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.


• Layer 6, the presentation layer, provides three main functions. Those functions are:– data formatting (presentation)– 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 - 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 : 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.


• 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


• 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 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.


• 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 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 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.


• 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

Layer 6 Presentation Layer

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