Just call it USB4 on the floor: 2021 will see a big wave of devices featuring a new USB standard that goes beyond the usual speed boosts. USB4, the newest version of USB, nixes the space (it's not "USB 4"—more on that in a moment) and papers over some of the wooly confusion that is USB 3. Plus, beyond the new speeds, it adds new nuances to device compatibility and charging over the port.
Where does USB4 come from? In part, from earlier USB specs; in part, from another interface. USB4 leverages the Thunderbolt 3 protocol to deliver speeds up to twice as fast as the USB version it replaces. In addition to its greater bandwidth, USB4 can more deftly shuttle and prioritize file and video data through its pipes than previous USB iterations could. And USB4 offers backward compatibility with all the old PC-connectivity crowd on its physical ports: Thunderbolt 3, USB 3.2, and USB 2.0 devices.
USB4 is poised to spread rapidly in 2021, but keen observers of the PC market know it's already here in limited fashion. The first computers with USB4 connectivity arrived near the end of 2020, and you can expect to see many more laptops and desktops equipped with it in 2021, along with peripherals dubbed USB4-compatible.
Although USB4 should help clear up some of the confusion wrought by the many flavors of USB 3 and their multiple, shifting monikers ("USB 3.2 Gen 2x2"...seriously, folks?), the new standard isn’t a completely straightforward, one-size-fits-all story. Here’s what you need to know about USB4 replacing USB 3.2 as the top dog in the Universal Serial Bus kennel.
Picking Up Speed: A New Peak of 40Gbps
One key thing to note up front: The USB Implementers Forum (USB-IF) styles USB4, indeed, as "USB4," with no space between the "USB" and the "4." We tell you this to potentially aid your online searches related to the term. If searching for the term "USB 4" term fails to turn up satisfying results, removing the space may help.
USB4 encompasses the Thunderbolt 3 specification, now matching that spec's top transfer speed, which is 40Gbps. That is double the peak speed (20Gbps) of the fastest, previous-generation version of USB, the dubiously named USB 3.2 Gen 2x2. That version of USB never got much uptake; only a few superfast external SSDs (like the Seagate FireCuda Gaming SSD and the SanDisk Extreme Pro Portable SSD v2) support it, and compatible USB 3.2 Gen 2x2 ports remain few and far between.
As a refresher on where USB has been, how the names have changed, and where it is going, here's a handy cheat sheet to all of the USB versions that remain relevant today in PCs old and new...
Because every USB rollout seems contractually obligated to sow some degree of confusion, here's USB4's: Not every USB4 device will support 40Gbps transfers. Some USB4 products will offer a top speed of only 20Gbps. You can expect lower-cost products to employ only the slower speed, but you’ll need to check the specs of a given USB4 device to see what USB4 capabilities it offers. Hardware makers will have a variety of logos at their disposal for illustrating USB4 capabilities...
In simple terms, to tell the speed difference with USB4, you’ll need to look for the "20" or the "40" in the logo on the box or spec sheet of the device you are considering.
One thing to note is that even the lower-speed (20Gbps) version of USB4 will offer faster transfer speeds than most implementations of USB 3.2. Although USB 3.2 (the last umbrella family of USB specs in recent computers and peripherals) has a maximum bandwidth of 20Gbps in its USB 3.2 Gen 2x2 variant, USB 3.2, too, has lower-speed flavors. And these are much more common, with top speeds of only 5Gbps (USB 3.2 Gen 1) or 10Gbps (USB 3.2 Gen 2).
About That Marriage of USB and Thunderbolt...
Thunderbolt has long been a proprietary Intel protocol that required its own, specialized chip and a royalty fee paid to Intel, which meant Thunderbolt never achieved the universal adoption that USB has. It tends to show up only in high-end computers. Now, in its latest Thunderbolt move, Intel has reversed course and opened up Thunderbolt to other chipmakers to use without taking its cut. (See our primer on Thunderbolt 4 for more, as well as our explainer on Thunderbolt 3 versus USB-C.)
Both USB4 and Thunderbolt 3 use the same physical connector—USB Type-C (also known as USB-C)—making this marriage a natural fit. The oval USB-C connector has two advantages over "classic" USB Type-A ports (the older-style rectangular ones). For one, USB-C ports are smaller, which allows for integration into thinner laptops and more compact devices. For another, there’s no up or down with a USB-C connector. You can plug a USB-C cable into a USB-C port either way, which removes the frustration of guessing which way is correct when attempting to plug in a USB device to your computer.
This union of USB4 and Thunderbolt 3 also means that new computers with USB4 ports could offer compatibility with your existing Thunderbolt 3 devices, which is potentially good news for gamers that have an external GPU in particular. But why do we emphasize "could," and say only “potentially” good news? Because incorporating Thunderbolt 3 compatibility into a USB4 port implementation is optional, according to the spec.
