Concept Notes – OSI Reference Model
⚙️ What is OSI Model?
The OSI (Open Systems Interconnection) Model is a 7-layer framework used to understand how data moves in a network. Each layer has a specific function in transmitting data from one device to another.
It helps in:
- Standardizing communication between devices
- Troubleshooting networks
- Understanding protocols
🔎 Layer-wise Explanation (with Functions & Examples)
1️⃣ Physical Layer (Layer 1)
- Deals with hardware transmission of raw bits (0s & 1s).
- Concerned with cables, switches, voltages, frequency.
- Functions: Bit transmission, data rate control, physical topology.
- Example Devices: Hubs, Repeaters, Cables, NIC.
- Example: Sending electrical signals over a LAN cable.
2️⃣ Data Link Layer (Layer 2)
- Provides error detection & correction.
- Ensures node-to-node delivery.
- Divided into two sublayers:
- LLC (Logical Link Control)
- MAC (Media Access Control)
- Functions: Framing, Error detection, Flow control.
- Example Devices: Switches, Bridges.
- Example: Ethernet, MAC address communication.
3️⃣ Network Layer (Layer 3)
- Responsible for source-to-destination delivery across multiple networks.
- Uses logical addressing (IP Address).
- Functions: Routing, Addressing, Packet forwarding.
- Example Devices: Routers.
- Example: IP protocol sending packets over the Internet.
4️⃣ Transport Layer (Layer 4)
- Ensures end-to-end delivery with reliability.
- Uses segmentation and reassembly.
- Functions: Flow control, Error control, Reliability.
- Protocols: TCP (reliable), UDP (fast but unreliable).
- Example: TCP for downloading a file, UDP for live video streaming.
5️⃣ Session Layer (Layer 5)
- Manages sessions (dialogues) between applications.
- Functions: Session establishment, synchronization, termination.
- Example: Login sessions, video conference connection.
6️⃣ Presentation Layer (Layer 6)
- Translates data into a format understandable by the application.
- Functions: Encryption, Decryption, Compression, Translation.
- Example: SSL/TLS encryption, JPEG/PNG image formats.
7️⃣ Application Layer (Layer 7)
- Closest to the end-user.
- Provides network services to applications.
- Functions: User interface, Resource sharing, Email, Browsing.
- Example: HTTP, SMTP, FTP, DNS.
⚙️ Quick Summary Table – OSI Layers
| Layer | Function | Example Protocols/Devices |
|---|---|---|
| 7. Application | User interaction | HTTP, FTP, DNS |
| 6. Presentation | Data translation, encryption | SSL, JPEG |
| 5. Session | Session management | NetBIOS, RPC |
| 4. Transport | Reliable delivery | TCP, UDP |
| 3. Network | Routing, addressing | IP, Router |
| 2. Data Link | Node-to-node, framing | Ethernet, Switch |
| 1. Physical | Bit transmission | Cable, Hub |
🔟 10 Most Expected MCQs – ECET 2026 (OSI Model)
Q1. Which OSI layer is responsible for encryption & decryption?
A) Session
B) Presentation
C) Application
D) Network
Q2. Which device works at Layer 3 (Network Layer)?
A) Switch
B) Hub
C) Router
D) Repeater
Q3. The OSI model has how many layers?
A) 5
B) 6
C) 7
D) 8
Q4. Which OSI layer ensures error detection and correction?
A) Physical
B) Data Link
C) Transport
D) Application
Q5. Which protocol works at the Transport Layer?
A) HTTP
B) TCP
C) IP
D) Ethernet
Q6. The session layer is responsible for:
A) Routing
B) Encryption
C) Session establishment
D) Error correction
Q7. In which layer does IP address work?
A) Data Link
B) Network
C) Transport
D) Session
Q8. Which OSI layer converts data into signals?
A) Physical
B) Data Link
C) Transport
D) Application
Q9. Which is the first layer from the user’s side in OSI model?
A) Session
B) Application
C) Network
D) Transport
Q10. Compression of data is handled by which OSI layer?
A) Application
B) Transport
C) Presentation
D) Session
✅ Answer Key
| Q.No | Answer |
|---|---|
| Q1 | B |
| Q2 | C |
| Q3 | C |
| Q4 | B |
| Q5 | B |
| Q6 | C |
| Q7 | B |
| Q8 | A |
| Q9 | B |
| Q10 | C |
🧠 Explanations
- Q1 → B: Encryption/Decryption happens in Presentation Layer.
- Q2 → C: Routers operate at Network Layer.
- Q3 → C: OSI Model has 7 layers.
- Q4 → B: Error detection/correction is in Data Link Layer.
- Q5 → B: TCP/UDP are Transport Layer protocols.
- Q6 → C: Session Layer establishes and manages sessions.
- Q7 → B: IP address belongs to Network Layer.
- Q8 → A: Physical Layer converts bits into signals.
- Q9 → B: Application Layer is user-facing.
- Q10 → C: Data compression is done in Presentation Layer.
🎯 Why This Practice Matters
OSI Model is a sure-shot topic in ECET. Every year, at least 1–2 direct questions are asked on OSI layers and their functions. By practicing these, you can easily secure 2–3 marks.
Also, understanding OSI layers helps in networking jobs, CCNA, GATE, and interviews.
📲 CTA
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