A device that increases the number of USB ports on a PC. However, since the hub plugs into one of the USB ports on the computer, the total number of additional ports is minus one. For example, a four-port hub adds three new ports. USB hubs are typically used to extend USB sockets to the top of the desk for conveniently connecting external peripherals. Self Powered Vs. Bus Powered Self-powered hubs derive their power from a wall outlet and usually provide up to 500 milliamps at each port. In contrast, bus-powered hubs plug only into the computer's USB bus and obtain all their power from the bus. As a result, they have to split the total 500 milliamps among all the ports and the hub itself, which generally draws 100 milliamps. For example, a four port bus-powered hub would have 100 milliamps of power at each port. When not plugged into an AC outlet, a self-powered hub may sometimes be able to function as a bus-powered hub. Keyboards and mice typically need less than 100 milliamps, but USB drives and external hard disks need more. See USB switch, PoweredUSB and USB.