System Administration Guide: IP Services

Protocol Layers and the Open Systems Interconnection Model

Most network protocol suites are structured as a series of layers, sometimes collectively referred to as a protocol stack. Each layer is designed for a specific purpose. Each layer exists on both the sending and receiving systems. A specific layer on one system sends or receives exactly the same object that another system's peer process sends or receives. These activities occur independently from activities in layers above or below the layer under consideration. In essence, each layer on a system acts independently of other layers on the same system. Each layer acts in parallel with the same layer on other systems.

OSI Reference Model

Most network protocol suites are 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. One or more protocols is associated with each layer. The layers represent data transfer operations that are common to all types of data transfers among cooperating networks.

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

Table 1–1 Open Systems Interconnection Reference Model

Layer No. 

Layer Name 



Consists of standard communication services and applications that everyone can use. 


Ensures that information is delivered to the receiving system in a form that the system can understand. 


Manages the connections and terminations between cooperating systems. 


Manages the transfer of data. Also assures that the received data are identical to the transmitted data. 


Manages data addressing and delivery between networks. 

Data link

Handles the transfer of data across the network media. 


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 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.