Networking Guide : The OSI Reference Model

The International Organization of Standardization (ISO) defined procedures for computer communications which was called Open System Interconnection (OSI) Reference Model or OSI Model for short. The OSI Model describes how data flows from one computer to another computer in a network.

Picture. The OSI Model
When computers A and B communicate,
data travels across the 7-layered protocol stack.

The OSI Model is defined as a protocol stack that consists of seven logical layers. Each layer has specific functions and handles a unique data format. When two computers communicate, data flows from the user-to-network interface (application) at the sending computer down through the protocol stack to the connecting physical medium (i.e. cable, radio, or infrared) and up through the protocol stack to the network-to-user interface (application) at the receiving computer. When data flows from an upper layer to a lower layer, it is converted to the lower layer data format and a lower layer header is added to it. This process is called encapsulation. Conversely, when data flows from a lower layer to an upper layer, it is converted to the upper layer data format and the lower layer header is discarded.