OSI stands for Open Systems Interconnection. It was created in 1984 by ISO –’International Organization for Standardization. ‘ It has seven layers. Each layer performs a different function. All seven layers transmit the data from one person to the other across the globe.

1. Physical Layer:

The physical layer, the lowest layer of OSI’s reference model, is the physical layer. It is responsible for the physical connection between all devices. The physical layer holds information in the form of bits. It can transmit individual bits from one node. The functions of this physical layer are:

Bit synchronization The physical layers provide the synchronization for bits through a clock. This clock is used to control both sender & receiver, thus synchronizing bits at bit level.

Bit rate controls The Physical Layer also determines the transmission rates, i.e., The number and speed of bits being sent each second.

Physical topologies This layer specifies how different devices/nodes in a network are arranged. Bus, star, mesh topology.

Transmission method A physical layer controls how data flows between the connected devices. There are three possible transmission modes: Simplex and half-duplex.

2. Data Link Layer:

The data link layer ensures the message is delivered from one node to the next. This layer has the primary function of providing that data transfers from one node to another are free of errors over the physical layer. Data Link Layer is broken into two sublayers.

  • Logical Link Control

  • Media Access Control

Data Link layer’s functions include:

Framing can be described as a function of the layer that links data. It allows senders to transmit bits that are meaningful to receivers.

Physical Address: When creating frames, the Data Link layer adds physical addresses (MAC addresses) to each frame’s header.

Error control: Data layer provides the error control mechanism. It detects frames that are damaged or lost and retransmits them.

Flow Control: Both sides must maintain a constant data rate, or the data might get corrupted. Therefore, flow control coordinates how much data can be sent to each other before the acknowledgment.

Access Control: If multiple devices use a single communication channel, the MAC sublayer of the data layer allows them to identify which device is in control.

3. Network Level:

It is responsible for transmitting data between hosts located in different networks. It also oversees packet routing.  The header is created by the network layer and includes the IP addresses for the sender/receiver.

The Network layer performs the following functions:

  1. Routing Routes: The most appropriate route from the source to the destination depends on which network layer protocol. This function is known as routing.

  2. Logical addressing: A network layer determines an addressing scheme to identify each device on the internetwork uniquely. The network layer inserts the sender and recipient’s IP addresses into the header.

4. Transport layer:

The transport layer services the application layer and receives services from the network layer. The Segments data is stored in the transport layers. The transport layer acknowledges the successful transmission of data and retransmits it if there is an error.

On the sender’s end: The transport layer receives formatted data and performs Segmentation. Flow control is also implemented to ensure data transmission. It also adds Destination and Source port numbers in its header and forwards segments to the Network Layer.

Transportation Layer reads its header to determine the port number and forwards the data it has received back to the application at the receiver’s side. It also performs sequencing, reassembling, and decomposition of segmented information.

The transport layer performs the following functions:

  • Segmentation, Reassembly, and Acceptance: This layer splits the message from the (session) layer into smaller units. Each segment has a header. Service Point Addressing: The transport layer header includes a type of address, called a service point or port address, to deliver the message correctly.

5. Session Layer:

This layer establishes a connection, manages sessions, authenticates, and ensures security.

The session layer functions are:

  • Session setup, maintenance, and termination: This layer allows two processes to create, use, and terminate a link.

  • Synchronization: This layer lets you add checkpoints to the data considered synchronization factors. These synchronization factors help identify an error and ensure that the data is adequately resynchronized.

  • Dialogue Controller: The session layers allow two systems to communicate in half-duplex or full-duplex.

6. Presentation layer:

The presentation layer is also known as the Translation Layer. The data extracted from the application layer can be manipulated to send it over the network.

The functions of the presentation layer include:

  • Translation: Using ASCII or EBCDIC as an example.

  • Decryption/ Encryption: Data encrypted transforms the data into a different form or code. The encrypted data is called the cipher text, and the decrypted is known simply as plain text. For both encryption and decryption, a key-value is required.

  • Compression: Reducing the number of bits needed to be transmitted over the network.

7. Application Layer:

The OSI Reference Model stack contains the Application layer, where the network applications are implemented. These applications create the data which must be transferred over the Internet

The Functions of the Application Layer are:

OSI model serves as a reference and is not being implemented on the Internet due to its late inception. TCP/IP is the current model.