USB and Thunderbolt
The dominant peripheral standards in 2022 are USB and Thunderbolt. These two standards have swept up nearly every other purpose inside their scope.
On a computer with just a USB 3 or USB4 port, you can pass data up to 40 Gbps, video, and power up to 240 W; on a device with Thunderbolt 4, you can also pass 40 Gbps of data, video, Ethernet, PCIe (for external GPUs and other high-speed purposes), and power up to 100 W. (Some day soon, you’ll be able to pass up to 80 Gbps in each direction or as much as 120 Gbps of video data in one direction.)
The big difference to you between the standards is whether, when you purchase a new computer or peripheral, it includes a USB4 or Thunderbolt 4 controller. It’s more likely your computer will include Thunderbolt 4, while peripherals other than docks and high-performance SSDs and drive arrays opt for USB, as it’s less expensive to build in.
In this chapter, I deal with the recent past and the present, in which slight variations and different generations of technology may leave you swearing at a port or cable.
Let’s avoid that by diving into where USB-C fits in and the innards the USB and Thunderbolt standards. Then we’ll move on to looking at the various ways to pass data and then power across cables, port adapters, and power adapters. But first, a few common scenarios you might encounter as a guide to understand the rest of the chapter.
Consider USB and Thunderbolt Scenarios
It always helps to ground technical stuff with practical concerns—and I know you’re reading this book because you have specific questions or are trying troubleshoot a connection. Let me lay out some typical scenarios people wrestle with:
“I plugged in a Thunderbolt cable into a USB-C jack on my computer and connected the other end to an SSD. The throughput is terrible! What’s happening?” Some USB 3.x-capable devices can’t pass data over some Thunderbolt 3 cables. If the cable is longer than 20 inches (50 cm) and uses active circuitry, it manages data with a USB controller on one or both ends at only USB 2.0 rates: 480 Mbps! Solve this problem with a Thunderbolt 4 cable or a USB 3.x or USB4 cable. See Thunderbolt Capabilities and Match Rates with Standards, Jacks, and Cables.
“I have an unmarked cable with USB-C on one end and a Type-A connector on the other. How fast can it transmit data?” Such a cable is limited to 5 Gbps with USB 3.1 standards and 10 Gbps with USB 3.2. It’s impossible to know without markings whether it can handle the higher speed. It will work at 5 Gbps between two Thunderbolt 3 jacks or USB 3.1 jacks, but if it’s capable of 10 Gbps, that speed will be supported by any combination of USB 3.2, USB4, and Thunderbolt 4 controllers. See USB Capabilities and Match Rates with Standards, Jacks, and Cables.
“This cable isn’t marked, but it has USB-C at both ends. What kind of cable is it?” You can know for sure only by plugging it in. Most such cables will allow 10 Gbps of throughput because they have USB-C on both ends, meaning they’re at least USB 3.1 Gen 2, or USB SuperSpeed 10 Gbps. The cable could, however, manage 20 Gbps or even 40 Gbps between ports with the right controllers. See USB Capabilities, Thunderbolt Capabilities, and Match Rates with Standards, Jacks, and Cables. But there is a rotten category of USB-C charging cables that are typically unmarked, may provide 15 W or 100 W of power, and only pass 480 Mbps of data. See Beware the USB-C Charging Cable: It’s Slow.
“I plugged in my laptop (or tablet) to a USB-C port—it’s only charging at a trickle, but the port says it should offer up to 100 W. What’s up?” A USB-C port might offer power between 7.5 W and 240 W. But if it’s rated at 100 W, the problem is likely the cable: you need a USB 3.x, Thunderbolt 3, or Thunderbolt 4/USB4 cable certified for 100 W. See USB and Thunderbolt Power.
Get To Know Bus Standards
The least-exciting sentence I could write at this point would be, “Let’s talk about standards!” But I promise I’m not digging down into wiring schematics, pinout diagrams, and 1,000-page protocol descriptions. Rather, let’s get up to speed on how USB and Thunderbolt work separately and together so you know how you can best use them—and troubleshoot them when incompatibilities arise.
