Presentation Layer

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

1. Integrity-Whatever message send by the sender, if  modified should be identified by the receiver.
2. Confidentiality-The data should be readable by only the intended receiver.
3.  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.