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At work, I want to plug all my USB devices into a single USB hub so I only need to plug one USB cable, external display, and the power cable into my laptop. I have some USB 3.0 devices and some slower USB 2.0/1.1 devices.

I'm aware that all the devices on a single USB Root Hub share the same bandwidth, but I'm curious whether plugging an older device into a hub causes the entire hub to fall back to a compatibility mode.

If I plug the slower devices into my USB 3.0 hub, will that hub and all its connected devices slow down to USB 2.0 speeds, or will the USB 3.0 devices continue to run at USB 3.0 speeds?

For example, suppose I have an USB 3.0 gigabit network adapter and an USB 2.0 keyboard. If I plug the keyboard into the same USB 3.0 hub that the network adapter is plugged into, will the network adapter's maximum theoretical throughput instantly drop to 480 Mbps or slower (USB 2.0's maximum throughput)?

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Short answer: No.

Long answer:

I stumbled across the answer to my question in a comment to an answer for a seemingly unrelated question. It turns out USB 2.0 and USB 3.0 are physically segregated in the wiring, so plugging USB 2.0 devices into an USB 3.0 hub won't affect the performance of USB 3.0 devices in the way that I was wondering.

  • @BenVoigt I had only considered the physical connectors and wires originally. Thanks for the clarification. – rob Jul 18 '14 at 18:09
  • @Ben Can you clarify something? If I have two USB2 webcams, normally each would saturate a USB2 link. If I connect both to a USB3 hub, are you saying that they would each be able to use 480 Mbps since the hub would aggregate traffic and resend over the 5 Gbps USB3 link to host? – Nicu Stiurca Sep 30 '15 at 14:16
  • @SchighSchagh: Yes, barring some major design problem in the hub, that's exactly the advantage. (Well, neither device will reach 480 Mbps, but they will each receive as much bandwidth as if there were no other device attached) – Ben Voigt Sep 30 '15 at 14:37
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    @clarkcox3 I'm saying that BenVoigt's comment is incorrect; the USB 2.0 signals are not translated to USB 3.0 signals, so all the USB 2.0 devices must share a single 480 Mbit/s bandwidth. If you plug two USB 2.0 devices into a USB 3.0 hub, the USB 3.0 wires from the hub will have no data on them. – endolith May 9 '16 at 16:51
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    I have a Lenovo E50-80 (model 80J2) - it features 2 USB 3.0 ports. I can use 2 1080p USB 2.0 webcams on these ports simultaneously even though each webcam draws about 15 megabytes/s alone and they will not work together when connected via a USB 2.0 hub. This means the USB 3.0 must do transaction translation from high-speed to super-speed. Surely such a transaction translation may not be a part of USB3.0 specs but in reality there seem to be devices that do perform this. I monitored the data traffic with usbtop utility. Anyone has similar experience? – Kozuch Jul 13 '18 at 12:37
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No.

Here is how to imagine speed in wires.

Think of it like a tunnel that transfers water.

Assume a big tunnel is split evenly into several similarly-sized tunnels (HUB). Next, you connect your 2.0 device (which is a smaller tunnel comparing to 3.0) to the hub.

What happens? Do the other tunnels get smaller? No, they do not. Your 2.0 device uses as much speed as it can.

This analogy really helped me to understand network problems.

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    This analogy is not correct for a wide variety of network (really "bus") situations. For example, USB 1 devices on USB 2 will slow the bus down by occupying a disproportionate amount of time for the data transferred (blocking other, faster devices from using the bus). Same for 10mBit devices on Gigabit networks. RS-485, SATA, and other serial buses only go as fast as the slowest device. Not true for USB3 however, because the cable contains DIFFERENT WIRES for v.2 and v.3 of the protocol. It's the difference between putting minivans and Ferrari's in the same lane vs each getting their own lane. – DrFriedParts Jun 22 '16 at 11:11
  • @DrFriedParts: You're not correct for Gigabit Ethernet networks though. You would have been correct if it was a 10/100 shared-medium Ethernet, but modern switched networks (which is required for Gigabit) no longer act like a "bus" of any sort. – grawity Feb 1 '17 at 6:55
  • @grawity -- You have to be more nuanced than that. Your statement is only true if the gigabit switch has enough lanes (i.e. most "managed" hubs) and if you can avoid the problem scenario typically involving simultaneous transfers from a single gigabit-equipped client to a mix of gigabit and Fast Ethernet clients. 802.3 flow control, switch fabric bandwidth, and other limits still affect gigabit switches. – DrFriedParts Feb 1 '17 at 9:38
  • Well, if you can actually find a Gigabit hub... – grawity Feb 1 '17 at 10:28
  • @grawity -- there are no "Gigabit hub's" because GigE requires point-to-point links. What there are are "unmanaged switches" and these typically have insufficient lanes to allow all ports to communicate with all ports (because that very rarely happens). So when ports use the same lanes in the switch they must wait for the slower transfer to complete before the lane becomes available and you have exactly the same scenario as in USB1/2. This occurs rarely, but it does happen. It most commonly occurs with "a single gigabit-equipped client to a mix of gigabit and Fast Ethernet clients." – DrFriedParts Feb 7 '17 at 0:37

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