I was curious myself on why the High Speed USB2.0 claims to transfer data at 480Mbps (mega-bits per second). A search got me into this post which comes with a short and simple explanation:
Are you perhaps worried that you are not moving data around at the claimed 480 Mbps. If so, you would be right. 480 Mbps is the bit data rate of all the communication that occurs over the USB bus. Out of that has to come a certain amount of house keeping - this is the USB host controller actually controlling all the hubs and peripheral devices, deciding what can talk and when. Of greater impact is that USB is implemented as a single bus, thus communication can only take place in one direction at any one time. The 480Mbps therefore represents the total data rate in *both* directions [1]. The remaining significant bottleneck is that the USB host port requires considerable processor support and if the processor isn't ready to service it, then it has to wait.
Contrast Firewire, where although it is only rated at 400 Mbps (in the common IEE1394a incarnation) the actual practical data rate is significantly higher because, being a dual bus system, you get 400 Mbps in *each* direction. Equally significantly the Firewire ports are able to operate more or less autonomously without processor support and thus don't have to wait. The housekeeping requirements are largely similar buthere is no need
to hand over data direction. In practice, the differences are usage dependant, but a 33% increase in speed is the sort of minimum that you can expect from Firewire.
[1] USB1 had another drawback in that communication with slow peripherals (1.5 Mbps) operated at that speed from end to end which had a major impact on the faster (12 Mbps) communications. USB2 solved this problem by requiring USB2 hubs to buffer the data and then up convert it to 480 Mbps for transmission to the host, and similarly for reception from the host.
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