Thunderbolt (originally codenamed Light Peak) is an interface for connecting peripheral devices to a computer via a peripheral bus. Thunderbolt was developed by Intel and brought to market with technical collaboration from Apple Inc. It was first introduced commercially on Apple's updated MacBook Pro lineup on 24 February 2011, using the same port and connector as Apple’s Mini DisplayPort.

Thunderbolt is intended to be a replacement for many or all of the current high-speed buses in modern PC platforms. In this respect, Thunderbolt can be thought of as a modern analog to USB, which replaced many lower-speed ports and greatly reduced port and cable confusion of earlier systems. Unlike USB, Thunderbolt has the performance needed to support the very latest high performance systems like eSATA and FireWire, and through the implementation of the driver chip, DisplayPort as well. Thunderbolt has more than enough bandwidth and low enough latencies to support any common protocol, typically many of them on a single daisy chained cable.

Originally known under the code-name “Light Peak,” the system was originally designed to use an optical cabling system based on flexible optical fiber. During development, a version using conventional copper wiring was also developed, and improved so that it met the targeted 10 Gb/s bandwidth at a lower cost. This also allowed the system to drive other copper-based ports, and was combined with the DisplayPort plug to allow a single expansion port to drive any external device. A single Thunderbolt port supports a daisy chain of up to seven Thunderbolt devices; up to two of these devices may be high-resolution displays. ^[5]

Thunderbolt essentially PCI Express into a new serial data protocol that can be carried over longer, less costly, cables, thus facilitating the connection of computers and peripherals. As most of Intel's modern chipsets include PCI Express support built-in, this allows Thunderbolt to be added to existing products with relative ease. Likewise, PCI Express is widely supported among device vendors, who can support the new standard with less development overhead. Other high performance ports, like eSATA, are often interface to the computer on the PCI Express bus, many times using a PCI Express expansion card; devices using this style of interface can be easily adapted to Thunderbolt. Intel's implementation of the port adaptor folds Thunderbolt and DisplayPort data together, allowing both to be carried over the same cable at the same time.

Intel first introduced Light Peak during a demonstration at the 2009 Intel Developer Forum (IDF). Their demonstrations were being run on a prototype Mac Pro motherboard, using Light Peak to run two 1080p video streams, LAN and storage devices over a single 30 m long optical cable with USB ends modified to carry Light Peak. ^[6] At the show, Intel claimed that Light Peak equipped systems will begin to appear in 2010. ^[7] The system was driven by a prototype PCI Express card, ^[8] with two optical buses powering 4 ports. At the time there was much speculation about Apple's role in the development of the system. Intel and Apple both commented on some sort of collaboration, but these comments were not specific.

On 4 May 2010, in Brussels, Intel demonstrated a laptop with a Light Peak connector (indicating that the technology had shrunk to small enough to fit inside such a device) and had the laptop send 2 distinct simultaneous HD video streams down the connection (indicating that at least some fraction of the software/firmware stacks and protocols are now functional). At the same demonstration, Intel maintained that it expected hardware manufacturing to begin around the end of 2010. ^[9]

In September 2010, some early commercial prototypes from manufacturers were demonstrated at Intel Developer Forum 2010. ^[10]

In December 2010 it was announced that Intel and AMD, with backing from various computer vendors, would stop supporting DVI-I, DVI-A, VGA and LVDS-LCD technologies from 2013/15 ^[11] [[d:Special:EntityPage/QIDC'S FIGURES SHOW DISPLAYPORT WAS ON 5.1 PERCENT OF COMMERCIAL DESKTOPS IN 2009, BUT THAT FIGURE WILL GROW TO 89.5 PERCENT OF THEM IN 2014. IN COMMERCIAL NOTEBOOKS, DISPLAYPORT'S PENETRATION WILL INCREASE FROM 2.1 PERCENT IN 2009 TO 95 PERCENT IN 2014. ONLY 24.5 MILLION OF THE 427 MILLION LAPTOPS IN USERS' HANDS IN 2014 WILL BE VGA-ENABLED, DAOUD STATED. ANOTHER 279 MILLION WILL USE HDMI, WHILE 167 MILLION WILL USE DISPLAYPORT.| (QIDC'S FIGURES SHOW DISPLAYPORT WAS ON 5.1 PERCENT OF COMMERCIAL DESKTOPS IN 2009, BUT THAT FIGURE WILL GROW TO 89.5 PERCENT OF THEM IN 2014. IN COMMERCIAL NOTEBOOKS, DISPLAYPORT'S PENETRATION WILL INCREASE FROM 2.1 PERCENT IN 2009 TO 95 PERCENT IN 2014. ONLY 24.5 MILLION OF THE 427 MILLION LAPTOPS IN USERS' HANDS IN 2014 WILL BE VGA-ENABLED, DAOUD STATED. ANOTHER 279 MILLION WILL USE HDMI, WHILE 167 MILLION WILL USE DISPLAYPORT.)]] ^[11]

