Thunderbolt 3 with USB-C Splitter

[Han1]

I've been agonising over adding some new external storage to my M1. I've arrived at what's probably the best solution, thanks to some really great advice on here.

I'm now at the point where I will connect via my Thunderbolt 3 port on my Startech dock. 10Gbps is probably enough, so I'm thinking of adding a 4-way splitter to make use of some old SSDs from the same Thunderbolt 3 port. The new SSD / NVMe will be my primary external storage and the older SDDs will mostly be idle, so I don't expect much contention.

However, assuming I might also have one of the older SSDs in use for TimeMachine, I'm thinking it would be ideal to have a splitter that presents 4 x USB-C to the drives and has Thunderbolt 3 back to the Thunderbolt 3 port on my dock, i.e. 22Gbps (theoretical) between the splitter and the dock. Does such a thing exist?

I've been looking on Amazon, but nearly every seller likes to put 'Thunderbolt' in the title (to demonstrate compatibility) even if it's USB-C in and out from the splitter. It makes the search results very misleading.

Thanks to any networking / storage gurus out there who might be able to help.

 

[PaulD-UK]

@Han1
Splitter isn't the right term. In computer terms, it's only called a splitter for power supply distribution in PCs.
For data or video cabling, with USB it's called a hub, and for Thunderbolt it's usually called a dock.

For your purpose you probably just need a self-powered four port USB 3.2 Gen2 USB-C uplink Hub.

So you already have a Startech dock. That will have a number of downstream output ports to other devices:
a downstream TB3-out port, and a number of USB-C or USB-A ports to connect devices to.

You can use these USB ports to plug SSDs into, one enclosure per port.
If you don't have enough USB ports you can also use the downstream TB port to plug an enclosure into.

You can plug the input port of a multi-port USB-(A or C) Hub into any TB3-out or USB port on your dock, to get further USB-out ports on the hub to plug a SSD into. A self powered USB hub will be needed if you have many devices connected to the dock.

The ports of a USB-A or C hub all share a total of about 900MB/s bandwidth - fine for HDs and SATA SSDs, but limiting for NVMe blade SSDs.

To get the highest bandwidth for NVMe enclosures use a Thunderbolt enclosure, and plug it into the downstream TB port of the dock.

If you still need more ports, then you can plug another Thunderbolt 3 dock into the downstream Thunderbolt port of your Startech TB dock.

 

[Han1]

Hi Paul - thanks a lot for taking the time to reply.

Yup, I'm no expert. I should have said hub. I'll have 1 or 2 USB (A) 3.0 and 1 TB3 port available to me. My thinking was that if I use the USB A ports a) I don't have enough empty ports for all the drives and b) I might be capping the performance (although I appreciate it won't be much higher than 5Gbps) for a single SSD, although that would drop off if any are sharing the USB A on a hub.

That's where I got my idea, which I think is the same as yours, that if necessary I should add a USB-C hub to the TB3 port. (I hadn't appreciated the possible need for a dedicated power unit - I think that might put me off the idea.) In a very uneducated way, the USB-C hub idea seemed like I would be negating the benefits of the TB3 port, i.e. I'd only get 10gbps USB-C between the TB3 port and the hub. Hence, my probably somewhat stupid question about whether there were hubs out there that would go TB3 from the port to the hub, before sharing out 4 x USB-C.

For example, that way I figured I might get 4 x 5Gbps if all four drives were in use. (Not sure if a hub would manage the bandwidth between drives this way - just a bit of easy math to show my thinking - or lack of it!)

I think I'm now leaning to a single NVMe or SSD (new) on the TB3 and a TimeMachine SSD (old) on the USB A. It's just with a single USB-C SSD on the TB3 it feels like I'm wasting the full bandwidth of the TB3.

Thanks for helping me get closer to the answer.

I'd love to know what would happen with bandwidth if I put my new SSD on a USB-C hub with 3 old SSD on the rest of the hub (connected but idle). Would my new SSD be able to mostly consume everything as if it were connected to the TB3 without the hub? I'm guessing a hub wouldn't be that intelligent and would just give it 25%.

