USB4

Type	USB
Production history
Designer	USB Promoter Group
Designed	29 August 2019; 4 years ago (2019-08-29)
Superseded	USB 3.2
Daisy chain	No
Audio signal	DisplayPort
Video signal	DisplayPort
Connector	USB-C
Electrical
Max. voltage	48 V (PD 3.1)
Max. current	5 A (PD)
Data
Data signal	Yes
Bitrate	20 Gbit/s 40 Gbit/s 80 Gbit/s 120 Gbit/s asymmetric

USB4 (Universal Serial Bus 4), sometimes erroneously referred to as USB 4.0, is the most recent technical specification of the USB (Universal Serial Bus) data communication standard. The USB Implementers Forum originally announced USB4 in 2019.

USB4 enables multiple devices to dynamically share a single high-speed data link. USB4 devices must support a data communication bit rate of at least 20 gigabits (Gbit/s). The current version allows bit rates of 40 Gbit/s (since USB4 version 1.0) and 80 Gbit/s (since USB4 version 2.0).^[1][2] USB4 is only defined for the USB-C connector and its Type-C specification^[3] regulates the connector, cables and also power delivery features across all uses of USB-C cables, in part^[4] with the USB Power Delivery specification.^[5]

The USB4 standard mandates backwards compatibility to USB 2.0, USB 3.x and DisplayPort connections^[6]. The dynamic sharing of bandwidth of a USB4 connection is achieved by carrying virtualized "tunnels" of other connections. This includes tunneling of USB3 and DP connections. Other protocols, such as PCI Express and Ethernet can also be tunneled, even without a way to access these directly directly from USB4 ports.

USB4 also incorporates some elements and shares principles with the Thunderbolt 3 protocol; however, interoperability with Thunderbolt 3 products is mandatory only for select USB4 device types.^[7]

Prior to USB4, Thunderbolt provided a way to dynamically share bandwidth between multiple DP and PCIe connections over a single cable. Thunderbolt originally used the mDP connector and was only backward compatible to DP connections and did not support power transfer.

The introduction of the Type-C connector in 2014 provided a connector that could support both USB data connectivity, power transfer as well as DP connections. It also allowed the static sharing of bandwidth between DP and USB connections over the same cable.

Thunderbolt 3 switched over to using the new Type-C connector and also added backwards compatibility for USB connections and power transfer features.

USB4 was announced in March 2019.^[8][9] 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 deliberate without a separating space, which is different from prior versions. Several news reports before the release of that version use the terminology "USB 4.0" and "USB 4".^[10][11] Even after publication of rev. 1.0, some sources write "USB 4", claiming "to reflect the way readers search".^[12]

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.^[13]

On 1 September 2022, the USB Promoter Group announced the pending release of the USB4 Version 2.0 specification, and the specification was subsequently released on 18 October 2022.^[14][15] It added 80 Gbit/s speeds with optionally asymmetric connections, a new, optional alternative to the existing "USB3 Gen T tunneling", removed PCIe overhead limitations and updated the support of DisplayPort to the then current Version 2.1.

Around the release of the new USB4 Version 2.0 specification, USB-IF also transitioned to new logos and names to simplify representing the maximum supported speeds (and wattages) to consumers.^[16] The new names are unified across all USB standards and removed the prior, explicit distinction between "SuperSpeed USB 20 Gbps" and "USB4 20Gbps" connections.

Similarly to how USB 3.x defined the new SuperSpeed protocols for faster connections, but also mandated that any USB3 port still include the pins and functionality for the previous USB2 connections, the USB4 specification describes 2 different aspects. The first one is what type of existing connections and compatibility a USB4 port guarantees. Since USB4 uses the Type-C connector, which was designed to be multifunctional and reversable, the term "host" port does not accurately reflect the situation. This is better denoted as a downward facing port (DFP). The peripheral side can similarly be described as upward facing port (UFP).

Any downward facing USB4 port is required to also implement USB 2.0, USB 3.2 and DP Alt mode support. Each according to their own specifications. As such a USB4 DFP is backwards compatible to all previous USB devices.

