What Is Thunderbolt?

Thunderbolt is a hardware standard that allows peripheral devices, such as smartphones and external hard drives, to connect to a computer. It was developed by Intel in collaboration with Apple.

Thunderbolt Versions

There are several versions of Thunderbolt, with newer iterations improving steadily in data transfer rates or speeds. The first version of Thunderbolt, initially called Light Peak, launched in 2011. The standard was first found on Mac computers, but it has since made its way to PCs, often competing with the USB standard. However, unlike USB devices, which don't need to be certified, Thunderbolt devices must be certified by Intel.

The fourth generation of Thunderbolt, called Thunderbolt 4, was announced in 2020, months after the announcement of USB4. USB4 is based on and compatible with Thunderbolt 3. Thunderbolt 3 was compatible with USB-C ports.

While Thunderbolt and USB standards are often compatible with each other, their specifications have historically been different. A USB device plugged into a Thunderbolt port may work, but it likely won't deliver Thunderbolt speeds. The transfer rate is limited by the slowest member. Traditionally, this was USB.

With the release of Thunderbolt 4, however, protocol and data rates are fully compatible with USB 4, which is backward compatible with Thunderbolt 3, USB 3.2, and USB 2.0. This convergence of compatibility makes USB the most cross-compatible standard, although USB4 devices are not likely to show up until 2021.

The History of Thunderbolt

During its early stages of development, Thunderbolt was called Light Peak. Light Peak was originally intended to be an optical interface standard. Thunderbolt dropped the objective in favor of more traditional electrical cabling.

This made Thunderbolt easier to implement. Rather than relying on a new connector, Thunderbolt was based on the existing DisplayPort technology and its mini-connector design. The idea was to allow a cable to carry a video signal and a standard data signal. DisplayPort was a logical choice among video interfaces because it had an auxiliary data channel built into its specification. The other two digital display connectors, HDMI and DVI, lacked this capability.

To achieve the data link portion of the Thunderbolt interface, Intel used the standard PCI-Express specification. Using the PCI-Express interface was a logical move because it was used as a connector interface for connecting internal components in a processor.

Thunderbolt, Meet Apple

For Apple, Thunderbolt was an exercise in reducing cord clutter. Ultraportable laptops like the MacBook offer limited space for external peripheral connectors. With Thunderbolt, Apple combined data and video signals into a single connector. The data signal portion of the Thunderbolt cable allowed the display to use USB ports, a FireWire port, and a Gigabit Ethernet over a single cable.

More Than One Device in One Port

Thunderbolt can run multiple devices from a single peripheral port because of its daisy chain functionality. For this to work, the Thunderbolt peripherals must have an inbound and an outbound connector port.

The first device on the chain is connected to the computer. The next device in the chain connects its inbound port to the first device's outbound port. Then, the pattern repeats for each subsequent device in the chain. Alternatively, you can use a Thunderbolt dock to connect multiple devices to your computer using a single port.

There are limits to the number of devices that can run on a single Thunderbolt port. The standard (including Thunderbolt 3 and 4) allows for up to six devices to be daisy-chained. If you connect too many devices, it can saturate the bandwidth and reduce the overall performance of the peripherals.

DisplayPort Compatibility

Thunderbolt ports are fully compatible with DisplayPort standards to maintain compatibility with traditional DisplayPort monitors. This means that any DisplayPort monitor can be attached to a Thunderbolt peripheral port. However, it renders the Thunderbolt data link on the cable inoperable.

Because of this, companies such as Matrox and Belkin designed Thunderbolt base stations for computers that allow for a DisplayPort pass through. This way, the PC can connect to a monitor and use the data capabilities of the Thunderbolt port for Ethernet and other peripheral ports.

PCI-Express

With the PCI-Express data bandwidths, a single Thunderbolt port can carry up to 10 Gbps in both directions. (Thunderbolt 3 and 4 support up to 40 Gbps overall bandwidth, which includes the DisplayPort signal.) This is more than enough for most peripheral devices that a computer would connect to. Most storage devices run below the current SATA specifications, and solid-state drives can't achieve these speeds.

Most local area networking is based on Gigabit Ethernet (1 Gbps), which is a tenth of the bandwidth provided by a 4-way PCIe connection. Consequently, Thunderbolt displays and base stations typically provide peripheral ports and pass through data for external storage devices.

How Thunderbolt Compares to USB and eSATA

USB 3.0 is the most prevalent of the current high-speed peripheral interfaces. It has the advantage of being compatible with all the backward USB 2.0 peripherals. However, it is limited to one port per device unless a USB hub is used.

USB 3 offers full bi-directional data transfers, but the speeds are roughly half that of Thunderbolt at 4.8 Gbps. It doesn't specifically carry a video signal the way that Thunderbolt does for DisplayPort. It is used for video signals through either a direct USB monitor or a base station device, which breaks out the signal to a standard monitor. The downside is that the video signal has a higher latency than Thunderbolt with DisplayPort monitors.

USB4 doubles the transfer speeds of USB 3.0. At 40 Gbps, it is on the same footing as Thunderbolt 3 and 4, both of which are compatible with USB4.

Thunderbolt is more flexible than the eSATA peripheral interface. External SATA is only functional for use with a single storage device. The current eSATA standards max out at 6 Gbps compared to the 10 Gbps of Thunderbolt.

Thunderbolt 3

Released in 2015, Thunderbolt 3 built on the ideas of previous versions. Rather than using DisplayPort technology, Thunderbolt 3 is based on USB 3.1 and its new Type-C connector. This opened up new possibilities, including the ability to transfer power as well as data signals.

Conceivably, a laptop using a Thunderbolt 3 port could be powered through the cable while also using the cable to send video and data to a monitor or base station. Transfer speeds for Thunderbolt 3 top out at 40 Gbps, which is more than enough to simultaneously power multiple devices.

Thunderbolt 4

Announced in early 2020, with devices appearing on shelves later in the year, Thunderbolt 4 did not add any speed to Thunderbolt 3. Still, it did improve the specs in several ways.

The Thunderbolt 4 protocol can support two 4K displays instead of one, or a single 8K display. Cords can be up to two meters long. It also includes several minimum standards for peripheral devices, including wake-from-sleep support for docks, power ratings for laptop charging, and protection against Thunderspy attacks.

Thunderbolt 4 is fully compatible with USB4 protocol and data rates. This cross-compatibility has generated confusion, namely that ports for Thunderbolt 4, Thunderbolt 3, and USB4 are visually indistinguishable.