 

[PaulD-UK]

@Han1 "...whether there were hubs out there that would go TB3 from the port to the hub, before sharing out 4 x USB-C."

There is exactly that sort of 'hub', and it's called a Thunderbolt 3 or 4 dock.
But some docks prioritise ports for displays, ethernet and audio etc.
The sort you want has multiple USB-A and C ports

"...if I put my new SSD on a USB-C hub with 3 old SSD on the rest of the hub (connected but idle). Would my new SSD be able to mostly consume everything as if it were connected to the TB3 without the hub? I'm guessing a hub wouldn't be that intelligent and would just give it 25%."

No, a 'splitter' is unintelligent. Hubs and docks are designed to allocate the bandwidth to whichever port needs it.
So 1 device gets 100%, 2 devices get 50% each, and 4 devices get 25% each.
That's in theory, in real-world use there are losses but that is the general principle.

Output ports from USB and TB hubs/docks only supply up to 15 watts of power.
In practice that is enough for 1 SSD but not enough for an HD.
So the hub/dock needs to have its own power supply because a 4 port hub needs to supply up to 60 watts.

 

[Han1]

@Han1 "...whether there were hubs out there that would go TB3 from the port to the hub, before sharing out 4 x USB-C."

There is exactly that sort of 'hub', and it's called a Thunderbolt 3 to 4 dock.
But some docks prioritise ports for displays, ethernet and audio etc.
The sort you want has multiple USB-A and C ports

"...if I put my new SSD on a USB-C hub with 3 old SSD on the rest of the hub (connected but idle). Would my new SSD be able to mostly consume everything as if it were connected to the TB3 without the hub? I'm guessing a hub wouldn't be that intelligent and would just give it 25%."

No, a 'splitter' is unintelligent. Hubs and docks are designed to allocate the bandwidth to whichever port needs it.
So 1 device gets 100%, 2 devices get 50% each, and 4 devices get 25% each.
That's in theory, in real-world use there are losses but that is the general principle.

Output ports from USB and TB hubs/docks only supply up to 15 watts of power.
In practice that is enough for 1 SSD but not enough for an HD.
So the hub/dock needs to have its own power supply because a 4 port hub needs to supply up to 60 watts.

Perfect. Thanks Paul. You've confirmed my amateur attempts to find the answer. I'm already using the TB3 port on the 'intelligent' dock (StarTech) and I can't use my spare TB3 port on my M1, so I'm going to use the dock's TB3 port solely for my new NVMe. Thanks a lot.

 

[joevt]

For example, that way I figured I might get 4 x 5Gbps if all four drives were in use. (Not sure if a hub would manage the bandwidth between drives this way - just a bit of easy math to show my thinking - or lack of it!)

Later Thunderbolt 3 (Titan Ridge) and Thunderbolt 4/5 hubs/docks limit total USB bandwidth to 9.7 Gbps because they use a single USB 10 Gbps hub (9.7 Gbps data). A USB hub is not a USB controller - meaning upstream and downstream are USB. A USB controller has PCIe for upstream and USB for downstream.

Early Thunderbolt 3 (Alpine Ridge) docks have multiple USB controllers to support total USB bandwidth of ≈22 Gbps (limited by Thunderbolt upstream bandwidth). For example, the CalDigit TS3+:
1) downstream Thunderbolt port: 9.7 Gbps USB or ≈22 Gbps PCIe over Thunderbolt - one port.
2) Fresco Logic FL1100: 4 Gbps (limited by PCIe 4 Gbps upstream connection) - multiple ports.
3) Fresco Logic FL1100: 4 Gbps (limited by PCIe 4 Gbps upstream connection) - multiple ports.
4) ASMedia ASM1142: 7.877 Gbps (limited by PCIe 7.877 Gbps upstream connection) - two ports, one of them is limited to 4 Gbps but it's slightly faster than the 4 Gbps of the Fresco Logic because the PCIe upstream is 7.877 Gbps).

TS3 Plus Interface Bandwidth Allocation and Diagram – CalDigit

Bandwidth allocation breakdown and diagram for the interfaces on the TS3 Plus.

www.caldigit.com

One benefit of the newer docks is that they have more 9.7 Gbps ports.

