The OSI (Open Systems Interconnection) 7-layer model is a conceptual framework that divides the process of network communication into seven distinct layers. Standardized by the ISO in 1984, it serves as the reference model for modern networks.
Layer 7 - Application Layer
The layer closest to the user, the Application Layer is where the user-facing applications we interact with operate.
Key Protocols:
- HTTP: Web service communication
- FTP: File transfer
- SMTP: Email transfer
- DNS: Domain name resolution
Practical Use Cases:
- Accessing websites with a web browser
- Sending and receiving emails
- Downloading/uploading files
Layer 6 - Presentation Layer
The Presentation Layer defines the format of the data, including encryption/decryption and encoding/decoding.
Key Functions:
- Image conversions (e.g., JPEG, GIF)
- Audio conversions (e.g., MIDI, WAV)
- Character encoding (e.g., ASCII, EBCDIC)
- SSL/TLS encryption
Practical Use Cases:
- Encryption during HTTPS communication
- Image file format conversions
- Character set conversions (e.g., UTF-8, EUC-KR)
Layer 5 - Session Layer
The Session Layer manages communication sessions, handling connection initiation, termination, and synchronization.
Key Functions:
- Session establishment and termination
- Session recovery
- Synchronization and checkpoints
Practical Use Cases:
- Maintaining login status
- Database connection management
- Real-time streaming connections
Layer 4 - Transport Layer
The Transport Layer ensures reliable data transfer between endpoints (end-to-end).
Key Protocols:
- TCP: Reliable, connection-oriented
- UDP: Fast, connectionless
Practical Use Cases:
- Web browsing (TCP)
- Video streaming (UDP)
- Game server communication (UDP)
Layer 3 - Network Layer
The Network Layer handles packet routing, determining the path packets take.
Key Protocols:
- IP: Packet delivery
- ICMP: Error reporting
- OSPF: Routing
Practical Use Cases:
- IP address-based communication
- Route determination via routers
- Inter-subnet communication
Layer 2 - Data Link Layer
The Data Link Layer ensures reliable transmission between adjacent nodes.
Key Protocols:
- Ethernet: Wired LAN
- WiFi: Wireless LAN
- PPP: Point-to-point connections
Practical Use Cases:
- MAC address-based communication
- LAN setup via switches
- Wireless network connections
Layer 1 - Physical Layer
The Physical Layer defines the physical medium over which data is transmitted.
Key Mediums:
- Fiber optic cables
- Twisted pair cables
- Radio frequencies
Practical Use Cases:
- Network cable connections
- WiFi antenna communication
- Optical fiber communication
Conclusion
The OSI 7-layer model provides a step-by-step understanding of the entire process of network communication. While each layer operates independently, they work collectively to enable data transfer and communication.