USB4 Specification│News

USB4 Specification

USB4 (Universal Serial Bus 4) refers to the USB system defined in the USB4 specification, first released by the USB Promoter Group on August 29, 2019 (version 1.0).

Unlike previous USB protocol standards, USB4 requires the use of a USB-C connector and support for USB Power Delivery (USB PD) for power transmission. Compared to USB 3.2, it allows for the tunneling of DisplayPort and PCI Express protocols. This architecture defines a method for dynamically sharing a single high-speed link among multiple device types, optimizing data transfer efficiency according to the type and application.

All USB4 products are required to support a minimum data throughput of 20Gbps, with optional support for 40Gbps or 80Gbps (symmetric for upstream and downstream). In asymmetric mode, speeds can reach up to 120Gbps in one direction, while the opposite direction is reduced to 40Gbps. Due to the tunneling capabilities, when transmitting mixed data types, the actual effective data rate can exceed that of USB 3.2—even when limited to 20Gbps.

The USB4 specification is based on the Thunderbolt 3 protocol specification.

USB4 Specification Version 1.0 (released on August 29, 2019) uses the terms "Universal Serial Bus 4" and "USB4." Prior to the release of this version, some news outlets referred to it as "USB 4.0" or "USB 4." Even after version 1.0 was published, some continued to use "USB 4" intentionally, claiming it reflected common search behavior among readers.

USB4 Specification

History
The USB4 standard was officially announced in March 2019.

Design Goals

The goals outlined in the USB4 specification include higher bandwidth and the unification of the USB-C ecosystem, aiming to “minimize end-user confusion.” Key approaches to achieve these goals include the use of a single USB-C connector type, while maintaining backward compatibility with existing USB and Thunderbolt products.

Data Transmission Method

Unlike USB 3.x, USB4 does not define a universal data transmission mechanism or device class. Instead, it primarily provides tunnels for transporting other protocols such as USB 3.2, DisplayPort, and optionally PCI Express (PCIe).

Although USB4 supports a native "host-to-host" communication protocol, as the name suggests, it is only used between two connected host devices and enables host-level IP networking. Therefore, if either the host or the device does not support optional PCIe tunneling, the maximum non-display bandwidth is limited to USB 3.2 at 20Gbps, with only 10Gbps USB 3.2 being mandatory.

USB4 supports the following tunneling protocols:

• USB 3.2 (Enhanced SuperSpeed) tunneling

• DisplayPort 1.4a-based tunneling

• PCI Express (PCIe)-based tunneling

USB4 also requires support for DisplayPort Alternate Mode, allowing DisplayPort signals to be transmitted either through USB4 tunneling or via the alternate mode.

The dedicated wires in the USB-C connector always support USB 2.0.

Data Transfer Mode Support

Some data transfer modes are mandatory for all USB4 devices, while support for others is optional and depends on the device type.

Data Transfer Mode Support

USB Data Transfer Modes

Name	icon	specification				Double Link	coding	speed
		USB4	USB 3.2	USB 3.1	USB 3.0			Gbit/s	GB/s
SuperSpeed USB 5Gbps			USB 3.2 Gen 1×1	USB 3.1 Gen 1	USB 3.0	NO	8b/10b	5	0.5
SuperSpeed USB 10Gbps			USB 3.2 Gen 2×1	USB 3.1 Gen 2		NO	128b/132b	10	1.2
not applicable			USB 3.2 Gen 1×2			YES	8b/10b	10	1.0
SuperSpeed USB 20Gbps			USB 3.2 Gen 2×2			YES	128b/132b	20	2.4
not applicable		USB4 Gen 2×1				NO	64b/66b	10	1.2
not applicable		USB4 Gen 3×1				NO	128b/132b	20	2.4
USB4 20Gbps		USB4 Gen 2×2				YES	64b/66b	20	2.4
USB4 40Gbps		USB4 Gen3×2				YES	128b/132b	40	4.8

USB4 Gen 2 and USB 3.2 Gen 2 are different technologies. While they share the same data transfer rate of 10 Gbps, they use different encoding methods at the electrical layer.

Although USB4 is required to support dual-lane mode, it initially operates in single-lane mode during the link initialization process. If a lane bonding error occurs during this phase, the single-lane connection can still be used as a fallback mode.

In Thunderbolt compatibility mode, as specified by the Thunderbolt protocol, lanes operate at higher speeds: 10.3125 Gbps (Gen 2) or 20.625 Gbps (Gen 3)

Power Delivery

Before a connection is established, USB Power Delivery (USB PD) negotiation must be completed. Each USB4 interface is required to provide at least 7.5W of power (5V, 1.5A).Before USB PD negotiation, a USB4 device must draw no more than:250mA (default) 1.5A, or3A at 5V,depending on the resistor configuration of the USB-C connector.USB PD supports up to 100W of power delivery.

Thunderbolt 3 Compatibility

According to the USB4 specification, a core design goal is to maintain compatibility with the existing USB and Thunderbolt ecosystems. However, Thunderbolt 3 compatibility is optional and applies only to hosts and peripheral devices, not mandatory across all USB4 devices.