At the time, it appeared that one port that Light Peak would not support DisplayPort, at least in the initial releases. The HDMI and DisplayPort connectors require more than twice the 10 Gb/s that Thunderbolt was proposed to include in its higher-end implementations. Given announcements that Intel would support DisplayPort 1.2 and HDMI 1.4a exclusively, it seems unlikely that Thunderbolt would compete with them in the near future:

Legacy interfaces such as VGA, DVI and LVDS have not kept pace, and newer standards such as DisplayPort and HDMI clearly provide the best connectivity options moving forward. In our opinion, DisplayPort 1.2 is the future interface for PC monitors, along with HDMI 1.4a for TV connectivity.

— AMD, Dell, Intel Corporation, Lenovo, Samsung Electronics and LG ^[12]

Originally, "Intel said it's working on bundling the optical fibre with copper wire so Light Peak can be used to power devices plugged into the PC" ^[13] and so seemed to compete with Power over Ethernet (PoE), USB and G.hn in this regard. However the clear announced intention as of 2010 ^[2] was "to have one single connector technology" that would allow "electrical USB 3.0 […] and, or, other protocols could, down the road, be run over optical" suggesting that Thunderbolt is a bus rather than an interface and further suggesting that Thunderbolt would piggyback on USB 3.0 or 4.0 DC power and possibly also (to support monitors) pass through AC (possibly IEEE P1901).

In January 2011, Intel's David Perlmutter told Computerworld that initial Thunderbolt implementations would be based on copper wires. ^{[14] [15]}

It was long rumoured that the early-2011 MacBook Pro update would include some sort of new data port ^[16], and most of the speculation suggested it would be Light Peak. ^[17] At the time there was no details on the physical implementation, and mock-ups appeared showing a system similar to the earlier Intel demos using a combined USB/Light Peak port. ^[18] Shortly before the release of the new machines, the USB implementers forum announced they would not allow this, stating that USB was not open to modification in this way.

In spite of these comments and speculation, the introduction came as a major surprise when it was revealed that the port was based on DisplayPort, not USB. As the system was described, Intel's solution to the display connection problem became clear: Thunderbolt controllers fold data from existing DisplayPort systems with data from the PCI Express port into a single cable. Older displays, using DisplayPort 1.1 or earlier, have to be located at the end of a Thunderbolt device chain, but newer displays can be placed anywhere along the line. ^[19] Thunderbolt devices can go anywhere on the chain. In this respect, Thunderbolt shares a relationship with the older ACCESS.bus system, which used the display connector to support a low-speed bus.

During a press conference after Apple's introduction, Intel stated that two cabling systems will be directly supported, the copper system introduced on the MacBook Pro, and an optical system for long runs. In the case of the copper cabling, 10 W of power is supplied to feed downstream devices, whereas the optical cabling will not support power of any sort. Copper wiring can extend about 3 m, fibre systems can extend "tens of meters". ^[19]

Thunderbolt is based on the Mini DisplayPort connector developed by Apple. This is electrically identical to "normal" DisplayPort connectors, but uses a smaller connector that is more suitable for use on laptops and other consumer devices. It is expected that Thunderbolt's use of this connector will drive wider acceptance, which is currently limited to Apple devices. ^[19] As Thunderbolt combines DisplayPort and PCIe data, it cannot be offered as a standalone PCIe card - the PCIe bus does not carry the video streams. Instead, Thunderbolt will have to be implemented on graphics cards, which do have access to the DisplayPort data, or on the motherboard of new devices, like the MacBook Pro. ^[19] It is expected that this will slow acceptance of the new system.

Thunderbolt controllers on the host and peripherals are responsible for folding the PCIe and DisplayPort data together and then unfolding them after they exit the cable. ^[1]

A future version is planned to use optical fiber cable containing two 62.5 micron wide fibers that can transport the infrared signal up to 100 metres. ^[20] The conversion of electrical signal to optical will be embedded into the cable itself, allowing the current display port socket to be future compatible, ^[21]but eventually Intel hopes for a purely optical transceiver assembly embedded in the PC.

