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 the following table.
Table 4-1 The Open Systems Interconnect Reference Model
Layer No. |
Layer Name |
Description |
---|---|---|
7 |
Consists of standard communication services and applications that everyone can use |
|
6 |
Ensures that information is delivered to the receiving machine in a form that it can understand |
|
5 |
Manages the connections and terminations between cooperating computers |
|
4 |
Manages the transfer of data and assures that received and transmitted data are identical |
|
3 |
Manages data addressing and delivery between networks |
|
2 |
Handles the transfer of data across the network media |
|
1 |
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.