以一个具体的例子讲述usb设备的各种描述符的请求与返回，并根据具体例子深入浅出地讲述各种描述符信息的含义。-With a concrete example about a variety of usb device descr iptor and return the request and, based on specific examples in layman' s language about the meaning of the various descr iptors of i