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I have question about USB 3.0 and USB 2.0.

1) For example, a motherboard we have has 4 USB ports on the back. Does that mean every USB port has 480 Mbits bandwidth, or are they connected onboard using a built-in USB hub so all of them have 480 Mbits?

2) When I get a USB 3.0 controller, and plug a USB 3.0 hub into that controller that has 4 USB 3.0 ports and get 12 USB 2.0 flash drives, each set of three plugged into a USB 2.0 hub and plug those 4 USB 2.0 hubs into 4 USB 3.0 ports. Will all flash drives work at FULL USB 2.0 speed? Or will all USB busses share a single USB 2.0 plug's worth of bandwidth?

Graph showing 12 USB 2.0 drives, 4 USB 2.0 hubs, and a USB 3.0 controller in the topology as described above.

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  • I hope someone edit my post, to show picture. I don't have right to do this... I have right to upload, but not show... wired
    – marc
    Commented Sep 19, 2011 at 15:23
  • We are a helpful bunch huh?
    – surfasb
    Commented Sep 19, 2011 at 15:43
  • I have a related question if someone is interested in answering: Maximizing speed of USB 2.0 clients on USB 3.0 hub
    – Kozuch
    Commented May 15, 2016 at 16:57

1 Answer 1

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The bandwidth is per Controller.

Per MSDN:

A transfer takes place every time data is moved between the host controller and the USB device. In general, USB transfers can be broadly categorized into control transfers and data transfers. All USB devices must support control transfers and can support endpoints for data transfers. Each type of transfer is associated with the type of USB endpoint (a buffer in the device). Control transfer is associated with the default endpoint and data transfers use unidirectional endpoints. The data transfer types use interrupt, bulk, and isochronous endpoints. The USB driver stack creates a communication channel called a pipe for each endpoint supported by the device. One end of the pipe is the device's endpoint. The other end of the pipe is always the host controller.

A connection with the USB device is to the USB controller. A device may also use as much bandwidth as they want. So thus the bandwidth is per controller.

edit

USB 3.0 Controllers will have both a USB 2.0 and and USB 3.0 controller. So USB 3 devices will not be affected by USB 2.0 devices when connected to the same controller.

Now lets see if I can find a reference in Wikipedia for this. . .

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USB 3.0

For SuperSpeed USB (defined since USB 3.0), connected devices can request service from host. Because there are two separate controllers in each USB 3.0 host, USB 3.0 devices will transmit and receive at USB 3.0 data rates regardless of USB 2.0 or earlier devices connected to that host. Operating data rates for them will be set in the legacy manner.

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  • Great, so first question is done.
    – marc
    Commented Sep 19, 2011 at 16:13
  • What about second, do i will have full usb 2.0 speed on all USB sticks ?
    – marc
    Commented Sep 19, 2011 at 16:14
  • Ah forgot about the second question.
    – surfasb
    Commented Sep 19, 2011 at 16:19
  • So, this will won't work in that way i show. Right ? I will got 480 mbit/s to share with all devices ? It's sad :-(
    – marc
    Commented Sep 19, 2011 at 16:34
  • @marc It's even worse. If I remember correctly, the controller in order to pass certification needs to provide around 50% of the bandwidth continuously!
    – AndrejaKo
    Commented Sep 19, 2011 at 16:40

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