USB is host-driven enumeration, endpoint-based data transfer, and layered link management.
In the USB 2.0 examples, the host schedules transactions addressed to device endpoints.
USB 2.0: distinguish full-speed frames from high-speed microframes. USB 3.x has its own link mechanisms.
Descriptors define device capabilities — firmware must match silicon.
Applies layering and packet concepts to external connectivity.
No. It transfers no accepted data; the later successful transaction uses the still-expected PID.