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]

The USB4 40Gbps logo

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 were: Apple Inc., Hewlett-Packard Inc., Intel Corporation, Microsoft Corporation, Renesas Corporation, STMicroelectronics, and 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, while retaining compatibility with existing USB and Thunderbolt products.[8]

Data transfer modes

USB4 by itself doesn't provide any generic data transfer mechanism or device classes like USB 3.x, but serves mostly as a way to tunnel other protocols like USB 3.2, DisplayPort and optionally PCIe. While it does provide a native Host-to-Host protocol, as the name implies it is only available between two connected hosts. It is used to implement Host IP Networking. Therefore, when the host and device do not support optional PCIe tunneling, the maximum non-display bandwidth is limited to USB 3.2 20 Gbps, while only USB 3.2 10 Gbps is mandatory.

USB4 allows tunneling:

  • USB 3.2 ("Enhanced Superspeed") Tunneling
  • DisplayPort 1.4a-based Tunneling
  • PCI Express (PCIe)-based Tunneling

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 always supported using the dedicated wires in the USB-C connector.

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
USB4 20 Gbit/s TransportYesYesYes
USB4 40 Gbit/s TransportOptionalYesOptional
Tunneled USB 3.2 (10 Gbit/s)YesYesOptional
Tunneled USB 3.2 (20 Gbit/s) [10]OptionalOptionalOptional
Tunneled DisplayportYesYesOptional
Tunneled PCI ExpressOptionalYesOptional
Host-to-Host communicationsYesYesN/A
DisplayPort Alternate ModeYesYesOptional
Thunderbolt Alternate ModeOptionalYesOptional
USB-C Alternate ModesOptionalOptionalOptional
USB data transfer modes
USB4 data transfer modes
Marketing name Logo Specifications Old specifications Dual-lane Encoding Nominal speed
Gbit/s GB/s
SuperSpeed USB 5Gbps USB 3.2 Gen 1×1 USB 3.0,
USB 3.1 Gen 1		 No 8b/10b 5 0.5
SuperSpeed USB 10Gbps USB 3.2 Gen 2×1 USB 3.1 Gen 2 No 128b/132b 10 1.2
N/A USB 3.2 Gen 1×2 Does not appear Yes 8b/10b 10 1.0
SuperSpeed USB 20Gbps USB 3.2 Gen 2×2 Yes 128b/132b 20 2.4
N/A USB4 Gen 2×1 No 64b/66b[lower-alpha 1] 10 1.2
N/A USB4 Gen 3×1 No 128b/132b[lower-alpha 1] 20 2.4
USB4 20Gbps USB4 Gen 2×2 Yes 64b/66b[lower-alpha 1] 20 2.4
USB4 40Gbps USB4 Gen 3×2 Yes 128b/132b[lower-alpha 1] 40 4.8
  1. USB4 can use optional Reed–Solomon Forward error correction (RS FEC). In this mode 12x 16B (128b) symbols are assembled together with 2B (12b+4b reserved) sync bits indicating the respective symbol types and 4B of RS FEC to allow to correct up to 1B of errors anywhere in the total 198B block.

USB4 Gen 2 is different from USB 3.2 Gen 2. They only signify the same speed, ie 10Gbps, but they are coded differently on the electrical layer.

Although USB4 is required to support dual-lane modes, it uses single-lane operations during initialization of a dual-lane link; single-lane link can also be used as a fallback mode in case of a lane bonding error.

In Thunderbolt compatibility mode, the lanes are driven slightly faster at 10.3125 Gbps (for Gen 2) and 20.625 Gbps (for Gen 3), as required by Thunderbolt specifications.

Power delivery

A USB4 connection requires a USB Power Delivery (USB PD) contract before being established. This is not a problem, as USB4 requires USB PD (and is exclusively USB-C) to negotiate USB4 mode in the first place. A USB4 source must at least provide 7.5W (5V, 1.5A) per port. A USB4 sink must require less than 250mA (default), 1.5A or 3A @5V of power (depending on USB-C resistor configuration) before USB PD negotiation. With USB PD up to 100W of power are possible.[11]

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.[12]

Software support

Linux 5.6, released on 29 March 2020, supports USB4.[13]

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.[14][15]

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.[16]

gollark: These are very relevant to your question.
gollark: I didn't say "were".
gollark: Wrong. You are wrong.
gollark: Wrong.
gollark: Well, this is probably down to the fact that deployment of new mobile network technology is quite slow, but also LTE is generally better than predecessors.

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. USB4 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. USB4 Spec 1.0 Sect 9 specifically mentions the requirement of "Superspeed Plus (=Gen 2x1) support for hubs and devices, the latter only if they optionally support USB3 tunneling. The rest of the spec doesn't mention the speed at all and more generically just refers to the whole USB3.2 architecture as "Enhanced Superspeed". Section 9.2.1 states the requirement to only support Gen 2x1 for the USB3 tunneling adapter, with Gen 2x2 being optional.
  11. USB Type-C Spec 2.0 Section 5.3
  12. 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.
  13. "Linux 5.6 Kernel Released With WireGuard, USB4, New AMD + Intel Hardware Support - Phoronix". Phoronix.com.
  14. "USB4 devices are clear to roll out next year". Engadget.
  15. Maislinger, Florian (June 14, 2019). "First USB 4 devices to be launched at the end of 2020".
  16. Shilov, Anton. "Cypress Announces USB 3.2 & USB4-Ready Controllers: EZ-PD CCG6DF & CCG6SF". www.anandtech.com.
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