[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.

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

[10][11] In the announcement press release, the USB Promoter Group mentions that USB4 is "based on the Thunderbolt™ protocol specification recently contributed by Intel Corporation".

[12] 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".

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.

[22] USB 2.0 defines 3 different bit rates (Low-, Full-, High-Speed), all are required to be supported.

The USB4 specifications make no reference to a minimum feature set for its DP Alternative Mode functionality, but Thunderbolt 3 does.

It outsources all requirements in terms of power to the Type-C[26] specification that underpins all USB, DP and other standards that use the USB-C connector.

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.

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.

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.

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).

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.

Support for this new feature requires every USB4 component / controller involved in the PCIe tunnel to implement USB4 Version 2.0.

USB4 connections can be expressed with consumer facing names that are also the basis for the official logos used on packaging and products.

USB commonly defines a "Lane" as a (bidirectional) connection, which for all recent transmission modes consists of one sending and one receiving wire-pair.

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.

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.

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.

connection USB4 networking (Low-, Full-, High-Speed)[d] The Type-C standard supports cable backward/downward compatibility in many situations.

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.

The USB4 specification states that a design goal is to "Retain compatibility with existing ecosystem of USB and Thunderbolt products."

The USB4 specification simply defines which features to disable, downgrade and which parameters to change to get to an implementation compatible with Thunderbolt 3.

[57] 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/replacing the USB3 tunnel with an integrated PCIe-USB3 Host controller attached via PCIe tunnel, switching back to the previous, slightly higher signaling rate of TB3 and its separate way of initiating a connection as an Alt Mode.

[63] The CC configuration channels have 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.

Older controllers had the connection manager implemented inside their firmware and thus required far less support from the OS.

AMD also stated that Zen 3+ (Rembrandt) processors will support USB4[71] and released products do have this feature after a chipset driver update.