Layer 6 Presentation Layer - SI-35-02

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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 highresolution, bit-mapped images
– JPEG (Joint Photographic Experts Group) - graphic
format used most often to compress still images of
complex pictures and photographs
The Presentation Layer : Presentation
layer functions and standards
• 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.
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

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