A while ago I wrote a “Demystifying USB-C and Thunderbolt” thread here, which addressed how USB-C and Thunderbolt worked, including in docking station scenarios, and how that related to supported display setups. Since then, Dell has released the WD19 dock family that includes support for the newer HBR3 standard, and the WD19TB dock in particular has some limitations on maximum resolutions with various display output combinations that might seem strange. So I decided to write this thread for anyone who was simply curious from a technical perspective about why those exist.
First of all, it’s important to note that although the WD19 dock family can take advantage of HBR3 support (DisplayPort 1.3 or 1.4) if the system has it available through its USB-C/TB3 port, the vast majority of systems on the market here in May 2019 still only support HBR2, for the simple reason that Intel GPUs today still only support HBR2 (DisplayPort 1.2). Even among systems that also have discrete NVIDIA/AMD GPUs, the USB-C/TB3 port is often still physically wired to the Intel GPU and is therefore subject to its limitations -- so at the moment, the only systems that have HBR3 support on USB-C/TB3 are those that have those ports driven directly by a discrete GPU. However, Intel’s upcoming “Ice Lake” family of CPUs will incorporate a new GPU that supports DisplayPort 1.4 and therefore HBR3. Those CPUs are slated to begin arriving in late 2019, as of this writing.
The main focus of this thread, however, is that the WD19TB has altered how it allocates display bandwidth to its various outputs compared to the TB16 that it replaces. That’s why if you look at the manual’s Display Resolution Table for a Thunderbolt system, you’ll find some limitations that might seem unintuitive or arbitrary. For example, when using an HBR2 system, running dual 4K 60 Hz displays requires that one of them be connected to the dock’s “downstream” Thunderbolt port, a limitation that didn’t exist on the older TB16 dock. But on an HBR3 system, that same Thunderbolt 3 port is limited to just QHD resolution whenever any other output is also in use. So what’s going on here?
There are two underlying causes for these limitations. The simple one is that the WD19 simply doesn’t support using its HDMI port and USB-C port for video output at the same time (although using the latter for a data device while using HDMI for video seems to be fine.) The second and much less obvious reason is that the WD19 family only allocates 4 of the incoming HBR lanes from the system to be shared across all of its “core” display outputs, i.e. all outputs except the Thunderbolt 3 port built into the removable attachment module. Any remaining HBR lanes coming from the system are only available to that Thunderbolt 3 port, regardless of whether it’s actually being used. This ends up accounting for both of the unintuitive and seemingly contradictory limitations relating to the Thunderbolt 3 port I mentioned earlier.
For the HBR2 system scenario, on a system that has two GPU outputs wired to its Thunderbolt 3 port (which to my knowledge all Dell systems have), an HBR2 connection over TB3 includes 8 HBR lanes, since a full DisplayPort link has always been defined as 4 HBR lanes, even before USB-C/TB3 arrived. But since the “core” display outputs only have access to half of those, which is equivalent to the bandwidth of a single full DisplayPort 1.2 link, you can only use those ports for display setups that fall within those bandwidth limits. That’s why even though the system is providing enough total bandwidth for dual 4K 60 Hz displays, for example, you’re limited to QHD if you want both displays on “core” outputs. However, if you instead connect only one display to a “core” port and the other to the Thunderbolt 3 port where the other 4 lanes are available, you can run dual 4K 60 Hz just fine.
For the HBR3 system scenario, there are at most 5 lanes coming from the system. The reason for this is that two full DisplayPort connections (i.e. 8 lanes) at HBR3 would require 64.8 Gbps of bandwidth, which is well beyond the 40 Gbps of Thunderbolt 3, and that’s before even considering any non-display data you might want to send across your Thunderbolt 3 connection to the dock, such as USB data for external hard drives, Ethernet data, etc. (If you're wondering, Thunderbolt 3 always prioritizes display traffic and throttles everything else when there isn't enough bandwidth to run everything at max performance. However, Thunderbolt 3 supports 40 Gbps in each direction simultaneously, and display traffic only ever runs one way, so depending on what else you're doing, high-bandwidth display setups might not bottleneck you.) In an HBR3 scenario where only 5 lanes are available, the first 4 get allocated to the “core” outputs, and then the Thunderbolt 3 port only gets access to that single remaining HBR3 lane – which is why it’s limited to QHD. The only exception seems to be if the Thunderbolt 3 port is the only one being used for display traffic, in which case it gets access to all 5 lanes, since the manual specifies that a single 8K 30 Hz display can be used from that port, just like all other ports.
One question not addressed by the manual is whether the dock supports DisplayPort DSC, i.e. Display Stream Compression. That’s part of the DisplayPort 1.4 spec, but I don’t know if it’s mandatory. But if the system and dock both support it, then higher-end display setups than indicated in the manual would be possible -- OR a given display setup would require less bandwidth, which would especially benefit Thunderbolt 3 connections because that would open up more bandwidth for other traffic. (On regular USB-C, currently half of the high speed lanes are allocated to video and half are allocated to USB, so reducing display bandwidth consumption doesn't benefit USB traffic -- although USB4 will be changing that to allow dynamic bandwidth allocation.) The higher-end display setup option could potentially even be achievable if the displays themselves didn’t support DSC as long the WD19 could “decompress” the DSC signal from the system and output a standard DisplayPort 1.4 signal to the attached display(s). And if the attached displays DID support DSC, then assuming all of the aforementioned support was still in place, even the maximum per-display resolution would increase, because at that point even the "normal" constraints on the DisplayPort 1.4 link between the dock and display could be exceeded. Hopefully we’ll find out through some testing once suitable systems and displays are more widely available.
