OSI REFERENCE MODEL

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Networking models such as the Open Systems Interconnection (OSI) Reference Model provide a framework for breaking down complex internetworks into components that can more easily be understood and utilized. The model defines networking functions not as a large, complicated whole, but as a set of layered, modular components, each of which is responsible for a particular function. The result is better comprehension of network operations, improved performance and functionality, easier design and development, and the ability to combine different components in a way that's best suited to the needs of the network.

The OSI Reference Model is composed of seven conceptual layers, each of which is assigned a number from 1 to 7 :

  1. Physical Layer
  2. Data Link Layer
  3. Network Layer
  4. Transport Layer
  5. Session Layer
  6. Presentation Layer
  7. Application Layer

The layer number represents the position of the layer in the model as a whole, and indicates how close the layer is to the actual hardware used to implement a network.

Physical Layer - The lowest layer of the OSI Reference Model is layer 1, the physical layer; it is commonly abbreviated PHY. This layer is the only one where data is physically moved across the network interface. All other layers perform functions to create messages that implement various protocols, but these messages must all be transmitted down the protocol stack to the physical layer, and they are eventually sent out over the network.
The physical layer technologies deal with the actual ones and zeros that are sent over the network. For example, repeaters, conventional hubs, and transceivers all operate at the physical layer. These devices have no knowledge of the contents of a message; they simply take input bits and send them as output.

Data Link Layer - The second layer in the OSI Reference Model is the data link layer. It is responsible for data framing, hardware addressing, error detection and handling, and defining physical layer standards. The data link layer is often conceptually divided into two sublayers : Logical Link Control (LLC) and Media Access Control (MAC). This is the layer where most LAN and wireless LAN technologies are defined.

Network Layer - The OSI Reference Model’s third layer is the network layer. It is responsible for the tasks that link together individual networks into internetworks. Network layer functions include internetwork-level addressing, routing, datagram encapsulation, fragmentation and reassembly, and certain types of error handling and diagnostics.

Transport Layer - The fourth and middle layer in the OSI Reference Model is the transport layer. This layer represents the transition point between the lower layers that deal with data delivery issues, and the higher ones that work with application software. The transport layer is responsible for enabling end-to-end communication between application processes, which it accomplishes in part through the use of process-level addressing and multiplexing or demultiplexing. Transport layer protocols are responsible for segmenting application data into blocks for transmission and may be either connection-oriented or connectionless. Protocols at this layer also often provide data delivery management services such as reliability and flow control.

Session Layer - The fifth layer in the OSI Reference Model stack is the session layer. As its name suggests, it is the layer intended to provide functions for establishing and managing sessions between software processes. Session layer technologies are often implemented as sets of software tools called application program interfaces (APIs), which provide a consistent set of services that allow programmers to develop networking applications without needing to worry about lower-level details of transport, addressing, and delivery.

Presentation Layer - The sixth layer in the OSI model is the presentation layer. Protocols at this layer take care of manipulation tasks that transform data from one representation to another, such as translation, compression, and encryption. In many cases, no such functions are required in a particular networking stack; if so, there may not be any protocol active at layer 6, so layer 7 may deal with layer 5.

Application Layer - The application layer is the seventh and highest layer in the OSI Reference Model. Application protocols that implement specific user applications and other high-level functions are defined at this layer. Since they are at the top of the stack, application protocols are the only ones that do not provide services to a higher layer; they use services provided by the layers below.



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