Understanding the OSI Model

In the late 1970s the International Standards Organization (ISO) began to develop a conceptual model for network communications called the Open Systems Interconnection Reference Model. It is commonly known as the OSI model .

In 1984 the model became the international standard for network communications, providing a conceptual framework that describes network communication as a series of seven layers. In the model, each layer is responsible for a different part of the process that takes place when two computers on a network establish a connection and move data between them. Figure 5.1 shows the layers of the OSI model. Table 5.1 provides a brief description of the role of each layer.

Figure 5.1. The OSI model is used to describe how a protocol stack initiates and controls network communication.


The OSI model is numbered from the bottom up. You will find, depending on what book or resource you are consulting, that the model layers are not always referred to by name . For example, the Network layer, which is the third from the bottom, is often referred to as layer 3 .

Table 5.1. The OSI Layers

Layer Number





Provides the interface and services that support user applications and provides general access to the network



Serves as the translator layer of the OSI model and is responsible for data conversion and encryption



Establishes and maintains the communication link between the sending and receiving nodes



Responsible for end-to-end data transmission, flow control, error checking, and recovery



Provides the logical addressing system used to route data on the network


Data Link

Responsible for the framing of data packets and the movement of the data across the physical link



Manages the process of sending and receiving bits over the physical network media (the wire and other physical devices)

Absolute Beginner's Guide to Networking
Absolute Beginners Guide to Networking (4th Edition)
ISBN: 0789729113
EAN: 2147483647
Year: 2002
Pages: 188
Authors: Joe Habraken

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