OSI 7 Layers Reference Model For Network Communication

Open Systems Interconnection (OSI) model is a reference model developed by ISO (International Organization for Standardization) in 1984, as a conceptual framework of standards for communication in the network across different equipment and applications by different vendors. It is now considered the primary architectural model for inter-computing and internetworking communications. Most of the network communication protocols used today have a structure based on the OSI model. The OSI model defines the communications process into 7 layers, which divides the tasks involved with moving information between networked computers into seven smaller, more manageable task groups. A task or group of tasks is then assigned to each of the seven OSI layers. Each layer is reasonably self-contained so that the tasks assigned to each layer can be implemented independently. This enables the solutions offered by one layer to be updated without adversely affecting the other layers. The OSI 7 layers model has clear characteristics. Layers 7 through 4 deal with end to end communications between data source and destinations. Layers 3 to 1 deal with communications between network devices. On the other hand, the seven layers of the OSI model can be divided into two groups: upper layers (layers 7, 6 & 5) and lower layers (layers 4, 3, 2, 1). The upper layers of the OSI model deal with application issues and generally are implemented only in software. The highest layer, the application layer, is closest to the end user. The lower layers of the OSI model handle data transport issues. The physical layer and the data link layer are implemented in hardware and software. The lowest layer, the physical layer, is closest to the physical network medium (the wires, for example) and is responsible for placing data on the medium.

The specific description for each layer is as follows:
Layer 7:Application Layer Defines interface to user processes for communication and data transfer in network Provides standardized services such as virtual terminal, file and job transfer and operations
Layer 6:Presentation Layer Masks the differences of data formats between dissimilar systems Specifies architecture-independent data transfer format Encodes and decodes data; Encrypts and decrypts data; Compresses and decompresses data
Layer 5:Session Layer Manages user sessions and dialogues Controls establishment and termination of logic links between users Reports upper layer errors
Layer 4:Transport Layer Manages end-to-end message delivery in network Provides reliable and sequential packet delivery through error recovery and flow control mechanisms Provides connectionless oriented packet delivery
Layer 3:Network Layer Determines how data are transferred between network devices Routes packets according to unique network device addresses Provides flow and congestion control to prevent network resource depletion
Layer 2:Data Link Layer Defines procedures for operating the communication links Frames packets Detects and corrects packets transmit errors
Layer 1:Physical Layer Defines physical means of sending data over network devices Interfaces between network medium and devices Defines optical, electrical and mechanical characteristics There are other network architecture models, such as IBM SNA (Systems Network Architecture) model . Those models will be discussed in separate documents. The OSI 7 layer model is defined by ISO in document 7498 and ITU X.200, X.207, X.210, X.211, X.212, X.213, X.214, X.215, X.217 and X.800. The protocols defined by ISO based on the OSI 7 layer mode are as follows: