1.2.1 Open Systems Interconnection Model

The International Organization for Standardization (ISO) developed the Open Systems Interconnection (OSI) reference model () to promote understanding of how components in a network system work. It does this by separating the functions of hardware and software components into seven discrete layers. Each layer performs a different group of tasks required for network communication.

The seven layers from bottom to top are as follows:

1. Physical.

2. Data link.

3. Network.

4. Transport.

5. Session.

6. Presentation.

7. Application.

The OSI model.

Although not all network systems implement layers using this precise structure, they all implement each task in some way. The OSI model is not a standard or a specification; it serves as a functional guideline for designing network protocols, software, and appliances and for troubleshooting networks.