Quick distinctions
| Evidence | What it establishes | What it does not establish |
|---|---|---|
| Port/device/cable declaration | Published ceiling or supported feature | Live negotiation |
| Negotiated link class | Current connection class where telemetry exposes it | Application throughput |
| Shared upstream budget | Finite planning pool for simultaneous clients | Equal per-port allocation |
| File-copy or benchmark result | End-to-end workload result | Which segment negotiated down |
Role map
| Layer | Roles | May be dual-role? |
|---|---|---|
| Power | Source / Sink | DRP when declared |
| USB data | DFP/Host / UFP/Device | DRD when declared |
| Display | Source / Sink | Only if both directions are explicitly supported |
Safe fallback evidence
- A segment declared below the target is a hard planning ceiling.
- A direct target pass plus an intermediary failure implicates the intermediary layer.
- An unknown cable claim remains unknown; connector fit is not a substitute.
- Missing USB4 enumeration is a reason to recheck the path, not proof of one defective item.
- A browser cannot automatically read the physical cable’s certification or the native negotiated USB link.