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Explaining Thunderbolt 3, USB-C, and Everything In Between

by Glenn Fleishman

The first Mac with Thunderbolt 3 [1] is now available, the non-Touch Bar 13-inch model from Apple’s new series of MacBook Pro laptops (see “New MacBook Pros Add Context-sensitive Touch Bar [2],” 27 October 2016); the rest of the models ship soon. Thunderbolt 3 relies on the USB-C physical connector and, with the appropriate adapters, supports nearly all common peripheral-connection and networking protocols, including USB 2, USB 3, FireWire, Thunderbolt 2, Ethernet, and DisplayPort, and by extension, HDMI, DVI, and VGA.

The reason confusion afflicts this space is that a USB-C port on another computer may support just USB, USB plus display and networking protocols, or all of that plus Thunderbolt 3. The 12-inch MacBook’s USB-C port, for instance, natively supports USB 2 and USB 3 along with DisplayPort and, via adapters, VGA, HDMI, and Ethernet connections, but not Thunderbolt 2 or FireWire.

The summary for potential late 2016 MacBook Pro owners is that all current USB-C devices, cables, and adapters should work when plugged into a MacBook Pro’s Thunderbolt 3 ports. However, Thunderbolt 3-specific devices won’t work with computers and other devices like the 12-inch MacBook whose USB-C ports are less capable. Now, let’s drill down into details.

How These Standards Relate to Each Other -- USB-C is a standard developed by the USB Implementors Forum [3] to modernize and coalesce USB into a single plug style that supports a variety of data, video, and power options for ports, cables, and adapters. It’s reversible, just like the Lightning plug Apple uses for iOS devices and recent peripherals like the Apple Pencil and Magic Mouse 2.

Support for high-wattage and high-amperage cables allows USB-C chargers to charge laptops and other devices. While previous versions of USB allowed for high power flow, USB-C is the first version in which laptops take advantage of that, allowing external charging from an AC adapter or external battery. USB-C also offers bidirectional power, so a MacBook Pro could recharge itself via an external battery while also charging an iPhone.

(Although we’re focusing here on USB-C, or more formally, USB Type-C, USB also supports other plug types [4]. These include USB Type-A, the standard rectangular USB plug we’re all accustomed to; USB Type-B, the squarish USB plug often used by large peripherals; and the smaller Mini-A, Mini-B, Micro-A, Micro-B, and Micro-B SuperSpeed plugs.)

USB-C appeared first in 2015 in the 12-inch MacBook, and shortly thereafter in equipment from Google and others. It has spread gradually, but it is still not the connection of choice for laptops and mobile phones. The third-party peripheral market, especially for docks and high-quality cables, started to accelerate only in early 2016.

Thunderbolt 1 and 2 use the same physical connector as Mini DisplayPort, which allowed a jack to fit easily into a thin laptop. Natively, they support two protocols: Thunderbolt (originally built on top of PCI Express) for general data and DisplayPort for video. With adapters, they can also carry USB, FireWire, and Ethernet.

Although Intel developed Thunderbolt jointly with Apple, Intel seems to control its future. In June 2015, Intel and the USB Implementors Forum announced that Thunderbolt 3 would rely on the USB-C plug style. That was good news: one fewer cable type and all ports would become multi-purpose! How this works has to do with abstraction: the hardware no longer defines a single associated communications protocol.

USB-C and DisplayPort exist both as a physical specification for connectors and cables, and also as a logical protocol that defines how data moves across a data bus. The USB-C data bus has a number of channels (called “lanes”) that can be assigned and configured to different protocols, depending on the USB-C controller hardware in the host computer or other device, including USB, DisplayPort, PCI Express, and Thunderbolt. DisplayPort can either use its own connector type or be encapsulated and carried by other standards, which include USB-C, whether or not the controller hardware supports Thunderbolt. This capability of DisplayPort made it possible for Thunderbolt to use the Mini DisplayPort connector type and be backward compatible with existing DisplayPort devices.

Thunderbolt allows daisy chaining — plugging one peripheral into the next — though DisplayPort monitors have to be at the end of such a chain. Though I can’t find a definitive answer, it appears that you cannot daisy chain USB devices connected via USB-C, although you can simulate daisy chaining with the addition of USB-C–connected hubs. Power, which is effectively a different kind of protocol in USB-C, can be passed through multiple devices.

When it comes to protocols, USB-C natively supports:

USB-C can create the appropriate electrical signals for these natively supported standards internally and pass them through a cable with a USB-C plug on one end and the native format, like DisplayPort or USB 3 Type-A, on the other. Some of these are in the form of a dongle with a jack on the non-USB-C end, but it’s still sending the signal straight through.

