Might help
Maximum Useful Signalling Distance
Although a single cable is limited to 5 meters, the USB specification permits up to five USB hubs in a long chain of cables and hubs. Consequently the maximum possible signalling distance is 30 meters, using six 5-meter cables and five hubs. In actual use, the last hub is a more convenient endpoint since some USB devices include built-in cables intended to directly connect to a hub, setting the maximum useful signalling distance at 25 meters.
Because USB is able to provide power for additional devices connected to the bus, a special type of USB extender cable was created which consists of a miniature one-port USB hub molded into one end of a 5 meter cable. These mini-hubs are fully self-contained within the cable, requiring no separate bulky hub device, and are as simple to use as plugging cables together, with each hub drawing power through all the previous single-port hubs in the chain. The bus power is limited however, so the most practical application is to use four single-port hub extender cables, one plain 5 meter cable, and then a powered multiport hub at the very end to support multiple additional USB devices.
or read here
http://en.wikipedia.org/wiki/Usb