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Protocol layering

An introduction to the OSI model
The OSI (Open Systems Interconnection) model is a model that describes what happens to some information when it is being sent from one computer across a network to another. The model organises the protocols and standards that are used into independent ‘layers’ of protocols and standards. As long as manufacturers of hardware and software products make products that conform to the standards and protocols in any one particular layer, then their product will slot into the model, will interface correctly with the layer above and below it and therefore will successfully enable communication to take place. For example, imagine you have written an email. What happens when you hit “Send”?

• • The message is passed to the Application layer, layer 7.
  • It then travels through each of the remaining independent ‘layers’ of protocols.
  • At each layer, something more happens to the message, something extra, that prepares it for the next layer.
  • Finally, the message is ready to leave the computer and be put on the network interconnections as a stream of packets of data. The packets travel to their destination, where they enter the Physical layer of protocols.
  • The packets move up the independent layers in reverse order, until the message has been reassembled.

The benefits of the OSI model
There are a number of key benefits to using the OSI model.

• • The OSI model provides standardisation with hardware and software involved with communication.
  • Manufacturers of hardware and software can be confident that their products can be used in successful communication if they follow the standards laid down in the OSI model. They may design products that use the standards and protocols in one layer. A new product, therefore, will interface correctly with the layers above and below it in the model, so long as they follow the protocols and standards in that particular layer. They can be confident that their products can be used successfully and so they will have happy customers!
  • If there is a communications problem, the model helps someone track down where the error is, in which layer and what caused it. This is because the layers are all independent of each other.
  • Protocols are updated from time to time. They only need to be updated in the layers where they occur.

The principle of layering
The term ‘layering’ is often discussed when the OSI model is being studied. It simply refers to taking all of the protocols and standards and grouping them into one of seven layers. This has a number of benefits:

• • It means that a product that uses a particular layer can be designed and tested totally separately from the other layers.
  • Layering describes the process where each layer of protocols builds upon the output from the previous layer, and provides the input for the next layer.
  • As long as the protocols and standards for that layer are followed, a manufacturer can be confident that it will work with other manufacturer’s equipment.
  • Each layer can be treated independently.
  • Layering helps problems to be identified because problems occur inside an independent layer.
  • It means that as long as everyone uses the OSI model, successful communication can take place. Before the model was specified, data communication between different applications and hardware was practically impossible.

More on the layers
As we know now, each layer is independent of the other layers. One layer builds upon what it receives from the previous layer and passes it on to the next one. Here is a table that describes briefly what each layer is responsible for.

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