Quick Answer
OSI model has 7 layers (bottom to top): Physical (bits, cables), Data Link (frames, MAC, switches), Network (packets, IP, routers), Transport (segments, TCP/UDP), Session (connections), Presentation (encryption, formatting), Application (HTTP, FTP, user services). Data is encapsulated going down, decapsulated going up. Mnemonic: "Please Do Not Throw Sausage Pizza Away."
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Why Interviewers Ask This
Fundamental networking concept
Tests understanding of network architecture
Required for any software/IT role
Basis for troubleshooting network issues
Shows structured thinking about communication
Concept Explanation
Simple Explanation (Start Here)
OSI model is like sending a letter: You write content (Application), put it in envelope (Presentation), address it (Session), choose courier (Transport), courier plans route (Network), puts in delivery van (Data Link), van drives on road (Physical). Each layer adds its own "wrapping" going down, removes it going up.
Real-World Analogy
Sending a package internationally: - Application: You decide what to send - Presentation: Pack it properly - Session: Fill customs forms - Transport: Choose express or economy - Network: Logistics plans international route - Data Link: Local delivery hub handles it - Physical: Actual truck carries the package
Detailed Technical Explanation
OSI (Open Systems Interconnection) - 7 layer reference model for network communication:
Layer 7 - Application: User interface (HTTP, FTP, SMTP, DNS) Layer 6 - Presentation: Data format, encryption, compression (SSL, JPEG, ASCII) Layer 5 - Session: Session management, authentication (NetBIOS, RPC) Layer 4 - Transport: End-to-end delivery, error recovery (TCP, UDP) Layer 3 - Network: Logical addressing, routing (IP, ICMP, routers) Layer 2 - Data Link: Physical addressing, frames (MAC, Ethernet, switches) Layer 1 - Physical: Bits on wire, cables, signals (hubs, cables, voltage)
Key Facts to Remember
- Mnemonic: "All People Seem To Need Data Processing" (7→1) or "Please Do Not Throw Sausage Pizza Away" (1→7)
- Application (7): End-user services (HTTP, FTP, DNS, SMTP)
- Presentation (6): Data translation, encryption (SSL, MIME)
- Session (5): Connection establishment, sync points (RPC)
- Transport (4): Segmentation, flow control (TCP-reliable, UDP-fast)
- Network (3): Logical addressing, routing (IP addresses, routers)
- Data Link (2): Physical addressing, frames (MAC addresses, switches)
- Physical (1): Binary transmission, cables, hubs
Formulas & Code
PDU Names by Layer:
Layer 7-5: Data
Layer 4: Segment (TCP) / Datagram (UDP)
Layer 3: Packet
Layer 2: Frame
Layer 1: BitsTCP/IP Mapping:
Application (OSI 5-7)
Transport (OSI 4)
Internet (OSI 3)
Network Access (OSI 1-2)Visual Explanation
Draw a stack of 7 boxes labeled 1-7 from bottom to top with layer names. Show protocols alongside each layer. Draw arrows showing data flow down (encapsulation) and up (decapsulation). Show PDU names: Data, Segment, Packet, Frame, Bits.
Pro tip: Draw this diagram while explaining to leave a strong impression.
Common Mistakes to Avoid
- ✗Confusing OSI with TCP/IP model (OSI has 7 layers, TCP/IP has 4)
- ✗Not knowing protocols for each layer
- ✗Mixing up Network (IP) and Data Link (MAC) addressing
- ✗Forgetting that data is encapsulated going down, decapsulated going up
- ✗Not knowing which devices work at which layer
Pro Tips for Success
- ✓Use mnemonics: "Please Do Not Throw Sausage Pizza Away" (1→7)
- ✓Know at least 2 protocols per layer
- ✓Remember: Routers = Layer 3 (IP), Switches = Layer 2 (MAC), Hubs = Layer 1
- ✓TCP/IP model is more practical; OSI is more conceptual
Expected Follow-up Questions
Key Takeaways
- 7 layers: Physical, Data Link, Network, Transport, Session, Presentation, Application
- Mnemonic: "Please Do Not Throw Sausage Pizza Away"
- Routers = Layer 3, Switches = Layer 2, Hubs = Layer 1
- TCP/UDP at Transport, IP at Network, MAC at Data Link
- OSI = 7 layers (conceptual), TCP/IP = 4 layers (practical)
Related Questions You Should Know
What is the difference between SQL and NoSQL databases?
SQL databases are like Excel spreadsheets—organized tables with rows and columns, strict structure. NoSQL databases are like folders with documents—flexible, each document can have different fields. SQL = predictable structure, strong relationships. NoSQL = flexibility, horizontal scaling.
Explain OOPs concepts with real-world examples
OOPs is like organizing a company: Encapsulation (departments keep their data private), Inheritance (junior roles inherit from senior roles), Polymorphism (same job title, different work in different departments), Abstraction (CEO sees high-level reports, not every detail).
Research Foundations
Our Information Technology interview guides are built on established pedagogical research and industry best practices. Here are the key sources that inform our approach:
Dr. HC Verma
Concepts of Physics (1992)
“Understanding fundamentals deeply enables solving complex problems by breaking them into basic principles.”
How We Apply This:
When answering technical questions, always start from first principles. Interviewers value candidates who understand WHY, not just WHAT.
Gayle Laakmann McDowell
Cracking the Coding Interview (2022)
“Technical interviews test problem-solving process, not just memorized answers.”
How We Apply This:
Think out loud, explain your reasoning, and show how you approach unfamiliar problems systematically.
Richard Feynman
The Feynman Technique
“If you cannot explain something simply, you do not understand it well enough.”
How We Apply This:
Practice explaining complex concepts in simple terms. Use analogies and real-world examples to demonstrate mastery.
NPTEL Faculty
National Programme on Technology Enhanced Learning
“Strong fundamentals in core subjects differentiate exceptional engineers from average ones.”
How We Apply This:
Revisit core subjects from your curriculum. Most technical questions test fundamental concepts, not advanced topics.
George Pólya
How to Solve It (1945)
“A systematic approach to problem-solving works across all engineering domains.”
How We Apply This:
Use a structured approach: Understand → Plan → Execute → Verify. Interviewers notice methodical thinking.
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