OSI Model Of Networking

OSI, Open Systems Interconnection model ,is open standard for communication among two systems. OSI model was developed  " Charles Bachman " which evolved to remove various difficulty faced in ARPANET. If you are interested ,the actual document is available free of cost at
TU-T/X.200-series of recommendations 

or

 http://www.itu.int/rec/T-REC-X.Imp200-200612-I/en


The Open System Interconnection (OSI) model is a set of protocols that attempt to define and standardize the data communications process. Its divides the communication channel into 7 layers ,each layer performing its own functionality. Numerous protocols at each layer provide security and encapsulation . each layer is at the top of another and transfer data to each other from SAP ( service access protocol)
The figure below show us the 7 layers of OSI

As in the figure the last three layers are  grouped as "Media  layers" and the upper four as "Host Layers". Media layers are  mainly responsible for transferring the data through the physical medium ( converting the data into bit streams , synchronizing it, etc) where as the host layer is mainly focused on how the data is to be represented  and various applications interfaces.
NOTE:  Segment and Presentation Layers are usually not very useful and hence were removed in TCP/IP MODEl. 



hope u understood and Best of Luck.

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