The USB2 family currently defines 3 different speeds (Low-, Full-, High-Speed), all are required to be supported. The newest available version of this specification is USB 2.0 Rev. 2.0.^[17] USB 2.0 abilities uses separate wires on the Type-C connector that are not used by USB 3.2 or USB4 connections. USB4, just as USB 3.2 before provides a parallel USB 2.0 connection to be present on the same cable to support backwards compatibility to USB 2.0 modes.

The USB3 family currently defines 3 different modes and therefore signaling rates ("5 Gbps" a.k.a. SuperSpeed, "10 Gbps" a.k.a. SuperSpeed+, "20 Gbps" a.k.a. SuperSpeed+ 20 Gbps). While the current USB 3.2 specification^[18] has been referenced since USB4 Version 1.0, only the 2 lower speeds (5 Gbit/s, 10 Gbit/s) are mandatory for USB4 DFPs to support.

The USB4 specifications make no reference to a minimum feature set for its DP Alt Mode functionality. It seems any support is enough. Although in practice, Intel's family of TB4 controllers support up to HBR3 speeds according to the DisplayPort 1.4a specification and DisplayPort Alt Mode specification.^[19]

The USB4 specification makes no explicit demands on power output. It outsources all requirements in terms of power to the Type-C^[20] specification that underpins all USB, Vesa and other standards that use the USB-C connector. This requires a USB4 DFP to supply at least 7.5W Type-C current. No power consumption features (e.g. charging of a notebook) are required, but can of course be supported following the USB PD specification.^[21] as well as supplying considerably more power. The USB PD protocol must always be supported (exchanging data according to the protocol. This is separate from any functionality of PD to negotiate actual power delivery other than 5V or > 15W).

Every USB4 port must support the new USB4 protocol, at least with the minimum speed of 20 Gbit/s.

USB4 Hubs and Docks are defined as their own category of USB4 devices, that include further requirements. For example, a USB4 Hub must also serve as a classic USB 3.2 hub with DP Alt mode passthrough with hosts that do not support USB4 connections. See USB4 Features by Device Type for more details.

Every USB4 port must support the USB4 protocol / connections, which is a distinct standard to establish USB4 links / connections between USB4 devices that exists in parallel to previous USB protocols. Unlike USB 2.0 and USB 3.x it does not provide a way to transfer data directly, but rather it is a mere container that can contain multiple "tunnels" / virtual connections.

Other specifications are referenced to define the contents and internal functionality of a tunnel. USB4 defines the following tunnel types:

• USB3 connections
• DisplayPort connections
• PCIe connections
• Ethernet/Network connections according to the included USB4Net and Cross-Domain specifications.^[22]

USB4 forms a tree-like topology of USB4 routers (each USB4 device includes a USB4 router to participate in this network). A tunnel can be end-to-end, where the route through the entire network of routers is preconfigured. But tunnels can also be single-hop, where it exists only for a single USB4 link (between 2 routers). In this case the tunnel will be "unpacked" by the recipient and will use some other, tunnel type specific means to identify where the data needs to be sent next. If the next hop is another USB4 router, the data will be ingested again into the next single-hop tunnel until it exits the USB4 network.^[23]

Accordingly, single-hop tunnels require specific support in each USB4 router to support even passing them through to further USB4 routers. End-to-end tunnels however only require specific support at the USB4 router where the data is ingested into the tunnel and at the target, the point where the tunnel ends.

A Protocol Input Adapter will ingest a connection according to whatever protocol it is based on and convert the contents into a USB4 tunnel. Protocol Output Adapters do the reverse. They extract a tunnel from the USB4 network and if needed recreate a regular connection from the tunnel contents.

The conversion into a tunnel typically entails removing any Phy/Electrical layer and encoding of the underlying connection standard and potentially losslessly compresses the contents, for example by leaving out empty filler data. A USB4 tunnel itself is virtual and need not conform to any fixed bandwidth or other limitations that stem from the Phy/Electric layer of the underlying connection standard. But since most tunnel types will eventually be converted back to a regular, physical connection again, most of those physical limitations, like max. bandwidth are still likely to apply in the end.

