Computer networks are a complicated affair. There are many moving parts that go into developing and deploying a network. Further, you have layers upon layers of abstraction, which makes some of the harder functions an easier task for professionals and the average person alike. The OSI model has been the basis for conventional computer networking for some time.
How does the OSI Model work? It relies on a series of layers that act as sequential steps, from taking the smallest basic unit of data used in a network and sending it all the way from a physical piece of hardware to your web browser. Let’s look at what the OSI model is, the layers that compose it, and how you interact with it on a daily basis.
What is the OSI Model?
The Open Systems Intercommunication, or OSI, Model has its origins well before the internet revolution. Developed in 1978 by French software engineer Hubert Zimmermann, the OSI model serves as a universal language shared by all networking principles. The International Organization for Standards standardized the model as IOS/IEC 7498-1.
Prior to its introduction, there was no set way to communicate between devices from different vendors. Networking itself was in its infancy, but the OSI Model gave a means and blueprint for designing and interfacing connectivity for products from different vendors. The standard, as established, is still in heavy use today, and serves as the backbone of all modern networks. Whether you are a professional designing a network for a massive corporation or enjoying streaming media, you are partaking in the OSI Model.
The model itself has seven different layers, each with their own distinct functions. In addition, each of these layers communicates data between each other in a process called encapsulation. This takes the raw data from the transmission and packages it with relevant information regarding how it should be routed, which client it’s going to, and ultimately which application is receiving it.
An Overview of the Seven Layers of the OSI Model
The OSI model is a top-down organization of connected layers, and it is numbered as if counting down.
|Presents transmissions to end user
|Encodes and decodes transmissions
|Establishes a connection for transmission
|Takes a transmission and breaks it into smaller chunks for reliable transit
|Breaks up data sent from a physical device
|Allows for communication between multiple networks
|Handles the transmission of data from actual physical components
Understanding the signal flow of the OSI Model is paramount to understanding how it directly transmits data between its endpoints. All data sent from one network to another, like your laptop accessing a website, passes through all seven layers before it is rendered in your browser. This is likewise applicable to every single network transaction you will see, from humble ping tests to streaming 4K media.
OSI Model Layer 7: The Application Layer
This is the most common layer the average person will interact with. The Application layer is the end goal of a data transmission. It is where data will end up inevitably serving some abstract application where the bare metal processes aren’t exposed. There is no specific protocol data unit or PDU for the Application layer. Instead, the fully encapsulated form from previous layers ends up here.
OSI Model Layer 6: The Presentation Layer
The Presentation layer is a midpoint, it isn’t opening a method of communication between different devices. Instead, this layer packages the received data transmission for the Application layer. This is a two-way method of transmission, with encoding or encryption and decoding or decryption happening. Every single transaction during data transmission has to be packaged and unpackaged. There isn’t a specialty PDU for this layer.
OSI Model Layer 5: The Session Layer
Inter-device communications need established sessions, and this is where the Session Layer comes into play. This works in close conjunction with Layer 4 to create a unique session for each connected user. This layer gives unique IDs to each user, allowing admins to have a more thorough accounting of each transaction. Sessions have to open and close immediately upon receipt of data, so this layer has quite a hefty job to handle. There is no personal data unit available on the session layer.
OSI Model Layer 4: The Transport Layer
This is the absolute backbone of modern networking. Layer 4 executes the TCP and UDP protocols. This layer functions by breaking up transmissions into smaller chunks, or segments. These segments allow for faster transmissions. Each of these individual segments also has something called a header. These headers tell the recipient the segments’ routing path. Segments are capable of error control, avoiding errant sessions being established in Layer 5.
OSI Model Layer 3: The Network Layer
The Network layer breaks down transmissions and reassembles them upon receipt. This is further aided by Layer 4. Instead, Layer 3 deals directly with networking hardware, as opposed to the software end of routing. You can think of the Network layer as the bare metal counterpart of the Transport layer, as is dealing with routers, switches, and other hardware directly. If you are in a networking profession, you will know Layer 3 and Layer 2 are where the nuts and bolts lie in terms of establishing a properly routed network in any environment. The PDU for Layer 3 is the packet.
OSI Model Layer 2: The Data-Link Layer
Local devices and their communications are the domain of the Data-Link Layer. However, this isn’t as nebulous as it seems, especially when accounting for how your router, modem, and computer are all different devices residing on the same local area network. This layer takes the raw data transmission received at Layer 1 and breaks it down into frames. In addition, these frames aid in this layer’s specialty, flow control. Local area networks use the Data-Link Layer to better control the flow of data, minimizing the risk of data collision on a network.
OSI Model Layer 1: The Physical Layer
The Physical Layer directly relates to the actual physical hardware on your network. For example, network interface cards, routers, cables, modems, and so on are all handled by the Physical Layer. Encapsulated data gets converted to bits here, as the data stream is in a raw state when entering this layer. The Physical layer’s raw data-stream is conventionally understood between different devices. Meanwhile, the subsequent layers and their encapsulation routes them to their correct destination.
The OSI Model makes networking possible. Further, without the standard established by this model, modern computing and inter-device communication would be reliant upon using vendor-specific hardware. Thanks to the OSI Model, users instead have a vast array of options to choose from for their own networks and even disparate devices like Linux and Windows clients can effectively communicate. In short, networking is a platform-agnostic means of communication, and having open and defined standards allows the modern world to keep on ticking.
The OSI Model is an elegant solution to a complex issue, and its clearly defined and constructed layers take much of the guesswork out of designing a network. Thus, even if you have no intention of joining the workforce as a network engineer, you still benefit from the standard with any of your day-to-day devices. For example, even if you’re just idly browsing the web, watching Hulu, or playing Call of Duty, you’re directly using and interacting with the OSI Model.
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