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Chapter 6 Internet Technology
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BITS 2513 – Internet BITS 2513 – Internet TechnologyTechnology
Lecture 6: Lecture 6: Presentation LayerPresentation Layer
Topics CoveredTopics CoveredIntroduction to Presentation
LayerData RepresentationData Compression
Image Audio Video
Encryption
Presentation LayerPresentation LayerThe presentation layer is the sixth
level of the seven layer OSI model. It responds to service requests
from the application layer and issues service requests to the session layer.
Concerned with syntax and semantics of the information exchanged between two systems.
Presentation LayerPresentation Layer
Data Data
From application layer To application layer
From session layerTo session layer
Presentation layer Presentation layer
Presentation FormatPresentation FormatIn the case of the sender and receiver
seeing the same data, the issue is one of agreeing to a message format, called a presentation format.
The presentation layer may represent (encode) the data in various ways (e.g., data compression, or encryption), but the receiving peer will convert the encoding back into its original meaning (decode).
Encoding/DecodingEncoding/DecodingThe sending program translates the
data it wants to transmit from the representation it uses internally into a message that can be transmitted over the network→ the data is encoded in a message.
On the receiving side, the application translates this arriving message into a representation it can process → the message is decoded.
Presentation formattingPresentation formatting
Application
data
Presentation
encoding
Application
data
Presentation
decoding
Message Message Message■ ■ ■
Presentation formatting involves encoding and decoding application data.
Presentation LayerPresentation LayerSpecific responsibilities of
presentation layer: Data Representation Data Compression Encryption
DATA DATA REPRESENTATIONREPRESENTATION
Data RepresentationData Representation
Different computers have different representations for characters.
If two dissimilar computers are to exchange text, say e-mail, they must agree on the representation to be used in the exchange. Then one must translate from, or into, the agreed upon representation.
Data RepresentationData RepresentationConverting the complex data
structures used by an application (strings, integers, structures, etc. ) into a byte stream transmitted across the network.
Representing information in such a way that communicating peers agree to the format of the data being exchanged.E.g., How many bits does an integer contain?, ASCII or EBCDIC character set?
Data RepresentationData RepresentationPopular network data
representations include:◦ASN.1 - an ISO standard◦XDR - used with SunRPC
ASN.1ASN.1Abstract Syntax Notation (ASN.1) is
an ISO standard that addresses the issue of representing, encoding, transmitting, and decoding data structures. It consists of two parts:1. An abstract syntax that describes data
structures in an unambiguous way. Use “ integers”, “character strings”, and “structures” rather than bits and bytes.
2. A transfer syntax that describes the bit stream encoding of ASN.1 data objects.
ASN.1ASN.1The main reasons for the success of ASN.1 is
that it is associated with several standardized encoding rules such as:◦ Basic Encoding Rules (BER) - X.209◦ Canonical Encoding Rules (CER)◦ Distinguished Encoding Rules (DER)◦ Packed Encoding Rules (PER) and◦ XER Encoding Rules (XER).
These encoding rules describe how the values defined in ASN.1 should be encoded for transmission, regardless of machine, programming language, or how it is represented in an application program.
ASN.1ASN.1Example of ASN.1’S abstract
syntax:
Student ::= SEQUENCE {name [0] IMPLICIT OCTET STRING OPTIONAL,grad [1] IMPLICIT BOOLEAN OPTIONAL
DEFAULT FALSE,gpa [2] IMPLICIT REAL OPTIONAL,id [3] IMPLICIT INTEGER,bday [4] IMPLICIT OCTET STRING OPTIONAL}
ASN.1ASN.1ASN.1 is currently used in ISO
protocol suite such as X.400 for electronic mail, X.500 for directory services, H.323 (VoIP) and SNMP
XDRXDRSun Microsystem's External Data
Representation (XDR) is much simpler than ASN.1, but less powerful. For instance:1. XDR uses implicit typing. Communicating
peers must know the type of any exchanged data. In contrast, ASN.1 uses explicit typing; it includes type information as part of the transfer syntax.
