The Presentation Layer
The presentation layer has three primary functions:
- Formats, or presents, data from the source device into a compatible form for receipt by the destination device.
- Compression of the data in a way that can be decompressed by the destination device.
- Encryption of the data for transmission and the decryption of data upon receipt by the destination.
As shown in the figure, the presentation layer formats data for the application layer and it sets standards for file formats. Some well-known standards for video include QuickTime and Motion Picture Experts Group (MPEG). QuickTime is an Apple computer specification for video and audio, and MPEG is a standard for video and audio compression and coding.
Among the well-known graphic image formats that are used on networks are Graphics Interchange Format (GIF), Joint Photographic Experts Group (JPEG), and Portable Network Graphics (PNG) format. GIF and JPEG are compression and coding standards for graphic images. PNG was designed to address some of the limitations of the GIF format and to eventually replace it.
The Session Layer
As the name implies, functions at the session layer create and maintain dialogs between source and destination applications. The session layer handles the exchange of information to initiate dialogs, keep them active, and to restart sessions that are disrupted or idle for a long period of time.