--- title: "Short note: OSI architecture" subject: "Telecommunications" chapter: 8 key: "semester_8/telecommunications:short-note-osi-architecture" source: https://www.bctengineer.com/q/semester_8/telecommunications/short-note-osi-architecture site: BCT Engineering --- # Short note: OSI architecture Telecommunications · Chapter 8: Data Communication - Asked in: 2065 Baisakh (old course), 8 marks - Video: [The OSI Reference Model (Part 1)](https://www.youtube.com/watch?v=qBXmbJZQ5rY) — Neso Academy - Answer checked by Claude Opus 5.5 (high effort) on 2026-10-14 - Page: https://www.bctengineer.com/q/semester_8/telecommunications/short-note-osi-architecture - Whole chapter: https://www.bctengineer.com/md/semester_8/telecommunications/chapter-8 ## Question Write a short note on OSI (Open System Interconnection) architecture in data communication system. ## Short answer The OSI reference model (ISO 7498) is a seven-layer framework for data exchange between open systems. Its layers are physical, data link, network, transport, session, presentation and application; each layer encapsulates data with headers, and lower three layers are network-oriented while upper four are end-to-end. ## Answer The **OSI (Open Systems Interconnection) reference model**, published by **ISO** (ISO 7498, 1984), is a seven-layer framework that describes how data is exchanged between open systems from different makers. Each layer does a defined job, uses the services of the layer below and serves the layer above. Peer layers on two machines communicate using **protocols**; actual data passes down the sender's stack, over the medium and up the receiver's stack. ```text Host A Host B +----------------+ +----------------+ | 7 Application |<-->| 7 Application | | 6 Presentation |<-->| 6 Presentation | | 5 Session |<-->| 5 Session | | 4 Transport |<-->| 4 Transport | | 3 Network |<-->| 3 Network | (also in | 2 Data link |<-->| 2 Data link | routers) | 1 Physical |<-->| 1 Physical | +-------+--------+ +-------+--------+ +====== medium =======+ ``` #### Functions of the layers 1. **Physical:** transmits raw bits; defines connectors, voltages, bit timing, line coding, data rate (RS-232, V.24, X.21). Unit: bit. 2. **Data link:** framing, MAC addressing, error detection (CRC) and correction by retransmission, flow control between adjacent nodes (HDLC, LAPB, PPP, IEEE 802; LLC and MAC sublayers). Unit: frame. 3. **Network:** routes packets from source to destination host across networks; logical addressing, routing, congestion control, internetworking (IP, X.25). Unit: packet. 4. **Transport:** reliable end-to-end delivery of whole messages; segmentation/reassembly, port addressing, end-to-end error and flow control, multiplexing (TCP, UDP, TP0–TP4). Unit: segment. 5. **Session:** sets up, manages and ends dialogues; dialogue control (who talks when) and synchronisation checkpoints for recovery. 6. **Presentation:** data representation – code conversion (ASCII/EBCDIC), encryption, compression (ASN.1, JPEG, TLS). 7. **Application:** network services for users – file transfer (FTAM, FTP), e-mail (X.400, SMTP), directory (X.500), virtual terminal, web. Layers 1–3 are network-oriented (also run in routers/switching nodes); layers 4–7 are end-to-end, running only in hosts. #### Encapsulation Each layer adds a header (data link also adds a trailer) to the data it receives, and the receiver strips them in reverse order: ```text L7 AH|Data L4 TH|...|Data segment L3 NH|TH|...|Data packet L2 DH|NH|TH|...|Data|DT frame L1 0101101001... bits ``` #### Merits - Breaks a complex job into small, independent layers; change in one layer does not affect others. - Open, vendor-independent standards allow interoperability. - Good tool for teaching, design and troubleshooting. The practical Internet uses TCP/IP, whose four layers map onto OSI (application ≈ 5–7, transport = 4, internet = 3, network access = 1–2). ## Check yourself The answer's key points as multiple-choice questions, with the correct option and why. 1. How do peer layers and actual data flow work in OSI? A. Data goes directly from layer 7 of A to layer 7 of B B. Peer layers talk by protocols, while real data passes down the sender stack, over the medium and up the receiver stack C. Only the physical layers communicate logically D. Each layer skips the layer below Answer: B. Communication between peers is logical; physical data movement is down, across and up the stacks. (Key point: Peer protocols vs actual data flow) 2. Which layer provides reliable end-to-end delivery using port addressing? A. Network B. Data link C. Session D. Transport Answer: D. The transport layer does process-to-process delivery with ports, segmentation, and end-to-end error and flow control; network does host-to-host routing. (Key point: Network vs transport responsibility) 3. Which set describes the data link layer? A. Framing, MAC addressing, CRC error detection and flow control between adjacent nodes B. Routing and logical addressing across networks C. Encryption and compression D. Dialogue control and checkpoints Answer: A. The data link layer makes a reliable link between adjacent nodes using frames, CRC/retransmission and flow control. (Key point: Data link functions) 4. Which task belongs to the presentation layer? A. Setting up dialogues B. Code conversion, encryption and compression C. Choosing paths for packets D. Sending raw bits Answer: B. Presentation handles data representation: ASCII/EBCDIC translation, encryption and compression. (Key point: Presentation layer) 5. During encapsulation, how are headers handled? A. Only the physical layer adds a header B. The receiver strips headers in the same order they were added C. Each layer adds its header and the receiver strips them in reverse order D. All headers are added at layer 7 Answer: C. Each layer adds its header on the way down; the receiver removes them in reverse order going up. 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