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Networking tutorial – part 3/the remaining OSI layers March 1, 2011

Posted by TelUS Consulting Services in Data Networking catagory.

Session Layer (Layer 5)

The session layer is responsible for managing and controlling the synchronization of data between applications on two devices. It does this by establishing, maintaining, and breaking sessions. Whereas the transport layer is responsible for setting up and maintaining the connection between the two nodes, the session layer performs the same function on behalf of the application.

Presentation Layer (Layer 6)

The presentation layer’s basic function is to convert the data intended for or received from the application layer into another format. Such conversion is necessary because of the way in which data is formatted, so it can be transported across the network. This conversion is not necessarily readable by applications. Some common data formats handled by the presentation layer include the following:

  • Graphics files JPEG, TIFF, GIF, and so on are graphics file formats that require the data to be formatted in a certain way.
  • Text and data The presentation layer can translate data into different formats such as American Standard Code for Information Interchange (ASCII) and the Extended Binary Coded Decimal Interchange Code (EBCDIC).
  • Sound/video MPEGs, QuickTime video, and MIDI files all have their own data formats to and from which data must be converted.

Another very important function of the presentation layer is encryption, which is the scrambling of data so that it can’t be read by anyone other than the intended recipient. Given the basic role of the presentation layerthat of data-format translatorit is the obvious place for encryption and decryption to take place.

Application Layer (Layer 7)

In simple terms, the function of the application layer is to take requests and data from the users and pass them to the lower layers of the OSI model. Incoming information is passed to the application layer, which then displays the information to the users. Some of the most basic application-layer services include file and print capabilities.

The most common misconception about the application layer is that it represents applications that are used on a system such as a Web browser, word processor, or a spreadsheet. Instead, the application layer defines the processes that enable applications to use network services. For example, if an application needs to open a file from a network drive, the functionality is provided by components that reside at the application layer.

When you have an understanding of the OSI model, it is possible to relate network connectivity devices to the appropriate layer of the OSI model. Knowing at which OSI level a device operates allows you to better understand how it functions on the network. The table below identifies various network devices and maps them to the OSI model.

Device OSI Layer
Hub Physical (Layer 1)
Switch Data-link (Layer 2)
Bridge Data-link (Layer 2)
Router Network (Layer 3)
NIC Data-link (Layer 2)
WAP Data-link (Layer 2)

You might find yourself working with a number of protocols in today’s networked environments. The primary function of these protocols is to facilitate communication between network devices. In my next blog I will review the main characteristics of the most widely used protocols.

Joe Buck, NCE

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