Suggest an editImprove this articleRefine the answer for “What is the OSI model?”. Your changes go to moderation before they’re published.Approval requiredContentWhat you’re changing🇺🇸EN🇺🇦UAPreviewTitle (EN)Short answer (EN)**The OSI model (Open Systems Interconnection)** is a *reference seven-layer model*, developed by ISO, that describes **how data travels through a network from an application on one device to an application on another**. **Key point:** the model breaks the process of network exchange into 7 layers, where each layer performs its own tasks, interacts with neighboring layers, and hides its internal workings from the other layers.Shown above the full answer for quick recall.Answer (EN)Image**The OSI model (Open Systems Interconnection)** is a *reference seven-layer model*, developed by ISO, that describes **how data travels through a network from an application on one device to an application on another**. Short formulation for an interview: > **OSI is a conceptual model of 7 layers that standardizes the interaction of network systems. Each layer is responsible for its part of network communication: from physical transmission to the work of applications.** OSI is NOT a protocol, but a **theoretical model**, on the basis of which protocols and networks are built. --- ## The main idea of OSI The OSI model answers the question: > **How does data turn from packets into electrical signals and back?** It breaks the process of network exchange into 7 layers, where each layer: - performs its own tasks, - interacts with neighboring layers, - hides its internal workings from other layers. --- ## The 7 layers of the OSI model: a breakdown of each layer ### 1. Physical Layer Transmits *raw bits* over the physical medium: - electrical signals - optical pulses - radio waves - connectors, cables, frequencies **Devices:** cables, connectors, hubs, transceivers **Example technologies:** Ethernet physical layer, Wi-Fi radio, fiber optics, USB --- ### 2. Data Link Layer Transmits *frames* between devices within one network (LAN). Responsible for: - MAC addressing - error detection (CRC) - media access control **Sublayers:** - MAC (Media Access Control) - LLC (Logical Link Control) **Examples:** Ethernet, Wi-Fi (802.11), PPP, VLAN (802.1Q) **Devices:** switch --- ### 3. Network Layer Moves *packets* between networks (routing). Functions: - IP addressing - routing - packet fragmentation **Protocols:** - IPv4 - IPv6 - ICMP - OSPF, BGP, RIP (routing) **Devices:** router --- ### 4. Transport Layer Ensures reliable or fast delivery of data between hosts. Functions: - port numbers - segmentation and reassembly - delivery control **Protocols:** - **TCP**: reliable, with acknowledgment - **UDP**: fast, without acknowledgment --- ### 5. Session Layer Manages *sessions* between applications. Functions: - creating/maintaining/terminating connections - dialog control - synchronization Rarely used directly in modern protocols. --- ### 6. Presentation Layer Responsible for *data format*: - encoding (ASCII, UTF-8) - compression - encryption (SSL/TLS partially belongs here) Example: converting data into a format suitable for transmission. --- ### 7. Application Layer The layer where the user and applications operate. Protocols: - HTTP / HTTPS - DNS - FTP / SFTP - SMTP / POP3 / IMAP - DHCP - SNMP These are not applications, but *network protocols for applications*. --- ## How data passes through the OSI layers #### On the sending computer: Application → Presentation → Session → Transport → Network → Data Link → Physical #### On the receiving computer: Physical → Data Link → Network → Transport → Session → Presentation → Application Each layer adds or removes its own header (**encapsulation / decapsulation**). --- ## Comparing OSI and TCP/IP    4 | OSI (7 layers) | TCP/IP (4 layers) | |---|---| | Application | Application | | Presentation | Application | | Session | Application | | Transport | Transport | | Network | Internet | | Data Link | Network Access | | Physical | Network Access | TCP/IP is a practical model, OSI is a theoretical one. --- ## Why the OSI model is needed - helps understand how networks work, layer by layer - useful for diagnostics (identify at which layer the problem is) - applicable when designing networks - structures the work of protocols - used in interviews and exams (Cisco CCNA, CompTIA Network+) --- ## A ready answer for an interview > **The OSI model is a seven-layer reference model for the interaction of network systems.** > **Each layer is responsible for its part of data transmission, from the physical medium to applications.** > **Layers: Physical, Data Link, Network, Transport, Session, Presentation, Application.** > **OSI helps understand network protocols, diagnostics, and network design.** --- ## Cheat sheet (5 seconds) **7 OSI layers:** 1. Physical: signals, cables 2. Data Link: MAC, frames, switch 3. Network: IP, routing 4. Transport: TCP/UDP 5. Session: session management 6. Presentation: encryption, encoding 7. Application: HTTP, DNS, SMTPFor the reviewerNote to the moderator (optional)Visible only to the moderator. Helps review go faster.