OSI Reference Model

Most network protocol suites are viewed as structured in layers. This is a result of the Open Systems Interconnect (OSI) Reference Model designed by the International Standards Organization (ISO). The OSI model describes network activities as having a structure of seven layers, each of which has one or more protocols associated with it. The layers represent data transfer operations common to all types of data transfers among cooperating networks.

The protocol layers of the OSI Reference Model are traditionally listed from the top (layer 7) to the bottom (layer 1) up, as shown in Table 2-1.

Table 2-1 The Open Systems Interconnect 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 it can understand.
5	Session	Manages the connections and terminations between cooperating computers.
4	Transport	Manages the transfer of data and assures that received and transmitted data are identical.
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 operations defined by the OSI model are conceptual and 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 and combines others. Other network protocols, such as SNA, add an eighth layer.