Explain briefly the function of different layers which are covered under end to end layer connectivity.

Different layer are as follows:

1. Transport Layer:

 This is responsible for establishing a network independent communication path appropriate for the particular terminal elements (for example: providing the suitable data rate and end-to-end error control). Therefore, it relieves the user from being related with such details.

Within a packet switched network, the transport entity breaks-up a long user message in packets to march the network abilities. These packets are reassembled at the receiving end to reconstruct the user message.

Also End-to-end flow control and end-to-end error recovery are the functions of transport layer.

2. Session Layer:

 The session layer is used to permit users to classify themselves when waiting access to the network.

 It is related with setting up and maintaining an operational session among terminals. For example: "signing on" at the commencement of a task and "signing off" at its end.

 The chief function of the session layer is to organize several sessions among cooperating entities and perform all concerned functions as synchronization, failure control and management for the successful execution of a session.

 The other facility given by the session layer is termed as Activity management.

3. Presentation Layer:

It is related with the format of the data represented, so as to overcome difference in representation of the information as supplied to one terminal and needed at other. Their reasons are to make communication over the network machine independent.

This resolves the syntax differences with encoding data into standard abstract notation which is valid in the network. Therefore file format differences, data structure and data representations are resolved by using a standard notation.

4. Application Layer:

 As the highest layer within the OSI reference model, the application layer gives services to the users of OSI environment. The layer gives services as follows:

a. Electronic mail or message handling service

b. Directory services

c. Cost allocation

d. Determination of quality of service

e. File transfer and management

f.  Editors and terminal support services

g. Telematic services like videotext.