OSI MODEL

OSI MODEL

The Open Systems Interconnection model (OSI 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. Its goal is the interoperability of diverse communication systems with standard protocols. The model partitions a communication system into abstraction layers. The original version of the model defined seven layers.

A layer serves the layer above it and is served by the layer below it. For example, a layer that provides error-free communications across a network provides the path needed by applications above it, while it calls the next lower layer to send and receive packets that comprise the contents of that path. Two instances at the same layer are visualized as connected by a horizontal connection in that layer.

LAYERS OF OSI MODEL

Layer 1: Physical layer

The physical layer refers to electrical and physical aspects of devices. In particular, it specifies how a device sends and receives information, such as using copper wires or fiber-optic cables. 

• Provides physical interface for transmission of information.
• Defines rules by which bits are passed from one system to another on a physical communication medium.

Layer 2: Data link layer

The Data Layer is mainly the method in which information from the network is broken down into frames and transmitted over the physical layer. This layer is also responsible for some Error detection and correction and some addressing so different devices can tell each other apart in larger systems.

• Data link layer attempts to provide reliability communication over the physical layer interface.
• Breaks the outgoing data into frames and reassemble the received frame.

Layer 3: Network layer

The Routing Layer works to coordinate related parts of a data conversation to ensure that large files are transferred. In other words, while the data link layer deals with the method in which the physical layer is used to transfer data, the network layer deals with organizing that data for transfer and reassembly. This layer also handles aspects of Routing Protocols, finding the available [best] path(s) from one network to another to ensure delivery of the data.

• Implements routing of frames (Packets) through the network.
• Define the most optimum path the packet should take from the source to the destination.
• Defines logical addressing so that any endpoint can be identified.

Layer 4: Transport layer

The Transport Layer is the level at which system reliability and quality are ensured. This layer manages traffic flow through the network layer to reduce congestion on a network, and performs error checking ensuring quality of service by resenting data when data has been corrupted. Some of the most popular methods of encryption and firewall security take place on this layer.

• Purpose of this layer is to provide a reliability mechanism for the exchange of data between two processes in different computers.
• Ensures that the data units are delivered error free.
• Ensure that data units are delivered in sequence.
• Ensure that there is no loss or duplication of data.

Layer 5: Session Layer

The session layer controls the dialogues (connections) between computers. It establishes, manages and terminates the connections between the local and remote application. It provides for full-duplex, half-duplex, or simplex operation, and establishes check-pointing, adjournment, termination, and restart procedures. The OSI model made this layer responsible for graceful close of sessions, which is a property of the Transmission Control Protocol, and also for session check-pointing and recovery, which is not usually used in the Internet Protocol Suite. The session layer is commonly implemented explicitly in application environments that use remote procedure calls.

• Session layer provide mechanism for controlling the dialog between the two end system.It defines how to start, control and end conversation (called session)between applications.
• This layer request for a logical connection to established on an end-user's request.                          

Layer 6: Presentation Layer

The presentation layer establishes context between application-layer entities, in which the application-layer entities may use different syntax and semantics if the presentation service provides a mapping between them. If a mapping is available, presentation service data units are encapsulated into session protocol data units and passed down the protocol stack.

• This layer provides independence from data representation by translating between application and network formats. The presentation layer transforms data into the form that the application accepts. This layer formats data to be sent across a network. 

Layer 7: Application  Layer

The application layer is the OSI layer closest to the end user, which means both the OSI application layer and the user interact directly with the software application. This layer interacts with software applications that implement a communicating component. 

• Application layer  interacts with application programs and is the highest level of OSI model.
• Application layer contains management functions to support distributed applications.