OSI Reference Model

The Open System Interconnection (OSI) model is the foundation for all communications that takes place between computer and other networking devices.

This model is based on proposal developed by International Standards Organizations (ISO) in 1977. ISO is the organizations dedicated to defining global communication and standards.

Open System Interconnection (OSI) reference model allows open communication between different systems without requiring changes to the logic of the underlying hardware and software.

There are seven layers in OSI reference model.

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

The seven layers in OSI model are organized into following subgroup :

Layer 1, 2 and 3. Physical, Data link and network layer are network support layers. They deal with physical aspect of the moving data from one device to another, such as physical specification, physical address, electrical specification.

Layer 4. Transport layer provides end to end reliable data transmission.

Layer 5, 6 and 7. Session, Presentation and Application layers are user support layers. These layers allow inter-operability among unrelated software system.

Layer 2. Data link layer ensures reliable transmission on a single link with a single link.

Each layer calls upon the services of the layer.

Organisation of Layers

Each layer in OSI model performs specific function for processing the data before it is passed into the layer above or below.

Each layer is independent from every other layer in its purpose and responsibilities.

At the top of OSI model is application layer (7 layer). This layer is the start and end point for communication with between a sending and receiving device.

The bottom layer of OSI model is physical layer (layer 1) is the one that actually sends the message on its way.

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