The OSI Networking Model

The OSI Networking Model

The Open System Interconnection (OSI) model, developed by the International Organization for Standardization, defines how the various hardware and software components involved in data communication should interact with each other.

A good analogy would be a traveler who prepares herself to return home through many dangerous kingdoms by obtaining permits to enter each country at the very beginning of the trip. At each frontier, our friend has to hand over a permit to enter the country. Once inside, she asks the border guards for directions to reach the next frontier and displays the permit for that new kingdom as proof that she has a legitimate reason for wanting to go there.

In the OSI model each component along the data communications path is assigned a layer of responsibilityin other words, a kingdom over which it rules. Each layer extracts the permit, or header information, it needs from the data and uses this information to correctly forward what's left to the next layer. This layer also strips away its permit and forwards the data to the next layer, and so the cycle continues for seven layers.

The very first layer of the OSI model describes the transmission attributes of the cabling or wireless frequencies used at each link or step along the way. Layer 2 describes the error correction methodologies to be used on the link; layer 3 ensures that the data can hop from link to link on the way to the final destination described in its header. When the data finally arrives, the layer 4 header is used to determine which locally installed software application should receive it. The application uses the guidelines of layer 5 to keep track of the various communications sessions it has with remote computers and uses layer 6 to verify that the communication or file format is correct. Finally, layer 7 defines what the end user will see in the form of an interface, graphical on a screen or otherwise. A description of the functions of each layer in the model can be seen in Figure.