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OSI Layers and its Functions


OSI Layer
OSI Layers

Open Systems Interconnection (OSI) model is developed by the International Standards Organization (ISO). It divides network communication into seven layers.

Physical Layer

The physical layer, the lowest layer of the OSI model, is concerned with the transmission and reception of the unstructured raw bit stream over a physical medium. It explains the electrical/optical, mechanical, and functional interfaces to the physical medium, and carries the signals for all of the higher layers.

Data encoding modifies the simple digital signal pattern (1s and 0s) used by the PC to better accommodate the characteristics of the physical medium and to aid in the bit and frame synchronization.

Data Link Layer

The data link layer supports an error-free transfer of data frames from one node to another over the physical layer, allowing layers above it to assume virtually error-free transmission over the link. This layer makes links across the physical network, putting packets into network frames. This layer has two sub-layers, the Logical Link Control Layer and the Media Access Control Layer.

Network Layer

This Network Layer controls the operation of the subnet, deciding which physical path the data should take based on network conditions, a priority of service, and other factors.
Routing: Routes frames among networks.
Subnet traffic control: Routers (network layer intermediate systems) can instruct a sending station to “throttle back” it is frame transmission when the router’s buffer fills up.
Frame fragmentation: It determines that a downstream router’s maximum transmission unit (MTU) size is less than the frame size, a router can fragment a frame for transmission and re-assembly at the destination station.
Logical-physical address mapping translates logical addresses, or names, into physical addresses.
Subnet usage accounting: This functions to keep track of frames forwarded by subnet intermediate systems, to produce billing information.

Transport Layer

Transport Layer decides if data transmission should be on the parallel path or single path. Functions such as Multiplexing, Segmenting or Splitting into the data are done by this layer. It gets messages from the Session layer and convert the message into smaller units and passes it on to the Network layer. This layer can be very complex, depending upon the network needs. Transport layer breaks the data into small units so that they are handled more efficiently by the network layer.

Session Layer

Session layer controls and synchronizes the conversation between two different applications. The stream of data transfer from source to destination session layer are marked and resynchronized properly so that the ends of the messages are not cut prematurely and data loss is averted.

Presentation Layer

Presentation layer sends the data in such a way that the receiver will understand the information and will be able to use the data. When receiving the data, this layer transforms the data to be ready for the application layer. Languages(syntax) can be different for the two communicating systems. In this condition, presentation layer plays a role of translator. It performs Data compression, Data encryption, Data conversion etc.

Application Layer

It is the topmost layer. Mail services, directory services, network resource etc are services provided by the application layer. Transferring of files disturbing the results to the user is also done in this layer. It primarily holds application programs to act upon the receiver end.

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