OSI Reference Model:

           The reference model is proposed by International Standard Organization (ISO) as a first step towards standardization of the protocols used in various layers in 1983 by Day and Zimmermann. This model is called Open System Interconnection (OSI) reference model. It is referred OSI as it deals with connection open systems. That is the systems are open for communication with other systems. It consists of Seven Layers.

Seven Layers OSI Model

           A layer should be created where a different level of abstraction is needed. Each layer should perform a well defined task. The function of each layer should define internationally standardized protocols. Layer boundaries should be chosen to minimize the information floe across the interface. The number of layers should not be high or too small. As such this model is not a network architecture as it does not specify exact services and protocols. It just tells what each layer should do and where it lies. The bottom most layer is referred as physical layer. ISO has produced standards for each layers and are published separately. Example: This layers are like onion layers as shown in figure below.

Like Onion Layer

                  When Packet leaving from system first Application layer encapsulate then presentation, Session, Transport, Network, data Link and the last layer is Physical.
                  The OSI reference model provides a number of benefits in understanding how networks function, by doing the following:

• Reducing complexity: The OSI model breaks network communications into smaller, simpler parts.

• Standardizing interfaces: The OSI model standardizes network components to allow multivendor development and support.
• Facilitating modular engineering: The OSI model allows different types of network hardware and software to communicate with one another.
• Ensuring interoperable technology: The OSI model prevents changes in one layer from affecting the other layers. Thus allowing for quicker development.
• Accelerating evolution: The OSI model provides for effective updates and improvements to individual components without affecting other components or having to rewrite the entire protocol.
• Simplifying teaching and learning: The OSI model breaks network communications into smaller components to make learning easier.

OSI Model Layers and Their Functions:

                 Each OSI layer has s specific function and associated software or devices. This topic describes each layer and its function.

Layer 1: The Physical Layer

                The physical layer defines the electrical, mechanical, procedural, and functional specifications for activating, maintaining, and deactivating the physical link for bit transmission between end devices. Physical layer specifications are defining characteristics such as voltage levels, timing of voltage charges, physical data rates, maximum transmission distances, physical connecors, and other similar attributes.

Layer 2: The Data Link Layer

                 The data link layer defines how data is formatted for transimission and how access to the physical media is controlled. This layer also typically includees error detection and correction to ensure reliable delivery of the data.

Layer 3: The Network Layer

                The network layer provides connectivity and path selection between two host system that may be located on geographically separated networks. The growth of the internet has increased the number of uses that access information from sites around the world. The network layer is the layer that manages the connectivity of these uses by providing logical addressing.

Layer 4: The Transport Layer

                The transport layer defines services to segment, transfer, and reassemble the data for individual communications between the end devices. For example, business users in large corporations often transfer large files from field locations to a corporate site. Reliable delivery of the files is important, so the transport layer will break down large files into smaller segments that are less likely to incur transmission problems.
                The transport layer shields the upper layers from transport implementation details. Specifically, issues such as reliability of transport between two hosts are assigned to the transport layer. In providing a communication service, the transport layer establishes, maintains, and properly terminates virtual circuits. transport error detection and recovery, as well as information flow control, ensure reliable service.

                The boundary between the transport layer and the session layer can be thought of as the border between application protocols and data flow protocols. Where as the application, presentation, and session layers are concerned with application issues, the lower four layers are concerned with data transport issues.

Layer 5: The Session Layer

                 The session layer establishes, manages, and terminates session between two communicating hosts. The session layer also synchronizes dialog between the presentation layers of the two hosts and manages their data exchange. For example, web servers have many users, so there are many communication processes open at a given time. It is  important, then, to keep track of which user communicates on which path. In addition to session regulation, the session layer offers provisions for efficient data transfer, class of service(CoS), and exception reporting of session layer, presentation layer, and application layer problems.

Layer 6: The Presentation Layer

                 The presentation layer ensures that the information that is sent at the application layer of one system is readable by the application layer of another system. For example, a PC program communicates with another computer. One PC is using extended binary coded decimal interchange code (EBCDIC) and the other one is using ASCII to represent the same characters. If necessary, the presentation layer translates between multiple data formats by using a common format.

Layer 7: The Application Layer

              The application layer is the OSI layer that is closet to the user. This layer provides network services to the applications of the user, such as email, file transfer, and terminal emulation. The application layer differs from the other layers in that it does not provide services to any other OSI layer. It provides services anly to applications outside the OSI model. The application layer establishes the availability of intended communication partners, and synchronizes and establishes agreement on procedures for error recovery and control of data integrity.