What is USB4?

The best way to think about USB4 is as a subset of Thunderbolt 4, where PCI Express tunnelling is optional (on Thunderbolt 4 it is required). Despite the fact that Intel is trying to downplay the capabilities of USB4 versus Thunderbolt 4, actual implementations on the host side are unlikely to differ much. For example, ASMedia's ASM4242 will support all the key features of USB4: PCIe tunnelling, USB4 speeds of up to 40 Gbps, USB 3.2 speeds of 20 Gbps, and DP Alt Mode, something other USB4 host controllers might not.

The minimal requirements, or common denominators, are a lot more basic as a USB4 host controller, hubs, and devices only need to support data speeds of 20 Gbps. With backwards compatibility to USB 3.2, 10 Gbps is enough, or in other words USB 3.2 Gen 2. This also applies to Thunderbolt, which is why it became the common "fallback" speed. PCIe tunnelling and Thunderbolt 3 support are also optional, alongside charging support and several other more or less important features Intel claims make Thunderbolt 4 the superior technology. However, in its full implementation, there's little difference between USB4 and Thunderbolt 4.

USB4 Tunnelling and DP Alt Mode

One of the key advancements with moving from USB 3.2 to USB4 is something referred to as Tunnelling. The short version of what tunnelling allows is a means of sending multiple different protocols simultaneously over the USB4 fabric. The longer version is that USB4 allows for not just USB data to be sent, but also PCI-Express and DisplayPort signals. Until the signal reaches an endpoint, such as a DisplayPort monitor, the signal is sent as encapsulated data within the USB data stream, and the actual DisplayPort signal only gets extracted once it reaches its destination. This is similar to how Thunderbolt works.



Unlike DP Alt Mode, which physically takes over two wire pairs of the four pairs in a USB Type-C cable for exclusive use for DisplayPort signals, tunnelling allows the DP 1.4 signal to be transmitted simultaneously with USB4 or PCIe signals. At higher resolutions and refresh rates, DP Alt Mode can even use all four data pairs in a USB-C cable, which means USB data transfers are limited to USB 2.0 speeds over a separate set of low-speed wires inside the USB-C cable. When tunnelling is used for DisplayPort 1.4 signals, DP will be prioritised over USB and PCIe data, due to a display signal being isochronous in nature, which means that an interruption to the signal would prevent a picture from being displayed on the screen. Although tunnelling doesn't solve this for high-resolution/high refresh rate displays today, future versions of USB4 should have a solution for this that still allows for high-speed data transfers. Note that Display Stream Compression (DSC) can be used to save bandwidth for the DisplayPort signal when tunneling is used.

Naming and Rated Speeds

A key thing to note is that the standard is called "USB4," not "USB 4.0" or "USB 4," and the USB-IF actually has two marketing names for USB4, namely "USB4 20 Gbps" and "USB4 40 Gbps." The technical names are "USB4 Gen 2x2" and "USB Gen 3x2," but there's also "USB4 Gen 2x1" and "USB4 Gen 3x1," both of which are fallback protocols in case of interference or a bad cable, and both rely on a single cable pair. Normally, both cable pairs are used, so using just one cable pair will cut speeds in half.

USB4 has to support at least one display output, but unlike previous USB standards, the DisplayPort signal is tunnelled rather than using DP Alt Mode. That said, DP Alt Mode is still supported for older devices, as well as for DP 2.0 devices. The advantage of tunnelling is that a single cable can in theory transmit USB4, DisplayPort, and PCIe signals at the same time, something that wasn't possible with previous versions of USB. This is also how Thunderbolt 4 works and why the two standards are so similar.



USB4 only requires 7.5 W power output by default, whereas Thunderbolt requires 15 W, although both standards are limited to 5 V by default. This is where USB Power Delivery (PD) comes into play, but we'll cover that in more detail later as it plays a key role when it comes to the USB4 ecosystem.