In order to realize the 480 Mbps bus operation for USB 2.0, a change in electrical specifications was necessary, but without sacrificing compatibility with USB 1.x. Furthermore, a change in protocol was required to allow 12/1.5 Mbps transfers to coexist with 480 Mbps transfers. The new specification also resolves issues that were evident in USB 1.x.
480 Mbps transfer could not be achieved with 3.3 V, so small-amplitude signals are used instead.
In order to implement bus operation at 480 Mbps, a 17.88 mA constant current in the buffer is directed to 45 ohm lines on both ends. This then achieves an output amplitude of 22.5 ohm x 17.88 mA = 402.3 mV.