OSI model and its 7 layers explained
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OSI model and its layers: This is how networks work
As a fundamental framework in IT, the OSI model explains how data is transmitted across networks. Although the framework dates back to the 1980s, it still serves relevant purposes today. It helps users to understand complex systems, improve network security, and serves as a foundational concept in IT education. It is indispensable for professionals performing network analysis or diagnosing transmission errors. In this article, we explain what the Open Systems Interconnection model (OSI) is and how it is structured.
What is the OSI 7 layer model?
The OSI model or ISO/OSI model is a reference framework for network communication developed in the 1980s. The abbreviation OSI stands for “Open Systems Interconnection” and describes the open connection between different computer systems.
OSI was developed by the ISO (International Organization for Standardization) with the aim of creating uniform standards for data exchange worldwide. Previously, many manufacturers used proprietary, incompatible solutions. This diversity led to problems in device interoperability and made networks difficult to expand.
The framework divides communication into seven logically structured layers, making the framework clear and transparent. This separation makes it easier to localize issues, expand networks in a targeted manner and develop new protocols.
By the way: Alongside the ISO/OSI model, the TCP/IP model also plays a central role as a practical reference framework that forms the basis of today’s Internet and differs from the Open Systems Interconnection model in structure and application. A comparison of the OSI and TCP/IP models helps to better understand networks and how they function.
The OSI layer model in practice
The Open Systems Interconnection model is not always directly implemented as a technical framework in real-world systems. Instead, it often serves as a conceptual and analytical tool. The purposes of the 7 layer model are to help users understand complex network operations in a structured way and to narrow down difficulties systematically. It helps identify the layer where an issue originates, whether it’s a physical connection failure, an addressing problem, or a malfunction at the application level.
Network administrators also rely on the OSI architecture to plan network designs, document systems, and categorize new services. The logical separation of layers allows protocols and devices to be assigned precisely according to their function. This simplifies both the maintenance and development of networks. In IT education, the model provides a clear and consistent framework that enhances technical understanding and supports structured learning.
How the OSI architecture is structured — an overview of the 7 layers
The Open Systems Interconnection model divides communication in networks into seven structured layers. This is why it is commonly known as the OSI 7 layer model. These can be identified as follows:
- Physical Layer
- Data Link Layer
- Network Layer
- Transport Layer
- Session Layer
- Presentation Layer
- Application Layer
Each layer of the OSI model architecture has a clearly defined task and interacts only with the layer directly above and below it. When data is transmitted, it passes through all layers from top to bottom, and when received, it moves in reverse from the bottom layer to the top. This structured separation enhances clarity and makes complex technical processes easier to understand.
The seven layers of the OSI model can be grouped into three categories: the lower ones handle physical transmission, the middle ones manage transport, and the upper OSI layers make data understandable and usable. In the following section of this article, we will take a closer look at each one of the Open Systems Interconnection model.
The lower OSI layers (1–3) as a technical foundation of communication
The foundation of the network connection is formed by the three lower ISO/OSI model layers. They are responsible for physical transmission, framing, and routing.
Layer 1: Physical layer
Individual bits are transmitted as electrical signals, light pulses, or radio waves. The physical layer of the OSI 7 layer model defines the physical aspects of a connection, such as cables, connectors, voltage levels, and frequencies. It ensures that devices are physically connected. At this stage, the actual content or meaning of the data does not matter, only the transmission of raw bits is important.
Layer 2: Data link layer
In the model, error-free data transmission between devices is ensured by the data link layer. It divides the data into small units called frames and controls access to the transmission medium to prevent collisions. Each device is assigned a unique identifier that enables its identification within the network.
Layer 3: Network layer
Through the network layer, it is determined which path the data takes through the network. It breaks information into packets and adds destination addresses. This OSI layer ensures that packets reach their destination, even when multiple intermediary systems are involved or when the destination is far away.
The middle OSI model layers (4–5): Managing and controlling data transmission
In the OSI architecture, the middle layers ensure that transmitted data arrives at the receiver complete, error-free, and in the correct order. They manage the transport and organization of the connection between communicating parties.
Layer 4: Transport layer
The complete, intact, and sequenced delivery of data to the receiver is ensured by the transport layer. Large data streams are divided into smaller segments and properly reassembled at the destination. By using ports, this OSI layer enables the simultaneous management of multiple network connections on a single device.
Layer 5: Session layer
The session layer is responsible for managing and controlling the logical connections between devices. It ensures that data is transmitted orderly, and that connections remain stable. In case of interruptions, this layer enables the resumption of the session, which plays a crucial role especially in longer or secure data transmissions.
The upper OSI layers (6–7): Making data usable and readable
Within the OSI model, the upper layers focus on processing data to make it understandable and usable for people or applications. They also provide the connection to the user interface.
Layer 6: Presentation layer
The task of the presentation layer is to convert data into a format that can be understood by different systems. It ensures the correct encoding and structuring of text, images, and numbers, and can also handle encryption and compression.
Layer 7: Application layer
The top layer of the OSI 7 layer model, the application layer, serves as the interface between network services and end users. It allows programs to directly access network functions such as web browsing, email communication, or remote data access, making the use of the network directly tangible for the user.
Additional layer: the OSI layer 8
In addition to the seven official layers of the OSI model architecture, the IT world jokingly refers to an eighth: the so-called “layer 8”. This refers to the human element, whether it's the user, administrator, or manager. Errors or problems are often blamed on this layer, such as incorrect operation, weak passwords, or a lack of security awareness. Behind the humor lies a serious truth: technology is only as secure as the person operating it. The OSI layer 8 serves as a reminder that human factors are an essential part of any functioning network.
