3 ms·

USB4 is basically Thunderbolt 3, i.e. USB3.x + PCIe + DisplayPort. See the diagram in the linked spec, "Figure 2-1 USB4/USB3.2 Dual Bus System Architecture"

djsumdog 7y ago
I remember hearing that on .. Dave2D I think? Or Linus Tech Tips .. one of them said USB4 is Thunderbolt 3. I assumed the standards were just combining and the two would be equivalent protocols, but I assume that's not true then? Does USB4 basically just add PCIe onto the USB bus? How will that work with AMD board that still don't officially have Thunderbolt/PCI Hotplug support?
- AceJohnny2 7y ago
  > Does USB4 basically just add PCIe onto the USB bus? Yes. Which is kinda what Thunderbolt 3 does (except encapsulated in the TB packet format, not USB). The silicon world has been gradually converging on its Serializer-Deserializer/PHY technology. I don't know quite the family history, but I believe the PCIe PHY is very similar to USB3 which is identical to Thunderbolt 3. Since the link layer was the same, it was then "just" a matter of protocol design to make everything just use the same link. And that's how you got Thunderbolt 3 / USB 4.
  - tarheelredsox 7y ago
    PCIe uses different rates than USB, but both at their basics use NRZ (Non-Return to Zero) https://en.wikipedia.org/wiki/Non-return-to-zero https://en.wikipedia.org/wiki/Non-return-to-zero. The bigger differences are in the channels they target. The channels characteristics drive the complexity of the signal processing circuitry required (eg Power and Cost) in the PHY devices. Hope that adds a bit of clarity to your PHY question.
- kalleboo 7y ago
  > How will that work with AMD board that still don't officially have Thunderbolt/PCI Hotplug support? They will have to add it. It will be easier now that Thunderbolt doesn't have to be licensed from Intel.
chx 7y ago
Not... quite. Thunderbolt 3 did not carry USB 3.x over its bus, it's just PCIe and DP signals. All the Intel TB3 controller chips used in enclosures have a PCIe switch and a USB root hub attached to that. This switch was ... not flawless ... to be diplomatic. It was utter shit, to be less politicaly correct. TB3 eGPUs famously struggled with their occassionally lagging and hiccuping USB ports so much so that the Visiontek Mini and the Razer 2 has two TB3 controllers to avoid this mess with the second TB3 controller solely providing the USB 3 root hub and almost all other eGPUs just gave up on them. USB 4 will carry 10gbps USB multiplexed with PCI Express and DisplayPort, native USB instead of hotplugged controller. Simpler.
- AceJohnny2 7y ago
  Thanks for the correction!

djsumdog 7y ago

I remember hearing that on .. Dave2D I think? Or Linus Tech Tips .. one of them said USB4 is Thunderbolt 3. I assumed the standards were just combining and the two would be equivalent protocols, but I assume that's not true then? Does USB4 basically just add PCIe onto the USB bus? How will that work with AMD board that still don't officially have Thunderbolt/PCI Hotplug support?

AceJohnny2 7y ago
> Does USB4 basically just add PCIe onto the USB bus? Yes. Which is kinda what Thunderbolt 3 does (except encapsulated in the TB packet format, not USB). The silicon world has been gradually converging on its Serializer-Deserializer/PHY technology. I don't know quite the family history, but I believe the PCIe PHY is very similar to USB3 which is identical to Thunderbolt 3. Since the link layer was the same, it was then "just" a matter of protocol design to make everything just use the same link. And that's how you got Thunderbolt 3 / USB 4.
- tarheelredsox 7y ago
  PCIe uses different rates than USB, but both at their basics use NRZ (Non-Return to Zero) https://en.wikipedia.org/wiki/Non-return-to-zero https://en.wikipedia.org/wiki/Non-return-to-zero. The bigger differences are in the channels they target. The channels characteristics drive the complexity of the signal processing circuitry required (eg Power and Cost) in the PHY devices. Hope that adds a bit of clarity to your PHY question.
kalleboo 7y ago
> How will that work with AMD board that still don't officially have Thunderbolt/PCI Hotplug support? They will have to add it. It will be easier now that Thunderbolt doesn't have to be licensed from Intel.

chx 7y ago

Not... quite. Thunderbolt 3 did not carry USB 3.x over its bus, it's just PCIe and DP signals. All the Intel TB3 controller chips used in enclosures have a PCIe switch and a USB root hub attached to that. This switch was ... not flawless ... to be diplomatic. It was utter shit, to be less politicaly correct. TB3 eGPUs famously struggled with their occassionally lagging and hiccuping USB ports so much so that the Visiontek Mini and the Razer 2 has two TB3 controllers to avoid this mess with the second TB3 controller solely providing the USB 3 root hub and almost all other eGPUs just gave up on them. USB 4 will carry 10gbps USB multiplexed with PCI Express and DisplayPort, native USB instead of hotplugged controller. Simpler.

AceJohnny2 7y ago
Thanks for the correction!