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This is mostly because Thunderbolt hub support was optional in Thunderbolt 3 spec, but mandatory in USB 4. Apple just added support for it in Big Sur.

by sirn 6y ago

Dylan16807 6y ago
Which leaves me deeply confused. Daisy-chaining had to be supported. From the host point of view, a hub is barely any more complicated. Why was it optional?
- sirn 6y ago
  My understanding is that daisy chaining doesn't require a mixed USB and Thunderbolt signal, only Thunderbolt. Before USB 4, USB-C spec itself disallows hubs to passthrough an Alternate Mode and even Thunderbolt daisy chain also have to terminate at any point where USB is plugged in. In this case, I believe you can have Thunderbolt 3 hub that only carries Thunderbolt signal, but it will only work with Thunderbolt devices and not USB-C.
  - Dylan16807 6y ago
    But you don't need to mix signals. When you use a thunderbolt dock right now, doesn't it use pure thunderbolt as a backhaul, and have a PCIe-based USB 3 controller inside of it? If that's right, then even without mixed signals you could have a pure-thunderbolt hub, and you could have a device with several thunderbolt ports and several USB 3 ports. And it should be straightforward to combine them into dual-purpose ports. The rules for passing through alternate modes don't matter because you're not passing through, you're not a USB hub, you have your own USB root.
    - sirn 6y ago
      That indeed could work, and it looks like Belkin's Thunderbolt 3 Express Dock HD[1] actually does this. Though in this case, Thunderbolt is limited to just 2 (1 for downstream and 1 for upstream). I guess the biggest benefit of USB 4 hub is that allows the hub to have more than 2 Thunderbolt ports (the upcoming OWC Thunderbolt hub has 1 upstream and 3 downstream) and no longer have to worry about unplugging a device breaking a chain (usability problem). [1]: https://www.belkin.com/us/p/P-F4U095/ https://www.belkin.com/us/p/P-F4U095/
chrischen 6y ago
Also doesn't exist for USB: https://superuser.com/questions/1381139/why-cant-i-find-a-usb-c-hub-with-multiple-usb-c-ports https://superuser.com/questions/1381139/why-cant-i-find-a-us...
- sirn 6y ago
  Before USB 4, USB-C spec itself say "see also USB 3.1" for USB hubs and disallow passing through Alternate Modes (e.g. Thunderbolt/DisplayPort) or Accessory Modes (e.g. audio dongle) which vastly limited the usability of USB-C port. USB 4 introduces a new discovery protocol with added support for passing through Alternate Mode in USB-C spec (as long as downstream is also USB 4).

This is mostly because Thunderbolt hub support was optional in Thunderbolt 3 spec, but mandatory in USB 4. Apple just added support for it in Big Sur.

by sirn 6y ago

This is mostly because Thunderbolt hub support was optional in Thunderbolt 3 spec, but mandatory in USB 4. Apple just added support for it in Big Sur.

Dylan16807 6y ago

Which leaves me deeply confused. Daisy-chaining had to be supported. From the host point of view, a hub is barely any more complicated. Why was it optional?

sirn 6y ago
My understanding is that daisy chaining doesn't require a mixed USB and Thunderbolt signal, only Thunderbolt. Before USB 4, USB-C spec itself disallows hubs to passthrough an Alternate Mode and even Thunderbolt daisy chain also have to terminate at any point where USB is plugged in. In this case, I believe you can have Thunderbolt 3 hub that only carries Thunderbolt signal, but it will only work with Thunderbolt devices and not USB-C.
- Dylan16807 6y ago
  But you don't need to mix signals. When you use a thunderbolt dock right now, doesn't it use pure thunderbolt as a backhaul, and have a PCIe-based USB 3 controller inside of it? If that's right, then even without mixed signals you could have a pure-thunderbolt hub, and you could have a device with several thunderbolt ports and several USB 3 ports. And it should be straightforward to combine them into dual-purpose ports. The rules for passing through alternate modes don't matter because you're not passing through, you're not a USB hub, you have your own USB root.
  - sirn 6y ago
    That indeed could work, and it looks like Belkin's Thunderbolt 3 Express Dock HD[1] actually does this. Though in this case, Thunderbolt is limited to just 2 (1 for downstream and 1 for upstream). I guess the biggest benefit of USB 4 hub is that allows the hub to have more than 2 Thunderbolt ports (the upcoming OWC Thunderbolt hub has 1 upstream and 3 downstream) and no longer have to worry about unplugging a device breaking a chain (usability problem). [1]: https://www.belkin.com/us/p/P-F4U095/ https://www.belkin.com/us/p/P-F4U095/

chrischen 6y ago

Also doesn't exist for USB: https://superuser.com/questions/1381139/why-cant-i-find-a-usb-c-hub-with-multiple-usb-c-ports https://superuser.com/questions/1381139/why-cant-i-find-a-us...

sirn 6y ago
Before USB 4, USB-C spec itself say "see also USB 3.1" for USB hubs and disallow passing through Alternate Modes (e.g. Thunderbolt/DisplayPort) or Accessory Modes (e.g. audio dongle) which vastly limited the usability of USB-C port. USB 4 introduces a new discovery protocol with added support for passing through Alternate Mode in USB-C spec (as long as downstream is also USB 4).