Open Systems Interconnections is a reference model which is used to design networks between machines. It is otherwise known as OSI model. Open Systems means the systems are free to connect to other systems or free to communicate with other existing open systems. The reference model consists of 7 layers namely, Physical, Data Link, Network, Transport, Session, Presentation, Application.
Different layers have different functions and some layers can be omitted in the network design for faster transfer of data from one machine to another. Physical layers deals with the hardware part of the network. Copper cables, opitical fibres, twisted pair cables are considered examples of physical medium of physical layer. In Data Link layer, the data is broken down into frames of information. Each frame is given a header to indicate the sequence of the frame. The sender sends a data frame where as an acknowledge frame is sent from the receiver's end indicating that the message has reached the destination.
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In network layer the frames are converted into packets and they are given addressing accordingly to reach the destination. Congestion control is taken care of in this layer. Transport layer is the one which plays the major role in end to end communication between two systems. At the source the transport layers fragments and breaks down the packet into frames and send it to the network layer. At the destination end these frames are added together to form a packet and sent to the transport layer. Routers come into play in this layer. They route the path of the packet being sent.
The next layer is the session layer. It takes care of the session between the source and the destination. If during the transfer the connection is lost, the session is lost. When the connection is re-established, the session layer is responsible for continuing the session from the place where the transfer has been left. These sessions can be tracked by the cookies present in the source and the destination which are responsible for the session to start and proceed.
Next layer is the presentation layer in which the semantics of the message is checked. At the receiving end the semantics are taken care of by the layer above it. In order to transfer the message between two different machines, the data structures are exchanged in a very abstract way. The next layer is the application layer. This forms the interface for the user to communicate. Web browser is one of the application.
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