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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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2 Answers 2

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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.

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  • @BenVoigt I had only considered the physical connectors and wires originally. Thanks for the clarification.
    – rob
    Jul 18, 2014 at 18:09
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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, 2016 at 16:51
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    @endolith: You can say that, but the better USB 3.0 hubs do have transaction translators (to SuperSpeed). See the VL670 for example, dated 2014.
    – Ben Voigt
    Jan 28, 2018 at 5:05
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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, 2018 at 12:37
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    Transaction translation is not built into the USB 3 spec as it was for USB 2, and you can assume the default configuration for typical hubs is that USB 2 communication is not translated to USB 3 speeds and re-transmitted, but instead is routed via the traditional USB 2 bus. The interesting benefit of this is that USB 2 and earlier devices occupy zero bandwidth of the USB 3 bus, so they will not slow down or demote USB 3 communication, even by a small amount. But the drawback is all USB 1/2 devices are relegated to share a combined 480Mbps bus. Apr 26 at 7:45
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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. Jun 22, 2016 at 11:11
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    @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.
    – user1686
    Feb 1, 2017 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. Feb 1, 2017 at 9:38
  • Well, if you can actually find a Gigabit hub...
    – user1686
    Feb 1, 2017 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." Feb 7, 2017 at 0:37

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