This is a single-hop tunnel that essentially can transport any Enhanced SuperSpeed connection according to the USB 3.2 specification. USB3 Gen X follows the Enhanced SuperSpeed Hub topology, where every USB4 router with more than one USB3 endpoint must include a USB3 hub as well. It is the default way USB3 connections through USB4 are made. Supporting it at 10 Gbit/s (SuperSpeed USB 10 Gbps, Gen 2x1) is mandatory on every USB4 DFP. The minimum supported speed for the USB3 connection being tunneled is 10 Gbit/s as every USB4 device already has to support this speed and USB3 Hubs handle converting this to 5 Gbit/s devices that may be connected.

This means, that a USB4 Hub will share a single upstream USB3 connection and distribute its bandwidth across all its downstream facing ports that make use of USB3 connections.

This is an optional alternative to USB3 Gen X tunneling that was introduced in USB4 Version 2.0. It is an end-to-end variant of USB3 Gen X tunnel.

Through this, it eschews the need for USB3 hubs in every USB4 router that can and will limit the throughput. It allows multiple separate USB3 Gen T tunnels even over shared links. Since it is an end-to-end tunnel, every USB4 hub will support passing it through. USB3 Gen T is intended as exclusively virtual, there exists no physical equivalent for it. Thus, it can only be used inside of a USB4 controller. This allows it to leave the limitations to 10 or 20 Gbit/s connections of USB 3.2 behind, while reusing most of the other parts of the Enhanced SuperSpeed protocol.^[24]

No known USB4 controller implements support for Gen T tunneling to date (August 2024).

DisplayPort is also tunneled as end-to-end connection. There can be multiple independent DP tunnels, but each will be delivered to a single protocol output adapter (at which point DisplayPort MST might be used to further split each connection up).

USB4 Version 1.0 only defines how to tunnel DP connections according to the DisplayPort 1.4a specification (up to HBR3 speeds). USB4 Version 2.0 updates this support to the full DisplayPort 2.1 specification (up to UHBR20 speeds).

DP tunneling has great understanding of the contents of DP connections, and will efficiently skip/transmit any filler data, reducing the actually utilized bandwidth of a DP tunnel. But since DP connections have real-time requirements, bandwidth must be reserved for them. USB4 mandates that in absence of any other information, the maximum possible bandwidth for the particular DP connection (DP lanes and speed) must be reserved. This reservation only applies to other real-time tunnels though. Reserved, but unused bandwidth can be used by non-real-time tunnels such as PCIe or USB3, but the reservation may still block other DP tunnels from being established.^[25]

Similar to USB3 Gen X tunneling, PCIe tunneling uses single-hop tunnels, requiring PCIe switches in every USB4 router that supports PCIe tunneling. USB4 has, from the start, referenced the PCI Express Specification Revision 4 and with USB4 Version 2.0 added references to PCI Express Specification Revision 5.0

PCIe tunneling has had a significant limitation in USB4 Version 1.0 and also Thunderbolt 3: PCIe Express has a variable maximum payload size, which applies end-to-end to a transmission. If any one component or PCIe Switch has a limited MPS, all packets passing through must be limited accordingly. Because USB4 uses a payload of up to 256 Byte per USB4 packet and a PCIe tunnel packet contains further PCIe headers and meta data, the MPS for PCIe tunnels was limited to 128 Byte. This limitation can reduce the efficiency of the PCIe connection greatly for all devices and systems that would otherwise support 256 Byte or even larger MPS.

USB4 Version 2.0 removes this bottleneck (mandatory for all implementers), by defining how a larger PCIe packet can be split across multiple USB4 packets. Support for this new feature requires every USB4 component / controller involved in the PCIe tunnel to implement USB4 Version 2.0.^[26]

Signaling refers to the lowest layer of the OSI Model, also called physical layer or phy. USB4 connections can be expressed with consumer facing names that are also the basis for the official logos used on packaging and products. These are the "20 Gbps", "40 Gbps", "80 Gbps" labels and they do not explicitly indicate how the connection is achieved on the physical layer. There are also more technical names based on the implementation and use of the USB-C cables. These usually consist of a speed per wire-pair expressed as Gen 1/2/3/4 (5 Gbit/s, 10 Gbit/s, 20 Gbit/s, 40 Gbit/s respectively) and some further information on how many wire-pairs are used in which combination.

