Host is Master
This is a fundamental USB concept. There is exactly one
master in a USB system: the host computer. USB devices
respond to host requests. USB devices cannot send information among themselves, as they could if USB were a
peer-to-peer topology.
However, there is one case where a USB device can initiate
signaling without prompting from the host. After being put
into a low-power ‘suspend’ mode by the host, a device can
signal a ‘remote wakeup’. This is the only case in which the
USB device is the initiator; in all other cases, the host makes
device requests and the device responds to them.
There is an excellent reason for this host-centric model. The
USB architects were keenly aware of cost, and the best way
to make low-cost peripherals is to put most of the ‘smarts’
into the host side, the PC. If USB had been defined as peerto-peer, every USB device would have required more intelligence, raising cost.
主机既是主设备
这个是USB的基础概念，在USB系统中只有一个主设备，就是主机电脑。USB设备相应主机的请求，但是USB设备不能发送同等的信息，如果能发送的话，那么USB就变成了同等的网络拓扑。
事实上只有在一种情况USB设备可以不需要主机的触发而发出信号，主机使其即进入低功耗的挂起模式后，USB设备发出远程唤醒信号。这是USB设备作为发起者的唯一案例。在其他场景中，USB设备都是等待主机的请求，然后进行回应。
USB不被构建成对等网络的原因就是因为降低成本和简化操作