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USB4 Specification Merges Thunderbolt 3 and USB With Transfer Speeds up to 40Gb/s


macrumors bot
Original poster
Apr 12, 2001

The USB4 specification for a new version of USB was today published by the USB Implementers Forum [PDF] giving us details on what to expect from the next-generation USB architecture following a preview back in March.

USB4 is a major update that "complements and builds upon" the current USB 3.2 2x2 (USB-C) and USB 2.0 architectures. According to the USB-IF, the USB4 architecture is based on Thunderbolt, doubling the maximum bandwidth of USB and allowing for multiple simultaneous data and display protocols.

The USB-IF outlined key specifications of the USB4 architecture, such as 40Gb/s speeds (twice the current 20Gb/s maximum) and backwards compatibility with USB 3.2 and Thunderbolt 3.

[*]Two-lane operation using existing USB Type-C cables and up to 40Gbps operation over 40Gbps certified cables
[*]Multiple data and display protocols that efficiently share the maximum aggregate bandwidth
[*]Backward compatibility with USB 3.2, USB 2.0 and Thunderbolt 3

USB4 will use the same USB-C connector design as USB 3, which means manufacturers will not need to introduce new USB4 ports into their devices.

Apple's newest Macs offer support for USB-C and Thunderbolt 3, which means most Mac users are already experiencing USB4 speeds when using Thunderbolt 3 cables and devices, but USB4 will make Thunderbolt-style speeds the new default and it will lower the cost of devices that use these faster transfer speeds.

USB Power Delivery will be required in devices built for USB4, which also means we can expect to see higher-powered chargers with multiple USB4 ports.

Though the USB4 specification is complete, it will still be some time before we can expect to see devices that take advantage of USB4. It typically takes at least a year for new products to come out following the finalization of a new specification, so it will be late 2020 or beyond before we begin seeing USB4 devices.

Article Link: USB4 Specification Merges Thunderbolt 3 and USB With Transfer Speeds up to 40Gb/s


macrumors 6502a
Nov 8, 2018
Interesting that USB4 will be no faster than Thunderbolt 3. I guess I’ll hold on to my Razer Core X eGPU for a while!
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macrumors 68040
May 14, 2012
Well at least 2019 MBP don't need a dongle for USB4. Older thunderbolt 1 and USB3.0 do.
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Internet Enzyme

macrumors 6502a
Feb 21, 2016
Congratulations Apple, you were four or five years early—you certainly earn that distinction. Too bad that the products in the meantime were entirely unflexible and annoying for anyone who wanted to conveniently use their laptop as a computer
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macrumors 68020
Dec 31, 2013
Any clue if Thunderbolt 3 will be forward compatible with USB4? (ie get 40gbps with USB4 devices connected to Thunderbolt 3 ports).

According to the spec, it's optional

A USB4 host or USB4 peripheral device can optionally support interoperability with Thunderbolt 3 (TBT3) products.

A USB4 hub is required to support interoperability with Thunderbolt 3 products on all of its DFP. A USB4-Based Dock is required to support interoperability with Thunderbolt 3 products on its UFP in addition to all of its DFP.

When interoperating with a TBT3 product, Thunderbolt Alt Mode is established on the link between products. The USB Type-C Specification describes how a USB4 product negotiates and enters Thunderbolt Alt Mode.

Skimming the relevant sections, there's a number of minor implementation details that are different between USB4 and TBT3.
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macrumors 65816
Jul 12, 2009
Finally. I can't wait for Thunderbolt 3, USB 4 devices to become mainstream (read - "cheap").

Planning on upgrading the iMac 5K with a Thunderbolt 3 DAS and 10 gb ethernet on top of the eGPU on one of the ports.
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macrumors 6502a
Sep 30, 2014
National Capital Region
Something is not said. There maybe a gotcha:

Thunderbolt supports more than USB. Thunderbolt is a superset of USB., since Thunderbolt supports PC Express (PCIe) and Display Port, natively. USB does not.

Hence, you can't use most Thunderbolt 3 devices from a USB host port, but the same isn't true the other way around. You can plug a USB device and cable into a Thunderbolt 3 host port whenever you want because Thunderbolt 3 fully supports USB.
At least that is my understanding.
Am I wrong?
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macrumors member
Sep 19, 2015
It was wise of Intel to finally open-source TB3.

I'd like to see a version with 8 lanes in each direction rather than just 2. 8 * 20Gb/s = 20 GB/s each direction raw. For short runs, that would beat 100G ethernet passive copper. Last time I checked, 100G 1m cables were $170, not including the chips on each end.


macrumors 65816
Nov 19, 2012
Just let's let USB-C be the end of it. At least for the foreseeable.

I don't want to start hearing talk of a D version...


macrumors 68020
Dec 31, 2013
I disagree. I think backwards compatibility is one of the keys to the success of USB. Keyboard and mouse makers can use a cheaper USB 1 or 2 controller instead of a more expensive chip. No need for high performance.

No. USB4 does not replace USB1/2 Protocols. The architecture does not change from a TBT3/USB-C port today.

When configured over a USB Type-C® connector interface, USB4 functionally replaces USB 3.2 while retaining USB 2.0 bus operating in parallel. Enhanced SuperSpeed USB, as defined in USB 3.2, remains the fundamental architecture for USB data transfer on a USB4 Fabric.

It's exactly the same as it is today, where we see all devices, including keyboards, go to USB-C.
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