higher-level protocols by layer in this Guide. (The OSI Reference Model was in fact designed to take into account the fact that differentiating between these layers might not make sense.)

  • The OSI Reference Model was designed primarily with LANs in mind. WAN technologies often fit very poorly into the OSI model, with lots of overlapping and partial layer coverage. However, it's still useful in most cases to look at these protocols in terms of their approximate fit in the OSI model, since parts of WAN technologies are sometimes interchanged.

  • The people who design products don't generally worry about ensuring that their latest inventions implement only specific layers of the model. Thus, sometimes new products come out that “break the rules” and implement functions across more than one layer that used to be done by multiple devices at the individual layers. This is usually progress, a good thing. J

Finally, an observation… I have noticed that people learning about networking—especially those trying to memorize easy answers to difficult questions so they can pass exams—often ask “at what layer does this piece of hardware operate?” The problem here is not the answer but rather the question, which is simplistic. With the exception of simple physical devices such as connectors and cables, pretty much all networking devices operate at many layers. While a router, for example, is usually associated with layer three; it has two or more device interfaces that implement layers two and one. A better question is what is the highest layer at which a device functions.

The bottom line is that the OSI Reference Model is a tool. If you use it wisely, it can be immensely helpful to you. Just remember not to be too inflexible in how you apply it, and you'll be fine.

Key Concept: It is just as much a mistake to assign too much importance to the OSI Reference Model as too little. While the model defines a framework for understanding networks, not all networking components, protocols and technologies will necessarily fall into the model’s strict layering architecture. There are cases where trying to use the model to describe certain concepts can lead to less clarity rather than more. One should remember that the OSI model is a tool, and should be used accordingly.

Previous Topic/Section
Why Understanding The OSI Reference Model Is Important To You
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Pages in Current Topic/Section
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Comparing the OSI Reference Model to Network Architectures and Protocol Stacks
Next Topic/Section

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Version 3.0 - Version Date: September 20, 2005

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