OSI Model its 7 Layers and Protocols

7 Layers in OSI Model

The OSI model is a conceptual model for understanding the communication between applications over a network. OSI stands for Open System Interconnection. It was developed by ISO - International Organisation of  Standardization in the year 1974. The main purpose of the OSI model is to guide the developers and vendors that how their application is going to communicate or interact with internet, what protocols it is going to use to communicate. It is a 7 layer architecture with each layer perform a specific functionality. All these layers work together to help in communication between applications. The layers starts from end.

Layer 1: Physical Layer

Physical layer consist of data in form of bits 0s and 1s with no interpretation. To transmit the data bits must be encoded into signals either electrical or optical. The physical layer defines how 1s and 0s are encoded into signals so that they can be transmitted. 
Physical layer also defines the data transmission rate and transmission medium between two devices.

Layer 2: Data Link Layer

Data Link layer is responsible for framing of data bits received from into manageable data units. It is also responsible for moving of frames from one node to another.
Error detection is also done in this layer, flow of data is also managed in this layer.

Layer 3: Network Layer

Network layer helps in delivery of data packets from source to the destination. Logical addressing and routing are two main functionalities done at this layer. This layer defines the route of the packets travelling to the destination, the packets follows the defined route.

Layer 4: Transport Layer

As the network layer routes the travelling pattern of data, this layer ensures that the message transmitted reaches the destination in order and without any errors. The main functionalities of this layer is to transmitting data/message from source to destination in specific order and to specific process of receiver, error control, flow control of data, connection control.

Layer 5: Session Layer

Session layer is responsible for maintaning and synchronizing the communication between the systems. It allows the communication between two processes either in half duplex or in full duplex mode.
It allows a process to add checkpoints to the stream of data so that if a system is sending 1000 pages then it can add checkpoints after every 100 page and if any error occure between sending 534th page then the system will rollback to the checkpoint and start sending data from there only, not from starting .

Layer 6: Presentation Layer

Presentation layer is responsible for encryption, compression and translation of data/message send by the sender.
Encryption means transforming the original data into another form mainly in hash form.  
Translation means decoding of the data that was encoded in the previous layer. It translates the data to the previous from in which the sender sended.
Compression to compress data by reducing the number of bits in data particularly in audio, video and text messages.

Layer 7: Application Layer

As the name of this layer suggests, the layer involves an application with which user or any software connects with internet. Application layer's main function is to provide services to the user like it provides user interface and supports e-mail, file transfer, DBMS and other distributed system services.