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How is bandwidth allocated?

Could someone shed some light on how the USB bandwidth is allocated in Mac OS X given a device with a specific number of configurations, interfaces/config, endpoints/interface? It seems that even if an end point within an interface is not being used, the system allocates bandwidth for it thus slowing everything down a bit. However, I have also noticed that even if a configuration has 2 interfaces and one of the interfaces is not opened, it still robs the other open interface from bandwidth, and this doesn't make sense to me. If I modify my device to have two configurations, such that I can switch between the two, will this guarantee that bandwidth is allocated only for the interfaces for the configuration in effect?

What is also puzzling is that I am using BULK endpoints, and to the best of my knowledge the bandwidth should be use on a first-come-first-serve basis, so if I don't use a BULK endpoint I don't see why it should rob bandwidth from the other endpoint, but that definitely is the case.

Thanks,
Juan

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