USB4

Functionality of USB4 ports

USB4 ports offer multiple different connection types. Some of them can coexist and work independently or in parallel with each other. This is further complicated, by USB4 connections themselves only being a container for other connections. Some connection standards are possible, both directly from the port as well as a contained within a USB4 connection. It is even possible for the functionality provided natively and within a USB4 connection to differ.

This section describes the possible non-USB4 connections, that mostly exist for backwards compatibility. For the USB4 connection and its contents refer to § USB4 protocol/connections.

The USB4 specification speaks of downstream facing ports (DFP) and upstream facing ports (UFP) rather than host and peripheral ports. Downstream facing ports includes host ports as well as any "outputs" of a USB4 hub, while upstream facing ports include anything that is connectable to a downstream facing port, like the ports of peripherals or the "input" port of a USB4 hub.

Any USB4 DFP port is required to also implement USB 2.0, USB 3.2 and DP Alternative Mode support, each according to their own specifications. As such, a USB4 DFP is backward compatible to all previous USB standards and DP output.

USB 2.0 DFP features

USB 2.0 defines 3 different bit rates (Low-, Full-, High-Speed), all are required to be supported. USB 2.0 abilities use separate wires and pins on the Type-C connector that are not used by USB 3.2 or USB4 or most other Alt modes.

USB 3.x DFP features

USB 3.2, the current version, defines 3 different bit rates 5 Gbit/s, 10 Gbit/s and 20 Gbit/s. While the USB 3.2 specification has been referenced by USB4 from the start, only the two lower speeds (5 Gbit/s, 10 Gbit/s) are mandatory for USB4 DFPs to support. As with any previous USB 3.x DFP, the USB 2.0 connection remains available in parallel to it.

DP Alt Mode DFP features

The USB4 specifications make no reference to a minimum speed or features for its DP Alternative Mode functionality, but Thunderbolt 3 does. In practice, Intel's family of TB 3 controllers requires at least DisplayPort 1.2 at HBR2 speeds to support 4K60 output, but is also available with up to HBR3 speeds according to the DisplayPort 1.4a specification. DP Alt Mode can always coexist with USB 2.0 connections. And it can be used in a halved mode, where half of the other high speed wires remain available for 5 or 10 Gbit/s USB 3.x connections.

Thunderbolt 3 Alt Mode DFP/UFP features

While a very common feature, it is optional for most USB4 ports to support the TB3 connections (at 20 or 40 Gbit/s). While those TB3 connections are technically an Alt Mode, similar to DP Alt mode, the principles of it are closest to USB4 connections. See Thunderbolt 3 Compatibility for more details. It is possible for USB4 devices to support TB3 connections (to other USB4 devices or to TB3 peripherals), but not support being recognized by TB3 hosts.

Power transfer features for DFP

The USB4 specification makes no explicit demands on power output. It outsources all requirements in terms of power to the Type-C specification that underpins all USB, DP 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 be supported following the USB PD specification, as well as supplying considerably more power. The USB PD protocol must always have support for exchanging data according to the protocol. This is separate from any functionality of PD to negotiate actual power delivery other than 5V or >15W.

USB4 hubs & docks

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 Alternative Mode passthrough with hosts that do not support USB4 connections. They are required to support Thunderbolt 3 connections. See USB4 capabilities by device type for more details.

USB4 protocol/connections

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, it is rather a mere vessel that can contain multiple virtual connections ("tunnels").

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

General principles of USB4

USB4 forms a tree-like topology of USB4 routers, where 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 (i.e., between 2 routers). In this case, the tunnel will be "unpacked" by the recipient and will use some other means specific to the tunnel type to identify where data needs to be sent next. If the next hop is another USB4 router, data will be ingested again into the next single-hop tunnel until it exits the USB4 network.

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

Note: USB 2.0 is not part of any tunnel. That functionality is provided by maintaining a physically and logically separate USB 2.0 connection in parallel to a USB4 connection over the dedicated wires, just as it was handled with USB 3.x and its backward compatibility to USB 2.0.

Protocol input/output adapters

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 doesn't need to conform to any fixed bandwidth or other limitations that stem from the Phy/Electrical 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.

USB3 Gen X tunneling

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 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.

USB3 Gen T tunneling

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.

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

DP tunneling

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).

