The OSI model

The Open System Interconnection model is a conceptual model that characterizes and standardizes the communication functions of a telecommunication or computing system without regard to their underlying internal structure and technology.

        The OSI model defines networking in terms of a vertical stack of seven layers.The upper layer of OSI model represent software that implements network service like encryption and connection management.The lower layer of the OSI model implements hardware-oriented functions such as routing ,addressing and flow control.All data that goes over a network connection passes through each of the seven layers.

       The OSI model was introduced in 1984.Designed to be an abstract model and teaching tool,the OSI model remains a useful tool for learning about today's network technologies such as Ethernet and Protocols.

Seven layers of OSI

1. Physical layer.
2. Data-link layer.
3. Network layer.
4. Transport layer.
5. Session layer.
6. Presentation layer.
7. Application layer.

1.Physical layer.

         OSI layer 1 conveys the bit stream electrical impulse,light or radio signal-through the network at the electrical and mechanical level.It provides the hardware means of sending and receiving data on a carrier.

RS232,Fast Ethernet and ATM protocols in physical layer.

2.Data-link layer.

        At OSI model, layer 2,data packets are encoded into bits.It furnishes transmission protocol knowledge and management and handles errors in the physical layer,flow control and frame synchronization.This layer divided in to two sub layers.

•                    The Media Access Control(MAC) layer.
•                    The Logical Link Control(LLC) layer.

3.Network layer.

        Layer 3 provides switching and routing technologies creating logical paths,known as virtual circuits,for transmitting data from node to node.Routing and Forwarding are functions of this layer as well as addressing,inter-networking,error handling,congestion control and packet sequencing.

4.Transport layer.

       OSI model,layer 4 provides transparent transfer of data between end systems or hosts and is responsible for end to end error recovery and flow control.It ensures complete data transfer.

5.Session layer.

        This layer establishes,manages and terminates connections between applications.The session layer sets up,coordinates,and terminates conversations,exchanges and dialogues between the applications at each end.

6.Presentation layer.

            This layer provides independence from difference in data representation by translating from application to network format and vice versa.The presentation layer works to transform data into the form that the application layer can accept.

7.Application layer.

            OSI model, layer 7 supports application and end-user-processes.Communication partners are identified.Quality of service is identified.User authentication and privacy are considered,and any constraints on data syntax are identified.Everything at this layer is application specific.