USB-C’s Alt Flexibility
USB-C became the ideal connector for both USB and Thunderbolt standards because the specification was designed for diversity. Instead of being designed to carry data only via USB 3.x, the USB-C standard includes something called “Alternate Mode Functional Extension,” often abbreviated to Alt Mode. USB-C’s Alt Mode allows other protocols to work over the same wiring at the same time.
A controller on a host computer or peripheral defines the kinds of Alt Modes that they can deliver and accept, like DisplayPort, HDMI, and Thunderbolt. Two more obscure Alt Modes are MHL (Mobile High-Definition Link), largely used for connecting mobile devices to home entertainment systems, and VirtualLink, an emerging standard for VR headsets.
Even though Thunderbolt is an Alt Mode within USB-C, it can manage its own separate types of data. The main Thunderbolt data standard is PCIe, noted above in An Internal Bus Detour. But it can also carry DisplayPort separately from the USB-C Alt Mode and pass Ethernet for networking.
The Alt Mode extension allows these different specs to use both the physical part of USB-C (its wires) and the logical part (how data is packaged onto the wires) to carry different kinds of data simultaneously and seamlessly.
Seamless is the key word: in most cases, you don’t need to know much about the controllers in your devices. Nearly any USB-C adapter provides the kind of output necessary for the jack or plug it incorporates without you having to fiddle with computer or peripheral settings. Plug a USB-C adapter into a port on the host, and then plug in a DisplayPort, Ethernet, or USB-C cable into the adapter’s jack—and it just works.
The USB Standard
USB was at one point the great hope of the future: Universal! Serial! Bus! All three words pointed in the right direction. Instead of many serial connectors and buses (and even some parallel ports), USB would unify many kinds of purpose into one controller with a limited set of jacks and plugs designed for different purposes. All USB devices could plug into any USB port, given the right cable.
A lovely idea, but one that was ruined by the large number of USB plug types that emerged. There are currently nine kinds of USB connectors. But that’s not the only issue: even though you may be most familiar with the rectangular USB Type-A plug and jack, a USB cable cannot have a Type-A plug on both ends, whether full-sized, Mini, or Micro.
In a chart from Wikipedia’s extensive USB entry, shown in Figure 23, the eight kinds of USB connectors run across the top and down the side. The following points are worth noting:
Type-A can’t connect to Type-A: it’s noted as “Proprietary, hazardous” (the pale red squares) as it’s that dangerous and bad.
The profusion of red squares reading “No” also help tell the story about the lack of like-to-like and even unlike-to-unlike options.
Across the top, you can see Type-A was once the most compatible, connecting to all the B types (four of them) and then to USB-C.
However, the final column reveals the truth and the evolution: USB-C can connect to most formats—mostly importantly, to itself. (Mini-A and Micro-A are so rare, it’s not a real lacuna that USB-C can’t work with them.) I’ll return to USB-C in a moment.
USB Standards Evolution
As USB connector types proliferated, the underlying technical standard evolved, too. The evolution drove the newer connection formats.
USB standards developed in this progression:
USB 1.0 (1996): Few computers incorporated this early version, which allowed either 1.5 Mbps or 12 Mbps connections.
USB 1.1 (1998): This 12 Mbps-only update became the first widespread flavor, with Apple adopting it in its revolutionary all-in-one 1998 iMac. It was largely used for keyboards, mice, and other input devices.
USB 2.0 (2000): Performance jumped to 480 Mbps, making it feasible for external storage (Figure 24).
Figure 24: The current packaging for USB 1.1 and 2.0 looks like the above left and right. “Hi-Speed” feels quite quaint. USB 3.0 (2008): Skyrocketing tenfold to 5 Gbps, USB 3.0 was the last standard that relied entirely on the Type-A connector. Marketed as SuperSpeed, it also bumped up power flow from 150 milliamps (mA) to 900 mA, allowing for mobile device charging and bus-powered peripherals.
USB 3.1 (2013, 2014): The 3.1 update had two versions. The first, 3.1 Gen 1, effectively rebranded 3.0: same format, speed, and specs. The second, not released until 2014, introduced USB-C as 3.1 Gen 2, and offered a 10 Gbps maximum data rate.
