Photoshop Designer

1 Mac 2011

Presentation Layer

Presentation Layer
•  is the sixth level of OSI model
•  sender and receiver seeing same data = presentation format.
  presentation layer may be represent(encode) the data in various way; ex: data compression,or encryption
•  convert into its original meaning






Specific Reponsibilities Of Presentation Layer

Data Representation

  • different computer=different representation for character
  • converting the complex data structures used by application into a byte stream trasmit across the network.
  • Popular network data reprentations include:
  •    • ASN.l - an ISO standard
  •    • XDR - used with SunRPC
Abstract Syntax Notation (ASN.l) that address the issue of representing, encoding , transmitting and decoding data structure
• Two part:
      1. Abstract Syntax: that describe data structure in an unambiguous way. use int; string;
      2. Transfer Syntax: that describes the bit stream encoding of ASN.l data object

The main reason for success of ASN.l is that it is associated with several standardized encoding rules such:
• Basic Encoding Rules(BER) - X.209
• Canonical Encoding Rules(CER)
• Distinguised Encoding Rules(DER)


XDR
• Sun Microsystem's External Data Representatin(XDR) is much simpler than ASN.l but less powerful.
     1. XDR uses implicit typing
     2. all data transfered in units of 4 bytes


 Data Compression

  • Lossless Compression: data is compressed and can be uncompressed
  • Lossy Compression: aim to obtain the best possible fidelity for given bit-rate. most suite to video and audio compression techniques
• Why Compression:
  • need to compress the data at the sender and decompress it at the receiver
  • storage can be efffectively increased
  • communication link can be effectively increased
step of data compression


Image Compression
 to represent image with less data in order to save storage or transmission time
 possible to reduce file size to 10% from original without noticeable loss in quantity
Lossless -image quality is not reduce: ex; drawing, graphic, logo 
              method: Run-Length Encoding(RLE), Huffman Coding
Lossy-    reduce image quality.cannot get the original image back and lose some information 
              ex: natural image such a photo landscape. method: Dicrete Cosine Transform(DCT)

Audio Compression
 a form of data compression design to reduce the size of audio files
• audio compression can be lossless or lossy compression
• audio compression algoritm are typically refered to as audio codecs
Lossless - allows one to preserve an exact copy of one's audio files
                codec: Apple Lossless, Free Lossless Audio Codec(FLAC)
                Lossless Predective Audio Compression(LPAC), (LTAC)
Lossy -     irreversible changes, achive for greater compresion
               codec: MP3, MP2, WMA


Video Compression
• storing and transmitting uncompressed raw video is not an efficient technique
  because it needs large amount of storage and bandwidth
• DVD, DSS use digital data: take a lot of space to store and large bandwidth to transmit
• less storage space=less bandwidth to transmit data
• inter frame=using its predecessor
• intra frame=coded independently




Video Compression Techniques
1. flow control and buffering
2. temporal compression
3. spatial compression
4. discrete cosine transform (DCT)

Encryption
• sender transform original information(plaintext) to another form(ciphertext)
• to carry sensitive information, a system must be abe to assure privacy
• to protect information
• done in presentation layer besides transport and physical layer


Decryption
• reverse the original process to transform the message(ciphertext back to original(plaintext)


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