USB 4 Version 2’s Secret 120 Gbps Mode Has a Catch
When the USB Promoter Group unveiled its USB 4 version 2 specification last week, which has data transfer speeds of up to 80 Gbps, it didn’t reveal any plans to run 80 Gbps over existing cables. But apparently, there’s more to it than that. USB 4 delivers even better performance, supporting up to 120 Gbps aggregate bandwidth.
The USB Type-C connector has 4 lanes configured in a 2 Tx + 2 Rx symmetrical arrangement and operates at 20 Gbps per lane for USB 4, providing 80 Gbps bi-directional bandwidth . USB 4 version 2 increases data transfer rate per lane to 40 Gbps and aggregate bandwidth to 80 Gbps with PAM-3 encoding (both directions). Anglonomics (opens in new tab) Citing a document from Keysight, a leading manufacturer of test equipment.
However, in addition to the symmetric 2 Tx + 2 Rx setup, Keysight’s M8040A Bit Error Rate Tester for USB 4 v2 can also be used in an asymmetric 3 Tx + 1 Rx configuration, or an aggregate bandwidth of 120 Gbps from host and device to host. 40 Gbps can be tested. Enough to carry a DisplayPort 2.0 UHBR20 signal (enough for 8Kp85 uncompressed and 8Kp144 with DSC) from the host to the display, USB 4 hub, 40 Gbps to handle the data consumed by the webcam ( both directions) remain. Built in display etc.
So far, the USB Promoter Group has not formally confirmed that support for 3 Tx + 1 Rx asymmetric configuration will be introduced in USB 4. Still, asymmetric configurations are nothing new to the USB specification. USB 4 (and Thunderbolt 3/4) are particularly easy to enable, as all signal types are multiplexed and demultiplexed on both ends of the interconnect.
Now, assuming that the 3 Tx + 1 Rx asymmetric setup is part of the USB 4 v2 specification, the main question is will 3 Tx + 1 Rx asymmetric support be mandatory for all USB 4 v2 controllers? Or is it reserved as an option? Some controllers only. Increasing the data transfer rate to 40 Gbps per lane and implementing PAM-3 encoding significantly increases the complexity and power consumption of USB 4 v2 controllers compared to USB 4 v2 controllers. Additionally, to increase bandwidth, you would need a USB 4 v2 controller to connect to the host at approximately 15 GB/s (equivalent to the bandwidth provided by a PCIe 5.0 x4 or PCIe 6.0 x2 interface). Supporting 120 Gbps upstream mode can complicate things further.
When will the first USB 4 v2 host controllers come out? Anglonomics (opens in new tab) Intel believes it will support USB 4 v2 on its 14th Gen Core ‘Meteor Lake’ platform in 2023. As for AMD, they are falling behind Intel and will only offer CPUs that support USB 4 v2 in 2025. As always, it’s hard to make predictions about Apple.
We have reached out to the USB Promoter Group to ask if the final version of USB 4 v2 will support a 3 Tx + 1 Rx asymmetric configuration. Unfortunately, so far, the organization has neither denied nor confirmed this.
“The USB Promoter Group announcement was intended only to cover high-level details as the specification is still in development,” the USB Promoter Group statement said. “We are targeting developers only at this time to facilitate in-depth training available at the USB DevDays events in Seattle and Seoul in November.”