USB 3.2 (2017): Just to add to the confusion, the 3.2 version supersedes 3.0 and 3.1, and comes in four flavors: 5 Gbps, two kinds of 10 Gbps, and 20 Gbps (Figure 25). These four standards have three separate names: a common name, a technical flavor in the 0x0 format indicates number of 5 Gbps data lanes or physical paths, and a marketing name. Here they are:
3.2 Gen 1 (3.2 1x1), SuperSpeed USB 5 Gbps: A Type-A connector carries 5 Gbps; it’s the same as 3.0 or 3.1 Gen 1.
3.2 Gen 2 (3.2 2x1 or 3.2 1x2), SuperSpeed USB 10 Gbps: A 3.2 Gen 2 Type-A connector can now carry 10 Gbps; for USB-C, this standard is more or less 3.1 Gen 2.
3.2 Gen 2x2 (3.2 2x2), SuperSpeed USB 20 Gbps: With USB-C only, a connection can pass up 20 Gbps.
Figure 25: The USB-IF uses the symbols from top to bottom (left, packaging; right, on device) for USB 2.0, 3.0/3.1 Gen (5 Gbps), 3.1 Gen 2/3.2 Gen 2x1 or 1x2 (10 Gbps), and 3.2 Gen 2x2 (20 Gbps). USB4 (2019): This is where I ask you to hold on to your socks and remove the space between USB and a digit. USB4 only allows for USB-C connectors and is an implementation of…Thunderbolt 3! (See USB and Thunderbolt Compatibility.) USB4 can operate at either 20 Gbps or 40 Gbps; in the latter form, it’s marketed and labeled as SuperSpeed USB 40 Gbps (Figure 26).
Figure 26: USB4 has separate 20 Gbps and 40 Gbps labeling (left, packaging; right, on devices). USB4 2.0 (2022): Announced in mid-2022, this update to USB4 offers the potential for controllers with 80 Gbps of data (and video) bandwidth. The standard also allows the use of 40 Gbps over older Thunderbolt 4/USB4 cables longer than 0.8 m that currently limit the maximum data rate to 20 Gbps. Between two USB4 80 Gbps controllers, these cables get a magic boost to 40 Gbps at up to 0.8 m; new cables are required for the 80 Gbps rate. Controllers will likely slowly appear in limited high-end devices in 2023.
The evolution of USB was therefore away from profusion and toward the USB-C single jack/plug type that could work everywhere—truly universal at last (Figure 27)!
The Emergence of USB-C
Finally, a single connection type that was used on both ends of a cable, reversible by 180° as a plug inserted into a jack across its long end, had a compact factor, and could carry up to 100 W of power (later, up to 240 W). Power flow could go either way: a laptop or desktop could charge a mobile device or power pack, or vice versa.
With Intel’s adoption of USB-C starting with Thunderbolt 3 and the near-complete convergence of USB on Thunderbolt standards, it’s all perfect, isn’t it?
Well, no. First, people had invested a lot into equipment that had USB Type-A connectors. Early computers with USB-C jacks tended to scrimp, and docks with many USB Type-A ports were in short supply. From 2015 to at least 2019, people complained endlessly—and largely rightly so—that they had to buy and keep handy a large array of cables, adapters, and mini-docks. By 2020, it seemed to settle down: peripherals switched to either be USB-C based or included a cable or adapter, less-expensive docks were widely available, and computer makers—particularly Apple—decided to include more and different kinds of jacks to reduce the hassle.
Second, during the awkward Thunderbolt 3, USB 3.1, USB 3.2, Thunderbolt 4, and USB4 transition, you could wind up buying a USB-C cable that wouldn’t properly connect two devices with USB-C ports, wouldn’t connect them at the highest possible data rate (dropping to 10 Gbps, say, instead of 40 Gbps), or would only pass 15 W or 60 W of power instead of 100 W. That problem still hasn’t gone away, as I explain in USB and Thunderbolt Compatibility, but it has decreased and will improve even more in the near future.
Third, as USB has moved from 3.1 to 3.2 to USB4, and added options for power offered by ports and carried by cables, the profusion of markings has become more than baroque (Figure 28). (See USB and Thunderbolt Power for more on the power component.)
Thunderbolt doesn’t replace USB, but the convergence of it with USB does help clear away some of the underbrush, as I explain next.
