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2 - 2 OSI (Open Systems Interconnection)

OSI (Open Systems Interconnection) is the network architecture of international standards that was established in the year 1983 mainly by ISO (International Organization for Standardization) and ITU-T (International Telecommunication Union - Telecommunication Standardization Sector). When network architecture was first taken into consideration, each manufacturer published unique network architecture. However, when a network is to be constructed with the products of different manufacturers, the network architecture that is unique to manufacturers is not useful. Thus, standard network architecture was considered necessary. According to OSI, the functions necessary for communication are arranged in seven hierarchies (i.e., layers). This is called the OSI basic reference model.

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Several function modules (i.e., entities) are present in each layer of OSI. (The entity present in the N-th layer is called the (N) entity.) The (N) entity is a service provided from the (N−1) entity that belongs one layer below, and provides service to the (N+1) entity that belongs one layer above. (The service provided by the entity of the N-th layer is called the (N) service.) Moreover, the protocol present between the (N) entities belonging to the same level of the transmitting side and the receiving side is called the (N) protocol. Data is exchanged between the (N) entities by using an (N) connection, which is a logical communication channel.

The service provided by each layer of OSI and the specific functions are as described below.

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