What Is Thunderbolt?

The high speed peripheral port for data and video

Thunderbolt is a hardware interface developed by Apple and Intel. The Thunderbolt interface allows users to connect peripherals such as iPhones and external hard drives to their computers.

Thunderbolt logo

Thunderbolt Defined

During its early stages of development, Thunderbolt was called Light Peak. Light Peak was originally intended to be an optical interface standard, but Thunderbolt dropped that in favor of more traditional electrical cabling. While this decision put limitations on how the cabling works, it made Thunderbolt much easier to implement.

Rather than relying on a new connector, the Thunderbolt technology was initially built upon the DisplayPort technology and its mini-connector design. The reason for doing this was so that a single cable could carry a video signal in addition to the data signal. DisplayPort was a logical choice among the video connector interfaces because it already had an auxiliary data channel built into its specification. The other two digital display connectors, HDMI and DVI, lack this capability.

Benefits of Thunderbolt

Ultraportable laptops such as the MacBook offer very limited space for external peripheral connectors. By using Thunderbolt, Apple was able to combine both data and video signals into a single connector. The data signal portion of the Thunderbolt cable allows for the display to utilize USB ports, a FireWire port, and a Gigabit Ethernet over a single cable. This goes a long way to reducing the overall clutter of cables coming out of the laptop and expands the overall capabilities as both physical Ethernet and FireWire ports are not featured on ultrathin laptops.

Thunderbolt and DisplayPort Compatibility

In order to maintain compatibility with traditional DisplayPort monitors, the Thunderbolt ports are fully compatible with the DisplayPort standards. This means that any DisplayPort monitors can be attached to a Thunderbolt peripheral port; however, it will render the Thunderbolt data link on the cable inoperable. Because of this, companies such as Matrox and Belkin are designing Thunderbolt base stations for computers that allow for a DisplayPort pass through. That way, the PC can connect to a traditional monitor and still utilize the data capabilities of the Thunderbolt port for Ethernet and other peripheral ports via the base station.

Use More Than One Device in One Port

Another feature that made its way into the Thunderbolt specification is the ability to utilize multiple devices from a single peripheral port thanks to the daisy chain functionality that was introduced with FireWire. In order for this to work, the Thunderbolt peripherals must have both an inbound and outbound connector port.

The first device on the chain is connected to the computer. The next device in the chain would connect its inbound port to the first's outbound port. Each subsequent device would be connected similarly to the previous item in the chain. Alternatively, you can use a Thunderbolt dock to connect multiple devices to your computer via a single port.

There are limits to the number of devices that can be put on a single Thunderbolt port. The Thunderbolt 3 standards allow for up to six devices to be put in a chain. If you connect too many devices, it can saturate the bandwidth and reduce the overall performance of the peripherals. This is most apparent with the current standard when multiple displays are attached to a single chain.

Thunderbolt and PCI-Express

To achieve the data link portion of the Thunderbolt interface, Intel decided to use the standard PCI-Express specifications. Essentially, Thunderbolt merges together a PCI-Express 3.0 x4 interface to the processor and combines this with the DisplayPort video and puts it over a single cable. Using the PCI-Express interface is a logical move as this is already used as a standard connector interface on processors for connecting to internal components.

With the PCI-Express data bandwidths, a single Thunderbolt port should be able to carry up to 10 Gbps in both directions. This is more than enough for most current peripheral devices that a computer would connect to. Most storage devices run well below the current SATA specifications, and even solid-state drives can't achieve such speeds.

Most local area networking is based upon Gigabit Ethernet, which is just a tenth of this overall bandwidth. Consequently, Thunderbolt displays and base stations are typically able to provide peripheral ports and still be able to pass through data for external storage devices.

How Thunderbolt Compares to USB 3 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, but it is limited to one port per device unless a USB hub is used.

USB 3.0 offers full bi-directional data transfers, but the speeds are roughly half that of Thunderbolt at 4.8 Gbps. While it does not specifically carry a video signal the way that Thunderbolt does for DisplayPort, it can be used for video signals either through a direct USB monitor or via a base station device, which can break out the signal to a standard monitor. The downside is that the video signal has a higher latency than Thunderbolt with DisplayPort monitors.

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

Thunderbolt 3

The latest version of Thunderbolt builds upon the concepts of previous versions. Rather than using DisplayPort technology, it is based on USB 3.1 and its new Type-C connector. This opens up a number of new capabilities including the ability to offer power over the cable in addition to the data signals.

Conceivably, a laptop using a Thunderbolt 3 port could be powered through the cable while also using it 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 power multiple devices simultaneously.