A guide to understanding USB Type-C

One port to confuse them all.

Apple has done it again. After courageously dropping the headphone jack from the iPhone, they have now continued their war against legacy ports and killed every port that was on the 2015 MacBook Pros (except, curiously, the headphone jack). Instead of HDMI, regular-sized USB, SD-card slots and Thunderbolt, there is now only one: USB-C.

Good arguments can be made (and certainly have been made) in favor of four multi-purpose ports instead of a singular-purpose legacy ports. One can cry over the omission of an SD card slot as a slap in the face of all creative professionals, something that Apple with all its engineering prowess should find space for. But find it they did not and here we are. With ports from the future causing frustration in the present.

The problem is: USB Type-C mostly relates to the form factor. It describes the reversible plug and port, but not necessarily what goes through. To add to the confusion, Apple sells these ports as Thunderbolt 3 ports on the 2016 MacBook Pro, but on the 2015-2016 Macbook they don’t even support Thunderbolt – while still using the same USB-C plug. So what’s going on?

Thunderbolt

First of all, there is Thunderbolt. First introduced in 2011, Thunderbolt 1 shared the same physical plug with Mini DisplayPort on the MacBook Pro. Two years later, Apple shipped Thunderbolt 2 with the 2013 MacBook Pro, which doubled the speed to 20 Gbit/s but still relied on the same Mini Displayport plug. They incorporate DisplayPort as their standard for video, with TB2 being able to drive a 4k Display (using Displayport 1.2).

The Thunderbolt 1/2 and Mini DisplayPort plug (Source: Apple).

Thunderbolt 3 again doubles the bandwith to 40 Gbit/s, but now abandons Mini DisplayPort in favor of the USB Type-C plug – the same plug that is meant to be the future of USB.

USB

USB has gone through similar transitions in the past, up to the point where USB 3.0 had become pretty much the standard for all current universal connections. The familiar standard Type-A plug with the blue colored insert comes with 5 Gbit/s of bandwidth. Since then USB 3.1 Gen.2 was introduced with a maximum bandwith of 10 Gbit/s, and in order to maximize confusion USB 3.0 (with the 5 Gbit/s bandwidth) is now called USB 3.1 Gen.1. But it’s still the same plug.

USB 2.0 on the left and USB 3.0, 3.1 Gen.1/Gen.2 on the right.

With the advent of mobile devices also came the smaller USB plugs, Micro USB being the most common smartphone port outside of the Apple universe. These however are usually USB 2.0 (maximum bandwidth 480 Mbit/s, or 0.48 Gbit/s), which is why there also exists a Micro-B plug that supports USB 3.0 (the flat plug that most external hardrives use).

USB 2 Micro on the left (USB 2.0) and USB 3 Micro-B on the right (USB 3.1).

Enter USB Type-C – the one plug that does it all. It’s now similar in form factor to Micro USB, but does so much more.

The future!

Let’s start with the 2015-2016 MacBook: the infamous single port notebook. That is a great place to start because, while being USB-C like the 2016 MacBook Pro, does not support Thunderbolt 3. In fact, it only supports USB 3.1 Gen.1 and maxes out at 5 Gbit/s. So how can a singular port that only supports USB protocol drive displays as well?

The answer is Alternate Modes, for which there are dedicated physical wires within the USB-C cables. USB Type-C supports various other protocols, most prominently DisplayPort Alternate Mode, HDMI Alternate Mode and (although not on the MacBook) Thunderbolt Alternate Mode. In addition, as part of the USB 3.1 standard, there are new rules for USB Power Delivery that support up to 100W being delivered over the cable, at various voltages and a current of 3A or 5A (the last part is important for cables).

The 2016 MacBook Pro supports all of these, including USB 3.1 Gen.2 (10Gbit/s) and Thunderbolt 3 (up to 40Gbit/s). In theory there might now be Thunderbolt 3 USB-C devices that will work seamlessly with the MacBook Pro, but not with the MacBook – even though they have the same plug. It remains to be seen how often that might be the case. Bar a few exceptions like external graphics that rely on direct PCIe access only Thunderbolt can provide, most devices will probably have USB 3.1 fallback (e.g. external storage) or the ability to use Alternate Modes for display. But there will be cases where some accessories simply won’t work on all devices with USB-C – the “if it fits it works” approach that made USB so popular is no more.

Cables

So what to do with all that information? Chances are you came here because you are looking into accessories and are confused by the plethora of options. Most of these come with the cables: one quick search on Amazon reveals that there a millions of USB-C cables with dozens of different specifications. What holds true for most cables (a cheap HDMI cable will work the same as a gold plated 200$ one) may not hold true here: cables with “low” specifications” actually lack the physical wiring or controllers inside to support certain transfer modes. The bandwidth-top down approach should work here: cables that support Thunderbolt 3 at 40 Gbit/s will generally also support USB 3.1 Gen2. and Alternate Modes (Thunderbolt being one of them), which in turn will work for 3.1 Gen.1 and so forth.

The top-of-the-line 40 Gbit/s USB-C cables are a good deal more expensive and shorter in length than the 20 Gbit/s cables. To support the full bandwith, the cables need an extra controller inside and are called active cables, whereas the slower ones are often marked as passive – and thus cheaper. But unless you plan on using the cable for external graphics cards like the Razer Core or connect 5k displays, the passive cables will support enough bandwidth for almost everything. No need to splurge.

Cables without Thunderbolt support (so neither 40 Gbit/s nor 20 Gbit/s) will usually be marked USB 3.1 Gen.1 (5 Gbit/s) and support Alternate Modes for video output, but no Thunderbolt. And then there are the USB 2.0 cables (480 Mbps). Unless you’re just want one charge your phone, don’t get these. They are slow and almost always the same price as the USB 3.1 cables.

But be careful though about power delivery: USB-C to USB-C cables are generally required to support at least 60 Watt (3A at 20V), cables that support 100 Watt will need to support 5A at 20V. That’s why a lot of USB-C to USB-C cables will be rated for 60W, but only a few for 100W. If you’re looking for a cable to charge your 15-inch MacBook Pro or other Quad Core-laptops, make sure they support 100W or 5A – or just get the one from Apple.

More problematic are adapter cables. USB-C to USB-A cables might not meet the Power Delivery rules and can actually damage your devices. These were never meant to support the high currents and that can lead to all sorts of problems where a USB-C phone might ask for more power than the other device or charger can handle. In that case it’s better to stick to USB-C to USB-C cables, because those will always be to spec (at least from the more respectable companies).

So, TL:DR, the current situation is a bit messy but once the standard becomes more established things will probably work out.