The OSI model describes the flow of data in a network, from the lowest layer (the physical connections) up to the layer containing the user’s applications. Data going to and from the network is passed layer to layer. Each layer is able to communicate with the layer immediately above it and the layer immediately below it. This way, each layer is written as an efficient, streamlined software component. When a layer receives a packet of information, it checks the destination address, and if its own address is not there, it passes the packet to the next layer.
The OSI model has seven layers, these are:
· The Application layer represents the level at which applications access network services. This layer represents the services that directly support applications such as software for file transfers, database access, and electronic mail.
· The Presentation layer translates data from the Application layer into an intermediary format. This layer also manages security issues by providing services such as data encryption, and compresses data so that fewer bits need to be transferred on the network.
· The Session layer allows two applications on different computers to establish, use, and end a session. This layer establishes dialog control between the two computers in a session, regulating which side transmits, plus when and how long it transmits.
· The Transport layer handles error recognition and recovery. It also repackages long messages when necessary into small packets for transmission and, at the receiving end, rebuilds packets into the original message. The receiving Transport layer also sends receipt acknowledgments.
· The Network layer addresses messages and translates logical addresses and names into physical addresses. It also determines the route from the source to the destination computer and manages traffic problems, such as switching, routing, and controlling the congestion of data packets.
· The Data Link layer packages raw bits from the Physical layer into frames (logical, structured packets for data). This layer is responsible for transferring frames from one computer to another, without errors. After sending a frame, it waits for an acknowledgment from the receiving computer.
· The Physical layer transmits bits from one computer to another and regulates the transmission of a stream of bits over a physical medium. This layer defines how the cable is attached to the network adapter and what transmission technique is used to send data over the cable.
James Bond meets Number One on the 7th floor of the spy headquarters building. Number One gives Bond a secret message that must get through to the US Embassy across town.
Bond proceeds to the 6th floor where the message is translated into an intermediary language, encrypted and miniaturized.
Bond takes the elevator to the 5th floor where Security checks the message to be sure it is all there and puts some checkpoints in the message so his counterpart at the US end can be sure he’s got the whole message.
On the 4th floor the message is analyzed to see if it can be combined with some other small messages that need to go to the US end. Also if the message was very large it might be broken into several small packages so other spies can take it and have it reassembled on the other end.
The 3rd floor personnel check the address on the message and determine who the addressee is and advising Bond of the fastest route to the Embassy.
On the 2nd floor the message is put into a special courier pouch(packet). It contains the message, the sender and destination ID. It also warns the recipient if other pieces are still coming.
Bond proceeds to the 1st floor where ‘Q’ has prepared the Aston Martin for the trip to the Embassy. Bond departs for the US Embassy with the secret packet in hand.
On the other end the process is reversed. Bond proceeds from floor to floor where the message is decoded. The US Ambassador is very grateful the message got through safely. “Bond, please tell Number One I’ll be glad to meet him for dinner tonight”.
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