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The OSI Model and TCP/IP

The International Standards Organization and the International Telecommunication Union has developed a seven layer model for network protocols entitled the Open Systems Interconnection (OSI) model. Whilst the dominant four-layer Transmission Control Protocol and Internet Protocol (TCP/IP) was already in existence when OSI was introduced and doesn't strictly conform to the OSI model, there are many similarities between the two.

The following provides a graphic illustration of the two models.

The Physical Layer in the OSI model converts the data to and from the electrical signals. The Data Link Layer provides an interface with the network adapter and logical links for the subnet. The Network Layer manages logical addressing and routing. The Transport Layer provides flow control and error checking. The Session Layer establishes sessions between the computers and shared applications. The Presentation Layer translates data to a standard format and manages encryption and data compression. Finally, the Application layer provides an interface for network applications.

In the TCP/IP model, the Network Access Layer provides an interface wit the physical network, formats the data for transmission and addresses data for the subnet based on the physical hardware addresses along with error checking. The Internet Layer provides logical addressing so that data can move through different physical architectures, along with routing between the internetwork (i.e., an interconnected network of LANs) and relating physical addresses with logical addresses.