2. In XDR, all data is transferred in units of 4 bytes. Numbers are transferred in network order, most significant byte first.
XDRXDR4 bytes of XDR message:
XDR (cont.)XDR (cont.)3. Strings consist of a 4 byte length,
followed by the data (and perhaps padding in the last byte).
4. Defined types include: integer, enumeration, boolean, floating point, fixed length array, structures, plus others.
One advantage that XDR has over ASN.1 is that current implementations of ASN.1 execute significantly slower than XDR.
DATA DATA COMPRESSIONCOMPRESSION
Data CompressionData CompressionReduces the number of bits contained
in the information. Lossless Compression -- data is
compressed and can be uncompressed without loss of information. These are referred to as bit-preserving or reversible compression systems.
Lossy Compression – aim to obtain the best possible fidelity for a given bit-rate or minimizing the bit-rate to achieve a given fidelity measure. Most suited to video and audio compression techniques
Why Compression?Why Compression?Sometimes programs need to send
more data in a timely fashion than the bandwidth of the network supports. (For example, a video stream that needs 10Mbps to transmit on a 1 Mbps network).
It’s hard to move data on the Internet at >1Mbps.
The Internet does not allow applications to use more than their “fair share” of the bandwidth on a congested link.
Need to compress the data at the sender and decompress it at the receiver.
Why Compression?Why Compression?In terms of storage, the capacity of a
storage device can be effectively increased with methods that compresses a body of data on its way to a storage device and decompresses it when it is retrieved.
In terms of communications, the bandwidth of a digital communication link can be effectively increased by compressing data at the sending end and decompressing data at the receiving end.
Steps of Data Steps of Data CompressionCompressionPerformed in the compression of still
images, audio and video data streams:1.Picture preparation – generates an
appropriate digital representation if the information in the medium being compressed.
2.Picture processing –make use of the various compression algorithms
3.Quantization – values determined in previous step are quantized according to a specific resolution and characteristic curve.
4.Entropy encoding – with a sequential of individual bits and bytes, different techniques are used to perform a final, lossless compression
Steps of Data Steps of Data CompressionCompression
Uncompressed Picture
Picture Preparation
Picture Processing
Quantization
Entropy Encoding
Compressed Picture
Major steps of image compression, can also be applied to audio and video data
IMAGE COMPRESSIONIMAGE COMPRESSIONDATA COMPRESSION
Image CompressionImage Compressionto represent images with less
data in order to save storage costs or transmission time.
possible to reduce file size to 10% from the original without noticeable loss in quality.
Image compression can be lossless or lossy.
Image CompressionImage CompressionLossless
- Image quality is not reduced. Use in: artificial images that contain sharp-edged lines such as technical drawings, textual graphics, comics, maps or logos. Methods: run-length encoding (RLE), entropy coding (Huffman coding) and dictionary coders (LZW).
Image CompressionImage CompressionLossy
- reduces image quality. Cannot get the original image back & lose some information. Use in: natural images such as photos of landscapesMethods: discrete cosine transform (DCT, used in JPEG) or wavelet transform (used in JPEG 2000), color quantization
FORMAT FILE EXTENTI
ON
TYPE OF COMPRESSI
ON
METHODS USAGE
BMP (bitmap) .bmp Cosiderably compressed with lossless
ZIP used to store bitmap digital images
JPEG (Joint Photographic Experts Group)
.jpg , .jpeg , .jpe
Lossy
Lossless
- Discrete Cosine Transform (DCT) & Chroma Subsampling- Run-Length Encoding (RLE)
For natural images
GIF (Graphics Interchange Format
.gif , .giff , .gfa
Lossless LZW (Lempel-Ziv-Welch)
For artificial images (sharp-edge lines and few colors) & support animation
PNG (Portable Network Graphics)
.png Lossless DEFLATE Better compression & features than GIF, but don’t support animation
TIFF (Tagged Image File Format)
.tiff , .tif Lossless RLE / LZW / DEFLATE / ZIP
Flexible file format, can store multiple images in a single file
JPEG2000 jp2, .j2c, jpc, j2k, jpx
Lossy & Lossless
Discrete Wavelet Transform (DWT)
Better image quality than JPEG (up to 20%), not widely used because of some patent issues.
