Civil Service Hospital2080 BSQuestion 4 of 10
What is OSI reference model? Describe each layer and compare it with TCP/IP model.
10 marks
Question source
- Exam year
- 2080 BS / 2023 AD
- Level
- Level 5 (Prabidhik, Engineering and Maintenance, Computer Technician)
- Paper
- Paper II – Technical Subject
- Section
- Section A
Model answer
OSI is ISO's 7-layer framework describing how data travels from app to wire. It is a reference, not implementation; TCP/IP is the practical 4-layer stack.
OSI layers (top → bottom) with example (opening hospital portal):
- Application (7) — User services: HTTP, email. Browser requests page.
- Presentation (6) — Format/encryption: JPEG, HTTPS/TLS.
- Session (5) — Dialog control: login session, reconnection.
- Transport (4) — End-to-end delivery: TCP (reliable) vs UDP (fast).
- Network (3) — Routing by IP: routers choose Kathmandu → server path.
- Data Link (2) — LAN framing by MAC: switch forwards to gateway.
- Physical (1) — Bits on cable/fibre/Wi-Fi.
OSI vs TCP/IP:
| OSI (7) | TCP/IP (4) | Note |
|---|---|---|
| Application + Presentation + Session | Application | Merged in practice |
| Transport | Transport (TCP/UDP) | Same role |
| Network | Internet (IP) | Same role |
| Data Link + Physical | Network Access | Merged |
Remember mnemonic: All People Seem To Need Data Processing (7→1).