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.