Page 26 - Handout Computer Network.

The OSI reference model is considered the best tool available for teaching people about sending and
receiving data on a network. The OSI reference model defines the network functions that occur at
each layer.

More importantly, it is a framework that facilitates an understanding of how information
travels throughout a network. In addition, the OSI reference model describes how information, or
data packets, travels from application programs (such as spreadsheets and documents) through a
network medium (such as wires) to another application program that is located in another computer
on a network, even if the sender and receiver have different types of network media.
The OSI reference model has seven numbered layers, each of which illustrates a particular
network function:

• Layer 7—Application layer
• Layer 6—Presentation layer

• Layer 5—Session layer

• Layer 4—Transport layer
• Layer 3—Network layer

• Layer 2—Data link layer
• Layer 1—Physical layer

This separation of networking functions is called layering.

Dividing the network into seven layers provides the following advantages:

• It breaks network communication into smaller, simpler parts.

• It standardizes network components to allow multiple-vendor development and
support.
• It allows different types of network hardware and software to communicate.

• It prevents changes in one layer from affecting the other layers so that they can be
developed more quickly.
• It breaks network communication into smaller components to make learning easier.

By working through the layers of the OSI reference model, you will understand how data
packets travel through a network and what devices operate at each layer. As a result, you will
understand how to troubleshoot network problems if they occur during data packet flow.
OSI Layers and Functions Each OSI layer has a set of functions that it must perform for data
packets to travel from a source to a destination on a network. The following sections briefly describe
each layer in the OSI reference model.2.7.2.1 Layer 7: The Application Layer
The application layer is the OSI layer that is closest to the user. It provides network services to
the user’s applications. It differs from the other layers in that it does not provide services to any other
OSI layer; instead, it provides services only to applications outside the OSI model. Examples of such
applications are spreadsheet programs and word-processing programs. The application layer

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