Typically only a small number of USB devices are attached to a system at one time, and on the average a USB device supports 3-4 endpoints, of which only a subset of the endpoints are active at the same time.
一般来说，只有少量USB设备被同时连接到一个系统，并且平均来说，一个USB设备支持3-4个端点，而其中部分端点在同一时间处于活动状态。