Osi Model

# OSI Model > Exam relevance: The OSI model is explicitly listed under Network Fundamentals, which carries 20% weight on the CCNA 200-301 v1.1 exam (120 minutes total). It appears in virtually every CCNA exam as both standalone questions and as the conceptual backbone for troubleshooting, protocol identification, and encapsulation questions. Expect 3–6 direct questions plus indirect application throughout the exam. --- ## Why the OSI Model Matters The Open Systems Interconnection (OSI) model is a 7-layer conceptual framework that standardizes how different network functions interact. Cisco uses it as the universal troubleshooting language — when a question asks "at which layer does a switch operate?" or "what PDU is used at Layer 4?", it is testing OSI fluency directly. --- ## The Seven Layers | Layer # | Name | Key Function | PDU Name | Devices / Protocols | |---------|------|-------------|----------|---------------------| | 7 | Application | User-facing services, HTTP/DNS/SMTP | Data | HTTP, DNS, FTP, SMTP, Telnet | | 6 | Presentation | Encoding, encryption, compression | Data | SSL/TLS, JPEG, ASCII | | 5 | Session | Session establishment/teardown | Data | NetBIOS, RPC | | 4 | Transport | End-to-end delivery, reliability, ports | Segment | TCP, UDP | | 3 | Network | Logical addressing, routing | Packet | IP, OSPF, ICMP | | 2 | Data Link | MAC addressing, framing, switch forwarding | Frame | Ethernet, 802.1Q, PPP | | 1 | Physical | Bits on the wire, cables, signals | Bit | RJ-45, fiber, hubs | > Memory aid: "All People Seem To Need Data…