OSI Model

Open Systems Interconnection Model : The Open Systems Interconnection (OSI) model describes seven layers that computer systems use to communicate over a network. It was the first standard model for network communications.

There are total 7 layers in the OSI Model and are divided into two parts according to their responsibilities :

1. Responsibility of The Host : Application Layer, Presentation Layer, Session Layer, Transport Layer
2. Responsibility of the Network : Network Layer, Data Link Layer, Physical Layer.

Application Layer :

1. Provide services to user
2. Network transparency, Resource allocation
3. Mail Service : (SMTP, POP3)
4. File transfer, access, and management

Presentation Layer :

1. Code Conversion
2. Encryption / Decryption
3. Compression
4. Translation

Session Layer

1. Authentication
2. Authorization
3. Session Restoration (using Session Beans : Saves the values of particular session)
4. Synchronization (Audio, Video in Webinar) : Adds some checkpoints when transmitting the data in a sequence.
5. Dialog control : Allows the communication between two processes

Transport Layer

1. End to end delivery of data (Port to Port Delivery)
2. In-order Transport of data (without any loss)
3. Error Control
4. Congestion Control
5. Flow control
6. Multiplexing / Demultiplexing
7. Segmentation and reassembly : Divides the message into multiple segments, and each segment is assigned with a sequence number that uniquely identifies each segment

Network Layer

1. Host to Host Delivery
2. Routing
3. Moving packets from source to destination
4. Adding the source and destination address to the header of the frame.

Data Link Layer

1. Hop to Hop Delivery (Node to Node)
2. Framing, Add a header to the frame that contains a destination address
3. Flow control for every single node : 1. Stop and Wait 2. Go-Back N 3. Selective Repeat
4. Providing a reliable and efficient communication between two or more devices.
5. Error Control
6. Access Control : 1. CSMA 2. ALOHA
7. Unique identification of each device that resides on a local network.

Physical Layer

1. Provides functionality to Data Link Layer
2. Provides a physical medium through which bits are transmitted
3. Establish, maintain and deactivate the physical connection.