When the USB Promoter Group announced its USB 4 Version 2 specification with a maximum 80 Gbps data transfer rate last week, it did not reveal how it planned to make 80 Gbps work on existing cables. But apparently, there is more than that. USB 4 can do even better and support aggregate bandwidth of up to 120 Gbps.
The USB Type-C connector has four lanes configured in a 2 Tx + 2 Rx symmetric arrangement, which in the case of USB 4 operates at 20 Gbps per lane and therefore provides 80 Gbps bi-directional bandwidth. With USB 4 version 2, per lane data transfer rate is increased to 40 Gbps, and aggregated bandwidth increases to 80 Gbps with PAM-3 encodings (in both directions), reports Angstronomics citing documents from Keysight, a leading maker of test equipment.
But in addition to symmetric 2 Tx + 2 Rx setup, Keysight's M8040A Bit Error Ratio Tester for USB 4 v2 can test asymmetric 3 Tx + 1 Rx configuration, or 120 Gbps aggregated bandwidth from the host and 40 Gbps from device to host. It should be enough to carry a DisplayPort 2.0 UHBR20 signal (enough for 8Kp85 uncompressed and 8Kp144 with DSC) from the host to the display and then 40 Gbps (in both directions) will remain to handle data consumed by a USB 4 hub, a webcam built into the display, and so on.
So far, the USB Promoter Group has not formally confirmed that a 3 Tx + 1 Rx asymmetric configuration support would come to USB 4. Still, asymmetric configurations are nothing new for USB specifications. They are particularly easy to enable in the case of USB 4 (as well as Thunderbolt 3/4), as all signal types are muxed and demuxed at each end of the interconnection.
Now, assuming that a 3 Tx + 1 Rx asymmetric setup is a part of the USB 4 v2 specification, a major question is whether support for 3 Tx + 1 Rx asymmetric will be mandatory for all USB 4 v2 controllers or will be optional and reserved for select controllers only. Increasing per lane data transfer rate to 40 Gbps and implementing PAM-3 encoding will significantly increase the complexity and power consumption of USB 4 v2 controllers compared to USB 4 v2 controllers. Furthermore, increased bandwidth will require USB 4 v2 controllers to connect to hosts at around 15 GB/s (equal to bandwidth offered by a PCIe 5.0 x4 or a PCIe 6.0 x2 interface). Supporting a 120 Gbps upstream mode might complicate things further.
When can we expect the first USB 4 v2 host controllers to arrive? Angstronomics believes that Intel will support USB 4 v2 with its 14th Generation Core 'Meteor Lake' platform in 2023 as the company has already implied support of 80 Gbps Thunderbolt input/output interface by its media. As for AMD, it will lag behind Intel and will only offer CPUs with USB 4 v2 support in 2025. As usual, it is hard to make predictions about Apple.
We have reached out to the USB Promoter Group and enquired whether the final edition of USB 4 v2 supports the 3 Tx + 1 Rx asymmetric configuration. Unfortunately, for now, the organization neither denies nor confirms this.
"The USB Promoter Group announcement was only intended to cover high-level details as the specification is still under development," a statement by the USB Promoter Group reads. "It is only targeting developers at this time in order to promote the detailed trainings that will be available at the upcoming Seattle and Seoul USB DevDays events in November."
