The USB4 40Gbps logo
|Designer||USB Promoter Group|
|Designed||29 August 2019|
|Max. voltage||48 V (PD 3.1)|
|Max. current||5 A (PD)|
|Bitrate||40 Gbit/s (4.8 GB/s)|
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. Support of interoperability with Thunderbolt 3 products is optional for USB4 hosts and USB4 peripheral devices, and is required for USB4 hubs on their downward facing ports and for USB4-based docks on their downward and upward facing ports.
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". Even after publication of rev. 1.0, some sources write "USB 4", claiming "to reflect the way readers search".
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, Intel, Microsoft, Renesas Electronics, STMicroelectronics, and Texas Instruments.
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.
Data transfer modes
USB4 by itself does not 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 specifies tunneling of:
- USB 3.2 ("Enhanced Superspeed") Tunneling
- DisplayPort 1.4a-based Tunneling
- PCI Express (PCIe)-based Tunneling
DisplayPort Alt Mode 2.0: USB 4 supports DisplayPort 2.0 over its alternative mode. DisplayPort 2.0 can support 8K resolution at 60 Hz with HDR10 color. DisplayPort 2.0 can use up to 80 Gbps, which is double the amount available to USB data, because it sends all the data in one direction (to the monitor) and can thus use all eight data lanes at once.
Legacy USB (1-2) 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.
|Legacy USB (1–2) (max. 480 Mbit/s)||Yes||Yes||Yes|
|USB4 20 Gbit/s Transport||Yes||Yes||Optional|
|USB4 40 Gbit/s Transport||Optional||Yes||Optional|
|Tunneled USB 3.2 (10 Gbit/s)||Yes||Yes||Yes|
|Tunneled USB 3.2 (20 Gbit/s)||Optional||Optional||Optional|
|Tunneled PCI Express||Optional||Yes||Optional|
|DisplayPort Alternate Mode||Yes||Yes||Optional|
|Thunderbolt Alternate Mode||Optional||Yes||Optional|
|USB-C Alternate Modes||Optional||Optional||Optional|
USB 3.x – 4.x data transfer modes
|Mode name||Old names||Encoding||Dual-lane||Lane speed
|Nominal speed||Marketing name||Logo|
|USB 3.2 Gen 1×1||USB 3.0,
USB 3.1 Gen 1
|8b/10b||No||5||5||0.5||SuperSpeed USB 5Gbps|
|USB 3.2 Gen 1×2||Does not appear||8b/10b||Yes||5||10||1.0||N/A|
|USB 3.2 Gen 2×1||USB 3.1 Gen 2||128b/132b||No||10||10||1.2||SuperSpeed USB 10Gbps|
|USB 3.2 Gen 2×2||Does not appear||128b/132b||Yes||10||20||2.4||SuperSpeed USB 20Gbps|
|USB4 Gen 2×1||64b/66b[a]||No||10||10||1.2||N/A|
|USB4 Gen 2×2||64b/66b[a]||Yes||10||20||2.4||USB4 20Gbps|
|USB4 Gen 3×1||128b/132b[a]||No||20||20||2.4||N/A|
|USB4 Gen 3×2||128b/132b[a]||Yes||20||40||4.8||USB4 40Gbps|
- USB4 can use optional Reed–Solomon forward error correction (RS FEC). In this mode, 12 × 16 B (128 bit) symbols are assembled together with 2 B (12 bit + 4 bit reserved) synchronisation bits indicating the respective symbol types and 4 B of RS FEC to allow to correct up to 1 B of errors anywhere in the total 198 B block.
USB4 Gen 2 is different from USB 3.2 Gen 2. They only signify the same speed, i.e. 10 Gbit/s, 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 Gbit/s (for Gen 2) and 20.625 Gbit/s (for Gen 3), as required by Thunderbolt specifications.
USB4 requires USB Power Delivery (USB PD). A USB4 connection needs to negotiate a USB PD contract before being established. A USB4 source must at least provide 7.5 W (5 V, 1.5 A) per port. A USB4 sink must require less than 250 mA (default), 1.5 A, or 3 A @ 5 V of power (depending on USB-C resistor configuration) before USB PD negotiation. With USB PD, up to 100 W of power is possible.
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 2.0 over USB4.
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 product compatible with USB4 is Intel's Tiger Lake processor, with more devices appearing around the end of 2020.
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.
At a November 10, 2020 event, Apple unveiled three new computers to their MacBook Air, MacBook Pro, and Mac Mini product lines, to take advantage of the new standard. They are amongst the world's first computers to come equipped with dual USB4/TB4 ports and single controller. The models Mac Mini (M1, 2020) (2 controllers), MacBook Air (M1, 2020), and MacBook Pro (13-inch, M1, 2020) are also highlighted by the fact that they are the first to come with Apple's own CPU offerings, the Apple M1 SoC.
- "USB Promoter Group USB4 Specification". usb.org. 29 August 2019.
- Bright, Peter (4 March 2019). "Thunderbolt 3 becomes USB4, as Intel's interconnect goes royalty-free". Ars Technica. Retrieved 4 March 2019.
- "With USB 4, Thunderbolt and USB will converge".
- Hagedoorn, Hilbert. "USB 4.0 Will Arrive In Late 2020". Guru3D.com.
- September 2019, Avram Piltch 03. "USB 4: Everything We Know So Far". Tom's Hardware.
- Hill, Brandon (4 March 2019). "USB4 Leverages Thunderbolt 3 Protocol Doubling Speeds To 40Gbps". HotHardware.
- "USB4 announced with 40Gbps bandwidth, it's based on Thunderbolt 3". GSMArena.com.
- USB4 Spec. p.1
- "USB4 - No more Mr. Nice Guy, your USB-C connector has to do it all! - #38". YouTube. 21 November 2019. Retrieved 1 May 2020.
- USB4 Spec 1.0 Sect 9 specifically mentions the requirement of "Superspeed Plus (= Gen 2×1) support for hubs and devices, the latter only if they optionally support USB3 tunneling. The rest of the specification 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 2×1 for the USB3 tunneling adapter, with Gen 2×2 being optional.
- USB Type-C Spec 2.0 Section 5.3
- USB4 Specification V1.0 August, 2019 Chapter 13: "A USB4 host and USB4 peripheral device may optionally support TBT3-Compatability. If a USB4 host or USB4 peripheral device supports TBT3-Compatability, it shall do so as defined in this chapter".
- 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.
- "Linux 5.6 Kernel Released With WireGuard, USB4, New AMD + Intel Hardware Support - Phoronix". Phoronix.com.
- "Introducing the next generation of Mac". apple.com. 10 November 2020. Retrieved 13 November 2020.
- "USB4 devices are clear to roll out next year". Engadget.
- Maislinger, Florian (14 June 2019). "First USB 4 devices to be launched at the end of 2020".
- Shilov, Anton. "Cypress Announces USB 3.2 & USB4-Ready Controllers: EZ-PD CCG6DF & CCG6SF". www.anandtech.com.
- "CApple Event November 10, 2020". www.apple.com.