Comparison of graphics file formats
Block Diagram of JPEG Block Diagram of JPEG CompressionCompression
Sourceimage
JPEG compression
DCT Quantization EncodingCompressed
image
Transformation coding
performed using the Discrete
Cosine Transform (DCT)
Quantization of all DCT
coefficients ( a lossy process)
Huffman coding and arithmetic
coding as entropy encoding
methods
AUDIO COMPRESSIONAUDIO COMPRESSIONDATA COMPRESSION
Audio CompressionAudio CompressionA form of data compression
designed to reduce the size of audio files
Audio compression can be lossless or lossy
Audio compression algorithms are typically referred to as audio codecs.
Audio CompressionAudio Compression Lossless - allows one to preserve an
exact copy of one's audio filesUsage: For archival purposes, editing, audio quality.Codecs: Free Lossless Audio Codec (FLAC) Apple Lossless MPEG-4 ALS Monkey's Audio Lossless Predictive Audio Compression (LPAC) Lossless Transform Audio Compression (LTAC)
Audio CompressionAudio Compression Lossy - irreversible changes , achieves far
greater compression, use psychoacoustics to recognize that not all data in an audio stream can be perceived by the human auditory system.Usage: distribution of streaming audio, or interactive applications Codecs: MP2- MPEG-1Layer 2 audio codec MP3 – MPEG-1 Layer 3 audio codec MPC Musepack Vorbis Ogg Vorbis AAC Advanced Audio Coding (MPEG-2 and MPEG-4) WMA Windows Media Audio AC3 AC-3 or Dolby Digital A/52
MPEGMPEGStands for Moving Picture Experts Group. MPEG is
an ISO/IEC working group, established in 1988 to develop standards for digital audio and video formats.
MPEG-1Designed for up to 1.5 Mbit/secStandard for the compression of moving pictures and audio. Most popular is level 3 of MPEG-1 (MP3). MPEG-1 is the standard of compression for VideoCD.
MPEG-2Designed for between 1.5 and 15 Mbit/secStandard on which Digital Television set top boxes and DVD compression is based. Designed for the compression and transmission of digital broadcast television
MPEG (cont.)MPEG (cont.) MPEG-4
Standard for multimedia and Web compression. MPEG-4 is based on object-based compression, similar in nature to the Virtual Reality Modeling Language. It also allows developers to control objects independently in a scene, and therefore introduce interactivity.
MPEG-7 (under development) - also called the Multimedia Content Description Interface. Contrary to the previous MPEG standards, which described actual content, MPEG-7 will represent information about the content.
MPEG Audio EncodingMPEG Audio Encoding
Uncompressed Audio
Signal
Division in 32 Frequency Bands
Psychoacoustic Model
Quantization
(if applicable) Entropy Encoding
Compressed Audi o
Data
controls
Audio Compression Formats Audio Compression Formats – MP3– MP3 Creation Process Stages:
Separation into Frames Spectral Analysis “Masking” ( Psychoacoustic Models ) Processing ~ Bit Rate & Joint Stereo Compressing ( Hoffman ZIP )
CBR & VBR (Constant Bit Rate and Variable Bit Rate)
Most Popular Many codec's and players
Audio Compression Formats Audio Compression Formats – Ogg Vorbis– Ogg Vorbis Creation Process Stages:
Same as those in MP3 Improvements over MP3
Designed for Streaming Connection Real Stereo ~ No Multiplexing Multi-Channel Support (more than Mono &
Stereo) Bit Rate Scaling
Preferred VBR New Codec
Free Source Code One codec and few players
Audio Compression Formats Audio Compression Formats – WMA– WMA Advanced System Format (ASF)
including: Support for Meta Data Scripting Security Features
CBR & VBR (only after version 9) Industrial Support
Free Codec Few Players
VIDEO COMPRESSIONVIDEO COMPRESSIONDATA COMPRESSION
Video CompressionVideo CompressionStoring and transmitting uncompressed
raw video is not an efficient technique because it needs large amounts of storage and bandwidth.
