Mak Hurst

The OSI model is fundamental in networking. This model provides a framework dictating how all networking devices will send, receive and comprehend data. In simple terms, it describes how computers communicate over a network. It was the very first standard model for network communications, and was widely adopted by everyone in the early 80s.

Throughout your IT journey you may have heard or seen some sort of reference to the TCP/IP model. TCP/IP stands for Transmission Control Protocol/Internet Protocol, and is a framework for communication and data transfer over the internet. It is very similar to the OSI model but is more of a practical suite of protocols used to actually connect devices on the internet. The TCP/IP model consists of 4 layers. The application layer, transport layer, internet layer, and network access layer. I won’t get too carried away, and may even decide to do a more in depth post about the TCP/IP model in the future. But for now, let’s shift our focus back to OSI.

OSI stands for Open Systems Interconnection Reference Model. It is made up of 7 layers, each having its own set of responsibilities. I am going to walking you through layers 1-7, discussing what takes place and what hardware is typically associated

Layer 1: Physical Layer

Layer one is arguably the easiest layer to comprehend. This layer references the physical components of the hardware that’s used in networking. Devices at this layer use electrical signals to transfer data between each other in binary code. A great example of a component you would see at this layer is an Ethernet cable. 

Layer 2: Data Link Layer

The data link layer establishes and terminates a connection between two physically connected nodes on a network. Within this layer, packets get broken up into frames, and sends them from the source to its destination. This layer can be broken up into two parts, Logical Link Control and Media Access Control. 

Logical Link Control (LLC) provides the logic for the data link. It controls synchronization, Flow control, and error-checking functions of the data link layer. Media Access Control (MAC) uses MAC addresses to connect devices and define permissions to transmit and receive data. A common hardware device found at the data link layer is a switch.

Layer 3: Network Layer

The network layer can be broken up into two main parts. One consists of breaking up segments into network packets to be sent, and  reassembling the packets on the receiving end. The other is routing packets by finding out the best path across a physical network. IP addresses are used by the network layer to route packets to a destination node. Network devices that can be found at the network layer are Routers and managed switches, 

Layer 4: Transport Layer

This layer plays a vital part in transmitting data across a network and can be a little bit difficult to grasp. When data is sent between devices, it follows one of two different protocols that are decided based upon several factors (TCP and UDP).

Layer 5: Session Layer

The session layer is where communication channels are created.  It is also responsible for opening sessions, and making sure that they remain open and functional while data is being transferred.

Layer 6: Presentation Layer

The best way to think about the Presentation layer, is to think of this layer as where the standardization starts to take place. Applications and websites are all enveloped differently but the data itself will all need to be handled the same way. This layer acts as a translator for data to and from the application layer. For example, when I send an email to a friend of mine, I may use gmail and they may be using Outlook. Although we both have different email clients, the contents of the email will still need to display the same.

In other words, the presentation layer defines how two devices should encode, encrypt and compress data so that it is received correctly on the other end.

Layer 7: Application Layer

The application layer is the layer the we (the user) are most familiar with.  This is because the application layer is the layer in which protocols and rules are in place to determine how the user should interact with data being sent or received. When looking at an application like gmail, an email client, they have a great GUI (graphical user interface). The GUI is what makes the app and service itself easy to use for users like us. A protocol that is used at this layer is DNS (domain name system), which is used to translate website addresses in a browser to an IP address.

Becoming familiar with the OSI model is very important. Whether you’re a hacker, network engineer, or just a weirdo who likes to learn, having a full understanding of each layer is essential.