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

How To Use The OSI Reference Model 1 Key OSI Reference Model Concepts

The OSI Reference Model is not the only model used to describe the structure of networks. There are several other models and systems that are used to describe various sets of networking technologies that work together. These are generally not theoretical models, but rather describe groupings of protocols that are actively used in actual networks. They are, therefore, more often called networking architectures and protocol suites than models.

As I mentioned in the previous topic, many technologies and protocols don't “fit” well into the specific layers used in the OSI model. Similarly, most of the protocol suites used in the “real world” don't fit the OSI model exactly. This happens, of course, because they were developed independently of the OSI model. Still, most of these architectures and suites still use layers—they are just different ones than the OSI model uses.

Since the OSI model is so often referenced, it can be very helpful in making sense of other architectures, and even comparing them. Regardless of what the individual layers and technologies are called, networking protocol suites all try to accomplish the same goals in implementing a network. Thus, even though the layers are not the same, they are often comparable.

In the case of TCP/IP, a special model called the DOD model or TCP/IP model is usually used in discussions of the suite. This model has many similarities to the OSI model, but also some important differences. In other areas in the field of networking, still other models are used, such as the IEEE 802 networking architecture model. These too are similar in some ways to the OSI model but have their own unique characteristics.

Even within the scope of some individual specific technologies, a layered structure of related protocols can be seen. There are technologies that are generally considered to implement a single level of the OSI model that actually have portions that overlap several layers; examples include Ethernet and ATM. In fact, some protocols even have subprotocols that are layered within the confines of what is considered a single layer under OSI; a good example is the TCP/IP Point-to-Point Protocol (PPP), which, despite the name, is not a single protocol but a protocol suite unto itself.

How To Use The OSI Reference Model 1 Key OSI Reference Model Concepts