The layers 4-7 of ISO-OSI reference model communicate with peer entities in the end systems. There is no communication with entities in the intermediate systems. In this sense, layers 4-7 are often called end-to-end layers. These are Transport layer, Session layer, Presentation layer and Application layer respectively.
1. Transport Layer:
It is responsible for establishing a network independent communication path suitable for the particular terminal equipments (e.g. providing the appropriate data rate and end-to-end error control). It thus relieves the user from being concerned with such details.
In a packet switched network, the transport entity breaks up a long user message into packets to march the network capabilities. The packets are reassembled at the receiving end to reconstruct the user message.
End-to-end flow control & end-to-end error recovery are also the functions of transport layer.
2. Session Layer:
The session layer is used to allow users to identify themselves when waiting access to the network.
This is concerned with setting up and maintaining an operational session between terminals. E.g. “signing on” at the commencement of a task and “signing off” at its end.
The main function of the session layer is to organize different sessions between cooperating entities and perform all related functions like synchronization, failure management , control etc. for the successful execution of a session.
Another facility offered by the session layer is known as Activity management.
3. Presentation Layer:
This is concerned with the format of the data represented, in order to overcome difference in representation of the information as supplied to one terminal and required at the other. Its purpose is to make communication over the network machine independent.
It resolves the syntax differences by encoding data into standard abstract notation that is valid throughout the network. Thus file format differences, data representations, data structure are resolved using a standard notation.
4. Application Layer:
As the highest layer in the OSI reference model, the application layer provides services to the users of OSI environment. The layer provides following services:
1. Electronic mail or message handling service
2. Directory services
3. Cost allocation
4. Determination of quality of service
5. File transfer and management
6. Editors and terminal support services
7. Telematic services like videotext.