The Thunderbolt Standard
Intel introduced Thunderbolt under the name Light Speed in 2010 and Apple—then a keen user of Intel CPUs—helped set the direction and adopted it in all their computers. The original version of Thunderbolt offered what was then a blazing 10 Gbps of data simultaneously in both directions—so blazing that it outstripped most storage and other hardware of the time.
Recognizing the shortage of ports on computers at the time, Thunderbolt was built from the start with support for daisy chaining, similar in concept to the earlier SCSI standard, and something that made sense for stringing together a series of high-performance optical drives, hard drives, or arrays of drives in a single enclosure (RAID). Up to six Thunderbolt devices could be daisy chained.
The original Thunderbolt also allowed DisplayPort to pass over the same connection, and could provide at least 10 W of power.
Intel doubled throughput to 20 Gbps in 2013 with Thunderbolt 2 and then again to a maximum of 40 Gbps in 2015 with Thunderbolt 3.
First-generation Thunderbolt and Thunderbolt 2 relied on a plug/jack style identical to Mini DisplayPort. This was conveniently what Apple had already adopted a few years before for external video connections, making the transition easier—at least within that customer segment.
Thunderbolt 3 Steps into USB-C and 20 Gbps
With Thunderbolt 3, Intel adopted the USB-C form factor in a completely standard fashion. Thunderbolt 3 brought several features and options to the communication standard. This is what you should expect from any Thunderbolt 3 port on any computer or device:
40 Gbps (jack only): Thunderbolt 2 maxed out at 20 Gbps; all Thunderbolt 3 ports have to support 40 Gbps. However, depending on the cable, you might achieve only 20 Gbps; see Thunderbolt Capabilities in Link Devices with Cables.
4K displays: Each Thunderbolt 3 controller must support at least a single 4K display at 60 Hz using DisplayPort 1.2. However, later versions of Thunderbolt 3 controllers could optionally incorporate DisplayPort 1.4 and handle up to two 4K displays at 60 Hz, one 4K display at 120 Hz, or one 5K display at 60 Hz. (The 5K option initially required a controller unique to Apple’s computers.)
Minimum 15 W power: Where Thunderbolt and Thunderbolt 2 had to deliver up to 10 W, Thunderbolt 3 starts at 15 W, more appropriate for charging a later generation of smartphones and tablets.
Peer-to-peer networks: Thunderbolt allows computers to daisy chain together and achieve up to 10 Gbps, as if they were connected by 10 Gbps Ethernet. (See Use Peer-to-Peer 10 Gbps Thunderbolt.)
Optional USB Power Delivery: With support for USB Power Delivery, a Thunderbolt 3 port can optionally push up to 100 W of power to a compatible device, like a laptop.
Optional hubs: Thunderbolt 3 could allow for up to four-port Thunderbolt hubs. This feature wasn’t generally supported until Thunderbolt 4 became available.
Mandatory Elements Shape Thunderbolt 4
Thunderbolt 4 amps things up a bit by increasing minimum requirements or making optional Thunderbolt 3 features mandatory in Thunderbolt 4. The two biggest changes that will affect you directly are cable and compatibility improvements.
First, the 480 Mbps USB 2.0 speed limitation of Thunderbolt 3 active cables—ones that are 20 inches (0.5 m) or longer—has been removed (see Thunderbolt 4 Capabilities). A Thunderbolt 4 cable—whether internally passive or active—offers 5 to 40 Gbps of USB throughput depending on the USB controller on one or both ends.
Second, the Thunderbolt 4 standard requires that Thunderbolt controllers be fully backwards compatible to at least 10 Gbps USB 3.1/3.2, but allow support for up to 40 Gbps USB4.
The result is the elimination of the major host/cable/peripheral compatibility that has dogged Thunderbolt, and which I’ve described throughout this book. Here’s what you can expect:
Minimum 10 Gbps USB 3.x/USB 4: Connect a Thunderbolt 4 cable between a USB 3/USB4 peripheral port over USB-C and a Thunderbolt 3 or 4 controller, and always get at least a 10 Gbps data rate, and potentially up to 40 Gbps.
