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OSI Model Explained

A person standing lecturing in front of a board containing strange symbols.  Source: Microsoft.

Each layer of the OSI model provides some service or action that prepares the data for delivery over the network to another computer. The lowest layers, i.e. 1 and 2 define the network's physical media and related tasks, such as putting data bits onto the network interface cards (NICs) and cable. The highest layers define how applications access communication services. The layers are separated from each other by boundaries called interfaces. All requests are passed from one layer, through the interface to the next layer. Each layer builds upon the standards and activities of the layer below it.

The sending end carries out a process called encapsulation on the information to be sent over the network. Each layer has a protocol data unit (PDU) that it prepares before giving it to the lower layer. Therefore, the information goes down from layer 7 to layer 1 and is then sent to the receiving end. At the receiving end, the process is reversed, i.e. de-encapsulation, with the PDU of each peer layer being processed before being moved to the upper layer.

With the exception of the lowest layer in the OSI networking model, no layer can pass information directly to its peer. Instead, information on the sending computer must be passed down through each successive layer until it reaches the physical layer and then passed back up at the receiving end.

Next: Layer 7: Application