OSI Model in Computer Networks: Concepts, Layers & MCQs (2025)

The OSI (Open Systems Interconnection) Model is one of the most important concepts in Computer Networks. It defines how data is transmitted between two devices over a network in a layered architecture.

In exams, technical interviews, and competitive tests, questions on the OSI model are very common. This post explains the 7 layers of the OSI model, their functions, examples, and includes MCQs for practice.

What is the OSI Model?

• Developed by ISO (International Standards Organization) in 1984.
• Provides a 7-layer framework for communication between systems.
• Each layer performs a specific function and communicates with adjacent layers.

7 Layers of OSI Model

1. Physical Layer

   • Deals with transmission of raw bits over a medium.

   • Example: cables, hubs, repeaters.

2. Data Link Layer

   • Ensures reliable transfer of data frames.

   • Handles error detection & MAC addressing.

   • Example: switches, Ethernet.

3. Network Layer

   • Provides logical addressing & routing.

   • Example: IP, routers.

4. Transport Layer

   • Provides end-to-end communication.

   • Ensures reliability using TCP/UDP.

5. Session Layer

   • Manages sessions between applications.

   • Example: authentication, synchronization.

6. Presentation Layer

   • Translates data formats, encryption & compression.

   • Example: JPEG, SSL/TLS encryption.

7. Application Layer

   • Closest to users, provides services like email, web browsing.

   • Example: HTTP, FTP, SMTP.

OSI vs TCP/IP Model

• OSI has 7 layers, TCP/IP has 4 layers.
• OSI is a reference model, TCP/IP is practical implementation.

Solved Example

Example 1: At which OSI layer does IP addressing occur?
Answer: Network Layer.

Example 2: Which layer ensures data encryption and compression?
Answer: Presentation Layer.

MCQ Practice

1. Which OSI layer is responsible for error detection?
   (a) Physical Layer
   (b) Data Link Layer
   (c) Transport Layer
   (d) Network Layer

2. Which of the following protocols works at the Application Layer?
   (a) HTTP
   (b) TCP
   (c) IP
   (d) Ethernet

3. Which OSI layer handles logical addressing and routing?
   (a) Transport
   (b) Session
   (c) Network
   (d) Data Link

4. Which OSI layer provides end-to-end delivery?
   (a) Physical
   (b) Network
   (c) Transport
   (d) Session

Tips to Master OSI Model

• Memorize layers using mnemonics like “All People Seem To Need Data Processing” (Application → Physical).
• Understand the difference between OSI and TCP/IP.
• Focus on protocols and devices at each layer.
• Practice MCQs frequently.

FAQs:

Q1. Why is the OSI model important?
It standardizes communication and helps understand networking.

Q2. What is the difference between physical and logical addressing?
Physical address = MAC address, Logical address = IP address.

Q3. Which OSI layer handles encryption?
Presentation Layer.

Q4. Which is the topmost layer of OSI?
Application Layer.