OSI (Open Systems Interconnection)

Open systems interconnection is a form of model that describes how applications can/will communicate over a network in seven layers. Autonomy of data is passed through each sequential layer (starting from the application on the first layer to the last layer). This is an outdated model as OSI was loosely defined which wasn't appealing to vendors that kept to those standards. Nowadays OSI model is used to teach about other types of protocols.

OSI Model image:

☀http://www.tech-faq.com/wp-content/uploads/2009/01/osimodel.png