From what I've read, USB4 defines three different and incompatible "up to 20 Gbps" USB transfer modes.
Two of them are new, and at least one (which has the marketing name "USB4 20 Gbps") is mandatory for USB4 host ports. USB 3.2 Gen 2x2 is optional for USB4 host ports.
Can you say "thrown under the bus"?
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Thunderbolt 4 may introduce new wrinkles for devices connected via Thunderbolt 4 hubs and docks. I just came across this article, and was surprised to read that with current chipsets, Thunderbolt 4's hubbing ability comes at this non-obvious cost.
Other World Computing – Blog – What’s the Difference Between Thunderbolt 3 and Thunderbolt 4?
"Because of the difference in how Thunderbolt 4 allocates data, every port on a Thunderbolt 4 device that isn’t Thunderbolt must share a single 10GBps slice of USB bandwidth."
So theoretically, that might mean that
- Apple could support USB 3.2 Gen 2x2 for USB devices and Thunderbolt 3 docks – but doesn't.
- There is no way to support USB 3.2 Gen 2x2 speeds on current Thunderbolt 4 docks, even if the host is capable of supporting those USB speeds in other contexts.
As for Thunderbolt 5, I don't have enough information to say.