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The OSI or Open System Interconnection was developed in 1984 by the ISO. It rapresent an abstract model and it is made up od 7 layers; thanks to its modularity each layers is independent but, at the same time, it can interact with the others and it offering same advantages: new protocols and network serverices can be added. Layer 7, or Application, communication partens and the quality of services are identificied. Moreover, application service for example file transports and other network services are provided. Layer 6, or Presentation, encrypts and formats data so that they can be accepted by layer 7 preventing compatibily problems. Layer 5, or Session, manages connections between applications. Layer 4, or Transport, translate date between hosts which are Pcs directly connected to the network. Moreover, it is also responsible for error recovery. Layer 3, or Network, manages switching and routing technologies by creating logical parts to trasmitted data from node to node; it handles error and packet sequencing. Layer 2, or Data Link, endecoded and decoded data packet into bits. It is divided into 2 sublayers: MAC which controls how a computers gets access to the data and trasmitted it; and LLC whhich handles packet syncronization flow control, and error checking. Layer 1, or physical, conveys the bit stream throw the Network providing the hardware means to send and receive data.

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