What is the OSI Model? OSI Model Layers Explanation and Mnemonic

8 Oct 2020 by Mike Allen

What do you think of when you hear the terms OSI model or seven layers? For those in IT, it might bring you back to the days of studying for the CCNA certification. In this article, I’ll look at the OSI model, the seven layers of the OSI model, and why it’s still relevant to all IT professionals today.

What is the OSI Model?

What is the OSI model? OSI is the abbreviation for Open Systems Interconnection. It’s a conceptional framework that describes the function of a network or telecommunications system.

The OSI model was conceived in the 1970s when computer networking was taking shape. Since then, the OSI has undergone several revisions over the years. Two additional variations of the OSI were merged in 1983 and published in 1984. The 1984 version of the OSI model is the model that most IT professionals are familiar with today.

Most descriptions of the OSI model begin at the bottom with all the numbers going up from Layer 1 to Layer 7. OSI includes the physical layer, data link layer, network layer, transport layer, session layer, presentation layer, and application layer.

OSI Layers Mnemonic

Before we get started, can you name all of the layers in order without referencing them above? If you can, that’s impressive. When trying to learn something new, I always like to start by creating a mnemonic. You may remember this from grade school or music lessons. Let’s get started.

For this exercise, let’s use the mnemonic “Please Do Not Touch Steve’s Pet Alligator.”

  • Layer 1: Physical = Please
  • Layer 2: Data Link = Do
  • Layer 3: Network = Not
  • Layer 4: Transport = Touch
  • Layer 5: Session = Steve’s
  • Layer 6: Presentation = Pet
  • Layer 7: Application = Alligator

7 Layers of the OSI Model

Next, let’s look at each layer of the OSI model and describe what it is, how it’s used, and examples.

Layer 1 – Physical Layer (Please)

At the bottom of the OSI model, we have the Physical Layer which is known as Layer 1. This layer reflects the electrical and physical representation of the system. This includes everything from the cable type to radiofrequency such as 802.11. It also includes the layout of pins, voltages, and other physical requirements. When a networking problem arises, many IT professionals go right to the physical layer first to check that all of the cables are probably connected, and the power plug hasn’t pulled out from the router, switch, or server.

Layer 2 – Data Link Layer (Do)

What is Layer 2 in the OSI model? It’s the Data Link Layer. This layer provides node-to-node data transfers between two directly connected nodes. It also handles error correction in the physical layer. Two sublayers exist here… They are the Media Access Control (MAC) layer and the Logical Link Control (LLC) layer. In networking, most switches operate at Layer 2.

Layer 3 – Network Layer (Not)

Are you ready for this? It’s one of my favorites… Layer 3 is the Network Layer. It’s where you will find most of the router functionality that most networking professionals care about and enjoy. In its most basic form, this layer is responsible for packet forwarding, including routing through different routers. What’s an example of Layer 3? Let’s say that you have a server in Boston, Massachusetts that wants to connect to another server in Los Angeles, California. There are millions of different paths to take and it’s the routers in this layer that help to achieve this efficiently.

Layer 4 – Transport Layer (Touch)

What is the Transport Layer of the OSI model? Also known as Layer 4, the Transport Layer involves the coordination of data transfer between end systems and hosts. Layer 4 involves how much data to send, at what rate, and where it goes, etc… What is an example of Layer 4? The best-known example of the Transport Layer is the Transmission Control Protocol (TCP). TCP is built on top of Internet Protocol (IP) and the combination is known as TCP/IP. It’s the TCP and UDP port numbers at work with Layer 4, whereas IP addresses operate at Layer 3 – the Network Layer.

Layer 5 – Session Layer (Steve’s)

What is needed when two devices, computers, or servers need to talk with one another? It’s Layer 5, also known as the Session Layer. A session needs to be created and this is achieved in Layer 5. What does Layer 5 involve? It involves setup and coordination. Setup is how long a system waits for a response. Coordination is how long before termination between the application at the end of each session.

Layer 6 – Presentation Layer (Pet)

What is the 6 Layer of the OSI model? If you guessed the Presentation Layer, you’re correct. The Presentation Layer represents the area that is independent of data representation in the application layer. Generally speaking, Layer 6 represents the preparation or translation of the application format to the network format, or from the network format to application format. In other words, it presents data for the application or the network. What are the examples of Layer 6? A good example of this is encryption and decryption of data for secure transmission.

Layer 7 – Application Layer (Alligator)

The Application Layer is the top layer and what most users see. In the OSI model, this is the layer that is closest to the end-user. Applications that operate at Layer 7 are the ones that users interact with directly. What are the examples of Layer 7 applications? Common examples include web browsers such as Google Chrome, Firefox, Safari, etc. and other office or collaboration suites such as Skype, Outlook, and Office.

Why is the OSI Model Important?

In the early day of computer networking, manufactures developed their own proprietary models to support their own products. As you can imagine, this caused a lot of issues. Only the devices produced by the same manufacturer could communicate with one another. An IBM device could not communicate with an HP device and vice versa.

The OSI model solved this by creating a vendor-neutral standard that all manufacturers and service providers could follow. Major manufacturers and vendors agreed to support the model in conjunction with their own proprietary networking model. Many have suggested that the OSI model is the reason why the Internet is not owned by a single company.

Today, the OSI model helps Network Administrators narrow down issues such as is it a physical issue or something with the application. For programmers developing an application, which layers does the application operate with? Many Solution Engineers and Network Architects often reference the OSI model to help others understand which layer their product or service operates within or whether it works throughout the stack.

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Author

Mike Allen

Mike Allen serves as VP of Solutions & Engineering and engages with clients directly to determine the best course of action for their IT infrastructure based on current and future requirements. Mike has an extensive technical background having worked for some of the largest carriers both domestically and Internationally. He specializes in data center, network, cloud and communications. He holds several certifications including CCNA and SSCA.

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