What is an OSI reference model ?

 OSI stands for Open systems Interconnection 

The Open systems Interconnetion model (OSI model) is a conceptual model that characterise and standarddises the communication functions of the telecommunication or computer system without regard to its underlying internal structure and technology. Its goal is the interoperability of diverse communication systems with standard communication protocols.

The model partitions the flow of data in a communication system into seven abstraction layers, from the physical implementaion of transmitting bis across a communications medium to the highest-level representation of data of a distributed application. Each intermediate layer serves a class of functionality to the layer above it and is served by the layer below it.