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Not a bad idea.
But for your Matebook you could test this with just a third display. As the LG TB Displays will take up 2 DP connections already. If the other TB port of the Matebook can still output DP at the same time you are good.
For the XPS 17 9710, the worst that could happen, is that you would need to attach the WD19TB / LG displays on different sides.
Other than that, your Variant #1 plans should work.
ok I'll just order one WD19TBS dock and do some testing before indulging into a bigger rollout.
I bookmarked this thread and will post my conclusions. Thanks so far for the interesting read!
A problem that might come up with your plan to use both TB4 ports on one side:
So far Intels TB-Host controllers have only ever had dual ports at most with at most 2 DP inputs.
So in previous devices, like the XPS 17 9700, there was one TB controller for each side of 2 ports and those shared only 2 DP ports between them. So you could either get 2 DPs tunneled through Thunderbolt or 1 off of each TB-Port. This limit still exists in Tiger Lake as far as I understand Intels specs. But Tiger Lake has 2 TB controllers built in, so theoretically, it could be possible for the manufacturers to wire up both neighboring ports to different controllers giving you access to up to 4 DPs on one side. I have no idea whether this is possible or practical with the needed Thunderbolt Redriver chips the mainboard has to have, let alone the XPS 17. I believe the Framework also has 1 controller wired to each side, instead of back and front ports.
Either way, without some proof that your Matebook is actually wired up this way, I would not rely on having access to 3 or more DPs on one set of dual TB ports. Which would mean you need to resort to MST anyway in order to connect 4 4K displays to 2 Displayports.
Regarding HBR2 mode on the WD19TB-docks (I am assuming the only difference between TBS and TB is the missing audio outputs as they share the same firmware updates, but I only have WD19TB myself):
The TB controllers only tunnel exactly those DP connections the DP devices want, up to its own capability and remaining, non-allocated bandwidth in the TB link.
Problem here is, the WD19TB has a MST-Hub built in, which powers all display outputs except the TB-out. And this, like all MST-Hubs always (in my experience) attempts to establish the maximum supported link possible, as soon as 1 sink is plugged into it. And in my observations, the MST-Hub is on the primary DP-Out of the TB controller, so if you plugin the dock into a host and the MST-Hub, as well as the TB-out have a display attached, the MST-Hub will always get first choice and then take an 4xHBR3 connection. This reserves too much bandwidth for another 4xHBR2 link to the TB-out. The speed of all outputs of the MST-Hub is completely independent of its source connection to the host however.
The only way to force this, is to plug in the monitor on the TB-out first, then the MST hub will take whatever bandwidth is left after that connection has already been made. This is rather impractical day-to-day.
But on the other hand, the MST-Hub has unofficial support for DSC, which allows it to actually run 3x4K displays off of a 4xHBR3 link, although you might have issues with connection stability if you try. This feature is completely undocumented by Dell and might be more buggy than in my experience.
This bandwidth distribution is just very hard to guess as the cause of most problems, if you do not have monitors that tell you what kind of link speeds they are currently using. And the link speed of the MST Hub inside the WD19 I could only guess, by comparing with a TB4 dock and running all kinds of monitor combinations, but I am quite confident in my results.
Also, monitors with built-in DSC are now coming to market (Dell U2723QE, U3223QE for example, I have one of those). Those have a built-in MST-Hub with official DSC support, that can still run 4K off of a 2xHBR2 link (what remains after already using a full HBR3 link through the same TB connection) or even 2 4K displays over 2xHBR3, which is the standard bandwidth you get with USB-C + USB3 without any Thunderbolt at all (and thereby only requiring 1 DP connection per port.
Although I should add, my U3223QE has been having black-screen issues on Nvidia graphics and Nvidia support's first reaction was "MST is not officially supported on GeForce GPUs, don't care", so it might not be the best horse to bet on, if those GPUs could be driving the MST-Hubs.
Since my TB-ports are only driven by Intel Alder Lake graphics, I could not test, whether or not the same issue exists with the MST-Hub inside the WD19TB...
@XPSnerd Ok that makes much more sense. For some reason when reading your post I thought that you intended to run the dual 4K display Thunderbolt daisy chain off the downstream TB3 port on the WD19TBS itself, but your proposed setup is certainly more sensible. And yes, I think that will work. HBR3 to HBR2 downconversion would have to be available because HBR3 systems are still subject to HBR3 allowed port configurations even when using HBR2 displays, which still represent the majority of DisplayPort displays in use today.
ok, let me clarify what I tried to say in my above (too lenghty?) post:
I intend to connect 4x 4K displays to a XPS17 9710 by using two of its four TB4 ports. So each TB4 port will drive two 4K displays @ 60Hz.
