OSI reference model or Seven Layer Architecture

The Open System Interconnection (OSI) model was developed by the International Organization for Standardization (ISO) to resolve problem regarding incompatibility among various network devices from variety of vendors.

OSI model is abstract description of layered network architecture to ensure compatibility and interoperability. This model divides the process of data transmission among computers in seven layers, each layer performing its own particular role in data transmission. Each layer provides service to the layer above it and uses services from layer below it using SAPs (Service Access points) or interfaces. Layer at particular level in one computer communicates to the layer at same level on another computer and are known as peers.

 Layered approach makes development easier, adds flexibility and reduces complexity in the system. Layers can be replaced as long as the interface provided to the above layer and required from the above layer remains intact. 

Seven layer of OSI reference model from top to bottom are as follows 

Application

Presentation

Session

Transport

Network

Data link

Physical

Data passes from one layer to the next, starting from application layer at one computer, proceeds to the bottom layer, through the network medium (Such as wire) to the target computer from bottom to top in hierarchy up to the application program.

The lower four layers (Physical, Data link, Network and transport) are mainly concerned for end to end connectivity while upper three layers (Application, Presentation and session) are concerned to provide services to the applications running on the system. Read Next article, to know Particular functions of each layer.

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