Seven responsibilities
Physical handles signals; data link frames and local delivery; network logical addressing/routing; transport end-to-end delivery; session conversation coordination; presentation representation/security concepts; application service protocols. The model is a reasoning tool, not a promise that every product or protocol belongs exclusively to one layer.
Encapsulation example
A web request becomes transport data, an IP packet and a link frame. At a router the link frame changes for the outgoing medium, while an ordinary no-NAT IP packet retains its final destination. Transport state remains end-to-end. Distinguish forwarding roles from endpoint application processing.
Evidence-driven layer isolation
A down link suggests physical or interface state; correct link with no same-VLAN connectivity points to L2/addressing; IP success with name failure points to DNS; a valid name and port with certificate error points to trust above IP. Several layers can fail together, so test rather than infer solely from one symptom.
AV application
Black video is not always an Ethernet issue: HDMI source selection, EDID, HDCP, power and media cabling can fail while IP control works. Network timing and discovery also matter in real AV systems. Match diagnostics to the actual signal path instead of calling every problem layer 1.
Signal / protocol flow
- 1Application
- 2Presentation / session
- 3Transport
- 4Network
- 5Data link
- 6Physical
Technical terminology
- Encapsulation: wrapping data in headers
- L2: local frame delivery
- L3: routed IP delivery
- PDU: protocol data unit
Worked example
PC-A can ping the projector but protected video is black. IP reachability does not validate HDCP across the media chain; investigate video authentication and compatible display capability.
Configuration reference
# Supported diagnostic references in the existing lab ping 192.168.10.20 nslookup intranet.local video status power status
Troubleshooting scenario
A technician changes VLANs to fix an expired certificate after successful HTTPS TCP connection. Preserve known-good transport and correct the certificate identity/validity issue instead.
Knowledge check
Which layer responsibility changes Ethernet headers at a routed hop?
Related practice
References
Educational model — not real hardware, traffic or official certification.