The Problems With USB-C Hubs

At first glance USB-C is a highly capable connector, providing:

  • Up to 100W of power, in either direction
  • 10Gbps USB 3.1 gen 2 connection
  • Dual 4K monitors
  • A reversible connector

However these cannot all be acheived at the same time. While power delivery functionality operates independently of the data lanes, USB3 and display signals must share these. In USB-C there are 4 differential pairs available for alternate modes to use, to continue to have USB3 functionality 2 of these must be left free. Luckily in DisplayPort alt-mode the DisplayPort signals can be configured to use 1,2 or 4 links. Multiple ports is then achieved on these docks using Multi-Stream Transport, where multiple displays can be multiplexed down a single connection as long as the total bandwidth doesn't exceed what is available.

As such most USB hubs set the DisplayPort to use 2 lanes so that USB3 is available on the dock, this allows for other interfaces such as gigabit ethernet and card readers to also be available. These hubs advertise either, dual 1080p @ 60Hz or 4K @ 30Hz. And there is the limitation, anyone like me who has a 1440p monitor and a 1080p monitor can no longer use a 2-lane Displayport 1.2 link.

Another interesting thing to note on hubs and dongles that support only a single output is that a DisplayPort connector is almost never seen on them despite the connection travelling over the USB-C link being DisplayPort. This is because DisplayPort over USB-C does not support Displayport dual-mode (DP++). Dual-mode is the technology that allows DisplayPort to be used with passive adapters for HDMI and DVI-D. Since this means you require an active DisplayPort to HDMI adapter, I suspect that dongle manufactures internally adapt the signal so that consumers aren't confused why none of thier DisplayPort to HDMI cables work.

Calculating Display Bandwidth

In order to figure out how many lanes are needed, you need to calculate the bandwidth your displays are occupying.

\[ Bandwidth = \left(Height + ceil\left(\frac{Height \times 0.00046}{T_{frame}-0.00046}\right)\right) \times (Width + 80) \times Color Depth \times Refresh Rate \] \[ Bandwidth \% = \frac{Bandwidth}{Link speed \times 0.8 \times Lanes \times 1000^3} \]

  • 0.8 comes from the 8b/10b encoding used in DP1.0-1.4
  • This is the raw display bandwidth and there is likely some overhead in MST mode, so be wary of marginal values.

DP1.2 (5.4Gbps per lane)

30-bit colour depth
Display Size Bandwidth Used (Gbps) Lane Percentage 4 Lane Percentage
1080p 4.01 92.8% 23.2%
1440p 7.00 162.1% 40.5%
4k @ 30Hz 7.74 179.2% 44.8%
4k @ 60Hz 15.61 361.3% 89.6%
24-bit colour depth
Display Size Bandwidth Used (Gbps) Lane Percentage 4 Lane Percentage
1080p 3.21 74.2% 18.6%
1440p 5.60 129.7% 32.4%
4k @ 30Hz 6.19 143.4% 35.8%
4k @ 60Hz 12.39 286.7% 71.7%

DP1.3/1.4 (8.1Gbps per lane)

30-bit colour depth
Display Size Bandwidth Used (Gbps) Lane Percentage 4 Lane Percentage
1080p 4.01 61.9% 15.5%
1440p 7.00 108.1% 27.0%
4k @ 30Hz 7.74 119.5% 29.9%
4k @ 60Hz 15.61 238.9% 59.7%
24-bit colour depth
Display Size Bandwidth Used (Gbps) Lane Percentage 4 Lane Percentage
1080p 3.21 49.5% 12.4%
1440p 5.60 86.5% 21.6%
4k @ 30Hz 6.19 95.6% 23.9%
4k @ 60Hz 12.39 191.1% 47.8%

So you may think the simple solution to this problem is just use DisplayPort 1.3 or above. However in order to use a DisplayPort version above 1.2 both the laptop and multi-stream transport hub must support it. This is especially difficult since there is no physical DisplayPort involved so manufacturers often don't mention the version in use.

As a rule of thumb (ignoring resolutions on thunderbolt docks) a DP1.3/1.4 link will specify:

  • 8k @ 30Hz or dual 4k @ 60Hz on the laptop connector (4 lanes)
  • 4k @ 60Hz and USB3+ on the dock (2 lanes)

Thunderbolt 3 Docks

Thunderbolt alt-mode does overcome most of these issues since a thunderbolt connection provides a single 40Gbps link using all 4 lanes. This link can encapsulate:

  • PCIe (up to 4x)
  • USB 3.1 gen 2 (10Gbps)
  • 10 Gbps ethernet
  • 8 lane DisplayPort 1.2

The most important feature here though is that the 8 lanes of DisplayPort can occur along side the USB 3.1 gen 2 connection since bandwidth is prioritsed for display. The are a number of issues though. Most notably the number of DisplayPort lanes available over Thunderbolt 3 depends on the device connected. For instance only the latest Lenovo X1 Carbon's support 8 lanes despite the 3 generations of thinkpads with Thunderbolt 3 ports. Additionally since Thunderbolt was, until recently, an Intel product, any AMD laptops only have USB-C ports. These should still be usable with Thunderbolt 3 docks since since the Thunderbolt 3 specification requires them to fallback to DisplayPort alt-mode if the device doesn't support Thunderbolt alt-mode. However this specification only requires the docks to provide DisplayPort 1.2 in this fallback mode so we end up in a similar situation to before. Furthermore Thunderbolt 3 docks currently add around £100 to the cost and paying approaching £200 just to avoid plugging in a few extra cables seems foolish.

See Also

Useful links for dock design