The Open Systems Interconnection (OSI) Model

The Open Systems Interconnection (OSI) Model

This defined a seven-layer model of data communications. Each layer of the OSI Reference Model provides a set of functions to the layer above and, in turn, relies on the functions provided by the layer below. Although messages can only pass vertically through the stack from layer to layer, from a logical point of view, each layer communicates directly with its peer layer on other nodes.

The seven layers are:

Application

The application layer gives the user access to all the lower OSI functions, and its purpose is to support semantic exchanges between applications existing in open systems. An example is the Web browser.

Presentation

The presentation layer is concerned with the representation of user or system data. This includes necessary conversations (for example, a printer control character), and code translation (for example, ASCII to EBCDIC).

Session

The session layer provides mechanisms for organizing and structuring interaction between applications and/or devices.

Transport

The transport layer provides transparent and reliable end-to-end data transfer, relying on lower layer functions for handling the peculiarities of the actual transfer medium. TCP and UDP are examples of a Transport layer protocol.

Network

The network layer provides the means to establish connections between networks. The standard also includes procedures for the operational control of internetwork communications and for the routing of information through multiple networks. The IP is an example of a Network layer protocol.

Data Link

The data link layer provides the functions and protocols to transfer data between network entities and to detect (and possibly correct) errors that may occur in the physical layer.

Physical

The physical layer is responsible for physically transmitting the data over the communication link. It provides the mechanical, electrical, functional and procedural standards to access the physical medium.