The OSI Model
At some point, everyone involved with networking comes across a reference to the Open Systems Interconnection (OSI) seven-layer model. Because this model provides the architectural framework for all of network and computing communication, it's a good place to start. Even if you don't ever plan on setting up your own network, being familiar with this model is essential to understanding how it all works.
The OSI seven-layer model describes the functions for computers to communicate with each other. The International Organization for Standardization (ISO) published this model in 1984 to describe a layered approach for providing network services using a reference set of protocols called OSI. The basis of the definition is that each of the seven layers has a particular function it must perform, and each layer needs to know how to communicate with only the layers immediately above and below it.
The advantages of the OSI approach may not be readily apparent. But this simple concept of having layers understand only those adjacent to themselves allows communications systems to be easily adapted and modified as technologies evolve. For example, as new technologies are introduced in a lower layer, such as Layer 1, upper layers do not necessarily need to be changed. Instead, the adaptations at Layer 2 allow the layers above to use the new technologies transparently. Imagine if all web browsers and e-mail programs had to be replaced every time a new wireless network standard were introduced.
When the OSI networking model was defined, there was little standardization among network equipment manufacturers. Customers generally had to standardize on a particular vendor's often proprietary hardware and software to have devices communicate with each other. As a result of the ISO's and other standardization efforts, networking customers can mix and match hardware when running open-standards protocols, such as Internet Protocol (IP).
Continue reading here: Open Versus Proprietary Systems
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