Why layered architecture is used in OSI reference model? Discuss.

Why layered architecture is used in OSI reference model? Discuss.
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The OSI reference model is as shown in the figure.There are seven layers in the model. Physical layer, Data link layer, Network layer, Transport layer, Session layer, Presentation layer, Application layer. Each layer performs unique and specific task and it offers services to the layer above it. Each computer on a network uses a series of protocols to perform the functions assigned to each layer. At the top of the stack is the application layer and at the bottom is the physical medium which actually connects the computers to form a network. The figure shows the OSI model with two hosts A and B, communicating with each other Interface: an interface defines the operations and services offered by lower layer to the upper layer. This is an interface between each pair of adjacent layers as shown in the figure. Peer: the entities comprising the corresponding layers on different machines are called as peers. The communication actually takes place between the peers using the protocols. Exchange of information: at the physical layer the communication is direct that is, machine A sends a stream of bits to machine B. at higher layers each layer in the sending machines adds its own information to the message it receives from the layer just above it and passes the whole package to the layer just below it. The information added by each layer is in the form of headers or trailers. At layer 1 the entire package is converted to a form that can be transferred to the receiving machine. 

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