USB4

USB4 is a USB system specified in the USB4 specification which was released in version 1.0 on 29 August 2019 by USB Implementers Forum.[1]

In contrast to prior USB protocol standards, USB4 requires USB-C connectors and for power delivery it requires support of USB PD. In contrast to USB 3.2, it allows tunneling of DisplayPort and PCI Express. The architecture defines a method to share a single high-speed link with multiple end device types dynamically that best serves the transfer of data by type and application. USB4 products must support 20 Gbit/s throughput and can support 40 Gbit/s throughput, but due to tunneling even nominal 20 Gbit/s can result in higher effective data rates in USB4, compared to USB 3.2, when sending mixed data.

The USB4 specification is based on the Thunderbolt 3 protocol specification.[2] Support of interoperability with Thunderbolt 3 products is optional for USB4 hosts and USB4 peripheral devices and required for USB4 hubs on its downward facing ports and for USB4-based docks on its downward and upward facing ports.

Overview

Name

The USB4 specification version 1.0, released 29 August 2019, uses "Universal Serial Bus 4" and "USB4". Several news reports before the release of that version use the terminology "USB 4.0" and "USB 4".[3][4] Even after publication of rev 1.0 some knowingly write "USB 4", claiming "to reflect the way readers search".[5]

Specifications

USB4 Specification

History

It was first announced officially in March 2019.[6][7]

Contributors

At time of publication of version 1.0 promoter companies having employees that participated in the USB4 Specification technical work group are: Apple Inc., Hewlett-Packard Inc., Intel Corporation, Microsoft Corporation, Renesas Corporation, STMicroelectronics, Texas Instruments.

Design goals

Goals stated in the USB4 specification are increasing bandwidth, helping to converge the USB-C connector ecosystem and "minimize end-user confusion". Some of the key areas to achieve this are using a single USB-C connector type, retain compatibility with existing USB and Thunderbolt products.[8]

Data transfer modes

USB4 allows tunneling:

  • (Superspeed) USB 3.2
  • the USB4 Display Tunneling protocol is based on the DisplayPort 1.4a Specification
  • PCIe Tunneling is based on the PCI Express (PCIe) specification

USB4 also requires support of DisplayPort Alternate Mode. That means, DP can be sent via USB4 tunneling or by DP Alternate Mode.[9]

USB 2.0 is supported, but not via tunneling but instead by dedicated wires

Support of data transfer modes

Some transfer modes are supported by all USB4 devices, support for others is optional. The requirements for supported modes depend on the type of device.

Support of data transfer modes[1]
Mode Host Hub Peripheral device
USB 2.0 (480 Mbit/s)YesYesYes
USB Gen 2x2 (20 Gbit/s)YesYesYes
USB Gen 3x2 (40 Gbit/s)OptionalYesOptional
DisplayPort Alternate ModeYesYesOptional
Host-to-Host communicationsYesYesN/A
PCI ExpressOptionalYesOptional
Thunderbolt Alternate ModeOptionalYesOptional
USB-C Alternate ModesOptionalOptionalOptional
USB data transfer modes
USB4 USB data transfer modes
Name Older specifications Dual-lane Encoding Nominal speed
USB 3.2 USB 3.1[10] USB 3.0 Gbit/s GB/s
USB 5 Gbit/s USB 3.2 Gen 1×1 USB 3.1 Gen 1 USB 3.0 No 8b/10b 5 0.500
USB 10 Gbit/s USB 3.2 Gen 1×2 N/A N/A Yes 8b/10b 10 1.0
USB 10 Gbit/s USB 3.2 Gen 2×1 USB 3.1 Gen 2 N/A No 128b/132b 10 1.2
USB 20 Gbit/s USB 3.2 Gen 2×2 N/A N/A Yes 128b/132b 20 2.4
USB 20 Gbit/s N/A N/A N/A No 20
USB 40 Gbit/s N/A N/A N/A Yes 40

Power delivery

For power delivery it requires support of USB PD.

Thunderbolt 3 compatibility

The USB4 specification states that a design goal is to "Retain compatibility with existing ecosystem of USB and Thunderbolt™ products ." But compatibility with Thunderbolt 3 is only optional for USB4 hosts and USB4 peripheral devices.

Alternate Mode partner specifications

DisplayPort Alt Mode 2.0

On 29 April 2020, DisplayPort Alt Mode version 2.0 was released, supporting DisplayPort version 2.0 over USB4.[11]

Software support

Linux 5.6, released on 29 March 2020, supports USB4.[12]

Hardware support

During CES 2020, USB-IF and Intel stated their intention to allow USB4 products that support all the optional functionality as Thunderbolt 4 products. The first products compatible with USB4 are expected to be Intel's Tiger Lake series and AMD's Zen 3 series of CPUs, due for release in late 2020.

Devices supporting USB4 are expected to appear in 2020.[13][14]

Brad Saunders, CEO of the USB Promoter Group anticipates that most PCs with USB4 will support Thunderbolt 3 but for phones the manufacturers are less likely to implement Thunderbolt 3 support.[5]

On 3 March 2020, Cypress Semiconductor announced new controllers supporting USB4, CCG6DF as dual port and CCG6SF as single-port.[15]

References
  1. "USB Promoter Group USB4 Specification". usb.org. 2019-08-29.
  2. Bright, Peter (2019-03-04). "Thunderbolt 3 becomes USB4, as Intel's interconnect goes royalty-free". Ars Technica. Retrieved 2019-03-04.
  3. "With USB 4, Thunderbolt and USB will converge".
  4. Hagedoorn, Hilbert. "USB 4.0 Will Arrive In Late 2020". Guru3D.com.
  5. September 2019, Avram Piltch 03. "USB 4: Everything We Know So Far". Tom's Hardware.
  6. Hill, Brandon (March 4, 2019). "USB4 Leverages Thunderbolt 3 Protocol Doubling Speeds To 40Gbps". HotHardware.
  7. "USB4 announced with 40Gbps bandwidth, it's based on Thunderbolt 3". GSMArena.com.
  8. USB 4 Spec. p.1
  9. "USB4 - No more Mr. Nice Guy, your USB-C connector has to do it all! - #38". YouTube. 2019-11-21. Retrieved 2020-05-01.
  10. "The New USB standard: USB 3.2 Explained". web.archive.org. February 20, 2020. Archived from the original on 2020-02-20.
  11. Association (VESA), Video Electronics Standards. "VESA Releases Updated DisplayPort™ Alt Mode Spec to Bring DisplayPort 2.0 Performance to USB4™ and New USB Type-C® Devices". www.prnewswire.com.
  12. "Linux 5.6 Kernel Released With WireGuard, USB4, New AMD + Intel Hardware Support - Phoronix". www.phoronix.com.
  13. "USB4 devices are clear to roll out next year". Engadget.
  14. Maislinger, Florian (June 14, 2019). "First USB 4 devices to be launched at the end of 2020".
  15. Shilov, Anton. "Cypress Announces USB 3.2 & USB4-Ready Controllers: EZ-PD CCG6DF & CCG6SF". www.anandtech.com.
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