# USB4, Thunderbolt and docks: what Windows can and cannot do over one cable

> How display, data and power share a USB-C link, why docks drop monitors or refresh rates, and how to size the cable you actually need.

**Tested on**

- Build: 26300.9457 / 26100.33650
- Edition: Pro (26H2) / Server
- Architecture: x64
- Evidence: E1
- Verified: 2026-10-29

## The short answer

One USB-C cable carries **four things at once**: display, USB data, PCIe (Thunderbolt/USB4 tunnelling) and power. They share a fixed budget.

That single fact explains almost every dock problem:

- monitors drop to a lower refresh rate when you plug in a USB 3 device;
- the second monitor does not light up on a laptop whose port is USB-C but not Thunderbolt;
- charging slows down when the dock also drives two displays;
- a dock works on one laptop and not another with an identical-looking port.

## Know what the port actually is

| Port | Display | Data | PCIe |
|---|---|---|---|
| USB-C (USB 3.2 only) | DisplayPort alt mode, often 1 stream | 5–20 Gbps | no |
| USB4 20/40 Gbps | DP tunnelling, 1–2 streams | up to 40 Gbps shared | yes (tunnelled) |
| Thunderbolt 3/4 | DP + PCIe | 40 Gbps shared | yes |
| Thunderbolt 5 | higher display/data budget | up to 80/120 Gbps | yes |

A USB-C shaped port can be any of these. The laptop's spec sheet and the port's icon (lightning bolt for Thunderbolt) are the only reliable indicators. Windows itself does not expose the link type in a supported UI.

## The bandwidth arithmetic

Approximate display bandwidth (uncompressed, 8-bit):

| Mode | Bitrate |
|---|---|
| 1080p 60 | ~3.2 Gbps |
| 1440p 60 | ~5.6 Gbps |
| 1440p 144 | ~14 Gbps |
| 4K 60 | ~12.5 Gbps |
| 4K 120 | ~25 Gbps |
| 4K 120 10-bit HDR | ~31 Gbps |

Subtract USB data and PCIe tunnelling from the link total, and the reason a dock "only does 4K 30" becomes obvious. **DSC** (display stream compression) is what makes 4K 120+ fit on 40 Gbps links — it is supported by the GPU, the dock and the monitor, or it is not used.

## Diagnose a dock problem

```powershell
# what Windows sees per display
Get-CimInstance -Namespace root\wmi -ClassName WmiMonitorID |
  Select-Object InstanceName,UserFriendlyName,Active
Get-CimInstance Win32_VideoController | Select-Object Name,CurrentHorizontalResolution,CurrentVerticalResolution,CurrentRefreshRate
Get-PnpDevice -Class Monitor | Select-Object FriendlyName,Status
```

Then:

1. **Compare with a direct connection.** If the monitor does full resolution/refresh directly but not through the dock, it is bandwidth or dock firmware.
2. **Remove USB devices** from the dock and re-check the display mode. If the refresh rate jumps back, you found the bandwidth conflict.
3. **Check the dock's power input.** A bus-powered dock has less budget than one with its own PSU.
4. **Update dock firmware** — many docks ship with bugs that only appear with specific laptops.

> **Caution: Cheap USB-C hubs are not docks**
>
> A hub that splits one USB-C port into HDMI + USB-A often supports only 4K 30 Hz and drops USB 3 speeds to USB 2 when video is active. If the listing does not state the alternate-mode display capability, assume the worst.

## Sleep, wake and docks

Docks are a common cause of "the laptop did not wake" or "the external monitor stays black after sleep":

- the dock's USB controller may not re-enumerate in time;
- **Modern Standby** laptops can enter a low-power state that drops the link;
- firmware updates for both laptop and dock usually fix it;
- as a workaround, unplug/replug the cable or disable "USB selective suspend" for the dock's hub.

## Choosing a dock

1. Count the **display modes** you need (resolution × refresh × number of monitors).
2. Add the **USB bandwidth** you need while those displays are active.
3. Check **power delivery** (watts) against the laptop's requirement.
4. Prefer Thunderbolt/USB4 if you need two 4K displays or PCIe devices.
5. Check the dock's **firmware update story** — a dock without updates is a dock you will replace.

## Related

- [Monitor choice and VRR](https://winbook.fyi/en/hardware/display/monitor-choice/)
- [Windows HDR chain](https://winbook.fyi/en/hardware/display/hdr-chain/)
- [Input polling rates](https://winbook.fyi/en/hardware/display/input-polling/)

## Lab boundary

E1. Link budgets and display modes are defined by the USB-IF/DisplayPort standards and documented at the Windows API level. The lab has no dock or external display, so no bandwidth test was run.

## Sources

- [Microsoft Learn — DISPLAYCONFIG_VIDEO_SIGNAL_INFO](https://learn.microsoft.com/en-us/windows/win32/api/wingdi/ns-wingdi-displayconfig_video_signal_info)
- [Microsoft Learn — Monitor drivers](https://learn.microsoft.com/en-us/windows-hardware/drivers/display/monitor-drivers)
- [Microsoft Learn — HID architecture](https://learn.microsoft.com/en-us/windows-hardware/drivers/hid/)

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Source: https://winbook.fyi/en/hardware/display/usb4-dock/