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). Notably, the USB4 specification explicitly carves out needing to support the UHBR13.5 DP speed, even if UHBR20 is supported. The DP specification is not public. It is unknown if it makes similar carve-outs.

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.

PCIe tunneling

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 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.

USB4 signaling modes

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 "USB 20Gbps", "USB 40Gbps", "USB 80Gbps" 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.

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 rounded speeds of USB4. It is 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 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 is no longer binary, the actual raw bit rates no longer match those numbers exactly.

USB4 capabilities by device type

USB4 hub

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

USB4-based dock

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.

USB4 peripheral device

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 considerably.

Cable compatibility

The Type-C standard supports cable backward/downward compatibility in many situations. The compatibility typically only breaks between the different families of standards (USB 2.0, USB 3.2, USB4). There are "passive" cables, which consist mainly of plain conductors that can be used and repurposed for any manner of signals in either direction and thus have the highest forward and backward compatibility. It also contains active or hybrid active (mix of active-conductive and optical) cables which the standard mandates to have vast backward compatibility support, so as to behave as if they were regular, passive cables in the eyes of the consumer. But forward compatibility is limited for active cables due the active electronics inside. Only optically isolated active cables (OIAC), which should be clearly distinguishable by price, design, cable thickness, and advertising, are allowed to strip most of the backward compatibility away.

The Gen 4 transmission mode with PAM-3 uses signalling very different from that of previous modes. Every active component needs to explicitly support this new signaling, but it stays within all signal quality requirements of existing, passive Gen 3 cables (USB4 and TB3).

The direction of signalling also poses another challenge to active cables. Most USB signalling uses predefined wire-pairs for sending and others for receiving. But alternate modes, such as DP Alt mode may require to use all of the wire-pairs in the same direction. This switching of the direction of wire pairs makes active cables more complicated and therefore support for it is optional.

Cable naming and relation to specification versions

USB-IF intends only for the new bandwidth-based logos and names to be used with consumers. For cables, the type (passive, active) and the highest supported bandwidth are usually enough to uniquely identify a cable and its supported features. Although some active types make clear distinctions where further details on the type are required. Formally, a cable type and properties are defined by a distinct specification version, which was used during the development/design of said cable model, so each cable would be a valid and possibly certified cable according to a specific set of USB specification versions, like "Type-C 2.3, USB 3.2, USB4 Version 2.0". But the standard is also designed to be interoperable, in that a newer specification version typically adds new modes of operation, new cable types, but does not restrict previously existing things. Because that would make existing things incompatible with new products. For this purpose, even the older USB logos and labels did not include a specification version, but only stated "Certified USB SuperSpeed+ 10 Gbps". This logo identified cables that could support the 10 Gbit/s connection speeds of USB3 across both the USB 3.1 and USB 3.2 version, because the requirements for the cables have not changed. Thus, a precise specification version is usually not relevant and would not make a difference.

Transmission modes such as Gen2×2 are also irrelevant to cables, as valid cables are either full-featured, having all the high speed wire-pairs for up to dual-lane connections at the stated speed or they are USB2-only or some other specific and restrictive type, as listed below.

USB4 cable compatibility

DP Alt Mode support for USB4 cables

The Type-C specification does not name specific DP speeds that it considers supported for passive cables where support is optional for active cables. The USB-C presentation on DP Alt mode calls out passive full-featured USB-C cables for their DisplayPort support and headroom for future DP speed increases. HBR3 was the highest available DP speed at the time.

Active cables may have additional complications, because the active electronics do not need to operate all high speed wire-pairs in the same direction for normal USB operations (but "USB 80Gbps" cables are mandated to support asymmetric connections, which includes at least some of the wire-pairs operating in either direction). Active cables can have further limitations, since the active electronics may only support specific signaling modes. There are 2 variants of active electronics. Linear ReDrivers only amplify the signal without any particular signaling mode or encoding in mind. ReTimers explicitly reconstruct the incoming signal for a higher quality result.

TB4 cables, even active ones, at least up to 2m in length, are guaranteed to support DP Alt mode. A specific maximum speed is also not mentioned, but the other requirements for TB4 all refer to DP 1.4 and its maximum speed of HBR3. TB5 renews the same guarantee for USB 80Gbps cables while referencing the DP 2.1 specification (up to UHBR20 speeds).

