When you browse a website on the internet, your browser renders the web page on your display for you to interact with the website. But what goes on behind the scenes is completely invisible to users.
The OSI Model defines how two computers interact with each other over a network. The model defines several protocols, allowing the transfer of information in the form of packets between two systems.
This article will be diving deep into what the OSI Model really is, along with a detailed description of each layer in the model.
What Is the OSI Model?
“OSI Model” stands for the Open Systems Interconnection Model. The OSI Model was introduced by the International Standards Organization (ISO).
ISO is a multinational organization that is responsible for setting global standards. Each potential international standard passes through six stages that determine its viability in the real-life world.
The model eases out the process of networking between two systems. The OSI Model allows two computers to interact with each other without knowing the underlying architecture of the machine. Since the protocols defined in the model are infallible and are in use throughout the world, this model is the global framework for digital communication over the internet.
The Open Systems Interconnection Model has several layers, each responsible for moving information across a network. For example, the Physical layer ensures that the data is properly transferred to the next layer, which is the Data Link layer. Similarly, the Data Link layer transfers information to the next layer and so on.
Layers of the OSI Model
Although the layers of the OSI model are distinct and handle different phases in data communication, they are related in some way or another.
During the model’s design, the developers found related functions in the processes and grouped them into common layers. The OSI Model contains seven layers, each one managing a specific phase in the data transfer process.
1. Physical Layer
The Physical layer is the first and probably the most important in the OSI Model. This layer is responsible for the transfer of data over a physical medium. Functions required to carry out the transfer are also defined within this layer.
In the sender’s machine, the data arrives from the Data Link layer. The packet is transferred to the receiver’s machine through a medium. Then, the Physical layer of the receiver interprets the packet and sends it to the Data Link layer for further processing.
The Physical layer covers the following aspects of the data transfer.
- Interface and medium of transfer (Wi-Fi or Ethernet cables)
- Stream of bits (the data that needs to be transferred)
- Data transmission rate
- Transmission Mode
- Bit synchronization
2. Data Link Layer
The Physical layer transfers the data directly to another machine.
The Data Link layer carries out the task of sending the data to the specific machine in the same network, a process known as Physical Addressing. In this layer, the data sent by the Physical layer is also interpreted and converted into manageable units known as frames.
In the Data Link layer, MAC (Media Access Control) addresses identify which system requested the information from the network of multiple systems. This layer is responsible for the following functions.
- Framing
- Physical addressing
- Flow of data
- Error control
- Access control
3. Network Layer
The Data Link layer oversees the transfer of data to a machine on the same network. In contrast, the Network layer is responsible for transferring the sender packet to the receiver on different networks. If two systems are on the same network, then the Data Link layer takes care of the transfer, and the Network layer is not required.
The Network layer treats the data packets independently. The system is not concerned about the order or the relationship between these packets.
The Network layer comprises the following responsibilities.
- Logical Addressing: Since the two devices are on a different network, logical addresses become crucial for the data transfer to take place.
- Routing: The packet is sent to devices on the network known as Routers. These devices forward or route the data packet to the system that requested the information.
4. Transport Layer
The Network layer transfers the data from one system to another. But a computer also runs various programs, and each program can request information from other systems. So how does your system know which packet belongs to which program?
Enter the Transport layer. This layer is responsible for the process-to-process delivery of the information. The system segments the data packets that arrive at this layer into different units and then reassembles them according to the sequence number.
The Transport layer takes care of the following functions in the OSI model.
- Service-point Addressing (deliver the packets to the correct program)
- Segmentation and reassembly
- Flow control
- Error control
5. Session Layer
When two systems communicate, they do not perform a quick transfer and then disconnect immediately. The systems enter into a dialog and perform the necessary transmission. The Session layer is responsible for initiating a common session between two or more systems to ensure the successful transfer of data.
This layer is also responsible for synchronization and checkpoints. For example, when you download a document file containing 1000 pages, the Session layer adds a checkpoint every 100 pages to maintain efficient data communication.
If the transfer fails on page 554, then instead of starting the download from the beginning, the transfer resumes from the last checkpoint, i.e., page 500.
The Session layer comprises two functions: Dialog control, which is responsible for creating a session, and Synchronization.
6. Presentation Layer
The data packet reaches the final destination. Now what? How will the program interpret the message? The Presentation layer takes care of the message translation, syntax, and semantics of the delivered packet.
This layer consists of functions that carry out the process of recognizing which part of the data is important and which is not. The Presentation layer has the following responsibilities in the model.
- Translation: Two computers or programs share information in the form of strings. And the system converts the data into streams of bits for better understanding.
- Encryption: To ensure privacy during the communication, the Presentation layer encrypts and decrypts the data that needs to be sent.
- Compression: To carry out efficient data transmission, the information is often compressed to reduce the number of bits in the stream.
7. Application Layer
At last, the Application layer is in charge of providing user interfaces for various network services. An internet browser, mailing platform, or graphical database management tool are examples of software that form this layer.
The Application layer provides the following services to the user.
- Network Virtual Terminal: NVT is software that emulates the working of a physical terminal. Users can connect to an NVT and control the remote host from their system.
- Mailing services
- File transfer and management
- Directory services
How Does the Internet Work?
The internet also uses the OSI Model in its working. When you request a packet from a server, the data passes through each layer in the model. The OSI model has formed the fundamental base of the massive network of systems that is the Internet.
There are a lot of services going on in your system background when you are browsing the internet. Network management protocols such as DHCP are responsible for providing a lightning-fast browsing experience to the users.