Key takeaways
The Open System Interconnection model — the OSI model, for short — shapes how we build digital environments. A conceptual framework, the OSI Model describes how different computer systems communicate with each other inside networks and cloud/internet environments.
Today, let’s look at how the OSI Model affects our digital lives, applications and networks.
The OSI model was first adopted in 1984 by the International Organization for Standardization (ISO) when networking was ruled by competing technologies such as Ethernet, Token Ring, FDDI and ARCNET. Like today, there were many different incompatible machine technologies and operating systems from companies such as IBM, Burroughs, Univac, Hewlett Packard, and more.
In the early 1980s, there were no agreed-on standards or blueprints for how two different computer systems could exchange data. The OSI Model was created to provide a framework for how diverse computer systems using different technologies can talk to each other.
The OSI Model abstracts and describes the activities, processes, and standard protocols used for cross-system communication.
It helps communicate and visualize how digital communication operates for a wide variety of uses including design, engineering, marketing, documentation and more. It is also used for troubleshooting and isolating network issues.
The OSI Model isn’t the only conceptual framework used in networks and the cloud and internet. The internet also comprehensively uses the TCP/IP framework model as another foundational set of communications protocols.
Unlike the seven layers of the OSI model, the TCP/IP model is typically described with fewer layers, often four or five. While the layers don't map perfectly one-to-one, the TCP/IP model's layers broadly correspond to functions found across multiple OSI layers. Many TCP and TCP/IP protocols are referenced in the OSI Model.
Understanding both models provides a more complete picture of how modern networks operate.
Even today, manufacturers, engineers, vendors, users and others still reference the model to determine what components are necessary to make their systems talk to other systems. Both the OSI Model and the TCP/IP Model have been absorbed and internalized into digital lives at such a basic level that they are both explicitly and implicitly used without even thinking about it.
The OSI Model is divided into seven layers that describe the activities and processes needed for disparate computer systems to communicate with each other over networks and the cloud/internet.
Often presented from top to bottom (layer 7 to layer 1), the OSI Model describes the path that data takes from the end-user level (layer 7: Application Layer) down through the stack to the physical transmission of bits across communication links and network cards (layer 1: Physical Layer), and all layers in between.
This top-down perspective aligns with how users interact with applications and how data begins its journey from a software process.
The OSI Model for how different computer systems communicate with one another
The network architecture needs to be comprehensible and interoperable. Integration with external network elements and services should be standardized. In a layered network model, each layer offers modularity and abstraction to other layers. That means that changing one layer element does not affect the functional performance of another layer element.
For example, if your page load speed is slow but your network hardware is performing optimally, you can troubleshoot, fix and modify the application layer (HTTP) without having to change the physical devices. Individual layer functions can be reused, modified and scaled — across topologies, functions, and differences that may exist in other layers.
So why the OSI layer model in particular? The industry has widely accepted the OSI model as their preferred choice and it comes down to the following reasons:
Besides this, like any popular open-source technology, early adoption of the OSI model by prominent players in the industry has had a knock-on effect, encouraged by large enterprises such as Cisco, HP, and IBM to build interoperable networking technologies. It emerged as a universal language of how data communication should take place across the global Internet and telecom expansion.
Looking at the OSI Model, notice that the seven layers can be grouped into three general categories. These groupings allow us to generally refer to OSI model function — software, Heart of OSI, hardware — based on their primary role in the communication process. We'll explore these layers from the top down (Layer 7 to Layer 1).
Let's now look at each individual layer, starting from the top (closest to the user) and moving down towards the physical network. The seven layers of the OSI Model reduce the design complexity of networked systems, each describing sub-functions within the Software, Heart of OSI, and Hardware groupings.
The application layer is the closest layer to the end user. It receives information from the end user and sends results back to the user. Despite its name, Layer 7 is not where client applications live. This layer provides the protocols that allow software/apps to transmit data, including:
The main functions are summarized as follows:
The presentation layer ensures the data is prepared in a usable form for the application layer (receiving side) or for the network layer (sending side). Layer 6 is responsible for:
The main functions are summarized as follows:
The session layer creates and maintains the sessions (connections) that two systems need to speak to each other. Layer 5 defines:
It also creates checkpoints to ensure and synchronize data transfer. The main functions are summarized as follows:
The transport layer uses transmission protocols including Transmission Control Protocol (TCP) and User Datagram Protocol (UDP), to manage network traffic between systems to ensure correct data transfers.
