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OSI model
At a
Glance
- 1. What is the OSI model?
- 2. Why was the OSI model developed?
- 3. How is the OSI model structured?
- 4. What are the tasks of the 7 OSI layers?
- 5. How does data transmission work in the OSI model?
- 6. How does the OSI model differ from the TCP/IP model?
- 7. Why is the OSI model still relevant today?
- 8. What security risks exist at each OSI layer?
- 9. Conclusion: A reference model for network communication
01
What is the OSI model?
The OSI model (also known as the OSI layer model or ISO/OSI model) is a conceptual framework for network communication.
OSI stands for Open Systems Interconnection
ISO stands for International Organization for Standardization
The model describes how data is exchanged between two systems across a network
It divides communication into 7 layers
Each layer has a clearly defined task
The OSI model is based on the concept of splitting a communication system into seven abstract layers that build on one another. Each layer performs a specific role and communicates with the layers above and below it.
03
How is the OSI model structured?
The OSI model consists of seven layers that build on each other. On the sender's side, data moves from Layer 7 down to Layer 1, is transmitted, and is then reassembled in reverse order on the receiver's side.
Mnemonic: "Please Do Not Throw Salami Pizza Away" – the initial letters stand for Physical, Data Link, Network, Transport, Session, Presentation, Application.
7 | Application Layer | Interface to applications |
6 | Presentation Layer | Data formatting, encryption |
5 | Session Layer | Connection setup and control |
4 | Transport Layer | Segmentation, transmission control |
3 | Network Layer | Routing, logical addressing |
2 | Data Link Layer | Node-to-node transmission |
1 | Physical Layer | Physical signal transmission |
04
What are the tasks of the 7 OSI layers?
Layer 7 – Application Layer
This is the only layer that interacts directly with user data. Software applications such as web browsers and email clients rely on the Application Layer to initiate communication. The Application Layer is responsible for the protocols and data processing on which the software depends.
Interface to the application (not the application itself)
Protocols: HTTP, HTTPS, FTP, SMTP, DNS
Attack surface for Layer 7 attacks (e.g., DDoS attacks on web applications)
Layer 6 – Presentation Layer
The Presentation Layer translates data into a uniform format.
Encryption and decryption (e.g., TLS/SSL)
Data compression
Character set conversion (e.g., ASCII, Unicode)
Data formatting
Layer 5 – Session Layer
Layer 5 establishes, coordinates, and terminates sessions between systems in a network. The Session Layer is responsible for identifying, establishing, maintaining, and closing communication sessions between applications on network nodes.
Session management
Synchronization of data exchange
Resumption of interrupted connections
Layer 4 – Transport Layer
The Transport Layer ensures reliable data transmission between end systems.
Segmentation of large amounts of data
Error correction and flow control
Connection-oriented (TCP) or connectionless (UDP)
End-to-end communication
Layer 3 – Network Layer
The Network Layer handles routing between networks.
Logical addressing via IP addresses
Path finding (routing) between different networks
Packet forwarding
Examples: IP, ICMP, routers
Layer 2 – Data Link Layer
The Data Link Layer ensures node-to-node data transmission between two directly connected nodes. It detects and, where possible, corrects errors that may occur at the physical layer. It also defines the protocol for establishing and terminating a connection between two physically connected devices, as well as the protocol for flow control.
Works with MAC addresses
Examples: Ethernet, switches, Wi-Fi (IEEE 802.11)
Layer 1 – Physical Layer
The lowest layer handles the physical transmission of data.
Responsible for sending and receiving unstructured raw data between a device and a physical transmission medium. It converts digital bits into electrical, radio, or optical signals.
Defines voltage levels, cables, connectors, and timing
Examples: Ethernet cables, fiber optics, Wi-Fi radio signals
06
How does the OSI model differ from the TCP/IP model?
The TCP/IP model is the dominant model used in today's internet. However, the OSI model remains the most important didactic and analytical reference framework. The biggest difference between the two models is that the OSI model separates several functions that the TCP/IP model groups into single layers. This affects both the application layer and the network access layer of the TCP/IP model.
The modern internet is not built on OSI but on the simpler TCP/IP model. Nevertheless, the OSI 7-layer model is still widely used because it helps visualize and communicate how networks function.
Number of layers | 7 | 4-5 |
Type | Reference model | Practical model |
Focus | Protocol-independent | Protocol-specific |
Usage | Teaching, troubleshooting | Real-world internet |
Flexibility | High | Medium |
08
What security risks exist at each OSI layer?
Every layer of the OSI model offers its own attack surface. Layer 7 attacks are particularly critical today because they target complex web applications and often bypass traditional firewalls.
Layer 7 (Application) | DDoS, SQL injection, XSS | DDoS Protection, WAF, Bot Management |
Layer 6 (Presentation) | SSL stripping, weak encryption | TLS, strong certificates, Post-quantum cryptography |
Layer 5 (Session) | Session hijacking | Session tokens, timeouts |
Layer 4 (Transport) | SYN flood, TCP reset | DDoS Network Protection |
Layer 3 (Network) | IP spoofing, ICMP flood | DDoS Network Protection |
Layer 2 (Data Link) | ARP spoofing, MAC flooding | Port security, VLANs |
Layer 1 (Physical) | Cable tampering, eavesdropping | Physical access controls |
FAQ – Frequently Asked Questions about the OSI Model
The OSI model is a reference framework that divides network communication into seven layers. Each layer has a clear task — from the cable to the application.
About the author
Stefan Bordel
Senior Editor
About the author
Stefan Bordel has been working as Editor and Technical Writer at Myra Security since 2020. He is responsible for the strategic development and editorial management of all content formats – from website content and specialist publications to whitepapers, social media communication, and technical documentation. In this role, he combines solid expertise from IT journalism with in-depth technical understanding in the field of cybersecurity. As a long-time Linux enthusiast, he closely follows developments in the IT industry both professionally and personally.