I'll now name the two TB ports of the XPS17 with "TB port #1" and "TB port #2" and describe for each port how two 4K monitors are connected to it.
TB port #1
A TB3 equipped 4K monitor is directly connected via TB3 cable and another 4K monitor daisy chained to it. This part of the setup already works with my current Tiger Lake laptop and should continue to work with the XPS17. I only mentioned it for the sake of a complete description.
Now, in order to complete my desired 4x 4K setup, I need to connect two more 4K monitors to the XPS17. As I described above, two 4K monitors are already connected to TB port #1.
I intend to connect two more 4K monitors by using a Dell WD19TBS dock because it will also charge the XPS17 at 130W. (With any other TB4 dock I'd need a seperate 130W power supply)
TB port #2
From the XPS17, a thunderbolt 3 cable connects to the Dell WD19TBS dock. Two DP1.2 4K monitors are connected to this dock.
My above post is mainly about that connection and what happens if the 4K monitors used are DP1.2 only, yet the TB3 connection between XPS17 and WD19TBS dock being HBR3/DP1.4
In my above post I concluded that the dock must either support HBR3/DP1.4 -> HBR2/DP1.2 signal conversion OR run in HBR2/DP1.2 mode in the first place which it obviously won't do when connected to a TigerLake Laptop that is HBR3/DP1.4 capable
@XPSnerd I’ll say right upfront that I can’t confirm any of what I’m about to say based on actual testing experience or even reading other anecdotal success stories, and also that not everything that SHOULD work on paper actually DOES work in practice. The WD19TB(S) dock has sometimes had reliability issues even running dual 4K 60 Hz displays, just as an example.
That said, am I correct that you are envisioning a scenario of having a WD19TBS connected to EACH of the TB4 ports on your system, with two 4K 60 Hz displays connected to each one? If so, I would imagine that would work, even without having to give the dGPU direct access to the display outputs, but it seems wasteful. Why not get a single WD19TBS dock for one pair of displays, and then a TB3 to Dual DisplayPort adapter, connected to another TB4 port on the system, for the other pair?
@Ray519 That's great information on all fronts. Always helpful to see real world tests since sometimes tech can perform above or below specs due to unadvertised/untested capabilities or firmware glitches.
I would expect DSC to pass through MST hubs and such as well, but I've also learned that not everything that SHOULD work in the tech world always DOES work. And if the display itself doesn't support DSC, then of course passthrough capability would not be useful.
Sadly I have a feeling that while DSC will enable some high-end display setups, it will also make spec sheets and support tables quite a bit more complicated. There are already separate tables/rows for things like Thunderbolt vs. regular USB-C DP Alt Mode and then for DisplayPort HBR2 vs. HBR3 support, the latter of which often isn't even clearly advertised in the specs of the system. But now with DSC, I imagine there will have to be at least one new table based on whether the user's system supports DSC. But even that will require some sort of caveat saying that you'll either need displays that support DSC natively or an intermediate dock/MST hub that can unpack that DSC signal for them -- which will of course send users down a rabbit hole of figuring out whether DSC support is available on those devices. And then what if your dock/hub doesn't support unpacking DSC and only certain displays you have support DSC? Will there be yet another table for "partial DSC support"?
I do sympathize for the everyday user who just wants to know if a given set of equipment will allow them to run a given display setup....
With regards to DSC,
This fits with the specifications of the actual chip doing the work (Synaptics VMM3550, specs: https://uploadcdn.oneyac.com/attachments/userfile/b4/13/1593672494627_4765.pdf)
And this specific chip can pass-through DSC arbitrarily and decompress display data for every one of its SST outputs separately.
But as far as I understand it, since Displayport is packet-based, I actually would expect pass-through to work through any MST-Hub, even old ones, predating the specification of DSC. But I do not have the HW to confirm this.
Also, DSC still works on the WD19TB, when the TB-Out is used as display output (HBR3 in my case) and the MST-Hub should only be getting a slower connection. DSC still allowed the Dock to run an additional UHD@60 display of the MST side.
@alexcapone I too saw instability issues, when maxing out the compression. 1 or more displays went dark and where regognized again after a few seconds (just like reconnecting). After a few of those I also saw it limiting bandwidth or no longer recognizing a monitor. For me, this seemed to be more or less proportional to amount of compression that was needed (compare with Dells specs, which only give resolutions possible without any compression).
So just a follow up to my last post. After coming back to my desk and trying to wake my computer from sleep it seems that both monitors defaulted to a horrendous 640x480 resolution and the only fix was to unplug from the laptop and replug and both monitors were back to 5120x1440. I can't even change the resolution in the display settings to fix it as it tells me that 640x480 is the maximum. Display Fusion couldn't correct it either, unfortunately.
So it seems the sticking point in the cable combos I've tried so far is that it doesn't remember or cannot display maximum resolution unless I unplug and replug back into the laptop. The only cable that has not downgraded the resolution after waking from sleep is the HDMI cable. Once I get that HDMI to DP active adapter I am going to try to see if HDMI + HDMI to DP will solve the problem.