USB4 is Coming! Here is What You Need to Know

 Oct 29, 2020   |    George Liu

The USB Promoter Group first announced the USB4 specification in March 2019. The USB4 architecture is based on the Thunderbolt protocol specification contributed by Intel Corporation to the USB Promoter Group.

Intel's latest 10th generation core processors are integrating Thunderbolt 3 into Intel CPUs. Thunderbolt 3 will underpin USB 4.0. Therefore, all USB 4.0 ports will be compatible with Thunderbolt 3 devices for 40-Gbps data transfer as well as device charging, load/store (PCIe based) transports, and display (DisplayPort based) through a single USB Type C cable.

Below is a table that details the differences between USB 3.2 and USB 4.

Type USB 3.2 USB4
Interface bandwidth on USB-C 20 Gbps (Gen2 x2) for USB only 40 Gbps (Gen3 x 2) for USB, DP, PCIe, TBT3
Protocol USB only for USB + DP Alt Mode USB4 link through tunneling architecture for USB, DP, PCIe
DP Alt Mode USB 3.2 Gen2 x 1 + DP 1.4a DP 1.4a through tunneling architecture
PCIe N/A PCIe 4.0 through tunneling architecture
TBT3 N/A Supported
Host-to-host N/A Supported
Sideband Alt Mode Alt Mode/USB4 configuration space

Key Features of USB4

Two-lane operation using existing USB Type C cables and up to 40-Gbps speeds

Both USB 3.2 20 Gbps (Gen2 x 2) and USB4 40 Gbps (Gen3 x 2) use two-lane operation to provide higher bandwidth. USB4 devices can support three speeds: 10 Gbps, 20 Gbps, and 40 Gbps. However, your actual speeds will vary depending on what devices you are using.

USB4 Device Transmission Speeds - VIA Labs, Inc.

Type 20G (Gen2 x 2) 40G (Gen3 x 2)
USB host Required Optional
USB4 hub Required Required
USB4 peripheral device Required Optional

Multiple data and display protocols that efficiently share the maximum aggregate bandwidth

USB4 uses DisplayPort alternate mode (VESA improved DisplayPort Alt Mode 2.0 spec). This display protocol allows USB4 connections to support the DisplayPort 2.0 standard natively by remapping the pins to work in a single direction. DisplayPort 2.0's Alt Mode will increase the available signal bandwidth to 80 Gbps to support 8K (7680 × 4320) video monitors over USB4.

Same form factor as USB Type C and backward compatible with USB 3.2 and USB 2.0

USB4 is backward compatible with USB 3.2 and lower specs. USB4 hubs must support Thunderbolt 3 Alt Mode, but USB4 peripheral devices and hosts are not required to support it.

USB4 Protocol Support in Different Devices - VIA Labs, Inc.

Type USB PCIe DP Alt Mode TBT3 Alt Mode
USB host Supported Optional Supported Optional
USB4 hub Supported Supported Supported Supported
USB4 peripheral device Supported Optional Optional Optional

The display protocol is based on DisplayPort and the storage protocol is based on PCI Express

DisplayPort Alt Mode is another important feature of USB4. VESA’s updated DisplayPort Alt Mode spec includes several under-the-hood developments including updates to interface discovery and configuration as well as power management to ensure seamless integration with the USB4 specification. Through DisplayPort Alt Mode, USB4 will improve the Multi-Stream Transport (MST) compatibility issues with USB 3.2.

USB4 full feature only works with cables with Type C connectors on both ends

There is only one Type C connector on a USB 3.2 Gen1 x 1 (USB 3.0) cable. USB 3.2 Gen2 x 2 (USB 3.2) and USB4 cables have a Type C connector on both ends. USB4 won’t support older Type A cables, but USB 3 is compatible with Type A and Type C. In addition, the Electronically Marked Cable Assemblies (EMCA)/E-Marker is mandatory for USB4 cable.


VIA Labs’ E-Marker for USB Type C Cable

USB4 Cable Compatibility Summary - www.usb.org

Type Rated Signaling USB4 Operation Response to Discover Identity
USB Type-C® Full-Featured Cables (passive) USB 3.2 Gen1 20 Gbps USB 3.2 Gen1 (001b) in Passive Cable VDO
USB 3.2 Gen2 20 Gbps USB 3.2 Gen1 (010b) in Passive Cable VDO
USB4 Gen3 40 Gbps USB4 Gen3 (011b) in Passive Cable VDO
Thunderbold™ 3 TBT3 Gen2 20 Gbps USB Gen 3.2 Gen1 (001b) or USB 3.2 Gen2 (010b) in Passive Cable VDO
TBT3 Gen3 40 Gbps USB Gen 3.2 Gen2 (010b) or USB 3.2 Gen2 (010b) in Passive Cable VDO + TBT3 Gen3 in Discover Mode VDO
USB Type C Full-Featured Cables (Active)1 USB4 Gen2 20 Gbps USB4 Gen2 (010b) in Active Cable VDO
USB4 Gen3 40 Gbps USB4 Gen3 (011b) in Active Cable VDO

1. SuperSpeed USB active cables do not support USB4-compatible operation whereas USB4 active cables do support SuperSpeed USB

Conclusion

The USB4 specification uses the USB Type C connector to perform a variety of functions. Through Intel’s contribution, USB4 can achieve 40-Gbps transfer speeds, making it the fastest port on a computer today. USB4 also supports high-quality video resolutions like 8K and even 16K with HDR.

But when can you expect USB4 products to be released? Labs are nearly finished with the last steps of the product certification program, so we can expect the first USB4 devices to reach shelves in 2021.

About the Author

George Liu

Product Engineer

George Liu has 12 years’ experience in the cabling, data, and video connectivity industry. As a Project and Product Manager at Black Box, he works directly with domestic and international OEM suppliers on new product launches. George is a certified PMP and CQE, and he is a master’s candidate in the industrial management program at National Taiwan University of Science and Technology.

 

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