OSI 7-Layer Model
Network Fundamentals
Overview
OSI separates responsibilities: application services, presentation encoding, session coordination, transport delivery, network addressing, data-link framing and physical signals. Troubleshoot from observed evidence, not by assuming every failure is a cable fault. TCP/IP combines some OSI responsibilities.
Protocol stages
- 1Application data
- 2TCP segment
- 3IPv4 packet
- 4Ethernet frame
- 5Physical signal
Interactive exercise
Knowledge check
A switch port is up and same-subnet ping succeeds, but an HTTPS certificate is expired. Which responsibility should you investigate?
Educational model — not real hardware, traffic or official certification.