For other protocols, you need an adapter, which performs internal signal conversion between USB-C and the adapter’s input port, like HDMI, VGA, Ethernet, and FireWire. (The HDMI trade group added USB-C as an option recently and says that monitors with native USB-C support will be out in 2017. For now, Ethernet is supported directly only in a 10 Gbps peer-to-peer version that’s part of Thunderbolt 3.)

USB-C can drive at least a single 4K monitor, depending on the display circuitry on the host device. A computer with Multi-Stream Transport (MST) [5] can drive two displays from a single USB-C port. Apple built MST into some Macs, but the 12-inch MacBook lacks it, and thus can only handle one external monitor. Thunderbolt 3 has more robust display support, enabling it to use higher refresh rates and manage significantly more pixels overall.

Thunderbolt 3 cables are labeled with the same lightning logo used by Thunderbolt 2 cables, while USB cables with USB-C connectors show the familiar USB logo and may also be branded with SS+ for SuperSpeed+. Older USB 3.0 (also known as USB 3.1 Gen 1) cables are sometimes also branded with an SS for SuperSpeed.

Happily, Thunderbolt 3, as supported by the new MacBook Pros, supports all the protocols handled by USB-C, plus Thunderbolt and FireWire. The end result is that nearly any device can be plugged into a Thunderbolt 3 port, with the correct cable, adapter, or dock, as I explain next.

(What kinds of ports are you using with a Mac laptop? You can fill out my survey [6] and see the current results.)

Adapt or Die! -- There’s both some FUD and reasonable caution about USB-C cables and adapters. Quality gear comes from the likes of Apple, Belkin, Google, and Kensington, although Apple’s products have historically been quite expensive. Apple recently cut prices of all its USB-C and Thunderbolt 3 accessories through 31 December 2016; see “Responding to Complaints, Apple Drops Adapter and Monitor Prices [7]” (4 November 2016).

You can often find less expensive gear made cheaply by little-known manufacturers and sold via Amazon (see “Be Careful When Buying Apple Accessories on Amazon [8],” 24 October 2016). Before buying from unfamiliar brands, I recommend that you consult Google engineer Benson Leung’s Amazon reviews [9], whether or not you make the purchase via Amazon. He has extensively tested cables, adapters, and other equipment in his personal time, and he can steer you towards and away from USB-C products.

Let’s assume you have or are planning to get a new MacBook Pro with Thunderbolt 3. Here’s a rundown of what you’ll need for each of the various connection types:

Real-World Scenarios -- Now that you know what the adapter options are, how do you apply them? There are two notable scenarios: moving data to a new MacBook Pro via the Setup Assistant or Migration Assistant, and working with Target Disk Mode.

Apple has published a support page explaining the possibilities [26] for migrating data to a 12-inch MacBook or late-2016 MacBook Pro; also be sure to read “How to Migrate to a New Mac [27]” (14 September 2016). Depending on the Macs in question, Apple suggests a few choices:

Target Disk Mode, a way to turn one Mac into a bootable drive for another Mac, is a somewhat different situation. Apple has updated its Target Disk Mode page [29] to include a mention of Thunderbolt 3. Apple is light on the specifics but seems to imply that you will need two matching ports and a cable to connect them. From this reading, only the 12-inch MacBook and the new MacBook Pros will be able to employ Target Disk Mode between themselves, using either a USB-only USB-C cable or a Thunderbolt 3 cable.

However, Target Disk Mode also reportedly worked with a Thunderbolt 2-to-FireWire adapter [30], so that combination may also work with an additional Thunderbolt 2-to-3 adapter, but we haven’t yet seen confirmation.

USB-Cing Stars -- This may all seem confusing initially, but it should pass quickly because everything on the market for USB and DisplayPort over USB-C today should work with Thunderbolt 3. The main group that will be disappointed are those who buy Thunderbolt 3 peripherals and expect them to work with a 12-inch MacBook, which doesn’t extend USB-C support to Thunderbolt. We can hope that Apple makes Thunderbolt 3 standard across the entire Mac line.

I anticipate that, now that Thunderbolt 3 is out and available in a mainstream Mac, other manufacturers will ship more new high-end computers with Thunderbolt 3 and USB-C. USB 3.1 Gen 2 tops out at 10 Gbps, which will be fine for lower-end systems, which don’t require 40 Gbps performance and aren’t intended to support more than two displays. Mobile devices outside of the Apple ecosystem will stick with and continue to adopt USB-C without Thunderbolt 3 for simplicity, power consumption, and controller cost.

With nothing else like either USB 3.1 Gen 2 or Thunderbolt 3 on the horizon and the broad industry support of the USB-C connector, USB is finally living up to the Universal part of its name — even when Thunderbolt is thrown in on top.