MAIN FUNCTIONS OF PRESENTATION LAYER
- At sender's end, it will change the user defined format received from the application to standard format and send it to session layer
- At receiver's end, it receives the data from session layer in a standard format and it converts into user dependent format.
- It is responsible for data security and data compression.
Security Goal
- Integrity-Whatever message send by the sender, if modified should be identified by the receiver.
- Confidentiality-The data should be readable by only the intended receiver.
- Availability - The data should be available with the user at all possible time.
Data Compression
→To reduce the number of bits before transmission, is the objective of Data Compression.
→data compression can save storage, speed up transfer of data, and decrease costs for storage hardware and network bandwidth.
there are two types of Data Compression techniques as follows:
1.Lossless Compression method
→Where the entire data can be reconstructed i.e. a data compressed can be decompressed back to its original size.
→In lossless data compression, the integrity of the data is preserved.
→No part of Data is lost in the process
→All the redundant data is removed while compression and added during decompression.
There are 2 techniques:
➤Run- length encoding (RLE)
➤Huffman coding
Run- length encoding (RLE)
→It is one of the simplest method of compression.
→This reduces your number of bits to be send tremendously.
→It can compress data of any combinations and symbols.
Huffman Coding
→It offers shorter codes to symbols that occur more frequently and longer codes to symbols that occur less frequently.
→For example,(A,B,C,D,E).Before we can assign bit patterns to each characters, we assign each character, we assign each character a weight based on its frequency of use.
2.Lossy Compression Method
→Sometimes, our eyes and ears cannot distinguish between subtle changes in such case we use lossy compression method.
→Some part of the data is lost permanently
→Multimedia
→Several Methods have been developed using lossy compression method.
ex:JPEG,MPEG,MP3,etc.