USB4 compatibility: Based on market research so far, all Thunderbolt 4 cables are fully USB4 compatible. When connecting any combination of Thunderbolt 3, Thunderbolt 4, USB 3.x, or USB4 jacks with such a cable, you will always achieve the maximum common data rate.
This change alone should bring a lot of relief from cable clutter, standards confusion, and frustration over low data rates or incompatibility. But there’s more! Here’s what else you can expect from Thunderbolt 4:
Always 40 Gbps: Thunderbolt 4 accepts no compromises. All jacks and cables must support 40 Gbps.
Two 4K displays: Thunderbolt 3 could optionally handle up to two external 4K displays at 60 Hz; that’s now the minimum requirement for Thunderbolt 4.
Required 100 W power on a host: Any computer with Thunderbolt 4 must have at least one jack that delivers up to 100 W.
Minimum USB support: As noted above, all Thunderbolt 4 controllers must support USB 3.2 Gen 2x1 and 1x2 (10 Gbps), but can optionally support up to USB4 (20 Gbps and 40 Gbps).
Wake from sleep: While not a marquee feature, Thunderbolt 4 requires that a host computer monitor the port for a “wake from sleep” signal. This allows a peripheral to wake the host. A similar feature was added to Ethernet in 1996, called Wake-from-LAN.
Thunderbolt ports on dock: Instead of being an option, operating systems and jacks must allow up to four external Thunderbolt 4 ports on a dock.
Thunderbolt 4 Docks May Net Few Ports
When you add a Thunderbolt dock that includes Thunderbolt ports, make sure to do the math about what you gain (Figure 29). A dock with three ports will only result in a net gain of one Thunderbolt jack; a dock with four, just two.
If you do the math, here’s what pencils out:
One Thunderbolt jack on your computer is occupied with the connection to the dock. That jack isn’t available to plug anything else into.
One Thunderbolt jack on the dock is similarly occupied with the connection to the computer.
On a dock with three Thunderbolt ports, two remain, but one replaces the port on your computer, netting you just one additional; on a four-port Thunderbolt dock, three remain, netting two more.
Of course, the point of the dock is typically to add other capabilities, such as direct charging of a laptop over the dock-to-computer connection, as well as jacks for other purposes or diversity, like adding USB Type-A that isn’t built into the laptop.
USB and Thunderbolt Compatibility
The biggest area of port and cable confusion I ever encounter is the matrix of compatibility between generations of USB and Thunderbolt. It’s natural to be confused: the two have converged, but in what ways?
The best path forward is to look at the five key steps from the introduction of the USB-C connector to the present:
USB 3.1: This evolution rebranded USB 3.0’s Type-A connector as 3.1 Gen 1 and introduced USB-C as 3.1 Gen 2. The difference? Only 5 Gbps could pass over the Type-A connector; USB-C could handle up to 10 Gbps.
USB 3.2: The USB-IF doubled the speed through more efficient use of the pins in the reversible connection. (In 3.1, only some pins carried data in each of the two possible cable orientations; in 3.2, the unused pins have been employed, too.)
Thunderbolt 3: The first version of Thunderbolt that used USB-C, the new standard doubled speed over Thunderbolt 2 while taking full advantage of the USB-C connection’s flexibility.
USB4: The USB-IF adopted Thunderbolt 3 as its data standard for 20 Gbps and optional 40 Gbps rates. A 2.0 update announced in mid-2022 brings the top optional rate to 80 Gbps.
Thunderbolt 4: As noted above, many Thunderbolt 3 optional features became mandatory. But the key improvement was requiring backward compatibility through USB 3.2 for 10 Gbps, a mandatory minimum of 40 Gbps. A “next-generation” version announced in October 2022 will bring that up to a mandatory 80 Gbps.
Here are the simplest ways to distinguish the capabilities of USB4 and Thunderbolt 4:
Thunderbolt 4 is almost entirely a superset of USB4: Thunderbolt 4 has support for USB Power Delivery specification only up to 100 W. Some Thunderbolt 4 controllers might not support 20 Gbps and 40 Gbps USB4, making it impossible for a USB4-based peripheral or host to exceed 10 Gbps between two devices.
