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Display Resolutions With USB-C Alternate Mode

Many monitor vendors sell 4K monitors, that connect via USB-C and also offer an USB3 hub. But sadly that’s not as simple as it seems.

DP modes and needed bandwidth

from https://en.wikipedia.org/wiki/DisplayPort#Resolution_and_refresh_frequency_limits :

1x 4K@60 Hz: 12.54 Gbit/s
1x 1440p@60 Hz: 5.63 Gbit/s , therefore 2x 1440p@60 Hz: 11.26 Gbit/s

from https://en.wikipedia.org/wiki/DisplayPort#Main_specifications :

DP1.2 HBR2 mode: 5.40 Gbit/s per lane
DP1.3 HBR3 mode: 8.10 Gbit/s per lane

Number of lanes: 4

from https://en.wikipedia.org/wiki/USB-C#Alternate_Mode_2 :

In the Alternate Mode one of up to four high speed links are used in whatever direction is needed. SBU1, SBU2 provide an additional lower speed link. If two high speed links remain unused, then a USB3.0/3.1 link can be established concurrently to the Alternate Mode.

What’s possible then?

That means: 4K@60 Hz or 2x 1440p@60Hz (daisy chained) need either 4 lanes with DP1.2 or 2 lanes with DP1.3.

If 4 lanes are used, only USB2 is possible.

But most monitors and Notebooks only supports DP1.2, therefore when using the full resolution of 4K@60 Hz the integrated USB3 hub is in USB2 mode.

Also for axample a Lenovo P24h-20 (with integrated gigabit ethernet) probably cannot do, what it’s specs implies: 2x 1440p daisy chain + gigabit ethernet at full speed.