DVD, DSS, and internet video, all use digital data → take a lot of space to store and large bandwidth to transmit.
Video compression technique is used to compress the data for these applications → less storage space and less bandwidth to transmit data.
Video CompressionVideo CompressionVideos are sequences of images displayed
at a high rate. Each of these images is called a frame.
Human eye can not notice small changes in the frames such as a slight difference in color.
Therefore, video compression standards do not require the encoding of all the details and some of the less important video details are lost. This is because lossy compression is used due to its ability to get very high compression ratios.
Typically 30 frames are displayed on the screen every second.
Video Compression Video Compression ProcessProcess
1. Start by encoding the first frame using a still image compression method.
2. It should then encode each successive frame by identifying the differences between the frame and its predecessor, and encoding these differences. If the frame is very different from its predecessor it should be coded independently of any other frame.
3. In the video compression literature, a frame that is coded using its predecessor is called inter frame (or just inter), while a frame that is coded independently is called intra frame (or just intra).
Video Compression Video Compression TechniquesTechniquesFlow Control and BufferingTemporal Compression Spatial Compression Discrete Cosine Transform (DCT)Vector Quantization (VQ)Fractal Compression Discrete Wavelet Transform
(DWT).
Video Compression Video Compression FormatsFormats
The ISO/IEC, or International Organization for Standardization and the International Electrotechnical Commission, have a group called the Moving Pictures Experts Group or MPEG. MPEG is responsible, for the familiar compression formats MPEG-1, MPEG-2 and MPEG-4.
The ITU-T standardizes formats for the International Telecommunications Union, a United Nations Organization. Some popular ITU-T compression formats include the H.261 and H.264 formats.
There are other compression formats, such as Intel Indeo and RealVideo (based on the ITU-T H.263 codec), AVI, DivX, Quicktime, Windows Media Video (WMV).
ENCRYPTIONENCRYPTION
EncryptionEncryption• To carry sensitive information, a
system must be able to assure privacy.• As the number of attacks increase and
as the public Internet is used to transmit private data, it is increasingly difficult to protect information.• One way to safeguard data from
attacks is encrypting the data.• Practically, encryption is suitably done
in presentation layer besides transport and physical layer.
EncryptionEncryption
Encryption – sender transform original information (plaintext) to another form (ciphertext) by a function that is parameterised by a key.
Decryption – reverses the original process to transform the message (ciphertext) back to its original form (plaintext).
EncryptionEncryption
Example of Encryption / Decryption Process
Plaintext
Plaintext
Ciphertext
Basic Terms and ConceptsBasic Terms and ConceptsCryptography – the science of transforming
information so that it is secure while it is being transmitted or stored. Data is scrambled so that it cannot viewed by unauthorized users.
Cryptosystem – a disguises message that allows only selected people to see through the disguise.
Cryptanalysis – the science of breaking a crptosystem
Key – a value that is used by an algorithm to encrypt and decrypt a message.
Cipher – an encryption/decryption algorithm tool that is used to create encrypted/decrypted text
Encrytption/Decryption Encrytption/Decryption KeysKeysSymmetric Keys – use same key
to encrypt and decrypt a message. Eg.: Data Encryption Standard (DES), Triple DES (3DES), Advanced Encryption Standard (AES)
Asymmetric Keys -2 keys are needed (public key and private key); 1 key to encrypt, another key to decrypt and vice versa. Eg.: RSA and Diffie-Hellman
How Encryption ProtectsHow Encryption ProtectsBecause cryptography is concerned
with the storage or transmission of information, five key security functions need to be fulfilled:
Protection Description
Confidentiality Allow only authorized users to access information.
Authentication Verify who the sender was and trust the sender is who they claim to be.
Integrity Trust the information has not been altered
Nonrepudiation
Ensure that the sender or receiver cannot deny that a message was sent or received.
Access Control Restrict availability to information.