Understanding power and data bandwidth when using USB 2.0 devices on a USB 3.0 controller

I believe that cde's response is incorrect. USB2 did indeed introduce Transaction Translators for attaching FullSpeed and LowSpeed devices to a HighSpeed hub. However not only did the USB3 spec fail to follow this precedent, but it requires that USB3 hubs use the old USB2 protocol over the two original signal wires of it's uplink for all downstream HS/FS/LS devices. All USB3 cables must carry both the old HS/FS/LS signal wires and the four new SS conductors. USB3 hubs are effectively two separate hubs for two separate buses. They may not actually be two separate chips, but since they're separate logic blocks, they may as well have been.

XHCI root hubs do not have this constraint: they may provide a full USB2 HS channel's worth of bandwidth to each port.

For bandwidth calculation, USB2 devices on an external USB3 hub might as well have been attached to a USB2 hub. Because, in essence, it is.

There are no insurmountable technical hurdles to a USB Transaction Translator; the designers just took a shortcut. VIA Labs owns some patents related to a USB3 Transaction Translator, which might explain why it didn't make it into the spec. They announced an actual implementation back in 2012 (see http://via-labs.com/en/resources/pressroom/2012/PR120104-usb2expressway.jsp) but I don't think it ever made it to a shipping product and I haven't heard anything on the topic since. At this point, implementing a USB2 controller driver on top of an XHCI stack would be non-trivial; maybe that's why we haven't seen one.

Depends on the HUB IC. If there is only a Single Transaction Translator, then the USB 2.0 (Or Worse 1.1) devices share bandwidth between the hub and the host, only one at a time. If the HUB IC has a Multiple Transaction Translator, the bandwidth should be multiplexed, so that all the information goes over a single xHCI channel.

There wouldn't be a boost in bandwidth between the camera and the hub. However, assuming the hub was connected over a USB 3.0 connection to a computer there would be an overall boost to the computer.

To get the higher speeds you need to use new USB 3.0 cables. See the wikipedia article for more info.