What is Presentation Layer ?

The presentation layer is concerned with the syntax and semantics of the information
exchanged between two systems. Figure shows the relationship between the presentation
layer and the application and session layers.

          The presentation layer is responsible for translation, compression, and encryption. Specific responsibilities of the presentation layer include the following:
Translation. 
          The processes (running programs) in two systems are usually exchanging information in the form of character strings, numbers, and so on. The infonnation must be changed to bit streams before being transmitted. Because different computers use different encoding systems, the presentation layer is responsible for interoperability between these different encoding methods. The presentation layer
at the sender changes the information from its sender-dependent format into a common format. The presentation layer at the receiving machine changes the common format into its receiver-dependent format.
Encryption. 
           To carry sensitive information, a system must be able to ensure privacy. Encryption means that the sender transforms the original information to another form and sends the resulting message out over the network. Decryption reverses the original process to transform the message back to its original form.
Compression. 
Data compression reduces the number of bits contained in the information. Data compression becomes particularly important in the transmission of multimedia such as text, audio, and video.