What is the OSI model?

The Open Systems Interconnection (OSI) model describes how computer systems communicate across networks. Published by the International Organization for Standardization (ISO) in 1984, it divides communications into seven layers, providing a common basis for developing networking standards.

It’s a reference model, a conceptual framework that outlines the structure and relationship between a network’s components without prescribing specific implementation methods. It provides a universal language for network operators, developers, and students to understand, design and implement complex networks.

The 7 layers of the OSI model

The OSI model organises network communications into a stack of seven layers:

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

Each layer performs a specific function and interacts with the layers above and below, ensuring efficient data transmission. Typically, the layers are listed from top to bottom (7 to 1) to reflect the data flow during communication, starting from the user’s application at the Application Layer and moving down to the Physical Layer.

7 layers of the OSI model

Here’s a breakdown of the seven layers and their roles in a network.

7 layers explained

How does the OSI model work?

When data is transmitted over a network, it moves sequentially through the seven layers, from the Application Layer at the top to the Physical Layer at the bottom.

For example, here’s what happens when you send an email:

1. You write a message in an email app like Gmail or Outlook and click “send”. The email client uses a protocol like SMTP to process your message (Application Layer).
2. The email client formats your message for transmission, including encryption if enabled (Presentation Layer).
3. A session is established between your device and the email server to manage the ongoing data exchange (Session Layer).
4. The email data is divided into smaller segments for transmission, and protocols like TCP and UDP ensure they’re delivered reliably (Transport Layer).
5. The email segments are assigned source and destination IP addresses to be routed across the network (Network Layer).
6. The IP packets are encapsulated into frames to be transmitted over the local network (Data Link Layer).
7. The frames are converted into electrical, optical, or wireless signals and transmitted through a physical medium like a network cable or Wi-Fi (Physical Layer).

At the receiving end, the process is reversed: the data travels up from the Physical Layer to the Application Layer, reconstructing the email for its recipient.

OSI model vs TCP/IP model

Another significant networking framework is the Transmission Control Protocol/Internet Protocol model (TCP/IP), also known as the Internet Protocol Suite. Unlike the OSI model, which is primarily theoretical, TCP/IP is a practical framework designed specifically for implementing internet communications.

Initially developed for the US military’s ARPANET, the forerunner of the internet, the TCP/IP model consists of just four layers:

1. The Application Layer combines the functions of OSI’s Application, Presentation and Session layers, managing high-level protocols and user interactions.
2. The Transport Layer uses protocols like TCP and UDP to ensure data transfer between end systems, like OSI’s Transport Layer.
3. The Internet Layer manages addressing and routing using the Internet Protocol, like OSI’s Network Layer.
4. The Link Layer handles data transmission over physical media, like cables and Wi-Fi, combining the functions of OSI’s Physical and Data Link layers.

OSI model vs TCP/IP model: comparing layers