USB 3.0 SuperSpeed (up to 5Gb/s) uses its own set of physical wires for full-duplex tx+rx. These are completely separate from the single pair used by USB 1.1 and 2.0 devices, which are limited to half-duplex communication as a result. Thus, USB 3.0 SS data doesn't timeshare _at all_ with devices on the same bus which adhere to earlier standards.

There are, however, *hub controller* contention issues regarding USB 1.1 devices being plugged into USB 2.0 ports. With USB 1.1 operating at 12Mb/s, and USB 2.0 up to 480Mb/s, the hub controller needs to be able to take the 1.1 signaling and integrate it into the 2.0 signaling before sending it upstream. Logically, this is done by what's called a Transaction Translator (TT). TTs are allocated to the downstream ports, and the most simplistic of hub controllers implement a *single* TT. This single TT is shared amongst all downstream devices, and as you might predict, this creates a processing bottleneck. More performant hub controller implement multiple TTs, usually with one per downstream port. This means that bottlenecks created by a mix of downstream 1.1 and 2.0 devices are completely minimized. As explained above, USB 3.0 devices don't see any of this - they have their own data signaling wires to start with.

So, in summary, USB 3.0+ hubs can be thought of two USB hubs in one, although it looks like one physical unit. The controller chips which comprise a USB 3 hub can be broken into two main logic areas, or PHYs: one servicing USB 1.1 and 2.0 devices using one or more TTs, and the other servicing USB 3.0+ devices. This is possible because USB 3.0 devices which are connected to the hub use their own separate wires. USB 1.1/2.0 devices are on their own, with any bottlenecks governed by how many TTs are available as explained above. This also explains why you can generally use a USB 3.0 hub on a system which only has USB 2.0 ports - the USB 3.0 portion just goes unused as unrecognized by the host.

On my setups and because astronomy gear tends to be a menagerie of USB 1.1, 2.0, and 3.0 devices, I make sure to use USB hubs which incorporate controllers which feature multi-TTs. One example of these is the StarTech ST7300U3M hub, which uses the ASMedia ASM1074 controller.

/dale