OSI Model – Complete & Simple Explanation
What is the OSI Model?
The OSI Model (Open Systems Interconnection Model) is a conceptual framework that explains how data travels from one device to another over a network.
It divides network communication into 7 layers, where each layer has a specific responsibility.
The OSI model helps engineers understand, design, build, and troubleshoot networks.
Why is the OSI Model Used?
The OSI model is used to:
- Standardize network communication
- Break complex networking into simple layers
- Help in debugging and troubleshooting network issues
- Allow interoperability between different vendors
- Make learning networking easier
The 7 Layers of the OSI Model
7️⃣ Application Layer
Purpose: User-facing network services
- Interfaces directly with applications
- Provides network services to user applications
Examples:
- HTTP, HTTPS
- FTP
- SMTP
- DNS
- SSH
6️⃣ Presentation Layer
Purpose: Data formatting and security
- Translates data formats
- Handles encryption and decryption
- Handles compression
Examples:
- SSL / TLS
- JPEG, MP3 encoding formats
5️⃣ Session Layer
Purpose: Session management
- Establishes, manages, and terminates sessions
- Maintains communication sessions
Examples:
- NetBIOS session
- RPC (Remote Procedure Call)
4️⃣ Transport Layer
Purpose: Reliable data delivery
- Breaks data into segments
- Ensures correct delivery
- Handles flow control and error correction
Protocols:
- TCP (reliable)
- UDP (fast, unreliable)
3️⃣ Network Layer
Purpose: Routing and logical addressing
- Determines best path for data
- Uses IP addresses
- Handles routing between networks
Protocols:
- IP (IPv4 / IPv6)
- ICMP
- Routing protocols (OSPF, RIP, BGP)
Devices:
2️⃣ Data Link Layer
Purpose: Physical delivery in local network
- Uses MAC addresses
- Frames data
- Error detection
- Switch-based forwarding
Protocols:
Devices:
1️⃣ Physical Layer
Purpose: Actual data transmission
- Transmits raw bits (0s and 1s)
- Defines cables, signals, voltages
Examples:
- Ethernet cables
- Fiber optics
- Wi-Fi signals
How Data Flows Through the OSI Model
Sending Device:
Application → Presentation → Session → Transport → Network → Data Link → Physical
Receiving Device:
Physical → Data Link → Network → Transport → Session → Presentation → Application
OSI Model vs TCP/IP Model
| OSI Model |
TCP/IP Model |
| 7 Layers |
4 Layers |
| Conceptual |
Practical |
| Used for learning |
Used in real networks |
Simple Analogy (Courier System)
- Application → What you want to send
- Presentation → Packaging and encryption
- Session → Starting the delivery
- Transport → Ensuring safe delivery
- Network → Choosing route
- Data Link → Delivery to correct house
- Physical → Road and vehicle
Why the OSI Model is Important?
- Makes networking easy to understand
- Helps isolate network problems by layer
- Improves communication between teams
- Used heavily in interviews and certifications