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

Institute of Technology Sligo - Dept of Computing
Sem 1 Chapter 14:
Layer 6 - The Presentation layer
Institute of Technology Sligo - Dept of Computing
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.
Institute of Technology Sligo - Dept of Computing
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
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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.
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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
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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.
Institute of Technology Sligo - Dept of Computing
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.
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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
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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.
Institute of Technology Sligo - Dept of Computing
14.1.7.1. 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.
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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.
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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