Layer 4 also handles flow control and error control, regulates transmission speed and requests retransmissions if needed. The main functions are:
The network layer decides which physical path the data will take across potentially multiple networks. It’s responsible for breaking up transport layer segments into smaller network packets for transmission and for reassembling those packets on the receiving system. This layer routes packets to their destination, mostly by using IP addressing.
Layer 3 processing is generally bypassed when the sending and receiving systems are on the same local network. The main functions are:
The data link layer defines the format of data on the local network segment. Like the network layer, the data link layer enables data transfer between two directly connected nodes or systems on the same network. Layer 2 also corrects errors that may have occurred at the physical layer (layer 1).
It uses media access control (MAC) processing for flow control and multiplexing between two systems on the same link. It also uses logical link control (LLC) to provide flow control and error control within the local network segment.
The main functions are:
The physical layer converts and transmits raw bit stream data (1s and 0s) over the physical medium. Layer 1 concerns the physical and electrical connections the system uses. It includes:
The physical layer also discusses network components such as hubs, repeaters, modems, network adaptors (NICs), etc. The main functions are summarized as follows:
Think about a common action you take every day, like clicking a link to visit a website or sending an email. While it seems instantaneous from your perspective, behind the scenes, your computer and the network are performing a complex series of steps defined by models like OSI.
The OSI Model is inherently bi-directional. It is used by both the data sender and data receiver, and the two sides change roles during the transmission process. Understanding this flow from the user's initiation to the final delivery is key to seeing how the layers work together.
When you, as a user, initiate an action that sends data from your system — like requesting a web page — that data starts at the Application Layer (Layer 7) on your system. It then travels down through each layer to the Physical Layer (Layer 1). At each layer, the data is processed and encapsulated, meaning the layer adds its own header (and sometimes footer) information to the data unit it received from the layer above. This added information is necessary for the corresponding layer on the receiving side to correctly process the data.
When data is received by your system — like the web page content arriving — it arrives first at the Physical Layer (Layer 1) and travels up through each layer to the Application Layer (Layer 7). At each layer, the data unit is processed, and the header/footer added by the corresponding layer on the sending side is removed (decapsulation). The data is then passed up to the layer above until it reaches the Application Layer and is delivered to the end-user application (your browser).
As an example, when your browser requested the URL for this article, it made the request first at the Application Layer (Layer 7). The requested data — URL and transmission information — was processed and modified as it traveled down the OSI Model stack, through Layers 6, 5, 4, 3, 2, until it was transmitted as raw bits at the Physical Layer (Layer 1) towards the Splunk web servers.
At Layer 1, the Splunk servers received the bits and processed the data up through their OSI model stack (Layers 2, 3, 4, 5, 6). The request was finally understood at the Application Layer (Layer 7). The servers then processed the request, retrieved the HTML code and transmission information, and transmitted the article back to your browser. This response data started at the Application Layer (Layer 7) on the server and traveled down the stack to Layer 1 for transmission. It was then received at Layer 1 on your system and traveled up your stack (Layers 2 through 6) until it arrived at your browser (Application Layer, Layer 7) where you’re reading it right now.
Because the model is bi-directional, both sides — your browser and the Splunk Web servers — function as sender and receiver:
This data transfer framework can be modeled and used by almost all computer systems in internal networks and in the cloud/internet.
And that’s how the OSI Model defines how data is transferred in both directions.
The OSI Model provides an invaluable framework for understanding how complex network communications function, breaking down the process into manageable, standardized layers. Its logical structure simplifies design, troubleshooting, and development across diverse technologies.
While technology constantly evolves and other models like TCP/IP are also fundamental, the OSI Model's core concepts remain a critical reference point for network professionals and engineers worldwide. So, while the specific implementations or dominant models might shift over time, the fundamental layered approach pioneered by OSI is likely to endure as a way to conceptualize digital communication.
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