The session layer ::
This layer offers facility to different users on different computer to establish session between them. A session allow an user to remotely log into a distant machine and transfer file between the two machines. Session layer perform token management to provide unidirectional communication. It also provides a service called synchronization.
The Presentation layer ::
This layer performs data presentation job by following syntax and semantics rules. Before presenting data to user, it transform data into their acceptable form.. we write date as dd/mm/yyyy, currency as Rs. While in western country it is written as mm/dd/yyyy and $ etc.
The application Layer ::
This layer is the nearest layer to all the network users it offers variety of protocols that are commonly needed. It helps to transfer file. Different file system have different meaning in different machines with different data format etc. When files are transferred from one machine to another with different file system, the application layer take necessary steps to resolve the abnormalities