The presentation layer has three primary functions:
Formatting, or presenting, data at the source device into a compatible form for receipt by the destination device
Compressing data in a way that can be decompressed by the destination device
Encrypting data for transmission and decrypting data upon receipt
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). Some 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.
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.