Most commentators^{[ who?]} believe that Thunderbolt, if it is to succeed, must replace single-use types of connectors such as HDMI, DisplayPort (which requires more than double the initial 10 Gb/s in its higher end incarnations – see list of device bandwidths), eSATA, at least some uses of Ethernet and ultimately the smaller and slower speed USB devices as cost of a Thunderbolt connector comes down.

USB 3.0 is the latest incarnation of the USB standard, supporting transfer speeds up to 5 Gbit/s. The higher speeds required changes to the cabling in order to reach higher speeds, but older devices can be plugged into a USB 3.0 port, and USB 3.0 devices can use older USB ports at those port's slower speeds. This means that USB 3.0 has backward compatibility with the enormous number of existing USB devices.

However, the USB protocol has a variety of problems when operating at high speeds. The system is host-driven, and relies on messages flowing from the host to keep the data flowing. This introduced high latencies into the system, so high that it seriously effects the maximum throughput. Even at USB 2.0's 480 Mbit/s speeds, throughput only reaches 50% of the maximum, and often much lower. These problems will be further exacerbated in USB 3.0.

In comparison, Thunderbolt offers twice the peak speed and two independent busses. In theory, a single Thunderbolt port thus has four times the throughput of a USB 3.0 port. In practice, the lower latency, on the order of 8 ns even at the end of the chain, ^[19] and very lightweight PCI Express protocol should offer performance much closer to the theoretical maximums. Intel demonstrated throughputs at 62.5% of the peak using prototype products. ^[19]

In performance terms, the most direct and relevant comparison is to 10 gigabit per second Ethernet. A typical offering such as that of Neterion ^[22] supports one or two ports on a PCIe 8× card. A short reach (300 m) or long reach (10 km) optical Ethernet cable is supported, and up to 100 m using copper twisted-pair cables, similar to Thunderbolt's promised 100 m. Power is a major factor limiting cable reach because regardless of how data is carried, a long, low-voltage, power cable suffers significant loss and is impractical much beyond 5 m.

However, 10 Gb Ethernet switching is extremely expensive and while it could come down in price (as 1 Gb Ethernet did) in response to competitive pressures (from Thunderbolt and USB 3.0), the price-insensitive backbone market tends to dominate 10 Gb.

Tunneling USB, SATA, HDMI/DVI, or other protocols over Ethernet is problematic. There is AoE, iSCSI and NBD, but support in regular Firmware is not commonplace. Storage devices with Ethernet ports need a network card with specialized firmware to be used as a boot device. Tunneling HDMI/DVI also currently requires hardware support, like what is offered by companies like AdderLink Infinity and Just Add Power HDMI over Ethernet. Some makers - like BeaconTech.com.tw - even go further and provide KVM solutions that can tunnel VGA/HDMI/DVI over Ethernet but also USB 2.0 devices of any kind, audio, serial and other connections. It is interesting that most of these ways of tunneling do not require a switch and both emitter and receiver can be connected together with cheap Cat-5 Ethernet cables. Another way of doing this is exploited by Teradici corporation, where a PCI-e card is added to the computer and then everything (up to 4 single-link DVI/HDMI, audio in/out, USB) is carried out over IP.

Because it is a general purpose bus, PoE (specifically IEEE 802.3at and IEEE 802.3af) at 10 Gb would be Thunderbolt's most direct competitor with some advantages over any new fiber-based non-power-carrying cable protocol:

• 10 Gb PoE will rely on widely available, cheap cabling ( Cat 6a).
• PoE provides up to 30 watts DC at variable voltage (1.5 V to as much as 80 VDC) which is sufficient for many devices including smaller monitors hooked up to PCs.
• PoE has an obvious upgrade path to 10 Gigabit Ethernet compatibility which is the same data rate as Thunderbolt – it is compatible with Gigabit Ethernet and will be compatible with 10 GbE and 100 GbE.
• PoE devices are already widely deployed.

Thunderbolt could turn out to be a security vulnerability of virtually unlimited exploit potential. What type of unlimited exploit potential exactly? ^[23]

• Optical interconnect
• Parallel optical interface
• Optical communication
• Interconnect bottleneck
• Optical fiber cable
• IEEE 1394 interface (FireWire)
• USB

• Thunderbolt technology at Intel.com