USB commonly defines a "Lane" as a (bidirectional) connection, which for all recent transmission modes consists of one sending and one receiving wire-pair. The "Gen AxB" notation refers to B Lanes of operation mode A. Since Gen 4 modes also introduced asymmetric connections with uneven numbers of wire-pairs dedicated to sending and receiving, the Lane-notation is no longer applicable.

The USB 3.x family has had the same technical notation retroactively added in the USB 3.1 and USB 3.2 specification versions. Though this shows common principles and the same generations refer to the same nominal speeds, "Gen A" does not have the same exact meaning in both USB 3.x and USB4 specifications. The overlap in naming mainly becomes relevant for cables as shown in Cable Compatibility, which is regulated by the Type-C specification shared across all users of Type-C connector.

Comparison of signaling modes

USB Family	Signaling Mode Name[a]	Introduced in	Encoding	wire-pairs sending / receiving	Raw Bit Rate (Gbit/s)		Net Data Rate[b] (Gbit/s)	USB-IF Current Marketing Name[27]	Logo[27]
					per wire-pair	total (per direction)
USB 2.x	High-Speed	USB 2.0	NRZI w/ bit stuffing	1 (shared)	0.480 (half-duplex)	0.480 (half-duplex)	?	Hi-Speed USB
USB 3.x	Gen 1x1	USB 3.0	8b/10b	1/1	5	5	4	USB 5Gbps
	Gen 2x1[c]	USB 3.1	128b/132b	1/1	10	10	~9,7	USB 10Gbps
	Gen 1x2	USB 3.2	8b/10b	2/2	5	10	8	(fallback)[d]
	Gen 2x2[c]		128b/132b	2/2	10	20	~19.39	USB 20Gbps
USB4	Gen 2x1[c]	USB4 v1.0	64b/66b[e]	1/1	10	10	~9,697	(transient/fallback)[f]
	Gen 2x2[c]			2/2	10	20	~19.39	USB 20Gbps
	Gen 3x1		128b/132b[e]	1/1	20	20	~19.39	(transient/fallback)[f]
	Gen 3x2			2/2	20	40	~38.79	USB 40Gbps
	Gen 4 symmetric	USB4 v2.0	PAM-3[28] 11b/7t	2/2	~40.58[g]	~81.15	~80.46	USB 80Gbps
	Gen 4 asymmetric 3:1			3/1		3x: ~121.725 1x: ~40.58	3x: ~120.69 1x: ~40.23	—[h]
	Gen 4 asymmetric 1:3			1/3				—[h]
—	TB3 Gen 2x2	—	64b/66b	2/2	10.3125	20.625	20	—
	TB3 Gen 3x2		128b/132b	2/2	20.625	41.25	40	—

Thunderbolt 3 Gen 2 and Gen 3 and the USB4 Gen 2 and Gen 3 modes use very similar signaling, however, Thunderbolt 3 runs at slightly higher speeds called legacy speeds compared to USB4' s rounded speeds.^[29] Thunderbolt 3's choices leads to the marketed bandwidth being the actual net data rate (after encoding overhead is removed). USB standards have mostly marketed the raw data rate instead.

USB4 Gen 4 is normally referred to as a speed of "40 Gbps" or 40 Gbit/s, with the full connections based on it being referred to as 80, 120/40, 40/120 Gbit/s. But since the actual signaling no longer is binary, the actual raw bit rates no longer match those numbers exactly.

A USB4 Hub is defined by having 1 USB4 UFP and one or more USB4 DFP.

A USB4-Based Dock is defined as a USB4 hub that also has more specialized outputs like HDMI or DP, but still keeping some USB4 DFP.

A USB4 Peripheral Device is defined by not having any USB4 DFP. This means devices that are colloquially called "USB-C Hubs" may use USB4 to support the dynamic bandwidth sharing or higher bandwidths of USB4. But they are not USB4 Hubs if they do not have any USB4 DFP. Not having any USB4 DFP allows the peripheral to only support exactly those USB4 features that it has uses for, potentially simplifying its implementation by a lot.