Responsibilities of different layers in the OSI model




Physical Layer:


1-     Establishment, maintenance and termination of point-to-point physical connections.

2-     Transmits row bits.



Data Link Layer:


1-     Provides protocols that deliver reliability for point-to-point phyiscal connections to upper layers.

2-     Organizes bit streams into structured frames with addressing and error checking information.

3-     Provides error detection, notification and recovery. (Error Control)

4-     Transmits frames sequentially.

5-     Processes Acknowledgements.

6-     Recognizes frame boundaries.

7-     Distinguishes between duplicate frames.

8-     Regulates between fast sender and slow receiver. (Flow Control)


Two sublayers:

1-     Media Access Control (MAC): Interfaces with Physical Layer.

2-     Logical Link Control (LLC): Interfaces with Network Layer.



Network Layer:


1-     Establishment, maintenance and termination of End-to-end network links.

2-     Responsible for addressing schemes.

3-     Routing and Internetwork routing.

4-     Recognizes frames as packets.

5-     Counting how many packets are sent (billing).

6-     Congestion control.

7-     Allows heterogeneous interconnections.

This layer provides upper layers with independence from transimission and switching issues.



Transport Layer:


1-     Responsible for providing reliability for the end-to-end network layer connections.

2-     End-to-end error recovery and flow control.

3-     Provides mechanism for sequentially organizing multiple network layer packets into a coherent message.

4-     In multiprogramming multiprocessor environments it is responsible for telling which message to which processor.

5-     Establishes and deletes connetion by naming.



Session Layer:


1-     Establishing, maintaining and terminating sessions between user application programs.

2-     User gives the remote address he wants to connect to. It is the user interface to the network.

3-     Token management.

4-     Synchronization.

5-     Allows user to remotely log on.



Presentation Layer:


1-     Interface between user applications and various presentation-related services required by those applications, like: Data encryption, decryption, compression, reformatting and encoding.

2-     Conversion between different kinds of terminals.



Application Layer:


1-     Provides utilities that support end-user application programs, like FTP, some email basic utilities and protocols.

2-     Provides virtual terminal.