USB4


Universal Serial Bus 4, sometimes erroneously referred to as USB 4.0, is the most recent technical specification of the USB 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 defines bit rates of 20 Gbit/s, 40 Gbit/s and 80 Gbit/s. USB4 is only defined for USB-C connectors and its Type-C specification regulates the connector, cables and also power delivery features across all uses of USB-C cables, in part with the USB Power Delivery specification.
The USB4 standard mandates backward compatibility to USB 3.x and dedicated backward compatibility with USB 2.0. The dynamic sharing of bandwidth of a USB4 connection is achieved by encapsulating multiple virtual connections of other protocols, such as USB 3.x, DisplayPort and PCI Express.
USB4 is based on the Thunderbolt 3 protocol. However, it is different enough that backward compatibility to Thunderbolt 3 is optional for many device types.

History

Prior to USB4, Thunderbolt provided a way to dynamically share bandwidth between multiple and PCIe connections over a single cable. Thunderbolt originally used the mDP connector and was only backward compatible to connections and did not support power transfer.
The introduction of the Type-C connector in 2014 provided a connector that could support USB data connectivity and power transfer as well as 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 backward compatibility for USB connections and power transfer features.

USB4 Version 1.0

USB4 was announced in March 2019 by the USB Promoter Group. The version 1.0 of the USB4 specification, released 29 August 2019, is titled "Universal Serial Bus 4 ". Several news reports before the release of that version sometime use the wrong terminology "USB 4.0" and "USB 4".
In the announcement press release, the USB Promoter Group mentions that USB4 is "based on the Thunderbolt™ protocol specification recently contributed by Intel Corporation".
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, to offer display and data transfer features, while retaining "compatibility with existing and Thunderbolt products".
Version 1.0 defined 20 Gbit/s and 40 Gbit/s connections, the required support of USB 2.0 and USB 3.x connections at up to 10 Gbit/s with support for tunneling connections according to the PCIe 4.0, USB 3.2, and DP 1.4a specifications. Optional backward compatibility to Thunderbolt 3 as well as Host-to-Host networking were also defined.
Compared to Thunderbolt 3, USB4 changed the raw bit rates slightly to bring them in line with other USB specifications, where the nominal bit rate matches the raw bit rate. USB4 also added support for USB3 tunnels and use of the USB2 wires for improved backward compatibility with previous USB standards and to allow for simpler USB4 devices without support for PCIe. USB4 also added support for hub topologies compared to Thunderbolt's previous restriction to daisy-chaining topology.
In July 2020 Intel announced Thunderbolt 4 as an implementation of USB4 40 Gbit/s with additional requirements, such as mandatory backward compatibility to Thunderbolt 3 and requirement for smaller notebooks to support being charged over Thunderbolt 4 ports. Publications such as Anandtech described Thunderbolt 4 as "superset of TB3 and USB4" and "able to accept TB4, TB3, USB4, and USB 3/2/1 connections".
Intel itself describes Thunderbolt 4 as "delivering increased minimum performance requirements, expanded capabilities and USB4 specification compliance" and as building "on the innovation of Thunderbolt 3".

USB4 Version 2.0

On 18 October 2022 the USB Promoter Group released the USB4 Version 2.0 specification.
It added a new transmission speed that allows 80 Gbit/s symmetric connections or asymmetric connections supporting 120 Gbit/s in one direction and 40 Gbit/s in the other. The new PAM3 encoding scheme enables this over existing, passive "USB 40Gbps" cables. Active cables are not forward compatible in the same way, instead a new speed grade of active cables was added.
It also upgraded the support of DP tunnels to DP 2.1, allowing the tunneling of DP connections with up to 80 Gbit/s. It also added a replacement of the previous tunneling of classic USB 3.2 connection speeds with "USB3 Gen T tunneling", which can exceed 20 Gbit/s and also removed PCIe overhead limitations.
Around the release of the new USB4 2.0 specification, USB-IF also mandated new logos and marketing names to simplify representing the maximum supported bit rates and wattages to consumers.
In September 2023, Intel announced the launch of Thunderbolt 5 as an implementation of USB4, using the new abilities of 80 Gbit/s connections and updated DP support Intel's own press release describes it as "built on industry standards – including USB4 V2".

Functionality of USB4 ports

Similarly to how USB 3.x specifications defined the new SuperSpeed protocols for faster signaling rates, they also mandated that USB 3.x physically and architecturally implement USB 2.0 specification with dedicated wires, where the USB4 specification describes 2 different aspects. The first one is what type of existing connections and compatibility a USB4 port guarantees. The USB4 specification speaks of downstream facing ports and upstream facing ports 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 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 is backward compatible to all previous USB standards and DP output.

USB 2.0 DFP features

defines 3 different bit rates, all are required to be supported. USB 2.0 abilities uses separate wires on the Type-C connector that are not used by USB 3.2 or USB4.

USB 3.x DFP features

, the current version, defines 3 different bit rates. While the USB 3.2 specification has been referenced by USB4 from the start, only the two lower speeds are mandatory for USB4 DFPs to support.

DP Alt Mode DFP features

The USB4 specifications make no reference to a minimum feature set 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.

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 to supply at least 7.5W Type-C current. No power consumption features 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. 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.
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. 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.

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.