Hubs

Physically there exist a number of USB ports at the rear panel of a computer. These ports can be used to attach normal devices or a hub. A hub is a USB device which extends the number of ports (i.e. 2-8) to connect other USB devices. The maximum number of attachable devices is reduced by the number of hubs on the bus. Hubs are self- and/or bus powered full speed devices.

Normally the physical ports of the host controller are handled by a virtual root hub. This hub is simulated by the host controller's device driver and helps to unify the bus topology. So every port can be handled in the same way by the USB subsystem's hub driver (see figure 1).