Understanding Thunderbolt ¾ Hubs

[BumpMan]

When you have a TB3/4 Hub how do the added TB ports work? AKA, how does it handle bandwidth coming from a single TB4 port and provide more?

Is each port allocated a certain static/permanent/dedicated split of the original 40Gbps bandwidth? So that say the 4 ports never get more than 10Gbps? (Ignoring overhead)

or

Is it dynamic, so that the 4 resulting ports can each receive the full 40Gbps bandwidth so long as its not simultaneous with any other ports asking for bandwidth?

Have a couple Sandisk G40 Pro SSD's and they are incredible and wondering if I can add more via a TB hub. It's only worth it if they can all achieve the proper speeds of 2700-3000Mbps independently understanding it's obviously not possible if they were all reading or writing simultaneously. But all connected at the same time...

Thanks.

 

[PaulD-UK]

Here’s an old thread with some info:

How do TB3/4 docks work at a low level? What signals are passed?

I haven't been able to find any good write ups on how TB3/4 works when it goes to a dock and the dock has multiple output devices. For this example lets assume a TB4 MacBook Pro connected to a TB4 dock and the dock has everything a TB4 dock can have as outputs. 1) Is there a single...

forums.macrumors.com

 

[BumpMan]

Thanks for the link to that thread. I’m still a little confused because of the added display into the mix in that conversation.

Is it possible to explain the scenario where it’s just TB 3 SSD’s connected to a TB4 hub connected to a TB4 port on the Mac Studio?

Where I understand if there is a monitor connected to the hub it is always on getting signal and taking up bandwidth. But if there is no display connected and it’s just SSD’s can each SSD utilize the full 40Gbps bandwidth UNTILL another one starts asking simultaneously then it splits the bandwidth in 2 or 3 depending on how many SSD’s as asking…?

I THINK that’s what the link you shared is explaining but didn’t quite get it.

 

[PaulD-UK]

TB4 always allows some bandwidth (for displays etc?) - reducing the data allowance for SSDs to about 32Gbps max.
TB3 only allows about 22Gbps for data.

Apple's original M1 Mac minis also reduced the data output of their USB-C ports to allow for display bandwidth.
If you plugged in a monitor often the data bandwidth increased, as Apple had over-estimated how much the monitor 'might' use if it wasn't plugged in.

With later Mx SOCs it isn't clear if this is still going on, but it might explain some of why USB-C performance is often worse when SSDs are connected directly to the Mac, and give faster performance connected to a TB3 connected dock.

On my M1 Mac mini with TB3 NVMe dock (WD D50 2TB SN750) I get 2800 MBps read and 2600 MBps write when the dock is connected alone, and 2800 MBps read, 1730 MBps write when my 5K monitor is plugged into the dock.

With a TB4 dock you will theoretically get 32Gbps SSD bandwidth to any SSD connected to the downstream TB4 ports, and the 32Gbps shared amongst any drives being written to or read simultaneously.

'Theoretical' being the important factor - individual SSD design and firmware (and cable performance) gives wide scope for variation...

 

[BumpMan]

Got it thanks PaulD. Wasn’t aware of the mandatory allocation to an even potential connected display.

So then any and all TB3 SSDs connected to a TB4 dock can achieve their full TB3 speeds (theoretically 3000-2700 Mbps-ish) if reading or writing alone, but if reading and writing simultaneously with other TB3 SSD’s connected to the additional TB4 hub ports it will then share the 32Gbps among whichever are reading/writing.

Is that right?

Also is it correct to understand that since there is upstream and downstream if one SSD is only reading and another SSD starts writing they won’t affect each other too much because they are on different channels or whatever you call it. So maintaining the highest speeds across multiple SSDs on the same TB4 hub is quite doable UNLESS they’re doing the same thing simultaneously.

Is that also right?

Thanks a lot for your patience.