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 |
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 |
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.
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 |
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.
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
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.
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.