Every USB 3.x controller has separate lines for USB 2.0 (480 Mb/s).

You can tell the difference by looking at how many USB controllers are added (in the System Information.app USB tab (USB bus) or PCIe tab (pci XHCI device)) when you connect a Thunderbolt dock.
A) Thunderbolt 4/5 dock connected to Apple Silicon: zero (it uses the USB controller of the Mac!)
B) New Thunderbolt 3 dock connected to Intel: one (just the USB controller of the Thunderbolt controller of the dock - it uses USB hub(s) for all the ports)
C) Old Thunderbolt 3 dock: 2 or 3 or 4 (as in the CalDigit TS3+ case)

So 1 device gets 100%, 2 devices get 50% each, and 4 devices get 25% each.

2 devices get 50% each only if they are trying to do the same thing at the same time.
A device can get 100% even if other devices are connected as long as the other devices are idle.
1 device could get 100% for transmit and another device could get 100% for receive at the same time because USB 3.x and later have separate lines for transmit and receive.

 

[Han1]

Later Thunderbolt 3 (Titan Ridge) and Thunderbolt 4/5 hubs/docks limit total USB bandwidth to 9.7 Gbps because they use a single USB 10 Gbps hub (9.7 Gbps data). A USB hub is not a USB controller - meaning upstream and downstream are USB. A USB controller has PCIe for upstream and USB for downstream.

Early Thunderbolt 3 (Alpine Ridge) docks have multiple USB controllers to support total USB bandwidth of ≈22 Gbps (limited by Thunderbolt upstream bandwidth). For example, the CalDigit TS3+:
1) downstream Thunderbolt port: 9.7 Gbps USB or ≈22 Gbps PCIe over Thunderbolt - one port.
2) Fresco Logic FL1100: 4 Gbps (limited by PCIe 4 Gbps upstream connection) - multiple ports.
3) Fresco Logic FL1100: 4 Gbps (limited by PCIe 4 Gbps upstream connection) - multiple ports.
4) ASMedia ASM1142: 7.877 Gbps (limited by PCIe 7.877 Gbps upstream connection) - two ports, one of them is limited to 4 Gbps but it's slightly faster than the 4 Gbps of the Fresco Logic because the PCIe upstream is 7.877 Gbps).

TS3 Plus Interface Bandwidth Allocation and Diagram – CalDigit

Bandwidth allocation breakdown and diagram for the interfaces on the TS3 Plus.

www.caldigit.com

One benefit of the newer docks is that they have more 9.7 Gbps ports.

Every USB 3.x controller has separate lines for USB 2.0 (480 Mb/s).

You can tell the difference by looking at how many USB controllers are added (in the System Information.app USB tab (USB bus) or PCIe tab (pci XHCI device)) when you connect a Thunderbolt dock.
A) Thunderbolt 4/5 dock connected to Apple Silicon: zero (it uses the USB controller of the Mac!)
B) New Thunderbolt 3 dock connected to Intel: one (just the USB controller of the Thunderbolt controller of the dock - it uses USB hub(s) for all the ports)
C) Old Thunderbolt 3 dock: 2 or 3 or 4 (as in the CalDigit TS3+ case)


2 devices get 50% each only if they are trying to do the same thing at the same time.
A device can get 100% even if other devices are connected as long as the other devices are idle.
1 device could get 100% for transmit and another device could get 100% for receive at the same time because USB 3.x and later have separate lines for transmit and receive.

That's great, thank you.

You've confirmed what I'd hoped - there will be very little contention from idle drives on a hub. I guess there might be some management inside the dock to share bandwidth back to the Mac, but the hub itself (hanging off the dock) won't have much intelligence and it'll be a bit random.

I have a StarTech TB3 dock. I'm guessing (didn't fully follow the science) this means I have the potential to get 20Gbps out my TB ports. If I did follow your explanation, it seems strange the newer docks cap out at 10Gbps.

 

[PaulD-UK]

@Han1 " ...this means I have the potential to get 20Gbps out my TB ports."

Yes, with a NVMe SSD in a Thunderbolt 3 enclosure. TB3 docks don't have faster USB 3.2 ports than 10Gbps.

