The OSI Model

The OSI (Open Systems Interconnection) Model is like a blueprint that helps us understand how computer networking works. It breaks down networking into seven layers, each with its own specific role, from managing physical connections to organizing data for transmission. While real-world networking is based on a more compact model called TCP/IP, the OSI model is easier to grasp initially because it provides a clear structure for understanding how different parts of networking interact.

The OSI model consists of seven layers

There are many mnemonics floating around to help you learn the layers of the OSI model — search around until you find one that you like.

I personally favour: A Penguin Said That Nobody Drinks Peps

Let’s briefly take a look at each of these:

Application Layer: The application layer is like a helper that lets your computer programs use the internet and other networks. It provides tools and options so apps can send and receive data. When an app wants to send data, it hands it to the application layer, which then passes it down to the next layer in the process.

Presentation Layer: The presentation layer acts like a translator and editor for data. It takes data from the application layer and converts it into a standard format that other computers can understand. It also handles tasks like encryption (for security) and compression (to save space). Once this is done, it passes the data to the session layer.

Session Layer: The session layer is like a coordinator for conversations between computers. It sets up and manages connections, ensuring each communication session is unique and doesn’t mix up data (like having multiple web browser tabs open). If it can’t establish a connection, it sends an error message. Once a connection is set up, it passes the data to the next layer, the transport layer.

Transport Layer: The transport layer decides how to send data and splits it into smaller pieces for easier transmission. It uses two main methods:

TCP (Transmission Control Protocol): Reliable and connection-based, ensuring all data reaches its destination correctly. It’s used for tasks needing accuracy, like file transfers or loading web pages.

UDP (User Datagram Protocol): Fast but less reliable, sending data without checking if it all arrives. It’s used for tasks needing speed, like video streaming or online gaming.

Once the method is chosen, the transport layer breaks the data into smaller parts called segments (for TCP) or datagrams (for UDP).

With a protocol selected, the transport layer then divides the transmission up into bite-sized pieces (over TCP these are called segments, over UDP they’re called datagrams), which makes it easier to transmit the message successfully.

Network Layer: The network layer acts like a GPS for data. When you request a webpage, it uses the page’s IP address to find the best path to get there. It handles logical addresses, like IP addresses, to organize and manage networks. It helps data travel between different networks. If devices are on the same network, this layer isn’t needed. The most common type of IP address is IPV4, such as 192.168.1.1, which is often used for home routers.

Data Link Layer: The data link layer manages the physical addresses of devices. When it gets a packet from the network layer with an IP address, it adds the device’s physical (MAC) address. Each device has a unique MAC address set by the manufacturer, found on the Network Interface Card (NIC).

This layer ensures data is in the right format for transmission and checks received data for errors to ensure it hasn’t been corrupted. It directs data to the correct device using MAC addresses and prepares it for the final physical layer transmission

Physical Layer: The physical layer deals with the actual hardware involved in data transfer. It converts the binary data into electrical signals (or light pulses, radio waves, etc.) to send over the network and then converts incoming signals back into binary data that the computer can understand. It handles the physical transmission of data.

The OSI model acts as a blueprint for creating and comprehending network communication protocols, enabling efficient communication between devices.