Concept Notes
1. Introduction
- OSI = Open Systems Interconnection model.
- It is a reference model developed by ISO (International Standards Organization) in 1984.
- Purpose → to standardize communication between different systems without hardware/software dependency.
OSI has 7 layers, each with specific functions.
2. OSI 7 Layers
(i) Physical Layer (Layer 1)
- Deals with raw bits (0s & 1s) transmission over medium.
- Functions:
- Transmission media (cables, radio waves)
- Data rate, voltages, synchronization
- Example: Ethernet cable, hubs, repeaters
(ii) Data Link Layer (Layer 2)
- Ensures error-free transfer of data frames between nodes.
- Functions:
- Framing
- Error detection (CRC, parity)
- Flow control
- MAC addressing
- Example: Switch, MAC address
(iii) Network Layer (Layer 3)
- Responsible for routing & addressing.
- Functions:
- Logical addressing (IP addresses)
- Routing (finding best path)
- Packet forwarding
- Example: Routers, IP protocol
(iv) Transport Layer (Layer 4)
- Ensures end-to-end communication between processes.
- Functions:
- Segmentation & reassembly
- Flow & error control
- Reliable/unreliable delivery (TCP/UDP)
- Example: TCP, UDP
(v) Session Layer (Layer 5)
- Manages sessions (dialogues) between systems.
- Functions:
- Establish, maintain, terminate sessions
- Synchronization checkpoints
- Example: API calls, Remote Procedure Call (RPC)
(vi) Presentation Layer (Layer 6)
- Handles data translation, encryption, compression.
- Functions:
- Data format translation (ASCII → EBCDIC)
- Encryption/Decryption (SSL, TLS)
- Compression (JPEG, MP3)
- Example: SSL, JPEG, MPEG
(vii) Application Layer (Layer 7)
- Closest to the user.
- Provides network services to applications.
- Functions:
- Email, File Transfer, Web browsing
- Example: HTTP, FTP, SMTP, DNS
3. OSI Mnemonic
👉 “Please Do Not Throw Sausage Pizza Away”
(P → Physical, D → Data Link, N → Network, T → Transport, S → Session, P → Presentation, A → Application)
4. OSI Model vs TCP/IP Model
| Feature | OSI Model | TCP/IP Model |
|---|---|---|
| Layers | 7 | 4 |
| Developed by | ISO | DARPA |
| Protocol dependency | Protocol independent | Protocol dependent |
| Common use | Theoretical | Practical (Internet) |
⚙️ Formulas
OSI model is theoretical (concept-based) → No direct formulas.
(But in Data Link Layer CRC, parity, error detection → separate topic later.)
🔟 10 MCQs
Q1. OSI model was developed by:
a) IEEE
b) ISO
c) IETF
d) ITU
Q2. How many layers does OSI model have?
a) 5
b) 6
c) 7
d) 4
Q3. Which OSI layer is responsible for routing?
a) Data Link
b) Transport
c) Network
d) Session
Q4. In which OSI layer does encryption take place?
a) Network
b) Session
c) Presentation
d) Application
Q5. TCP and UDP operate at which OSI layer?
a) Transport
b) Session
c) Network
d) Data Link
Q6. Which layer converts data into frames?
a) Physical
b) Data Link
c) Network
d) Presentation
Q7. HTTP works at which OSI layer?
a) Transport
b) Application
c) Presentation
d) Network
Q8. A router works at which layer of OSI model?
a) Network
b) Data Link
c) Transport
d) Application
Q9. Which layer is also called the syntax layer?
a) Transport
b) Presentation
c) Application
d) Network
Q10. Which of the following is a mnemonic for OSI layers?
a) All People Seem To Need Data Processing
b) Please Do Not Throw Sausage Pizza Away
c) Eat An Apple Every Day
d) None
✅ Answer Key
| Q No | Answer |
|---|---|
| Q1 | b |
| Q2 | c |
| Q3 | c |
| Q4 | c |
| Q5 | a |
| Q6 | b |
| Q7 | b |
| Q8 | a |
| Q9 | b |
| Q10 | b |
🧠 Explanations
- Q1: OSI was developed by ISO.
- Q2: OSI = 7 layers.
- Q3: Routing = Network layer.
- Q4: Encryption, translation = Presentation layer.
- Q5: TCP/UDP → Transport layer.
- Q6: Frames = Data Link layer.
- Q7: HTTP = Application layer protocol.
- Q8: Router = Network layer device.
- Q9: Presentation = Syntax layer.
- Q10: Mnemonic is “Please Do Not Throw Sausage Pizza Away.”
🎯 Motivation
OSI Model is the foundation of Networking.
- If you know functions of each layer, you can easily solve ECET, GATE, and job interviews.
- Remember: Each layer adds services → together they make communication reliable.
👉 Mastering OSI → You can confidently handle any network-related MCQ in ECET 2026.
📲 CTA
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