OSI

The Open Systems Interconnection Basic Reference Model (OSI Reference Model or OSI Model for short) is a layered, abstract description for communications and computer network protocol design, developed as part of the Open Systems Interconnection (OSI) initiative. It is also called the OSI seven layer model.

The layers, described below, are, from top to bottom:
1. Application
2. Presentation
3. Session
4. Transport
5. Network
6. Data Link, and
7. Physical.