The International Organization for Standardization (ISO) and the CCITT (Consultative Committee on International Telephony and Telegraphy), which is now the ITU (International Telecommunications Union), worked on a reference model at the same time that DARPA was doing research in networking protocols. The effort resulted in the OSI (Open Systems Interconnection) model (ISO 7498), which defined a seven-layer model of data communication with the physical layer at the lowest layer and application protocols at the upper layer. This model has been widely accepted as the basis for the understanding and design of a network protocol stack. It is also the basis against which other networking protocols are compared. The OSI reference model has seven layers , as shown in Figure 2-1. Figure 2-1. OSI reference model.
The functionality of the seven layers of the OSI model is as follows :
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