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USB Type C was envisioned as the universal connector, though USB-C cables are far from universal. USB4 cables are limited to a maximum length of 80 cm.
Following the publication of USB4 standards, the specifications for USB Type-C were revised, bringing with it two additional cables to support the 40 Gbps speeds of USB4. USB4 Gen3x2 cables are limited to a maximum of 80 cm, which “is just a practical consequence of physics and signal integrity when it comes to passive cables,” according to noted Google engineer Benson Leung, famed for Amazon reviews of faulty, non-standard USB-C cables that flooded the market in 2016.
The introduction of new cables does not necessarily require users to toss their current cables to support the existing standards, though certain older cables will not support 40 Gbps speeds when used with USB4-capable hardware.
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From a physical standpoint, the USB4 Gen3 cables have the same number of wires as the USB 3.2 cables in the table above, though they “are built to tolerances such that they can sustain 20Gbps per set of differential pairs, or 40Gbps for the whole cable,” Leung noted in a blog post.
Aside from USB 2.0 Type-C cables—which are more commonly utilized for power than for data transfer—USB 3.x cables are forward-compatible, to an extent. The 2015-era USB 3.1 Gen1 cables marketed as capable of supporting 5 Gbps, and 2017-era USB 3.s Gen2 cables labeled for 10 Gbps can reach 20 Gbps when used on USB4-compatible hardware.
Accordingly, “a passive 40Gbps TBT3 cable you bought in 2016 or 2017 will just work at 40Gbps on a USB4 device in 2020,” according to Leung, due to Intel’s opening of the Thunderbolt 3 specification, and the USB-IF’s adoption of that standard for USB4.
If it is confusing for engineers, it is confusing for end users
With the USB4 standards now public, engineers are saddled with the task of implementing support for them in software and hardware. Leung describes how these are enumerated in Linux, in order to graphically indicate to users what the cable is capable of doing.
“In the real world, your average user will pick a cable and will simply not be able to determine the capabilities of the cable by looking at it. Even if the cable has the appropriate logo to distinguish them, not every user will understand what the hieroglyphs mean,” he said.