avoid this problem completely. Length limitations USB cables are limited to 3 metres (10 feet) for low-speed USB 1.1 devices. A hub can be used as an active USB repeater to extend cable length for up to 5 metre (16 feet) lengths at a time. Active cables (specialized connector-embedded one-port hubs) perform the same function, but since they are strictly bus-powered, externally powered USB hubs would likely be required for some of the segments. Power A bus-powered hub (passive hub) is a hub that draws all its power from the host computer's USB interface. It does not need a separate power connection. However, many devices require more power than this method can provide and will not work in this type of hub. It may be desirable to use a bus-powered hub with self-powered external hard-disks, as the hard-disk may not spin down when the computer turns off or enters sleep mode while using a self-powered hub since the hard disk controller would continue to see a power source on the USB ports. All USB ports operate at 5 volts, but may draw or supply differing amounts of electric current. A USB's electric current is allocated in units of 100 mA up to a maximum total of 500 mA per port. Therefore, a compliant bus powered hub can have no more than four downstream ports and cannot offer more than four 100 mA units of current in total to downstream devices (since the hub needs one unit for itself). If a device requires more units of current than the port it is plugged into is able to supply, the operating system usually reports this to the user. In contrast, a self-powered hub (active hub) takes its power from an external power supply unit and can therefore provide full power (up to to every port. Many hubs can operate as either bus powered or self powered hubs. Power equals voltage times current. A USB port that draws 500 mA (0.5 A) at 5 Volts is drawing 2.5 Watts of power. However, there are many non-compliant hubs on the market which announce themselves to the host as self-powered despite really being bus-powered. Equally, there are plenty of non-compliant devices that use more than 100 mA without announcing this fact. These hubs and devices do allow more flexibility in the use of power (in particular, many devices use far less than 100 mA and