Apple has co-developed several systems for connecting peripherals to Macs, starting with Apple Desktop Bus (ADB) in 1986. At the same time, it was working on a more generic high-speed interface which became FireWire then IEEE 1394. Despite its widespread adoption for digital video cameras, USB became more popular. Recognising the need for something faster and more capable, and with Apple’s involvement, Intel developed what it initially termed Light Peak prior its release in 2011, when Apple also released its first MacBook Pros with Thunderbolt 1 ports.
Thunderbolt is designed to be easily supported, as it’s fundamentally a combination of PCI Express (PCIe) and DisplayPort, with substantial power supply. As many Macs and PCs already offered separate interfaces compliant with those standards, and its connectors were Mini DisplayPort standard too, it required minimal hardware redesign to accommodate Intel’s new Thunderbolt controllers. Even so, it still hasn’t proved as popular as USB on PCs or peripherals.
Thunderbolt 1 and 2 offer PCIe 2.0 x4 and DisplayPort 1.1a (TB1) or 1.2 (TB2), for a total of 20 Gb/s throughput over its Mini DisplayPort connectors. Thunderbolt 3 enhances those to PCIe 3.0 x4, two streams of DisplayPort 1.2, and USB 3.1 Gen 2, for a total of 40 Gb/s in both directions, or 80 Gb/s one way, and changes to USB-C connectors. Thunderbolt 4 improves DisplayPort support to 2.0, with USB4, which is essentially Thunderbolt 3. Although this is simpler and clearer than USB in terms of numbering and compliance, its support for the multiplicity of USB standards has become recursive and confusing. Greatest clarity comes in its bus power support, which delivers up to 18 V and 550 mA for a total of 9.9 W.
As a hybrid, few peripherals benefit from the whole of its available 40 Gb/s, as performance is constrained by PCIe and DisplayPort capabilities. For Thunderbolt 3, now the most widely supported alternative to USB, this means PCIe 3.0, which should deliver 985 MB/s for each of its four lanes, giving a maximum data transfer rate of 3.94 GB/s for all four.
DisplayPort support is also not straightforward. In theory, it should have a maximum total bandwidth at HBR2 of 21.6 Gb/s over four lanes, which represents a data rate of 2.16 GB/s. In practice, as priority is given to DisplayPort connections, they tend to be maintained at the cost of PCIe data transfer rates, and can account for some of the performance effects seen on heavily loaded Thunderbolt buses.
The original design of Thunderbolt envisaged connections being made over expensive optical cables, which also couldn’t deliver power to peripherals. By January 2011, Intel accepted the compromise to deliver shorter cables using copper wire, ensuring the delivery of power and modest cost using (now) the USB-C format. Cables up to a metre in length, including the de facto standard of 0.8 m, are passive and don’t require active signal conditioning, which is only required on longer ones. Optical cables are available in lengths up to 50 m. Certified Thunderbolt 4 cables are also fully compatible with USB 3.x and USB4 devices.
Until relatively recently, one limitation with Thunderbolt was the absence of hubs which allowed the connection of multiple Thunderbolt devices to a single port on the Mac. Several models are now available, although they’re considerably more expensive than USB hubs. Inevitably, these are constrained by the bandwidth available in the single Thunderbolt connection made to the Mac. Connect a couple of external displays and those will limit the performance of SSDs connected to the same hub.
With only two Thunderbolt standards (3 and 4) being actively supported, they’re simpler for users to negotiate than USB standards. Certification and marking programmes and Intel’s support chips have also led to greater uniformity.
Unfortunately, like all peripheral buses, Thunderbolt doesn’t deliver the transfer rates you might think it promises. You could naïvely assume that, at 40 Gb/s, it might be possible to read external storage or other devices at rates approaching 5 GB/s. As explained above, the absolute maximum possible would be slightly less than 4 GB/s because data transfer is performed to PCIe 3.0. In practice, even when using an SSD compliant with PCIe 4.0 and with all four lanes available, transfer rates are usually limited to 2.5 GB/s, and 3 GB/s appears impossible. Perhaps this is all the fault of the two words which invariably appear in such performance claims, “up to”.
You should also be wary of the fact that many Thunderbolt 3 peripheral interfaces don’t support transfer using all four PCIe lanes. Less expensive SSD cases and enclosures usually only support a maximum of two lanes, which limits transfer rates to well below 2 GB/s, and may be as low as 1.6 GB/s. It’s that “up to” again.