What is Thunderbolt 4?

Posted by Ēriks Jaunzems on

You got or are thinking of getting the new M1 mac mini for your new DIT station, because of its compactness, affordability and power, but don’t exactly know what is Thunderbolt 4, and I would not blame you for thinking that Thunderbolt 4 is a replacement to Thunderbolt 3 for Apple silicon, because they have basically the same spec sheet: 40Gb/s, two 4K monitors, 32Gb/s PCIe (4 lanes of PCIe Gen 3), so what is the difference, and why pro stuff is still on Thunderbolt 3?  In this small article, I will try and demystify and maybe give you better understanding the differences between Thunderbolt 3 and Thunderbolt 4. From what Thunderbolt 4 is, to some differences in both technologies. 

As most professionals know Thunderbolt evolutions were well defined: TB 10Gbps, TB2 20Gbps, Thunderbolt 3 40Gbps. Every generation of Thunderbolt, the bandwidth has doubled and because of that many of us, before Thunderbolt 4 was revealed, were hoping for another doubling of bandwidth, but it did not. So, is it another generation of Thunderbolt, like the name implies or is it something else? Thunderbolt 4 is a stand-alone product that harmoniously lives with Thunderbolt 3 in the Thunderbolt family. As they are intended to be used interchangeably and can be mixed and matched. Both uses the same protocol, share the same bandwidth, tunnels the same amount of PCIe lanes and can be daisy chained up 6 devices.

So, what is the difference, and why did we need Thunderbolt 4? To explain the difference between them I will use the as example 2 of Sonnet technologies docks, Echo 11 TB3 and Echo 11 TB4. Looking at the diagrams I made, using Sonnet technologies given data, for representing both docks, you can see, that Thunderbolt 3 controller allows only for one Thunderbolt 3 downstream with 4 PCIe lanes for device it self, but this dock does not use them, a NVMe dock would you them for high speed SSD.

(graphic from Sonnet Technologies)

In contrast to that, Thunderbolt 4 controller can downstream 3 Thunderbolt 4 ports. This is due to Thunderbolt 4 controller’s superior hub functionality, and it is great for docks, as it provides more options. But this flexibility is at cost, looking at the diagram, you can see that Thunderbolt 4 controller provides only 1 PCIe lane in the device itself, meaning expansion chassis, eGPU, professional media readers, other professional Thunderbolt equipment will suffer if made with Thunderbolt 4, because they rely on bandwidth provided by 4 PCIe lanes. This might discourage some D.I.T.s and data managers, as it seems backwards, going from 4 lanes to 1, but rest assured, if you connect Thunderbolt 3 product after Thunderbolt 4 dock, it will pass-through all 4 lanes to intended Thunderbolt 3 product, so you can still use professional expansion chassis and media readers you are used to, not worrying if they get all the lanes.  These are the main differences of Thunderbolt 3 and Thunderbolt 4 in products.

But one more difference is in specification enforcement. Thunderbolt 3 is quite loose standard, as it was not enforced. Thunderbolt 3 goal standards: PD-15W, 4 PCIe lanes, Two 4K display capability, 40 Gbps cable, those are suppose to be the specs of Thunderbolt 3 but many of you know, that in a lot of the cases, mostly windows side of things, that’s not the reality, Thunderbolt 3 specs could go as low as half of the “Golden specs”: 7.5w PD, 2x PCIe, One 4K display and 20 Gbps cable and sill get the “Thunderbolt 3”name. But Thunderbolt 4 is strictly enforced, and that means if it has Thunderbolt 4 logo, it is full spec, and you do not need to worry if your Thunderbolt 4 cable is full 40Gbps or 20Gbps.

And for those D.I.T.s and data managers who are thinking that they will need to change all the Thunderbolt cables for your shiny new Thunderbolt 4 computer or dock, don`t, as they are interchangeable, as long as they are full 40Gbps cables, you can use them with Thunderbolt 4 products and vice versa. And if you have Intel powered Apple computer with native Thunderbolt 3 port (Thunderbolt 2 with adapter will not work), running Big Sur, you can use Thunderbolt 4 products with your Thunderbolt 3 mac. Unfortunately, for now Windows machines are not capable of running Thunderbolt 4 dock with Thunderbolt 3 machine.

So, in conclusion Thunderbolt 3 and 4 are two different products in the same family, so Thunderbolt 3 is not going to be replaced by Thunderbolt 4 as they serve different purposes. One is for professional equipment and giving your computer new capabilities with expansion slots and high speed raid storage, and the other is to give you the ultimate docking solution and flexibility you need to enhance your workflow. Together they give you best flexibility and best performance for your D.I.T. station, so you can work more efficiently and faster.


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