--- title: "Explain OSI model and its layers" subject: "Computer Networks and Security" chapter: 1 key: "semester_7/computer_networks_and_security:osi-model-layers" source: https://www.bctengineer.com/q/semester_7/computer_networks_and_security/osi-model-layers site: BCT Engineering --- # Explain OSI model and its layers Computer Networks and Security · Chapter 1: Introduction to Computer Network - Asked in: 2070 Asar, 8 marks; 2074 Chaitra, 5 marks; year not given (asked 2 times) - Video: [What is OSI Model?](https://www.youtube.com/watch?v=Ilk7UXzV_Qc) — RealPars - Answer checked by Claude Opus 5.5 (high effort) on 2026-10-14 - Page: https://www.bctengineer.com/q/semester_7/computer_networks_and_security/osi-model-layers - Whole chapter: https://www.bctengineer.com/md/semester_7/computer_networks_and_security/chapter-1 ## Question Explain the layers of the OSI reference model with an appropriate figure. ## Short answer OSI has seven layers: physical transmits bits, data link frames with MAC and error control, network does IP addressing and routing, transport provides process-to-process delivery, session controls dialog and sync, presentation handles translation and encryption, and application gives user services. Data is encapsulated down and decapsulated up. ## Answer The **OSI (Open Systems Interconnection) model** by ISO divides network communication into seven layers. Each layer uses the services of the layer below and gives services to the layer above. Peer layers on two machines talk through protocols. ```text Sender Receiver +----------------+ +----------------+ | 7 Application |<--------->| 7 Application | | 6 Presentation |<--------->| 6 Presentation | | 5 Session |<--------->| 5 Session | | 4 Transport |<--------->| 4 Transport | | 3 Network |<--------->| 3 Network | | 2 Data link |<--------->| 2 Data link | | 1 Physical |<--------->| 1 Physical | +----------------+ +----------------+ \_______ transmission medium _______/ ``` #### 1. Physical layer Transmits raw bits over the medium. It defines voltage levels, bit timing (data rate), synchronization, connectors, topology and transmission mode (simplex, half duplex, full duplex). #### 2. Data link layer Moves frames node to node. Functions: framing, physical (MAC) addressing, flow control, error detection and correction (CRC), and media access control. #### 3. Network layer Delivers packets from source host to destination host across networks. Functions: logical (IP) addressing, routing, fragmentation and congestion control. #### 4. Transport layer Process-to-process (end-to-end) delivery. Functions: port (service-point) addressing, segmentation and reassembly, connection control, end-to-end flow control and error control (TCP, UDP). #### 5. Session layer Establishes, manages and terminates sessions. Functions: dialog control (who transmits when) and synchronization using checkpoints for recovery. #### 6. Presentation layer Deals with syntax and semantics of the data. Functions: translation between character codes, encryption and decryption, and compression. #### 7. Application layer Gives the user (or application) access to the network. Services: file transfer (FTP), mail (SMTP), web (HTTP), remote login (Telnet), directory services. Data is wrapped with a header at each layer on the sending side (encapsulation) and removed in the reverse order, layer by layer, at the receiver (decapsulation). ## Check yourself The answer's key points as multiple-choice questions, with the correct option and why. 1. Which is the correct order of OSI layers from bottom (1) to top (7)? A. Physical, Data link, Network, Transport, Session, Presentation, Application B. Physical, Network, Data link, Transport, Session, Presentation, Application C. Physical, Data link, Network, Transport, Presentation, Session, Application D. Data link, Physical, Network, Transport, Session, Presentation, Application Answer: A. The stack from layer 1 to 7 is Physical, Data link, Network, Transport, Session, Presentation, Application. (Key point: Layer order) 2. Which layer is responsible for process-to-process (end-to-end) delivery using port addresses? A. Network layer B. Data link layer C. Transport layer D. Session layer Answer: C. The transport layer uses port (service-point) addressing to deliver data between processes; the network layer only delivers host to host. (Key point: Delivery scope of layers) 3. Which set of functions belongs to the presentation layer? A. Routing and fragmentation B. Translation, encryption/decryption and compression C. Dialog control and checkpoints D. Framing and CRC Answer: B. The presentation layer handles syntax and semantics: code translation, encryption and compression. (Key point: Presentation layer functions) 4. What happens to data as it passes down the layers on the sending side? A. Each layer removes a header B. Each layer adds its own header, and the receiver removes them in the same order going up C. Only the physical layer adds a header D. Headers are added only at the application layer Answer: B. Encapsulation adds a header at each layer on the sender; the receiver unwraps them as data moves up. (Key point: Encapsulation) ## Related in Computer Networks and Security - [Tasks done at OSI layers (set 2)](https://www.bctengineer.com/question.md?key=semester_7%2Fcomputer_networks_and_security%3Aosi-layer-task-mapping-2) — Computer Networks and Security, chapter 1 - [Significance of OSI layers](https://www.bctengineer.com/question.md?key=semester_7%2Fcomputer_networks_and_security%3Aosi-significance-layers) — Computer Networks and Security, chapter 1 - [Tasks done at OSI layers (set 1)](https://www.bctengineer.com/question.md?key=semester_7%2Fcomputer_networks_and_security%3Aosi-layer-task-mapping-1) — Computer Networks and Security, chapter 1 - [Compare OSI and TCP/IP models](https://www.bctengineer.com/question.md?key=semester_7%2Fcomputer_networks_and_security%3Aosi-vs-tcpip) — Computer Networks and Security, chapter 1, asked 4× ## Related in other subjects - [SS7 vs OSI; MTP-L2 signal units](https://www.bctengineer.com/question.md?key=semester_8%2Ftelecommunications%3Ass7-vs-osi-mtp2-units) — Telecommunications, chapter 5 - [Book store process model](https://www.bctengineer.com/question.md?key=semester_5%2Fsoftware_engineering%3Abook-store-process-model) — Software Engineering, chapter 2