DP 2.1 aligned itself to the USB4 PHY layer, according to VESA, the creator of DisplayPort. It is unclear how complete this alignment is. However, the UHBR10 DP speed matches USB4 Gen 2 in bit rate and encoding, whereas the UHBR20 DP speed matches USB4 Gen 3 in bit rate and encoding. A USB and DP certification service lists USB 5Gbps cables (USB 3.2 Gen 1) as supporting UHBR10 speeds, which would fit for having the same requirements as USB 20Gbps (USB4 Gen 2x2) connections.

Anandtech reports that "this also means that DP Alt Mode 2.0 should largely work with USB4-compliant cables, although VESA is being careful to avoid promising compatibility with all cables".

There are linear redrivers and retimers available that are advertised for USB4 Gen 3 speeds and all current DP speeds up to UHBR20 and including UHBR13.5.

Thunderbolt compatibility

Thunderbolt 3

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, where this is optional for USB4 hosts and USB4 peripheral devices. Compatible products need to implement 40 Gbit/s mode, at least 15W of supplied power and a different clock. Implementers need to sign the license agreement and register a Vendor ID with Intel.

The USB4 protocol is based on and related to the operating principles of Thunderbolt 3. The USB4 specification simply defines which features to disable, downgrade and which parameters to change to get to an implementation compatible with Thunderbolt 3. This includes, for example: limitation to daisy-chain topology (a hub must expose at most one USB4 DFP), downgrade of DP capabilities to DP 1.2, disabling any USB3 tunnel or the parallel USB 2.0 connection and switching back to the previous, slightly higher signaling rate of TB3. Any USB 3.x or USB 2.0 connection - if needed at all - is handled by PCIe-USB controllers and via the PCIe tunnel.

The TB3 connection is formally an Alt Mode and not a native USB operating mode. For this the extensible "vendor defined" mechanisms of USB PD are used to query cables and devices for support and to initiate the connection.

Thunderbolt 4

During CES 2020, USB-IF and Intel stated their intention to allow USB4 products that optionally support any or all of the same 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.

Thunderbolt 4 is a "solution brand" around USB 40Gbps. Thunderbolt 4 mandates some features that are optional in USB4, including backward compatibility to Thunderbolt 3, minimum PCIe ("32 Gbps") and DP capabilities (2 DP tunnels, "4K60 each", HBR3+DSC).

For cables, it similarly mandates above-minimum features of at least 100W support and the less well defined "universality" for up to 2 meters.

Thunderbolt 4 even requires certain notebooks to support charging via Thunderbolt ports, which is otherwise independent of any USB4 support.

Thunderbolt 5

Thunderbolt 5 is an upgrade over Thunderbolt 4 and likewise a solution brand around USB 80Gbps. It mandates even higher minimum PCIe ("64 Gbps") and DP capabilities (2 DP tunnels, "6K60 each", unclear min. DP speed). It also mandates support for asymmetric 120/40 Gbit/s connections from hosts to docks, but does not mention the reverse.

For cables, Thunderbolt 5 mandates 240W support and "universality" up to 2 meters.

Notably, other features that are part of the advertising of Thunderbolt 5, like USB3 20Gbit/s or support for 3 DP tunnels at speeds of up to UHBR20 are optional even for Thunderbolt 5. While Intel's own controllers widely support these features and some are even available from newer Thunderbolt 4 controllers, not every Thunderbolt 5 device or controller does. See Known USB4 Controllers.

Pinout

USB4 uses the Type-C connector with its 24 pins in a symmetrical shell. Type-C has 12 A pins on the top and 12 B pins on the bottom.

The CC configuration channel is used per the Type-C standard to detect connections, plug orientation (due to the reversible USB Type-C shell) and establishing USB PD communication to negotiate power and query the cable (eMarker) and opposing side for USB4 or TB3 capability. If all participants support USB4, the entering of USB4 mode is initiated. If not, the port may fall back on other alternative modes such as Thunderbolt 3 Alt mode, DP Alt mode or USB3 connections.