USB4 is a superset of Thunderbolt 3, not including the mandatory elements of Thunderbolt 4: USB4 requires a minimum of only 20 Gbps, not 40 Gbps, making a USB4 controller on a host or peripheral potentially slower than a pair of Thunderbolt 4 devices, and always slower if the cable is longer than 0.8 m between two USB4 40 Gbps controllers (see USB Capabilities). It can deliver up to 240 W of power.
These differences largely apply if you’re have a high-performance environment in which the difference among 10, 20, and 40 Gbps throughput is crucial. For most people, owning a computer with a Thunderbolt 4 port and purchasing a 10 Gbps USB 3.x or USB4 device won’t have a high impact. But I’ll look at when it does next.
USB and Thunderbolt Data
For all the discussion of compatibility above, your biggest questions at this stage are probably the following:
Given my particular devices, how fast a connection can I achieve between any two, and based on what?
How do I reach the highest possible data rate between two devices?
How fast can I transfer data to and from a storage device?
The answers are straightforward now that we’ve done all the preparatory work.
Match Rates with Standards, Jacks, and Cables
Here how you can achieve the following data rates with USB and Thunderbolt jacks and cables given the devices you have, along with what the slowest rate you could wind up with.
Let’s go fastest to slowest:
40 Gbps (Thunderbolt 3 or 4, USB4): These combinations can achieve 40 Gbps:
A pair of Thunderbolt 4 controllers connected by a Thunderbolt 4 cable or Thunderbolt 3 active cable: All Thunderbolt 4 connections over a Thunderbolt 4 cable must be at 40 Gbps per the specification. An active Thunderbolt 3 cable can also carry 40 Gbps between these two controllers.
A pair of Thunderbolt 3 or 4 controllers connected by a 20-inch (0.5 m) Thunderbolt 3 passive cable, or by a 6.6 feet (2 m) or shorter active cable: A short, passive Thunderbolt 3 cable or longer Thunderbolt 3 active cable is required to reach 40 Gbps.
A Thunderbolt 4 controller and 40 Gbps USB4 controller using a Thunderbolt 4/USB4 cable: A Thunderbolt 4 controller with optional 40 Gbps USB4 support can connect over a high-speed cable to a USB4 port with optional 40 Gbps capability.
A pair of USB4 40 Gbps controllers over a Thunderbolt 4/USB4 cable that’s 0.8 m or shorter: Two USB4 controllers, both with 40 Gbps support, can communicate at that data rate over a Thunderbolt 4/USB4 cable using USB4 protocols—functionally no different than two Thunderbolt 3 controllers.
20 Gbps (Thunderbolt 3 or 4, USB4): A smaller set of possibilities allow no more than 20 Gbps throughput:
A pair of Thunderbolt 3 or 4 controllers connected by a 3.3 feet (1 m) or longer Thunderbolt 3 passive cable: Passive cables at this length with a Thunderbolt 3 or 4 controller restrict the maximum rate to 20 Gbps.
A Thunderbolt 4 controller connected to a 20 Gbps USB4 controller by a Thunderbolt 4/USB4 cable: A Thunderbolt 4 controller with 20 Gbps USB4 support can connect over a high-speed cable to a USB4 port at 20 Gbps.
A pair of USB4 controllers connected by a Thunderbolt 4/USB4 cable: Two USB4 20 Gbps controllers can communicate at 20 Gbps over a Thunderbolt 4/USB4 cable of any length; two USB 4 40 Gbps controllers are limited to 20 Gbps with a cable longer than 0.8 m.
10 Gbps (USB 3.1, 3.2): You can reach 10 Gbps with a USB SuperSpeed 10 Gbps controller to a like USB port or any Thunderbolt 4 to USB SuperSpeed 10 Gbps connection. All USB-C to USB-C cables and some USB-C to Type-A cables support this data rate.
5 Gbps (USB 3.0, 3.1, 3.2): With a USB 3.0 controller and a USB 3.0 Type-A to Micro-A or other compatible plug end cable, you can reach this minimum data rate. All USB 3.0 Type-A cables support this data rate.
480 Mbps (USB 2.0): Three possibilities exist in which you could wind up using 480 Mbps or throttled to that rate:
2.0 on one end: If one end of one connection is a USB 2.0 Type-A or Type-B jack of any size, then Thunderbolt 3 and 4 and USB 2.0 and later will work at this slow, outdated data rate. USB 2.0 is still used by input devices, like keyboards.
