The OSI Reference Model
(Open Systems Interconnection)
BACKGROUND
In 1978, the International Standards Organization (ISO) released a set of specifications that described network architecture for connecting dissimilar devices. The original document applied to systems that were open to each other because they could all use the same protocols and standards to exchange information.
In 1984, the ISO released a revision of this model and called it the Open Systems Interconnection (OSI) Reference Model. The 1984 version has become the standard and serves as a guide for networking.
This model is the best known and most widely used guide to describe networking environments. Vendors design network products based on the specifications of the OSI model. It provides a description of how network hardware and software work together in a layered fashion to make communications possible. It also helps with troubleshooting by providing a frame of reference that describes how components are supposed to function.
A LAYERED ARCHITECTURE
The OSI model is an architecture that divides network communication into seven layers. Each layer covers different network activities, equipment, or protocols. The layers are as follow:
7. Application Layer
6. Presentation Layer
5. Session Layer
4. Transport Layer
3. Network layer
2. Data Link Layer
1. Physical Layer
Layering specifies different functions and services at different levels. Each OSI layer has well-defined networking functions, and the functions of each layer communicate and work with the functions of the layers immediately above and below it. Each layer provides some service or action that prepares the data for delivery over the network to another computer. The layers are separated from each other by boundaries called interfaces.
DESCRIPTION OF LAYERS
Application Layer:
Layer 7, the topmost layer of the OSI model, is the Application layer. It serves as the window for application processes to access network services. This layer represents the services that directly support user applications, such as software for file transfers, for database access, and for e-mail. The lower levels support these tasks performed at the application level. The Application layer handles general network access, flow control, and error recovery.
Presentation Layer:
Layer 6, it determines the format used to exchange data among networked computers. It can be called the network's translator. At the sending computer, this layer translates data from a format sent down from the Application layer into a commonly recognized, intermediary format. At the receiving computer, this layer translates the intermediary format into a format useful to that computer's Application layer. The Presentation layer is responsible for protocol conversion, translating the data, encrypting the data, changing or converting the character set, and expanding graphics commands. The Presentation layer also manages data compression to reduce the number of bits that need to be transmitted.
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