What Is The OSI Model? Definition and Layers

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Written By Nidhi Sharma

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The OSI model outlines rules that govern how devices communicate with each other in a network environment.

Open systems interconnection (OSI), is a standard model that describes the flow of information between computing devices operating in a network environment. This article will explain the OSI model, its seven layers, and its importance in today’s networked world.

What is the OSI Model?

Open systems interconnection (OSI), is a standard model that describes the flow of information between computing devices operating in a network environment. This model defines the rules and requirements that govern data communication and interoperability among different products and software within a network infrastructure. The OSI model can be divided into seven basic layers (bottom-to-top): Physical, DataLink, Network, Transport, and Session.

The International Organization for Standardization (ISO), published the OSI framework in 1984 to standardize network design. Before OSI, network architecture did not have the standard protocols required for data communication and design infrastructure.

Network administrators often found it difficult to install, configure, or set up new equipment within existing networks. Even more difficult was the integration of such devices with other networks. Administrators were now able to create a network infrastructure that allowed equipment to communicate with other universal networks using the OSI reference model.

OSI Model Layers

Each layer of the OSI model has a specific function that is essential for data flow within a network. It communicates with the layers below and above it to enable physical and virtual data communication through a network architecture. Let's start with layer 1 to layer 7.

1. Physical Layer

The physical layer of the OSI last layer manages physical hardware as well as network components like cables, switches, or routers that transmit data.

Layer 1 transmits data as ones- and zeros in the context of data. This layer takes bits from the sender, encodes them into signals, then sends them over the network. The receiver receives the decoded signal. Without layer 1, it is impossible to transmit data bits over network devices using physical media.

Key functions

  • Synchronizes data bits
  • Modulation (conversion of one signal form to another for data transmission).
  • Defines data transmission rate (bits/sec)
  • This document describes the arrangement of network devices in different topologies like bus, tree, star, or mesh.
  • Defines transmission modes like simple or half-duplex mode
  • Layer 1 transmits data bits of 0s or 1s via network systems using electrical, mechanical, and procedural interfaces.

2. Data Link Layer

The data link layer transmits data to two nodes that are either directly connected or operate over the same network architecture. This layer typically takes data packets from Layer 3 and breaks them into frames before sending them on to their destination.

Layer 2 can be divided into two layers: media access control, (MAC), and logical link control, (LLC). The MAC layer encapsulates data frames that are transmitted over the network connecting media like wires and cables. LLC manages packet retransmission in cases where data transmission is interrupted.

The Address Resolution Protocol (ARP), a well-known protocol for the data link layer, converts IP Addresses into MAC addresses to establish communications between systems whose addresses differ in bit length (32 bits vs.48 bits).

Key functions

  • Recognizes broken or lost frames and sends them back
  • Frames are used to subdivide data from layer 3 into smaller units called frames.
  • By adding the MAC addresses of the receiving and sending devices, you can update the headers of newly created frames
  • Layer 2 is responsible for establishing and terminating physical connections among participating network nodes.

3. Layer Network

The network layer allows communication between multiple networks. It receives data segments that have been sent by the layer below and then breaks them down into smaller packets on the sender side. This layer assembles the data on the receiver side.

The network layer handles routing functionality. This means that data transmissions are made by selecting the best route or path to connect different networks. This ensures data transfer is efficient. The internet protocol is used by this network layer for data delivery.

Key functions

  • Routes are handled to identify suitable routes from the sender and receiver
  • Performs logical address that assigns unique names for each device on the network
  • Layer 3 is responsible for the following: dividing the segmented data into packets that can be used to transmit data packets; reassembling them at the receiver's end, and identifying the most secure and efficient path.

4. Transport Layer

The transport layer is used to ensure safe message transfer between sender and receiver. It breaks down the data received from the layer below into smaller segments. It also reassembles data at the receiver to enable the session layer read it.

Layer 4 serves two crucial functions: flow control, and error control. Flow control is the ability to regulate data transfer speeds. This ensures that data is not sent at higher speeds to devices connected to the internet. Devices with slower connections may have difficulty handling this information. The error control function ensures that data is complete. This layer asks the system to send incomplete data back if it is possible.

Examples of the transport protocol are user datagram protocol and Transmission Control Protocol (TCP).

Key functions

  • This ensures that each message sent between the source and destination is complete
  • Maintaining proper data transmission via flow control and error management
  • Performs data segmentation, reassembling, and data fusion
  • Layer 4 is responsible for the transmission of an entire message from a sender to a receiver application.

5. Session Layer

The session layer establishes communication sessions between communicating entities. It is necessary to maintain the session at a reasonable time interval to ensure data transmission is efficient and not waste computing resources.

This OSI layer also handles data synchronization to ensure data flows are smooth. Layer 5 can be used to break down large amounts of data by adding checkpoints.

Let's take, for example, the 500-page document you wish to send to someone. This layer can add checks at 50 and 100 pages. This happens if a document transfer is disrupted by a network or system problem. After the issue with the system is fixed, the document transfer will resume from the previous checkpoint. This system saves time and does not start a new file transfer.

Key functions

  • Communication sessions are open, maintained, and closed
  • Data synchronization is made possible by adding checkpoints in data streams
  • Layer 5 establishes and maintains, synchronizes, terminates, and synchronizes sessions between end-user applications.

6. Presentation Layer

As it converts application data into network format, the presentation layer is also known as a syntax layer or translation layer. This layer encrypts and then decrypts data before it is transmitted over the network. Layer 6 for instance encrypts data at the source and then decrypts it at its destination, providing secure data transmission. This layer can compress data from layer 7 to reduce its size.

Key functions

  • Data translation is performed based on an application's data semantics
  • Secure data can be encrypted and decrypted over communication channels
  • Data compression is used to reduce the number of bits in exchanged data
  • Layer 6 ensures that all information sent is in the format requested by the receiving application.

7. Application Layer

The OSI model's top layer is the application layer. This layer establishes communication between an application on the network and the end user who uses it. It defines the protocols that allow for user interaction. Web browsers are a great example of this layer.

The software can direct data flow and present it to the user using application layer protocols. Hypertext Transfer Protocol, Simple Mail Transfer Protocol, and File Transfer Protocol are some of the most well-known protocols.

Key functions

  • The Application layer provides user interfaces (UI), which are crucial for user interaction
  • Supports remote file transfer and e-mail.
  • Layer 7 allows applications to communicate effectively across different networks and computing systems.


OSI models play a critical role in standardizing a network system and assist in solving many networking problems. This framework allows equipment manufacturers to make products that communicate with and interact with any type of software.

OSI models improve interoperability among devices and allow for smooth communication between computer networks. TCP/IP and other advanced standards may replace OSI in the future, but network administrators will likely continue to use OSI to protect their computing systems.

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