A Device can be accessed directly from the Host with its PortIdentifier, or can be found by traversing the USB Bus starting from the root hub.
可以用一台Device的PortIdentifier直接从Host访问它，也可以通过从root hub开始遍历usb Bus找到它。
The program as written simply looks at the root hub for available USB devices, but it would be easy to improve it to traverse the whole USB tree.
这个程序编写为只是查看 root hub 上可用的 USB 设备，但是很容易将它改为遍历整个 USB 树。
A hub is at the center of each star, and the USB host is considered the root hub.
一个hub是每个星形的中心，usb主机被认为是root hub。
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