CertiGuide to A+ (A+ 4 Real)  9  Chapter 13: Basic Networking Terminology       9  OSI Reference Model and Networking Protocols and Technologies

Layer 5 - Session Layer 1 Encryption

The presentation layer is responsible for presenting data in a form that the receiving device can understand. The presentation layer serves as the translator for devices that need to communicate over a network. Layer 6 provides three main functions. These functions are:

• Data formatting
  
  
• Data encryption
  
  
• Data compression

After receiving data from the application layer, the presentation layer performs one, or all, of its functions on the data before it sends it to the session layer. At the receiving station, the presentation layer takes the data from the session layer and performs the required functions before passing it to the application layer.

To understand how data formatting works, imagine two dissimilar systems. The first system uses Extended Binary Coded Decimal Interchange Code (EBCDIC) to represent characters onscreen. The second system uses American Standard Code for Information Interchange (ASCII) for the same function. (Note: Most personal computers use ASCII, while mainframe computers traditionally use EBCDIC.) Layer 6 provides the translation between these two different types of codes.

Layer 6 standards also determine how graphic images are presented. Three of these standards are as follows:

• PICT is a picture format used to transfer QuickDraw graphics between programs on the MAC operating system
  
  
• TIFF (Tagged Image File Format) - a format for high-resolution, bit-mapped images
  
  
• JPEG (Joint Photographic Experts Group) - graphic format used most often to compress still images of complex pictures and photographs

Other Layer 6 standards guide the presentation of sound and movies. Included in these standards are the following:

• MIDI (Musical Instrument Digital Interface) - for digitized music
  
  
• MPEG (Motion Picture Experts Group) is a standard for the compression and coding of motion video for CDs and digital storage
  
  
• QuickTime is a standard that handles audio and video for programs on both MAC and PC operating system.

Layer 5 - Session Layer 1 Encryption