OSI Model

The OSI Model, created in the 1980s, is a framework that explains how devices communicate across a network, step by step through seven layers.

Its strength is in its layered structure. This divides communication into seven layers, each with its own responsibilities.

Figure 1 – OSI Model

The OSI model helps us understand network communication and troubleshoot issues. But it serves as a theoretical reference above all else. The TCP/IP model is the standard in networking. It outlines the protocols for transferring data between devices.

The OSI model is built on seven layers, each with a specific role in communication.
To understand how data moves through the network, you need to know two key processes: encapsulation and decapsulation.

Encapsulation

When you send data (for example, a file or a message), it travels down through the OSI layers on your device.

Figure 2 – Encapsulation Process in the OSI Model

At each layer, extra information is added such as addresses, port numbers, and delivery details. This ensures that the data can reach the correct destination.

Decapsulation

When the data arrives at its destination, the process is reversed. The data moves up the OSI layers, and each layer removes and interprets the information that was added by its counterpart on the sender’s side.

Figure 3 – Decapsulation Process in the OSI Model

Why It Matters

• Encapsulation ensures the data contains all the information needed to reach its destination.
  

• Decapsulation ensures the receiving device can interpret the data correctly and deliver it to the right application.

Now that you understand encapsulation and decapsulation, let’s look at the seven OSI layers and the role each one plays in communication.

Seven layers make up the OSI model, and each has a specific function. Let’s look at them from the top (closest to the user) down to the bottom (closest to the physical network).

Layer 7 – Application

The Application Layer is where you interact with the network. You use applications like web browsers, email clients, and messaging apps. When you visit https://pingmynetwork.com, this layer starts the request with the correct protocol.

Figure 4 – OSI Layer 7: Application Layer

Key points:

• Closest to the end user.

• Manages application-level protocols such as HTTPS, HTTP, FTP, and SMTP.

Layer 6 – Presentation

The Presentation Layer formats the data in a way that the receiving application can use. It converts data from the application format to the network format. It can also handle encryption.

Figure 5 – OSI Layer 6: Presentation Layer

Key points:

• Translates and formats data for the application.

• Encrypts and decrypts data for secure communication (e.g., TLS/SSL for HTTPS).

• Example: Encrypting a message before sending it, then decrypting it upon receipt.

Layer 5 – Session

The Session Layer handles starting, maintaining, and closing communication sessions between devices.

Figure 6 – OSI Layer 5: Session Layer

Key points:

• Establishes, maintains, and terminates communication sessions.

• Allows many applications to run and communicate at the same time.

Upper Layers Overview

The Upper Layers consist of Layers 7, 6, and 5. They focus on preparing data for transmission.

Figure 7 – OSI Upper Layers (Layers 7, 6, 5)

The lower layers (4–1) are responsible for actually delivering the data across the network.

Layer 4 – Transport

The Transport Layer provides end-to-end communication between devices. It splits large streams into smaller segments and uses TCP or UDP to manage delivery.

Figure 8 – OSI Layer 4: Transport Layer

Key points:

• Segmentation: breaks data into smaller units.

• TCP: reliable, ordered delivery.

• UDP: faster, connectionless delivery.

• Port numbers: identify services (TCP 443 for HTTPS).

Example: When streaming a video, TCP guarantees ordered delivery, while UDP prioritizes speed even if some segments are lost.

Layer 3 – Network

The Network Layer handles logical addressing and routing, ensuring data can move across different networks.

Figure 9 – OSI Layer 3: Network Layer