OSI Reference Model

Most network protocol suites are viewed as structured in layers. The International Organization for Standardization (ISO) designed the Open Systems Interconnection (OSI) Reference Model that uses structured layers. The OSI model describes a structure with seven layers for network activities. Each layer associates one or more protocols with the layer. The layers represent data transfer operations common to all types of data transfers among cooperating networks.

The OSI Reference Model lists the protocol layers from the top (layer 7) to the bottom (layer 1). The following table shows the model.

Table 2–1 Open Systems Interconnection Reference Model

Layer No.	Layer Name	Description
7	Application	Consists of standard communication services and applications that everyone can use.
6	Presentation	Ensures that information is delivered to the receiving machine in a form that the machine can understand.
5	Session	Manages the connections and terminations between cooperating computers.
4	Transport	Manages the transfer of data. Also assures that the received data are identical to the transmitted data.
3	Network	Manages data addressing and delivery between networks.
2	Data Link	Handles the transfer of data across the network media.
1	Physical	Defines the characteristics of the network hardware.

The OSI model defines conceptual operations that are not unique to any particular network protocol suite. For example, the OSI network protocol suite implements all seven layers of the OSI Reference Model. TCP/IP uses some of OSI model layers. TCP/IP also combines other layers. Other network protocols, such as SNA, add an eighth layer.