The USB hot-swap feature is very convenient, but it is not foolproof. USB Hubs and Controllers USB allows for multiple peripherals to be plugged into a single USB port and addressed concurrently. The USB port on the computer is both a controller and a hub. While many computers have two USB ports, they may be connected to the same controller. When multiple devices are connected to a single USB controller, each shares the controller s bandwidth.
Hubs allow the physical connection of multiple peripherals to the USB controller.
Hubs also provide power for peripherals. External hubs are connected to the USB port on a computer to provide additional ports. Active hubs provide 500 mA to each USB port. Passive hubs divide power among the USB ports and are useful only for low power peripherals. USBPre functions best when it is the only device connected to a host controller, giving it full USB bandwidth.