How Do USB 3.0 Hubs And USB Extension Cables Effect Latency, If At All

latencyperipheralsrepeaterusbusb hub

I currently have three peripherals (mouse, keyboard, Wacom tablet) all wired to an individual USB port on my computer. They also each have their own 6ft USB extension cable (passive) because the computer is a little far away. They are each USB 2.0 devices.

To cut down on the cables, I kind of wanted to simplify things. Run a single 6-10ft USB 3.0 extension cable from the PC to a powered 4-port USB 3.0 hub that I have lying around, and plug all three USB 2.0 peripherals into it.

I'm curious about a few things:

  1. I know plugging the 2.0 devices into a 3.0 extension won't take any advantage of 3.0 speeds. But what if they're all connected to a 3.0 hub, connected to a 3.0 cable? Would that cause those 2.0 devices to use the extra wires in a 3.0 cable, theoretically?

  2. Might forcing the event calls from all three devices through a single cable theoretically add latency?

  3. I've been told that powered USB hubs act as repeaters. Is that the case, and if so, could that reduce latency? Or is that not how a repeater works?

Answer one or some or all. I realize this might seem like such an inconsequential thing but I care about the details. Thanks very much for reading.

Best Answer

  1. I know plugging the 2.0 devices into a 3.0 extension won't take any advantage of 3.0 speeds. But what if they're all connected to a 3.0 hub, connected to a 3.0 cable? Would that cause those 2.0 devices to use the extra wires in a 3.0 cable, theoretically?

USB 3.0 includes the USB 2.0 connections for backwards compatibility, but the traffic is kept separate. There is a USB 2.0 controller that handles USB 2.0 traffic passing through the USB 3.0 infrastructure. So you can use USB 3.0 cables and hubs, but USB 2.0 devices get USB 2.0 treatment the entire way. The extra USB 3.0 infrastructure is ignored.

  1. Might forcing the event calls from all three devices through a single cable theoretically add latency?

The key word there is "theoretically". Yes, theoretically, it could add latency to combine the signals through a single path, but you would never know it if it did. Your devices (mouse, keyboard, and tablet), have very low data rates. Even compared to USB 2.0 bandwidth, the combined data from all three is like spitting in the ocean. Statistically, there will be occasions where there will be simultaneous attempts to move data, and some data will be delayed. But the delay will be in the microsecond or shorter order of magnitude. Humans can't perceive delays even in milliseconds.

That said, it wouldn't change anything because it is already happening. Groups of USB ports on your computer are connected to an internal hub (the root hub). So feeding those ports separately results in any collisions happening inside your computer. If you combine the devices on an external hub, you're just moving the location of where those collisions occur.

  1. I've been told that powered USB hubs act as repeaters. Is that the case, and if so, could that reduce latency? Or is that not how a repeater works?

Repeaters are used to extend the distance limits of a connection. A hub can be used as a repeater. But you can only add latency, you can't reduce it. Each time the signal is handled, there is the potential to introduce additional delay (and you can't send it back in time). But again, such delays are many orders of magnitude too short to affect anything you would be aware of.

That said, you could potentially reduce theoretical latency by using a hub (which could be a USB 2.0 hub), for a different reason. The USB 2.0 distance limit is 5 meters when using cables designed for that distance.

Say you have a device with a 5' cable and you add a 10' extension. The wire in the device cable may be sized for a total run of 5', and the wire in the extension cable may be sized for a total run of not much over 10', and the design quality of the cables may be similarly targeted to their length. In that case, each cable has already introduced the allowable losses for the connection, and you are combining the losses of the two cables.

So your setup may already be contributing latency, and it just isn't in a range that you are aware of. For this reason, the USB spec specifically defines the use of extension cables as non-compliant, although the unreliability in actual practice may be at levels acceptable for your purpose, especially with low data rate, human input devices.

Thus, combining the devices at a hub and using a long cable to connect the hub to the computer may reduce the latency of your current setup (not that you would know it), by ensuring that each connection is well within the USB limits.