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)