Layer 6 Presentation Layer

Overview Now that you have learned about Layer 5 of the OSI model, it is time to look at Layer 6, the presentation layer. This layer is typically a pass-through protocol for information from adjacent layers. It allows communication between applications on diverse computer systems in a manner that's transparent to the applications.

Overview The presentation layer is concerned with the format and representation of data. If necessary, this layer can translate between different data formats. In this chapter, you will learn how the presentation layer provides code formatting and conversion, which is used to make sure that applications have meaningful information to process. Layer 6 is also concerned with the data structures that are used by applications. To better understand this, you will learn how Layer 6 arranges and organizes data before it is transferred.

The Presentation Layer : Presentation layer functions and standards 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.

The Presentation Layer : Presentation layer functions and standards Layer 6, the presentation layer, provides three main functions. Those functions are: – data formatting (presentation) – data encryption – data compression

The Presentation Layer : Presentation layer functions and standards 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.

The Presentation Layer : Presentation layer functions and standards 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

The Presentation Layer : Presentation layer functions and standards Layer 6 standards also determine how graphic images are presented. Three of these standards are as follows: – PICT - 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) - standard for the compression and coding of motion video for CDs and digital storage – QuickTime - a standard that handles audio and video for programs on a MAC operating system The Presentation Layer : Presentation layer functions and standards

The Presentation Layer : File formats ASCII and EBCDIC are used to format text. ASCII text files contain simple character data, and lack any sophisticated formatting commands, such as boldface or underline. Notepad is an example of an application that uses and creates text files.

The Presentation Layer : File formats They usually have the extension.txt. 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

The Presentation Layer : File formats Another common file format is the binary format. 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. Networks use many different types of files. A previous section briefly touched on graphic file formats.

The Presentation Layer : File formats The Internet uses two binary 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.

The Presentation Layer : File formats Some programs can read multiple image types as well as convert files from one type to another. Web browsers have the ability to display graphic files in either of these two formats without any additional software

The Presentation Layer : File formats The multimedia file format is another type of binary file, which stores sounds, music, and video. Sound files generally operate in one of two ways. They may be completely downloaded, first, and then played, or they may download while they are playing.

The Presentation Layer : File formats The latter method is referred to as 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.

The Presentation Layer : File 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 to hyperlink to another URL. HTML is not a programming language, but is a set of directions for displaying a page

The Presentation Layer : Data encryption and compression Layer 6 is also responsible for data encryption. Data encryption protects information during its transmission. 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 : Data encryption and compression 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 files. The algorithm searches each 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.

Summary Determines how graphic images, sound and movies are presented Provides encryption of data Compresses text and converts graphic images into bit streams so they can be transmitted across a network