|Designer||USB Promoter Group|
|Designed||29 August 2019|
|Max. voltage||48 V (PD 3.1)|
|Max. current||5 A (PD)|
|Bitrate||40 Gbit/s (5 GB/s)|
USB4 is a specification by the USB Implementers Forum (USB-IF), which was released in version 1.0 on 29 August 2019. The USB4 protocol is based on the Thunderbolt 3 protocol; the Thunderbolt 3 specification was donated to the USB-IF by Intel Corp. The USB4 architecture can share a single high-speed link with multiple end-device types dynamically, best serving each transfer by data type and application.
In contrast to prior USB protocol standards, USB4 requires USB-C connectors, and for power delivery, it requires support of USB PD. 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. In contrast to USB 3.2, it allows tunneling of DisplayPort and PCI Express.
Support of interoperability with Thunderbolt 3 products is required for USB4 hosts and USB4 peripheral devices, and is optional for USB4 hubs on their downward facing ports and for USB4-based docks on their downward and upward facing ports. On the other hand, support for USB4 is required in Thunderbolt 4.
The USB4 specification version 1.0, released 29 August 2019, uses "Universal Serial Bus 4" and specifically "USB4", that is the short name branding is deliberately without a separating space versus the prior versions.
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".
On September 1, 2022, the USB Promoter Group announced the pending release of the USB4 Version 2.0 specification. All of these specification updates are expected to be published in advance of this year's series of USB DevDays developer events planned for November.
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 Gbit/s, while only USB 3.2 10 Gbit/s 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 and can use up to 80 Gbit/s which is same amount available to USB data, but just unidirectional.
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 Name||Encoding||Dual-Lane||Lane Speed
|Nominal Speed||USB-IF Marketing Name||Logo|
|USB 3.2 Gen 1×1||USB 3.0,
USB 3.1 Gen 1
|8b/10b||No||5||5||0.625||SuperSpeed USB 5Gbps|
|USB 3.2 Gen 1×2||Does not appear||8b/10b||Yes||5||10||1.2||—|
|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||—|
|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||—|
|USB4 Gen 3×2||128b/132b[a]||Yes||20||40||4.8||USB4 40Gbps|
|USB4 Gen 4×1||No||40||40||4.8||—|
|USB4 Gen 4×2||Yes||40||80||9.6||USB4 80Gbps|
- 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 (these are called legacy speeds and rounded speeds). After removal of 64b/66b encoding, those also become round, 20.625/66*64 = 20.000 Gbit/s.
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 240 W of power is possible with 'Extended power range' (5 A at 48 V). For 'Standard Power range' up to 100 W is possible (5 A at 20 V).
Thunderbolt 3 compatibility
The USB4 specification states that a design goal is to "Retain compatibility with existing ecosystem of USB and Thunderbolt products." Compatibility with Thunderbolt 3 is required for USB4 hubs; it is optional for USB4 hosts and USB4 peripheral devices. Compatible products need to implement 40 Gbit/s mode, at least 15 W of supplied power, and the different clock; implementers need to sign the license agreement and register a Vendor ID with Intel.
Alternate Mode partner specifications
On 29 April 2020, DisplayPort Alt Mode version 2.0 was released, supporting DisplayPort 2.0 over USB4.
USB4 has 24 pins in a symmetrical USB type C shell. USB4 has 12 A pins on top and 12 bottom B pins. 
USB4 has two lanes of differential SuperSpeed pairs. Lane one uses TX1+, TX1-, RX1+, RX1- and lane two uses TX2+, TX2-, RX2+, RX2-. USB4 transfers data at 20 Gbp/s per lane. USB4 also keeps the differential D+ and D- for USB 2.0 transfer.
The CC configuration channels have the roles of creating a relationship between attached ports, detecting plug orientation due to the reversible USB type C shell, discovering the VBUS power supply pins, determining the lane ordering of the SuperSpeed lanes, and finally the USB protocol makes the CC configuration channel responsible for entering USB4 operation. 
|A2||SSTXp1 ("TX1+")||SuperSpeed differential pair #1, TX, positive|
|A3||SSTXn1 ("TX1-")||SuperSpeed differential pair #1, TX, negative|
|A6||Dp1||USB 2.0 differential pair, position 1, positive|
|A7||Dn1||USB 2.0 differential pair, position 1, negative|
|A8||SBU1||Sideband use (SBU)|
|A10||SSRXn2 ("RX2-")||SuperSpeed differential pair #4, RX, negative|
|A11||SSRXp2 ("RX2+")||SuperSpeed differential pair #4, RX, positive|
|B11||SSRXp1||SuperSpeed differential pair #2, RX, positive|
|B10||SSRXn1||SuperSpeed differential pair #2, RX, negative|
|B8||SBU2||Sideband use (SBU)|
|B7||Dn2||USB 2.0 differential pair, position 2, negative[a]|
|B6||Dp2||USB 2.0 differential pair, position 2, positive[a]|
|B3||SSTXn2||SuperSpeed differential pair #3, TX, negative|
|B2||SSTXp2||SuperSpeed differential pair #3, TX, positive|
- There is only a single non-SuperSpeed differential pair in the cable. This pin is not connected in the plug/cable.
USB4 is supported by:
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 were Intel's Tiger Lake processors, 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.
List of Apple devices featuring USB4 ports include:
- MacBook Pro (13-inch, M2, 2022)
- MacBook Pro (14-inch, 2021)
- MacBook Pro (16-inch, 2021)
- MacBook Pro (13-inch, M1, 2020)
- MacBook Air (M2, 2022)
- MacBook Air (M1, 2020)
- Mac mini (M1, 2020)
- iMac (24-inch, M1, 2021)
- Mac Studio (2022)
- iPad Pro 12.9-inch (5th generation)
- iPad Pro 11-inch (3rd generation)
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