These functions may be implemented differently from vendor to vendor, but they will generally all share a few basic characteristics. The Open Systems Interconnection Reference Model was created by the International Standard Organization (ISO) to provide a basic model on which all protocols can be modeled.

Having this type of standard architectural model provides several benefits. It provides a general framework to guide programmers who develop protocol stacks. It allows all members of the computer networking community, both proprietary and non-affiliated, to be on the "same page" when discussing the common layers of functionality inherent to all protocol systems. Also, in an ideal situation, the very modularity of a layered protocol concept would allow for one obsolete functional block of code to be replaced with an improved version of the same functional block-without having to reworked the entire stack. This interchangeable, modular quality can help to interface networking software supplied by two different vendors.

Before the OSI Reference Model was recognized as a valid model for protocol design, most vendors created their own proprietary protocols with very little concern for interoperability with other vendors' products. However, as more and more consumers began to embrace the idea of open standards, proprietary standards began to lose out in the market. Interoperability became increasingly important; hence, the need for a standard model, such as OSI, became obvious. For example, if two vendors both decided to develop their own versions of a protocol, and they both decided to use the OSI Model as a reference, at the very least, both protocols would be relatively modular in design, with distinct responsibilities assigned to each of the defined functional blocks of both protocols. And although the OSI Model itself might not guarantee interoperability between two implementations of the same protocol, it is a good framework within which to begin development.

The Layers of the OSI Model

The OSI Model is divided into seven layers, and each layer has a specific responsibility. This is not to say that a protocol based on the OSI Model has seven distinct parts, or only seven specific functions. As mentioned earlier, these layers merely represent the types of functions that a protocol should support, and they arrange the functional blocks in a logical fashion, with a top and a bottom. Note that the top of the model represents functions that occur nearest the user or the application, and the bottom of the model represents those functions that occur nearest the physical network or network interfaces.

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