We’d expect Thunderbolt 3 compatibility to be widespread among USB4 PCs, since manufacturers no longer need to pay a tithe to Intel to include it. If, however, you are shopping for a new laptop to pair with, say, your Thunderbolt 3 eGPU, it would be a good idea to check the laptop’s specs to ensure compatibility before making any assumptions.
Video-Signal Handling With Brains and Brawn
In addition to working with Intel on adding the Thunderbolt 3 spec to USB4, the USB-IF worked with the Video Electronics Standards Association (VESA) to bring increased display capabilities to USB4. The relevant part of the USB4 spec is support for DisplayPort Alt Mode 2.0, which offers a tremendous amount of overhead for powering very high-resolution displays.
With DisplayPort Alt Mode 2.0, a USB4 port can transmit up to 80Gbps of signal data, allowing you to power multiple DisplayPort 2.0 displays with an 8K resolution at a 60Hz refresh rate or, in the more distant future, a single 16K display at 60Hz. (Or more 4K displays than you likely own.) This is possible because the display data being sent is unidirectional.
It’s not just seemingly endless video bandwidth you are getting with USB4, though. USB4 can allocate resources over the interface, to let you drive, say, an HD display up with up to 40Gbps while handling a data transfer from an external SSD at the same time. On previous versions of USB, it could send video down one of its two lanes and data down the other for a 50/50 split at best. With USB4, there’s a dynamic allocation of resources. The lanes can be shared between video and data, giving the display the bandwidth it needs, allowing the rest to be used to transfer data. Just think of what a USB4 hub could offer: the ability to power all the 4K displays you can fit on your desk, with plenty of bandwidth left over for high-speed data transfers.
Power Delivery for All
One spec you won’t need to look up or hunt for on USB4 devices is USB Power Delivery (USB PD). USB PD lets you charge USB devices via your computer instead of needing to find a wall outlet. All USB4 connections will offer USB PD for charging your devices.
That's a change from the existing state of affairs. Right now, some USB 3.2 ports offer Power Delivery, and some don’t. With USB4, you can charge your phone on any USB4 port on a laptop, and you won’t need to hunt around for the specific USB PD-capable port or ports with the tiny lightning-bolt logo. (If, indeed, the PC maker opted to include the logos in the first place.) That—guessing at the power capabilities of a given unmarked USB port—is probably the second-biggest USB annoyance after always guessing wrong which way is up when attempting to plug something into a USB Type-A port.
USB PD can deliver up to 100 watts of power, offering more than enough juice to charge (inbound) most laptops in addition to charging (outbound) smaller devices like an iPhone. Plus, USB PD delivers only as much power as the device that you are charging can handle, so you don’t have to worry about overloading and ruining your phone’s battery.
USB4's Arrival Date (Hint: It's Already Here)
We mentioned that a few USB4-capable laptops have already trickled out. Intel’s 11th Generation “Tiger Lake” Core CPUs will deliver, among other benefits, native USB4 support. A few Tiger Lake laptops have passed through PC Labs, starting with the Asus ZenBook 13 (UX325EA) and the Dell XPS 13 (9310) late in 2020. You can expect every major laptop line to receive a Tiger Lake update in 2021.
Apple’s new M1 MacBook Air, MacBook Pro, and Mac mini were released toward the end of 2020 and also feature USB4 ports. You can expect some of Apple’s other Macs to get an M1 makeover this year that adds USB4 support.
Just remember that although backward compatibility means your current USB 3.x devices will work with a new, USB4-equipped laptop, these legacy devices won’t be able to hit 40Gbps data transfers. They’ll be able to communicate with your new laptop, but they will be unable to exceed the maximum throughput of the older USB protocol they support.
We’ve yet to see a USB4 SSD or other peripheral in the wild, but existing portable drives with Thunderbolt 3 support can provide theoretical transfer speeds of up to 40Gbps—the shared maximum throughput of the two standards—with a USB4-equipped PC. Again, though, mind that big caveat from earlier: This kind of peak speed is a possibility only if the PC offers the 40Gbps flavor of USB4 (not just the 20Gbps one), and it offers specific backward compatibility with Thunderbolt 3.
By the end of the year, USB4 devices should outnumber Thunderbolt devices, and it’ll be far easier to match a new laptop with a new SSD for the fastest possible data transfers, especially if those external SSDs use swift PCI Express based innards. That, plus the ability to drive one or more high-def DisplayPort 2.0 monitors off the same interface, foreshadow a new world of fast connectivity—at least, for those ready to invest in new systems and hardware that can go "4" it.
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