Composite USB devices 组合设备
Compound USB devices 复合设备
Composite USB devices expose multiple functions to a host by
using multiple interfaces.
A compound device is an external hub that is combined with
other devices. Compound devices appear to the host as a hub with one or more
non-removable devices that are connected to the hub’s ports.
对于compound device，它内置的hub和其它设备都会有各自的device address， 而compoiste device只有一个device address。实际上，从host的角度来看，compound device与一个
hub上挂多个设备是等效的。WUSB不推荐将device实现为compound device，参见WUSB AM FAQ。