Breaking down how USB4 goes where no USB standard has gone before

USB has come a long way since the 12Mbps days of the '90s. It has waved goodbye to USB-B and is inching away from USB-A in favor of the slim, reversible USB-C connector. Data transfer rates have increased so dramatically that we can run powerful setups with high-resolution monitors, speedy external storage, and numerous other devices from the USB Implementers Forum's latest open standard, USB4.

USB4 unifies the USB and Intel Thunderbolt protocols for the first time, expanding USB's capabilities while further dividing the technology into different performance classes. Adding features like dynamic bandwidth allocation ensures that USB4 is by far the most advanced USB generation. While there are PCs, docks, and cables supporting USB4, we've yet to see everything the protocol can do, like powering an eGPU without Intel branding.

With all that going on, we thought it would be a great time to put a spotlight on the latest and greatest generation of USB. We've whipped up a handy refresher for all things USB4, breaking down the various key aspects of the spec, from how it differs from other specs to protocol tunneling, Alt Mode, and power delivery.

USB4 vs. USB 3 vs. Thunderbolt at a glance

We'll be diving deeper into USB4 and related standards shortly. But if you want a quick overview of how currently available USB generations compare against each other and Intel's more recent iterations of Thunderbolt, refer to the table below (note that speeds are max theoretical transfer rates):

*Doesn't include the upcoming USB4 Version 2.0 (see next section). 

Update 10/4/2022: Originally, the above chart was published with SuperSpeed naming. But this quarter, the USB-IF said goodbye to SuperSpeed USB branding. We've updated the above chart accordingly. 

Original story, 9/2/2022 continues: 

USB4 Version 2.0

The table above doesn't include the upcoming "USB4 Version 2.0," since the specification isn't released yet. Announced on September 1, it's supposed to enable up to 80Gbps over USB-C through a new physical layer architecture that uses "newly defined," as the USB Promoter Group put it, 80Gbps USB-C active cables and the 40Gbps USB-C passive cables that already exist today.

USB4 Version 2.0 will also boost protocol tunneling performance by leveraging the larger bandwidth, with expected improvements including USB 3.2 tunneling past 20Gbps.

The USB-IF expects to publish the spec, as well as relevant updates to the USB-Type-C and USB Power Delivery (USB PD) standards, by November 2022.

USB4 vs. USB-C

Now, some basics. How does USB4 differ from USB-C?

You can think of USB Type-C (or USB Type-A, etc.) as the physical shape of the port and plug, and USB4 (or USB 3.2, Thunderbolt 4, etc.) as the underlying technology. All USB4 products use the USB-C connector, but not all USB-C cables and ports use the USB4 protocol.

Just because it's USB-C doesn't mean you know which USB standard it uses. 

Credit: Scharon Harding

USB4 is the latest generation of USB, which is maintained by the USB Implementers Forum (USB-IF). The USB4 architecture and prior generations of USB, like USB 3.2 and 2.0, refer to the cable bus supporting data exchange between a host computer or device and peripherals, as the USB-IF puts it in the USB4 specification.

A USB-C cable or port could use USB4 technologies or older generations of the USB architecture, like USB 3.2.

The USB-IF has a detailed specification for USB4 and another for USB-C cables and connectors.

What is USB4?

The USB-IF released the USB4 spec in 2019, building on USB 2.0 and 3.2, as well as the Thunderbolt 3 spec that Intel made royalty-free.

USB4 is compatible with USB 2.0 and 3.2 and can support Thunderbolt 3 and 4. Remember that you'll get the lowest data transfer rate available. For example, if you plug a USB4 40Gbps cable into a USB 3.2 Gen 1 port on your laptop, expect the cable to work at 5Gbps.

USB4 differs from USB 3.2 in that it can operate at 40Gbps; however, that max theoretical transfer rate is optional. At a minimum, USB4 requires 20Gbps, just like USB 3.2 Gen 2x2.

USB4 also introduces the ability to use multiple data and display protocols simultaneously and allocate total available bandwidth based on current needs.

40 gigabits per second

USB4 must operate at 20Gbps, but it can also support 40Gbps speeds using a pair of bidirectional cable lanes.

Jeff Ravencraft, president and COO of USB-IF, told Ars that USB4's multi-monitor support depends on the computer "and is a question of how many streams a host can support, but "from a bandwidth perspective, the maximum is four 1K monitors at [a refresh rate of] 120 Hz, two 4K monitors at 120 Hz, and one 8K monitor at 30 Hz."