Advantages and disadvantages of the OSI 7 layer model
The Open Systems Interconnection model is widely used in IT education and network analysis because it provides a helpful conceptual framework. It’s especially valuable for systematically understanding complex processes and conducting targeted troubleshooting. Therefore, it’s worth examining its strengths and weaknesses in comparison to real-world applications:
| Advantages of the OSI model: | Disadvantages of the OSI model: | |
|---|---|---|
| Clear separation of functions across seven layers | Highly theoretical; rarely implemented fully in practice | |
| Each layer has a clearly defined role | Partial functional overlaps between layers | |
| Errors can be isolated and analyzed more precisely | Strict separation of layers is difficult to maintain in real networks | |
| Modular structure simplifies maintenance and development | In practice, the more streamlined TCP/IP model is often preferred | |
| Ideal for training, documentation, and network planning | Ambiguity in assigning functions like encryption or compression | |
Advantages of the OSI model:
- Clear separation of functions across seven layers:
- Each layer has a clearly defined role:
- Errors can be isolated and analyzed more precisely:
- Modular structure simplifies maintenance and development:
- Ideal for training, documentation, and network planning:
Disadvantages of the OSI model:
- Clear separation of functions across seven layers: Highly theoretical; rarely implemented fully in practice
- Each layer has a clearly defined role: Partial functional overlaps between layers
- Errors can be isolated and analyzed more precisely: Strict separation of layers is difficult to maintain in real networks
- Modular structure simplifies maintenance and development: In practice, the more streamlined TCP/IP model is often preferred
- Ideal for training, documentation, and network planning: Ambiguity in assigning functions like encryption or compression
The OSI 7 layer model is most useful as a tool for analysis and learning. Its clear assignment of responsibilities to specific layers brings structure to areas such as protocol selection, troubleshooting, and system integration. At the same time, it's important to be aware of its theoretical limitations: Many processes in real networks don't follow the model's clean separation between layers. Nevertheless, the ISO/OSI model remains a proven reference framework that continues to play a significant role in modern IT environments, from network solutions to IoT-based services.
The OSI model protocols in each layer
In practice, a general assignment of typical protocols to the individual OSI model layers has become established. This helps to better understand where specific functions take place within the network. Due to this clear structure, communication processes can also be systematically analyzed and optimized:
- Layer 1 (Physical): DSL, Wi-Fi, Bluetooth
- Layer 2 (Data Link): Ethernet, PPP, MAC
- Layer 3 (Network): IP, ICMP, ARP
- Layer 4 (Transport): TCP, UDP
- Layer 5 (Session): NetBIOS, PPTP
- Layer 6 (Presentation): SSL/TLS, JPEG, MP3
- Layer 7 (Application): HTTP, FTP, SMTP, DNS
At the top layer, the Application Layer, you'll find many well-known protocols that directly interact with user-facing programs such as web browsers, email clients, or cloud services. This is where network functionality becomes directly tangible for users. But also in lower OSI layers important protocols play a key role: On the Transport Layer, TCP and UDP ensure that data is either delivered reliably or with maximum speed, depending on the use case. The IP protocol on the Network Layer handles addressing and ensures that data packets reach their destination, even when they must pass through multiple network nodes.
Technologies like the Internet of Things (IoT) also follow the OSI model. IoT protocols such as MQTT or CoAP are typically assigned to the Application Layer and are specifically designed for lightweight, resource-efficient data exchange between devices. Their position within the OSI 7 layer model helps IT professionals better understand, test, and secure them in the broader context of network infrastructure.
The ISO/OSI model as a proven framework for the future
The Open Systems Interconnection model explains network communication in a clear and understandable way, supporting learning, planning, and the analysis of technical processes. Its structured layout enhances the understanding of complex systems, and the layered approach allows network protocols to be logically categorized and more easily evaluated. Even though it is rarely implemented in its entirety, it remains a valuable tool for education and troubleshooting.
Frequently asked questions about the OSI model
What is the OSI model?
The OSI layer model is a conceptual guide for understanding how communication happens in networks.
How is the ISO/OSI model generally structured?
OSI consists of seven logical layers, each performing a distinct role – from physical hardware up to user-facing applications. The layers are clearly separated, forming a structured system for analyzing and designing networks.
What are the 7 layers of the OSI model?
The seven layers of the Open Systems Interconnection model are: 1. Physical, 2. Data Link, 3. Network, 4. Transport, 5. Session, 6. Presentation, 7. Application. These layers build on one another logically, with each playing a specific part in data communication.
How does the OSI model work?
When sending data, a message moves through all seven OSI layers from top to bottom. When receiving, the process is reversed – the data moves back up from the bottom to the top. Each layer handles the data independently and passes it along to the next.
What is the OSI layer 8?
The layer 8 is a humorous extension and refers to humans. It’s often mentioned when errors occur due to incorrect operation or poor configurations.
Is there a mnemonic for the OSI model?
Yes, a popular mnemonic for remembering the OSI model in order is
"Please Do Not Throw Sausage Pizza Away”. Each first letter stands for a layer:
- P – Physical (Layer 1)
- D – Data Link (Layer 2)
- N – Network (Layer 3)
- T – Transport (Layer 4)
- S – Session (Layer 5)
- P – Presentation (Layer 6)
- A – Application (Layer 7)
This mnemonic for the OSI 7 layer model makes it easier to remember the correct order of the layers.
Where is the OSI layer model used in practice?
The Open Systems Interconnection model is widely used in troubleshooting, network design, and IT education. It provides a structured reference point for understanding how data flows and for categorizing network protocols effectively.