Thunderbolt 4 and USB4 share the same physical layer specification, both referring to TBT4, and theoretically have the same rate compatibility. However, they differ in the protocols used. USB4 employs the tunneling protocol, while Thunderbolt 4 uses the DP and PCIe protocols. In practical use, the functional differences between them are hardly noticeable, and the same testing software can be used for Thunderbolt 4, Thunderbolt 3, and USB4.
Thunderbolt 4, the latest version of the Thunderbolt interface, evolved from Thunderbolt 3 and still uses the USB Type-C connector. Intel officially announced Thunderbolt 4 in early 2020 and began shipping it on Core-based 11th-generation laptops in the fall of that year. Compared with Thunderbolt 3, Thunderbolt 4 increases the minimum PCIe link bandwidth requirement from 16 Gbps to 32 Gbps and adds support for dual 4K monitors (60 Hz) using the DisplayPort 1.4 protocol. Other improvements include the ability to wake up the PC from sleep, support for accessories with up to four Thunderbolt ports, and support for Intel VT-d DMA protection for virtual machines.
Thunderbolt 4 supports a bidirectional bandwidth of up to 40 Gbps, making it an ideal interface for PC-to-PC data transfer, optimal SSD storage solutions, and external GPU docks like the Razer Core X. At least one Thunderbolt 4 interface on a laptop can charge the device, and its universal compatibility extends to support all current and previous USB standards, including USB 4. USB 4 peripherals can be directly inserted into the Thunderbolt 4 port and run at full speed. Thunderbolt 4 can also accommodate older USB peripheral devices using USB Type-C or Type-A connectors (via adapters or docking stations). The docking solution is especially suitable for laptops with limited external ports. In addition, Thunderbolt 4 can provide up to 98 watts of charging power for laptops.
Nevertheless, Thunderbolt 4 has some inherent limitations. Although its theoretical transmission rate is 40 Gbps, a portion of the bandwidth is reserved for video output to ensure better compatibility with DisplayPort (DP), leaving only 32 Gbps for data transmission, which is roughly equivalent to the PCIe 3.0 x4 standard. Therefore, the theoretical maximum speed of Thunderbolt 4 is only 3200 MB/S, and there is a certain performance loss when using an external graphics card docking station, with the loss being greater for higher-end graphics cards. When Thunderbolt 4 interfaces are paired with storage peripherals that do not support the Thunderbolt protocol, the maximum transmission speed is only at the level of USB 3.2 Gen2, which is only 10 Gbps, or less than 1000 MB/s.
USB4, short for Universal Serial Bus Generation 4, was first introduced in 1996 with the release of the USB 1.0 specification, which supported low-speed 1.5 Mbps and full-speed 12 Mbps transmission speeds. Subsequently, versions with supported speeds of 480 Mbps, 5 Gbps, 10 Gbps, 20 Gbps, etc. were successively released. In September 2019, the latest generation of USB4 specification was released, supporting 20 Gbps and 40 Gbps.
USB4.0 directly adopts the "Thunderbolt" Thunderbolt3 protocol standard introduced by Intel and Apple on high-end laptops in 2015, based on the Type-C USB interface. The data transmission rate supports two rates: 10 Gbps/lane and 20 Gbps/lane, and selectively supports the TBT3-compatible 10.3125 Gbps/lane and 20.625 Gbps/lane rates. At the same time, it supports DisplayPort, PCIE and other signal standards through the alternate mode (ALT mode). To avoid confusion, Intel has unified the naming of the Thunderbolt interface to be deployed on high-end laptops in the future as Thunderbolt4.
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