But if USB4 can be 20Gbps or 40Gbps, how do you know what you're getting?

Your best bet is to carefully examine spec sheets. But to simplify things, in 2021, the USB-IF introduced a logo program that included USB4 transfer rates. The badges look like this:

Speed is also limited by distance. Ravencraft said that max passive cable length is 3.3 feet (1 meter), while active copper cables can reach 13.1 feet (4 meters) and operate at 40Gbps, and "active optical cables can be much longer."

The USB-IF is moving away from USB sub-generation names like USB 3.2, with USB4 Version 2.0” being the next big update. Meanwhile, USB-IF is pushing branding like "USB 40Gbps," "USB 20Gbps," and "SuperSpeed USB 20Gbps," aka USB 3.2 Gen 2x2. However, the accompanying logos are optional and require USB-IF certification. You'll still see plenty of products simply labeled as USB4, which doesn't tell you the data transfer rate.

Dynamic bandwidth allocation

USB4's other headline feature is its protocol tunneling, including USB 3.x, DisplayPort, and, optionally, PCIe. Briefly, this feature allows USB4 to allocate bandwidth based on current needs. As an example, imagine using a USB4 dock to connect your PC, monitor, and external SSD. USB4 will allocate the necessary bandwidth to the monitor—say 10Gbps—and give the remaining 30Gbps to the external drive.

This differs from prior USB versions, which can support DisplayPort and HDMI over Alt Mode but can only split bandwidth between DisplayPort and USB equally, and not between USB and HDMI, per Tom's Hardware.

Protocol tunneling: USB 3.x, DisplayPort, and PCIe

According to the USB4 spec, "protocol tunnels are interfaced via protocol adapters specific to each protocol. For USB and PCIe protocols, native USB hubs and PCIe switches are required to handle protocol-related packet routing and buffering."

"For display tunneling, no intermediate DisplayPort-specific logic is required with display tunnels being established as an end-to-end link," the spec says. "Time synchronization across a USB4 Fabric [data transmission protocol] uses distributed time management units (TMUs) associated with each router" or the component that manages USB Fabric traffic and time synchronization.

USB4 is the first generation of USB to tunnel the DisplayPort protocol instead of relying on DisplayPort over Alt Mode. USB4's DisplayPort tunneling is based on the DisplayPort 1.4a spec (we'll get to USB4's support for DisplayPort 2.0 over Alt Mode shortly).

PCIe tunneling, meanwhile, is something we hear about less often with USB4. That's partially because the feature is optional, except in USB4 docks. PCIe tunneling makes USB4 a candidate for the likes of external storage drives and eGPUs.

However, we've yet to see any eGPUs that use USB4 exclusively. Granted, eGPUs are a more niche product. "USB4 supports PCIe tunneling and hence can/will support eGPUs. We believe we will see the market adoption in this product category begin to take off," Ravencraft told Ars.

According to the USB4 spec, a device with a USB4 port that supports PCIe tunneling either has an internal PCIe switch or connects to a PCIe root complex through root ports or other means supported by the PCIe spec.

When it becomes available, USB4 Version 2.0 should "enable higher performance USB 3.2, DisplayPort, and PCIe data tunneling to best use the higher available [80Gbps] bandwidth," the USB Promoter Group said when announcing the spec. Thanks to USB data architecture updates, USB4 Version 2.0 is expected to support USB 3.2 data tunneling at over the spec's max data transfer rate of 20Gbps. Version 2.0 should also be up to date with the current DisplayPort and PCIe specs. 

USB4 Alt Mode

A USB4 port can still support Thunderbolt and DisplayPort over Alt Mode. Ravencraft confirmed to Ars that USB4 doesn't support HDMI over Alt Mode.

According to the USB-IF's specification for USB Type-C (2.1), all of a computer's downward-facing USB-C ports must support DisplayPort Alt Mode for it to be USB4-certified. "The USB4 host shall support the first connected DisplayPort display on any of its USB Type-C ports," the spec says. "Support for subsequently connected DisplayPort displays is optional."

To be a true USB4 dock, the spec also calls for all downward-facing USB-C ports to support DisplayPort Alt Mode. This wasn't a requirement for prior USB generations. More specifically, USB4 can support DisplayPort 2.0 Alt Mode. VESA released the spec to the USB-IF in 2020, but DisplayPort 2.0 products just started rolling out slowly this year.

