Concept Notes – OSI & TCP/IP Layers
🌐 What is OSI Model?
- OSI (Open Systems Interconnection): A theoretical 7-layer model by ISO for standardizing communication between systems.
- Each layer has a specific role, making networking modular.
7 Layers of OSI Model (Top → Bottom):
- Application – Provides network services to applications (HTTP, FTP, SMTP).
- Presentation – Data translation, encryption, compression.
- Session – Establish, maintain, and terminate sessions.
- Transport – Reliable delivery (TCP/UDP), error checking, segmentation.
- Network – Logical addressing, routing (IP).
- Data Link – Framing, MAC addressing, error detection.
- Physical – Transmission of raw bits via medium (cables, wireless).
🌐 TCP/IP Model
- Practical model used in real networks (like the Internet).
- Has 4 layers, mapping roughly to OSI.
4 Layers of TCP/IP Model:
- Application – Combines OSI’s Application + Presentation + Session (Protocols: HTTP, DNS, SMTP).
- Transport – Same as OSI Transport (TCP/UDP).
- Internet – Equivalent to OSI Network (IP, ICMP, ARP).
- Network Access (Link/Host-to-Network) – Equivalent to OSI Data Link + Physical.
🔁 OSI vs TCP/IP
| Feature | OSI | TCP/IP |
|---|---|---|
| Layers | 7 | 4 |
| Approach | Reference model | Practical model |
| Protocols | Generic | Defines protocols (TCP, IP, HTTP, etc.) |
| Usage | Theoretical learning | Internet, real-world |
⚙️ Formulas
\text{Delay} = \text{Transmission Delay} + \text{Propagation Delay}
\text{Transmission Delay} = \dfrac{\text{Packet Size (bits)}}{\text{Bandwidth (bps)}}
\text{Propagation Delay} = \dfrac{\text{Distance}}{\text{Propagation Speed}}
🔟 10 MCQs – GATE 2026 CN (OSI & TCP/IP)
Q1. How many layers are there in the OSI model?
A) 5
B) 6
C) 7
D) 4
Q2. Which layer of OSI is responsible for encryption?
A) Application
B) Session
C) Presentation
D) Transport
Q3. Which protocol works at Transport layer in TCP/IP?
A) HTTP
B) TCP
C) IP
D) Ethernet
Q4. IP addressing is part of which layer?
A) Network (OSI) / Internet (TCP/IP)
B) Data Link
C) Transport
D) Application
Q5. Which of the following is NOT part of TCP/IP layers?
A) Transport
B) Internet
C) Presentation
D) Application
Q6. Match OSI to TCP/IP:
Application (OSI) → ?
A) Application (TCP/IP)
B) Transport (TCP/IP)
C) Internet (TCP/IP)
D) Network Access
Q7. Which OSI layer handles framing and MAC addressing?
A) Data Link
B) Network
C) Transport
D) Physical
Q8. What is the correct order (top to bottom) in OSI model?
A) Physical → Data Link → Network → Transport → Session → Presentation → Application
B) Application → Presentation → Session → Transport → Network → Data Link → Physical
C) Application → Session → Presentation → Transport → Network → Data Link → Physical
D) Session → Application → Presentation → Transport → Physical → Data Link → Network
Q9. In TCP/IP, ICMP belongs to which layer?
A) Transport
B) Internet
C) Application
D) Network Access
Q10. If packet size = 10,000 bits, bandwidth = 1 Mbps, distance = 200 km, speed = 2×10^8 m/s → Find Transmission Delay.
A) 0.001 sec
B) 0.01 sec
C) 0.1 sec
D) 1 sec
✅ Answer Key
Q1 → C
Q2 → C
Q3 → B
Q4 → A
Q5 → C
Q6 → A
Q7 → A
Q8 → B
Q9 → B
Q10 → B
🧠 Explanations
- Q1: OSI has 7 layers.
- Q2: Encryption, compression at Presentation layer.
- Q3: Transport uses TCP/UDP.
- Q4: IP = Network/Internet layer.
- Q5: TCP/IP has no Presentation layer.
- Q6: OSI’s top three layers merge into TCP/IP Application.
- Q7: Data Link = Framing, MAC.
- Q8: OSI top-to-bottom = Application → Presentation → Session → Transport → Network → Data Link → Physical.
- Q9: ICMP belongs to Internet layer.
- Q10: Transmission Delay = 10,000 / 10^6 = 0.01 sec.
🎯 Motivation / Why Practice Matters
- Questions on OSI & TCP/IP mapping are very common in GATE.
- Networking fundamentals are often tested in coding interviews too (system design, networking roles).
- Mastering this topic helps in understanding protocol layers, packet flow, and performance metrics.