Once USB4 connection has been requested, USB4 uses the SBU channel to manage the overall connection, initialization, disconnect. All SuperSpeeed differential pairs are used for the main USB4 link. For Gen 2 and Gen 3 connections, TX1+, TX1−, RX1+, RX1− form lane one and TX2+, TX2−, RX2+, RX2− form lane two, each in sending and receiving direction respectively. Gen 4 connections can also use three pairs in one direction for the asymmetric modes. The differential D+ and D− pair is used for USB 2.0 transfer that runs independent of the other USB4 communication. While USB3 communication happens via a "tunnel" inside the main USB4 link, any USB2 communication with potential USB2 devices continues to happen over the same wires as it would without USB4.

USB4 only requires that PD communication occurs and that the participants have agreed upon an initial power contract, before proceeding to USB4 negotiation. They are otherwise independent and power can be renegotiated at any point (including direction changes) without disturbing the USB4 link and is only guided by the Type-C and PD specifications, not by USB4 itself.

Software support

USB4 is supported by:

• Linux kernel 5.6, released on 29 March 2020
• macOS Big Sur (11.0), released on 12 November 2020
• Windows 11, released with support for USB4 Version 1.0 on 5 October 2021
  • upgraded to USB4 Version 2.0 support including 80 Gbit/s around March 2024

Connection manager

Connection manager is the part of a USB4 host that manages connections across the entire USB4 topology, establishing of tunnels, handling any bandwidth reservations and data prioritization, like which DP tunnels can be established and at what speed. The USB4 driver in Windows 11 implements native OS support of USB4, where the connection manager is part of a driver that only works with matching controllers. Older controllers had the connection manager implemented inside their firmware and thus required far less support from the OS.

On Linux, the USB4 and Thunderbolt driver (named "thunderbolt") supports both firmware-managed and also OS-managed controllers via the same tools.

Hardware support

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.

On 3 March 2020, Cypress Semiconductor announced new Type-C power (PD) controllers supporting USB4, CCG6DF as dual port and CCG6SF as single port.

In November 2020, Apple unveiled MacBook Air (M1, 2020), MacBook Pro (13-inch, M1, 2020) and Mac mini (M1, 2020), featuring two USB 40Gbps (USB4 Gen 3) ports.

AMD also stated that Zen 3+ (Rembrandt) processors will support USB4 and released products do have this feature after a chipset driver update. However, AMD has only announced support for USB 20Gbps (USB 3.2 Gen 2×2) in Zen 4 processors that were released in September 2022. Intel supported Thunderbolt 3 and USB-C with their 8th generation mobile processors in 2018. For example, the Lenovo P52 has dual TB3 ports on the rear. TB/USB has evolved as Intel is able to refine logic design.

Known USB4 controllers

The following table of USB4 controllers lists what features the controllers could support. The boards or products in which they are used can often influence the specific supported features, for example by not connecting all inputs, configuring the controller differently for compatibility or power saving or by combining it with other, lower tier components.

For example, many USB4 controllers Intel builds into their CPUs require further external components such as ReTimers to reach the full 40 Gbit/s speeds. They can also be used in simpler configurations for only 20 Gbit/s.

Features of alternate connections, such as the native DP or USB3 speeds - which can differ from the speeds supported for DP and USB3 tunnels inside USB4 connections - may also depend on other components, such as the ReTimers used.

Specification References

• "USB Type-C® Cable and Connector Specification Release 2.3 | USB-IF". usb.org. USB-IF. Retrieved 2024-08-05.
• "USB4® Specification v2.0 | USB-IF". usb.org. USB-IF. Retrieved 2024-08-05.
  • "USB4® Specification v2.0, Adopters Agreement | USB-IF". usb.org. USB-IF. Retrieved 2024-08-05.
  • "USB4® Specification v2.0, Connection Manager Guide | USB-IF". usb.org. USB-IF. Retrieved 2024-08-05.
  • "USB4® Specification v2.0 Inter-Domain Service | USB-IF". usb.org. USB-IF. Retrieved 2024-08-05.
• "USB4® Specification v2.0 with Errata and ECN through September 25, 2024 | USB-IF". usb.org. USB-IF. Retrieved 2026-03-25.
• "USB Power Delivery | USB-IF". usb.org. USB-IF. Retrieved 2024-08-05.
• "USB 2.0 Specification | USB-IF". usb.org. Retrieved 2024-08-05.
• "USB 3.2 Revision 1.1 - June 2022 | USB-IF". usb.org. USB-IF. Retrieved 2024-08-05.