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.