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If I connect all peripherals to a USB-C hub (or docking station) that is connected to one USB-C port of the PC, will the USB ports on docking station support the full USB speeds? Assume it exposes USB 3.0 ports.

If it will share the bandwidth of the USB-C, what if we are using just one USB for data transfer and the others are for small things like mouse, keyboard?

What if we have two USB 3 flash drives connected to the USB-C hub and we transferring files from them one to the other? Will it be half the speed of connected directly to the PC?

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There's two kinds of USB-C docks. There's the kind that use USB 3.0, 3.1, or 3.2, which I will refer to as USB 3.x from here on, to communicate with the host. There's the kind that use Thunderbolt 3, or TB3, to communicate with the host. There's USB4 docks now too but those are not all that different than TB3 since USB4 is mostly just rolling in TB3 with USB 3.x. The portions of the spec that differentiate USB4 from TB3 are all optional, an existing TB3 hub or dock does not have to change anything to meet the USB4 spec. That is unless I'm missing something important.

USB-C has four "superspeed" data lanes which can be used for USB 3.x, DisplayPort, or TB3 data. There are other alternate modes that a dock could use but I have not seen any and using any protocols other than those three don't make much sense for a number of reasons. Reasons that can get complicated quickly.

A USB 3.x dock will most likely use one pair of these "superspeed" lanes for USB 3.x data and the other pair for DisplayPort video. There might be a HDMI port on the USB-C dock but it will use two "superspeed" lanes for DisplayPort video and convert that into HDMI internally. This is because the HDMI alternate mode on USB-C requires use of all four "superspeed" lanes, leaving nothing for USB 3.x devices.

It's possible for a USB-C dock to use all four "superspeed" lanes for USB 3.x data but I have yet to see such a dock. For one this leaves no "superspeed" lanes for video. Another issue is that the USB 3.x spec did not define how a USB-C port on a USB-C dock would work except for providing power. USB4 fixes this, and TB3 docks avoid this, by putting a separate USB controller in the dock. With no defined means to support a downstream USB-C port on a dock there's not much to gain with using all four "superspeed" lanes for USB 3.x data.

The USB 2.0 lanes on a USB-C dock will act like the USB 2.0 would on any USB 2.0 hub. This is because the USB 2.0 data path is for the most part independent from the "superspeed" data lanes. A keyboard, mouse, or other USB 2.0 device is not likely to impact anything on the "superspeed" data lanes. There might be some impact if a device uses the USB 2.0 lane for negotiating the data on the "superspeed" lanes. A drive might do this. A DisplayPort display might use USB 2.0 for making video adjustments since DisplayPort is a unidirectional protocol.

A USB-C hub that lacks support for USB4 or TB3 will have only USB-A ports for USB 3.x devices. A USB 3.2 device with a USB-C jack will not be able to even plug in to such a dock, or if it does get plugged in then it will be a power only USB-C port and the host won't even know it's there. All USB 3.x devices will then split time on the USB "superspeed" lines to the host. Each time slice it has will be at the full data rate. If the other devices don't have much to say then one device will get nearly all the bandwidth. If there's a file copy from one USB 3.x device to another on the same dock then the time, and therefore total average bandwidth, gets split.

TB3 can share "superspeed" data lanes with DisplayPort, splitting the 40 Gbps total bandwidth with it. As stated above USB devices on a TB3 dock will work by the dock having a USB controller inside for USB 3.x devices. This means a USB 3.2 device with a USB-C jack can plug in and get the full 20 Gbps bandwidth to the host. Two USB 3.x devices could then not have to split time on the "superspeed" lanes with a dock that uses USB 3.x to communicate with the host. This is because TB3 has more bandwidth to share, taking full advantage of the 40 Gbps that USB-C is capable of providing. Because a TB3 or USB4 dock will have a USB controller it's possible that data transfers from one USB device to another may not have to pass to the host and back over the USB-C connection.

So, there's four ways a USB-C dock could connect to a host. There's using two "superspeed" lanes for USB 3.x data. There's using two "superspeed" lanes for USB 3.x data and two "superspeed" lanes for DisplayPort video. There's using all four "superspeed" lanes for USB 3.x data. And there's using all four "superspeed" lanes for TB3 and DisplayPort. When using a pair of "superspeed" lanes for USB 3.x data the maximum bandwidth per pair is 10 Gbps. Thunderbolt and DisplayPort can attain 20 Gbps per pair of "superspeed" lanes. The tradeoff being USB 3.x can use longer cable lengths than TB3 before having to resort to expensive active cables.

Using TB3 to connect to the host and a putting a USB 3.x controller in the dock to speed up USB 3.x devices means a more expensive dock, a bigger dock, and more power consumption from the dock.

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All devices downstream of the USB hub share the same bandwidth to the PC's USB controller, which can reduce the speed of connected devices, depending on the actual bandwidth used. Unused bandwidth is available to other devices.

Considerations:

  • Not all USB devices are capable of fully saturating the link
  • Each USB controller on a PC may be connected the multiple USB ports through an internal USB hub
  • Other bandwidth limits may limit the speed of your USB devices
    • A fast flash drive copying files through a hub into a slow HDD will be limited by the HDD
    • A USB controller connected through a southbridge may also be sharing southbridge bandwidth

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