OSI Model | Open System Interconnection Layers | Computer Network OSI Model

OSI ( Open System Interconnection) Model:- 

• OSI Stands for Open System Interconnection.
• OSI Model was developed by the International Organization for Standardization (ISO) in 1984.
• OSI consist of seven layers.
• Each layer performs a particular network function.

whenever we want to a data  then firstly this data is generated by Network application, and actual data is transmitted by physical layer. so at sender end data is flow from application layer to physical layer and at receiver end data is flow from physical layer to application layer(as mentioned in below figure). 

Application Layer: 

Network applications produce the data, which has to be transferred over the network. This layer also serves as a window for the application services to access the network and for displaying the received information to the user. Ex: Application – Browsers, Skype Messenger etc.

Presentation Layer:

whenever we receive the data from application layer, it is not transferable. Presentation layer firstly translate this data so Presentation layer is also called the Translation layer. The functions of the presentation layer are :

• Translation ( To make data transferable).
• Encryption/ Decryption( For data security data is encrypted and decrypted by network application).
• Compression( so that less bandwidth will require to transmit of data).
  
  

Session Layer:

This layer is responsible for establishment of connection, maintenance of sessions, authentication and also ensures security. The functions of the session layer are :

• Session Establishment
• Session Maintenance 
• Session Termination
• Synchronization

Transport Layer:

The data in the transport layer is referred to as Segments. It is responsible for the End to End Delivery of the complete message. The transport layer also provides the acknowledgement of the successful data transmission and re-transmits the data if an error is found.

At sender’s side:

Transport layer receives the formatted data from the upper layers, performs Segmentation . It also adds Source and Destination port number in its header and forwards the segmented data to the Network Layer.

At receiver’s side:

Transport Layer reads the port number from its header and forwards the Data which it has received to the respective application. The functions of the transport layer are :

• Segmentation and Reassembly

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Network Layer:

Network layer works for the transmission of data from one host to the other located in different networks. It also takes care of packet routing i.e. selection of the shortest path to transmit the packet, from the number of routes available. The sender & receiver’s IP address are placed in the header by the network layer. The functions of the Network layer are:

• Routing( Decide the best path for data transmission)
• Logical Addressing( Add IP address in data segment)

Data Link Layer (DLL):

The main function of this layer is to make sure data transfer is error-free from one node to another, over the physical layer. When a packet arrives in a network, it is the responsibility of DLL to transmit it to the Host using its MAC address.
        The packet received from Network layer is further divided into frames depending on the frame size of NIC(Network Interface Card). DLL also encapsulates Sender and Receiver’s MAC address in the header. The functions of the data Link layer are :

• Framing( convert data packet into data Frame)
• Physical addressing( Add MAC Address or Physical address in the header of data packet)
• Error control ( Use error Checksum to control Error)
• flow Control( decide frame unit flow)

Physical Layer:

The lowest layer of the OSI reference model is the physical layer. It is responsible for the actual physical connection between the devices. The physical layer contains information in the form of bits. It is responsible for transmitting individual bits from one node to the next. The functions of the physical layer are :

• Bit synchronization
• Bit rate control
• Physical topologies
• Transmission mode

Note:-

• Application layer, Presentation layer, Session layer also known as Software layer because function of these layer is done Network software application.
• Network layer, Data link layer and Physical layer also known as Hardware layer because it deal with hardware devices ( Router, switches, hub etc.) to transmit data.
• Transport layer also known as Heart of OSI Model. 

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