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I am wondering if a USB3 hub connected to a USB3 motherboard could sustain 10 USB2 flash drives being used simultaneously to transfer data at full bandwidth. Theoretically USB 2 speed is 60 MB/sec and USB3 is 625 MB/sec, so in principle you could have 10 USB 2 flash drives hooked up to a USB3 hub.

The real question here is "what is a USB3 hub actually doing at the hardware level?" How many USB2 controllers is it capable of running? What are the bottlenecks to full bandwidth, and where will they occur?

I am guessing that not all USB hubs are created equally.

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Unfortunately, no. When the host is communicating with one device (at USB 2 speeds), the rest of the devices need to wait their turn. Since all devices cannot communicate faster than USB 2, then you have a USB 2 network with one host capable of faster speeds (but cannot use it).

It is the same with using a 1 GB network card with a 10 MB network card; it can only go as fast as the slower component it is currently communicating with.

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  • Without knowing actual details, I'm tempted to think in terms of a gigabit ethernet switch, which is perfectly capable of handing all 10 connections simultaneously, at whatever speed each connection can handle. So it's not beyond the realms of possibility.
    – Tetsujin
    Nov 7, 2014 at 8:11
  • "It is the same with using a 1 GB network card with a 10 MB network card" -- Bad example for comparison. If you have a switch that uses store-and-forward buffering (and most SOHO switches do), then each Ethernet host runs at its full negotiated speed with the switch. A 10Base-T NIC would not slow down a Gigabit NIC when each is connected to a Gigabit switch.
    – sawdust
    Nov 7, 2014 at 8:21
  • I am guessing the key word here is 'serial', which is what USB is. LDC3 is correct in that the slowest 'signal' train is going to set the rate of traffic on the track. And just like on real-world switch-yards, a slower train can get out of the way and let a faster train pass before resuming.
    – arch-abit
    Nov 7, 2014 at 9:59
  • @sawdust I didn't mention a switch because I wasn't certain if each connection was independent. I was aim more at the 100 MB hubs with 10 MB cards in the computers which will have the same problem.
    – LDC3
    Nov 8, 2014 at 1:04
  • "100 MB hubs" -- 100 Mbps hubs are rare, and actually is more likely to be a switch (but labeled/sold as a hub). The Wireshark site has a story about that.
    – sawdust
    Nov 8, 2014 at 1:19
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A USB3 hub connected to a USB3 motherboard uses only ONE USB 2.0 channel.

USB3 hub is simply sitting in parallel with USB2 hub, Tx and Rx diff.pairs running over the standard D+/D- wires. This is how the "backward compatibility" is achieved. The USB3 architecture doesn't have any "transaction tranlsators", unlike USB2 having for FS and LS devices. Therefore all 10 USB 2.0 devices will share the same 45 MBytes/s bandwidth, and will have something like 4 MBytes/s per flash drive.

In this sense all USB 3.0 hubs are created equal, with an exception of internal USB 2.0 hub, which can be "single-TT" or "multi-TT".

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