TB4 docks only support USB 3.2 Gen 2x2 20 Gbps with Windows PCs.
MacOS doesn't support USB 3.2 Gen 2x2 20 Gbps, only USB 3.2 Gen2 10Gbps.

Recent Apple Silicon Macs with USB 4 (TB4/USB 4) ports support USB4 40Gbps enclosures, which can also be connected through a TB4 dock.

 

[joevt]

I have a StarTech TB3 dock. I'm guessing (didn't fully follow the science) this means I have the potential to get 20Gbps out my TB ports. If I did follow your explanation, it seems strange the newer docks cap out at 10Gbps.

Which StarTech Thunderbolt 3 dock? The TB3CDK2DP claims to have USB-C compatibility, which means it uses Titan Ridge Thunderbolt controller (confirmed by product page tech specs) and a single USB connection for all USB ports. It uses a RealTek RTL8153 USB to Gigabit Ethernet Adapter. Old Thunderbolt 3 docks that use Alpine Ridge usually use a PCIe Ethernet controller.
Titan Ridge docks don't usually use PCIe controllers so that all their devices can be used by a non-Thunderbolt host.

The TB3DK2DPM2 and TB3DKM2DPL and some others use Alpine Ridge. Check the tech specs for extra PCIe USB XHCI controllers. Usually they are only 4 Gbps USB controllers though. Some might one 8.77 Gbps USB controllers.

TB4 docks only support USB 3.2 Gen 2x2 20 Gbps with Windows PCs.

True if USB tunnelling is involved (Thunderbolt 4 or USB4). You can maybe disable USB tunnelling by putting a Thunderbolt 3 dock between the Mac host and the Thunderbolt 4/5 dock. A Thunderbolt 5 dock might support Gen 2x2 20 Gbps in that case.

MacOS doesn't support USB 3.2 Gen 2x2 20 Gbps, only USB 3.2 Gen2 10Gbps.

macOS supports USB3.2 Gen 2x2 20 Gbps since Sonoma. There's a couple ways you might add gen 2x2 USB.
https://forums.macrumors.com/thread...-at-full-speed-on-a-mac.2443940/post-33591251
https://forums.macrumors.com/thread...-m4-series-macbook-pros.2413354/post-33594639

 

[theluggage]

I'm guessing (didn't fully follow the science) this means I have the potential to get 20Gbps out my TB ports. If I did follow your explanation, it seems strange the newer docks cap out at 10Gbps.

First - forget 20Gbps over USB 3.2 - see @PaulD-UK's post.

Even old-style TB3 hubs with "dedicated" PCIe-to-USB controllers often use internal hubs to share them between multiple USB-A or USB-C ports, SD card readers, audio interfaces etc. so if you're really concerned about every device getting maximum USB bandwidth you'll have to go digging through the manufacturers' websites to find the internal schematics anyhow. Or there's this:

Thunderbolt 3 to USB 3.1 Host Controller - Thunderbolt Hubs & Adapters | USB Hubs | StarTech.com United Kingdom

External Thunderbolt 3 to USB Controller - 3 Host Chips - 1 Each for 5Gbps Ports, 1 Shared on 10Gbps Ports - Self Powered | United Kingdom

www.startech.com

The advantage of the hubs now sold as TB4 or USB4 is that they usually have 3 downstream TB4/USB4 ports which - while USB 3 devices connected to those have to share < 10Gbps - can all accept Thunderbolt devices which share close to 40 Gbps of bandwidth. So if you really want/need the fastest external SSDs (you probably don't) you can pay the premium for one or more Thunderbolt SSDs and connect those to a TB4 hub. With an "old style" hub you only get a single downstream Thunderbolt SSD (you can chain further devices like that if the SSD has it's own daisychain ports, but many of the cheaper ones don't)

...but check that they really are 20 or 40Gbps Thunderbolt/USB4 devices, not just "Thunderbolt compatible" which often just means USB 3 over a Thunderbolt connection.

All that said, if you can connect a fast USB 3.1 Gen 2 SSD (or stump up for a TB one) direct to your Mac and hang everything else off the Hub that will almost certainly be fine. Old SSDs and TM Backups won't care if they're sharing USB via a hub.