The OSI Model

The OSI (Open Systems Interconnection) model is a 7-layer framework that describes how data travels across a network, from the physical cable to the application a user interacts with.

The 7 Layers (top to bottom)

7. Application   - The app itself (browser, email client)
6. Presentation   - Data formatting, encryption (SSL/TLS)
5. Session        - Managing connections/sessions
4. Transport      - TCP/UDP, reliable delivery, ports
3. Network        - IP addresses, routing
2. Data Link      - MAC addresses, switches
1. Physical       - Cables, WiFi signals, hardware
    

Easy Way to Remember

“Please Do Not Throw Sausage Pizza Away” (Physical, Data Link, Network, Transport, Session, Presentation, Application)

Why IT Support Needs This

The OSI model helps structure troubleshooting from the bottom up:

• Layer 1: Is the cable plugged in? Is WiFi on?
• Layer 2: Is the network switch/adapter working?
• Layer 3: Does the device have a valid IP address?
• Layer 4-7: Is the application/service itself working (e.g., website down vs. network down)?

Common Interview Talking Point

“When troubleshooting connectivity, I think in terms of OSI layers – starting from the physical connection, then checking the IP configuration, and finally testing the specific application or service the user is trying to reach.”