DisplayPort 2.0 Alt Mode, as Ravencraft explained, "defines the use of the high-speed interface signals over the USB Type-C connector differently than USB4, and, in some configurations, DisplayPort Alt Mode can support higher than 40Gbps, depending on the mode in which it is operating."

By taking over both pairs of bidirectional lanes (four lanes total), VESA says DisplayPort 2.0 Alt Mode can operate at up to 80Gbps.

USB4 240W Power Delivery

A USB4 cable can supply up to 240 W of power, per the USB PD Revision 3.1 specification. The USB-IF announced that in late 2021 (upping max support from 100 W), so 240 W USB-C power delivery is limited. There are, however, some 240 W USB-C power meters available, as well as a cable from Club 3D. Note that a USB-C cable doesn't have to be USB4 to support 240 W power delivery; it can use an older generation of USB instead.

240 W power delivery is also an optional feature of USB-C cables, so to ensure you know what you're getting, you can look for the USB-IF's optional logo, which has 240 W and 60 W USB PD branding.

And while we're discussing power, any USB4 port can output at least 7.5 W, up from 4.5 W with USB 3.x.

USB4 vs. Thunderbolt 4

USB4 was built on the Thunderbolt protocol when Intel finally made it royalty-free in 2019. As a result, there are numerous similarities between USB4 and the current-gen Thunderbolt 4. But there are also key differences—and they go beyond the Thunderbolt's lightning bolt logo.

Thunderbolt 4 (second from the left) is like USB4 ramped to its max potential.

Credit: Scharon Harding

In the simplest terms, Thunderbolt 4, which also requires USB-C, can do everything USB4 can and is fully compatible with USB4. But there are some things Thunderbolt 4 requires that USB4 does not. Thunderbolt 4 targets high-performance computers, while USB4 may be found in other gadgets, like smartphones and tablets, making some of Intel's more stringent requirements unnecessary.

One big difference is that Thunderbolt 4, which can also use dynamic bandwidth allocation across data and video and started rolling out with products in 2021, always operates at 40Gbps. 40Gbps is optional for USB4; a cable can run at 20Gbps and still be considered USB4. When USB4 Version 2.0 comes out, though, it should be able to surpass Thunderbolt 4 by hitting 80Gbps. Update 10/4/2022: Intel quickly followed the USB-IF's announcement of USB4 Version 2.0 by announcing an upcoming, 80Gbps version of Thunderbolt. Intel didn't specify a release date.

Original story, 9/2/2022 continues:

The USB-IF told us that USB4 could support multiple monitors, depending on the computer. In contrast, Thunderbolt 4 promises support for up to two 4K DisplayPort or HDMI monitors, each running at 60Hz and "through a compatible dock or an adapter."

As mentioned, PCIe support is optional for USB4, but operation at 32Gbps is mandatory for Thunderbolt 4 (Thunderbolt 3 requires 16Gbps). This is a big reason why you'll find products like eGPUs and video capture cards relying on the Thunderbolt protocol.

When it comes to power, a USB4 port must be able to output at least 7.5 W, but a Thunderbolt 4 (or Thunderbolt 3) port must supply at least 15 W. So if you want to charge a device from your laptop as quickly as possible, use a Thunderbolt port.

Further, if an ultralight laptop can charge at under 100 W and wants to include a Thunderbolt 4 port, it must be able to charge over USB. There's no such requirement for USB4 laptops, nor do USB4 devices need to be able to wake up when a device is connected through a dock.

Intel promotes Thunderbolt 4's ability to connect two computers in a peer-to-peer network via a Thunderbolt 4 cable, and USB4 can perform the same feature identically, according to Ravencraft. This is helpful if you have a lot of data you need to move from one system to another. Both protocols also support a 10 Gigabit Ethernet connection via an adapter.

Additionally, Thunderbolt 4 requires the use of Intel's VT-d feature, which is supposed to prevent virtual access to the host system's physical memory. This forces more Intel technologies upon partners but is also a security layer for people running virtual machines.

In terms of cable length, Intel says passive cables can be up to 6.6 feet (2 meters) while operating at 40Gbps. However, we've seen active Thunderbolt 4 cables, namely Apple's Thunderbolt 4 Pro Cable, go up to 9.8 feet (there are no optical options yet, though there have been Thunderbolt 3 offerings reaching 32.8 feet/10 meters).

Finally, while Intel says it doesn't charge royalties or licensing fees for third parties to use Thunderbolt, the technology does require an Intel-sold controller, and laptops using rival AMD processors rarely have Thunderbolt ports. This isn't the case with an open standard like USB4.