Layers of OSI Model

In this article we will discuss about the OSI model in details .

History 

 The OSI model was developed in the late 1970s to support the computer networking methods that were competing for application in the large national networking efforts in France, the United Kingdom, and the United States. In the 1980s, the model became a product of the Open Systems Interconnection. This model is developed by ISO (‘International Organization of Standardization).

Full form of OSI is Open system interconnection.

OSI reference model consist seven layer each of the layer have their own functions. So let us discuss in details about the OSI model. 

7 layer of OSI Model are given below

1. Physical Layer
2. Data Link Layer
3. Network Layer
4. Transport Layer
5. Session Layer
6. Presentation Layer
7. Application Layer

Transport layer is heart of OSI model. 

Physical Layer

The physical layer is responsible for the transmission of unstructured raw data between a device and a physical transmission medium. It converts the digital data into electrical, radio, or optical signals. Layer specifications define characteristics such as voltage levels, physical data rates, maximum transmission distances, modulation scheme, channel access method and physical connectors.

The functions of the physical layer are :

Bit synchronization: The physical layer provides the synchronization of the data by providing a clock. This clock controls both sender and receiver and providing synchronization at data level.

Bit rate control: The Physical layer also defines the Baud  rate i.e. the number of bits sent per second.

Physical topologies: Physical layer specifies the path in which the different, devices/nodes are arranged in a network i.e. bus, star or mesh topology.

Transmission mode: Physical layer also defines the path in which the data flows between the two connected devices. The various transmission modes possible are: Simplex, half-duplex and full-duplex.

Data Link Layer

The data link layer provides node-to-node data transfer—a link between two directly connected network. It detects and corrects errors that may occur in the physical layer. It defines the protocol to establish and terminate a connection between two physically connected devices. It also defines the rules for flow control between them.

Functions of this layer is given below

Flow Control: The data rate of data transfer must be constant on both sides else the data may get corrupted thus , flow control coordinates that amount of data that can be sent before receiving acknowledgement.

Access control: When a single communication channel is shared by multiple devices, MAC sub-layer of data link layer helps to determine which device has control over the channel at a given time.

Framing: Framing is a function of the data link layer. It provides a path for a sender to transmit a set of data that are meaningful to the receiver. This can be accomplished by attaching special bits to the beginning and end of the frame.

1. Physical addressing: After creating frames, Data link layer adds physical addresses (MAC address) of sender and/or receiver in the header of each frame.

Network Layer

The network layer provides the functional means of transferring packets from one node to another connected in "different networks". A network is a medium to which many nodes can be connected, on which every node has an address and which allow nodes connected to it to transfer data to other nodes connected to it by providing the content of a data and the address of the destination node and letting the network find the way to deliver the data to the destination node, possibly routing it through intermediate nodes. If the data is too large to be transmitted from one node to another on the data link layer between those nodes, the network may implement message delivery by splitting the message into several fragments at one node, sending the fragments independently, and reassembling the fragments at another node. 

This layer also determine the routing of the data and also responsible for local addressing.

Transport Layer (Layer 4) :

Transport layer provides services to application layer and takes services from network layer. The data in the transport layer is termed as Segments.

The transport layer also provides the message of the successful data transmission and sends the next data if no errors occurred. The transport layer creates segments out of the data received from the application layer. Segmentation is the process of dividing a long data into smaller data. Transport layer also control the flow of data. This layer is also called as heart of OSI model.

Session Layer

The session layer controls the dialogues between computers. It manages and terminates the connections between the local and remote application.

Functions of Session Layer

Session establishment, maintenance and termination: The layer allows the two processes to done, use and terminate a connection.

Synchronization : This layer allows a process to add checkpoints which are considered as synchronization points into the message. These synchronization point help to identify the error so that the data is re-synchronized properly, and ends of the data are not cut prematurely and data loss is avoided.

Dialog Controller : The session layer allows two systems to start communication with each other in half-duplex mode or full-duplex mode.

Presentation Layer

Presentation layer is also called the Translation layer. This layer is do following functions.

Translation : For example, Low to High Level

Encryption/ Decryption : Data encryption convert the data into another form or code. The data i.e. encrypted  is known as the cipher text and the decrypted data is known as plain text. A key value used for encrypting as well as decrypting data.

Compression: Reduces the number of bits or data that need to be transmitted on the network.

Application Layer

 The application layer is the OSI layer closest to the end user, which means both the OSI application layer and the user interact directly with the software application. This layer interacts with software applications that implement a communicating component. Such application programs fall outside the scope of the OSI model. Application-layer functions typically include identifying communication partners, determining resource availability, and synchronizing communication

This layer also serves as a window for the application services to access the network and for displaying the received information to the user.