Sem 1 v2 Chapter 14: Layer 6 - The Presentation layer

Explain the presentation layer in simple terms The presentation layer is responsible for presenting data in a form that the receiving device can understand. To better understand the concept, use the analogy of two people speaking different languages. The only way for them to understand each other is to have another person translate. The presentation layer serves as the translator for devices that need to communicate over a network.

Layer 6, the presentation layer, provides three main functions. Data formatting (presentaion) 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 them to the session layer

The first system uses Extended Binary Coded Decimal Interchange Code (EBCDIC) to represent characters onscreen, and the second system uses American Standard Code Internet Interchange (ASCII). (Note: Most personal computers use ASCII, while mainframe computers traditionally use EBCDIC.) Layer 6 provides the translation between these two different types of codes that represent text.

Layer 6 standards also determine how graphic images are presented. PICT - a picture format used to transfer QuickDraw graphics between programs on the MAC operating system TIFF - a format for high resolution, bit-mapped images JPEG - Joint Photographic Experts Group format GIF - Still images Layer 6 standards guide the presentation of sound and movies. MIDI – (Musical Instrument Digital Interface) for digitized music MPEG – (Motion Picture Experts Group) standard for the compression and coding of motion video for CDs, digital storage, and bit rates up to 1.5 MBPS. QuickTime - a standard that handles audio and video for programs on a MAC operating system

ASCII and EBCDIC are used to format text. ASCII text files contain simple character data, and lack any sophisticated formatting commands that word processors would typically apply to document. Notepad is an example of an application that uses and creates text files.

EBCDIC is very similar to ASCII in that it also does not use any sophisticated formatting. The main difference between the two is that EBCDIC is primarily used on mainframes and ASCII is used on personal computers. Another common file format is binary. Binary files contain special coded data that can only be read by specific software applications. Programs such as FTP use the binary file type to transfer files.

The Internet uses two file formats to display images, Graphic Interchange Format (GIF) and Joint Photographic Experts Group (JPEG). Any computer with a reader for the GIF and JPEG file formats can read these file types, regardless of the type of computer. Readers are software programs designed to display an image of a particular file type

Binary file is the multimedia file format, which stores sounds, music, and video. Sound files operate in one of two ways. They may be completely downloaded, first, and then played, They may download while they are playing ( streaming audio). Windows uses the WAV format for sound, and the AVI format for animation files. A few of the more common video formats are MPEG, MPEG2, and Macintosh QuickTime.

Describe markup language formats Another type of file format is markup language. This format acts as a set of directions that tell a web browser how to display and manage documents. Hypertext Markup Language (HTML) is the language of the Internet. HTML directions tell a browser whether to display text, or a hyperlink to another URL. HTML is not a programming language, but a set of directions for displaying a page.

Layer 6 is responsible for data encryption. Data encryption protects information during its transmision. Financial transactions (e.g. credit card information) use encryption to protect sensitive information as it traverses the Internet. An encryption key is used to encrypt the data at its source and then to decrypt the data at its destination.

The presentation layer is also responsible for the compression of files. Compression works by using algorithms (complex mathematical formulas) to shrink the size of the file. The algorithm searches the file for repeating bit patterns, and then replaces them with a token. A token is a much shorter bit pattern that represents the long pattern. A simple analogy might be the name Cathy (the nickname), the token, to refer to anyone whose full name is Catherine. The End