Active Thunderbolt 3: You can also be throttled to 480 Mbps over an active Thunderbolt 3 cable connected between a Thunderbolt 3 port and a USB 3.1 or 3.2 devices using USB-C. See Thunderbolt Capabilities. (This problem disappears with a Thunderbolt 4 cable.)
USB-C charging cable: An oddball form of USB-C cable is designed for charging, not high-speed data. It may have a maximum power capacity of 15 W or 100 W, but carries data at only USB 2.0 rates. See Beware the USB-C Charging Cable: It’s Slow.
Use Peer-to-Peer 10 Gbps Thunderbolt
Thunderbolt has a special peer-to-peer mode that I’ve mentioned in passing throughout the book, but I wanted to call it out here. Connect two Thunderbolt 3 or 4 jacks with a Thunderbolt 3 or 4 cable, and the operating systems establish a 10 Gbps peer-to-peer connection. It’s like you were using 10 Gbps Ethernet. You can also daisy-chain computers through their Thunderbolt ports to create a peer-to-peer network.
This peer-to-peer method is best known as something Apple suggests for performing a system migration between two of their computers (Figure 30).
How this mode gets invoked varies by operating system. In general, you should just be able to plug in and the operating system establishes a local Internet Protocol link. You have to then set up or access local services, like file sharing, to transfer data.
Pick a Drive To Match the Standard
There’s an additional factor in the above speeds: how fast do your peripherals operate? They might be capable of faster rates than the one you’ve set up with your jack/cable combination, or they may be far slower than the rate you can establish.
SSDs and RAIDs are the best examples here, as they are the highest throughput devices you’ll connect to a USB-C port. The only other would be another computer, but the computer would be constrained by the fastest data rate of its internal or external drives.
Drive transfer rates are usually cited in megabytes or gigabytes per second (MB/s or GB/s), so we should convert USB and Thunderbolt maximums to those terms:
40 Gbps: 5 GB/s
20 Gbps: 2.5 GB/s
10 Gbps: 1,250 MB/s
5 Gbps: 625 MB/s
480 Mbps: 60 MB/s
You can purchase SSDs that are priced based on the standard they support. For some uses, an older, slower standard may offer enough performance in comparison to the higher cost to achieve higher throughput. With an SSD, you can choose among:
SATA III (or 3): This interface evolved for use with hard drives and shows that constraint. It’s capable of 4.8 Gbps or about 600 MB/s. That fits well with USB’s 5 Gbps or 10 Gbps flavors.
NVMe: Paired with the PCIe bus standard, an NVMe card can connect directly to the equivalent of an external PCIe connector with a Thunderbolt 3 or 4 enclosure. The most common flavors allow up to 2.4 GB/s, 3.5 GB/s, or 7 GB/s for sequential reads; writes are about 25% slower. That requires 28 Gbps and 56 Gbps to deliver full performance, making the faster flavor a slight overkill with Thunderbolt 4. (The super-fast flavor can operate at full speed when plugged into an internal bus.)
Reading that comparison, you might think you should always opt for NVMe paired with Thunderbolt—why not get the faster version where possible? A few factors might affect your choice:
Cost: NVMe SSDs with a Thunderbolt enclosure can cost two or more times the same SATA III/USB pairing:
A 1 TB NVMe SSD in the M.2 form factor with 2.4 GB/s or 3.5 GB/s read speed can cost from about $80 to $120; The 7 GB/s flavor runs closer to $140 to $180. Add $80 to $100 for a Thunderbolt enclosure.
A 1 TB SATA III SSD can cost as little as $40 to $50, plus another $50 or so for a USB 3.1 or 3.2 drive case.
Needs: You may want higher throughput for some needs and not for others. I bought a 1 TB NVMe SSD (3.5 GB/s) and enclosure for about $300 in September 2020 (prices have since dropped a bit) to act as the startup volume on my Mac. However, my photo library remained on a hard drive. In mid-2021, I bought a 1 TB SATA III in an enclosure for about $110 to hold my 600 GB library. The price and speed tradeoff were perfect.
Available ports: A Thunderbolt enclosure requires a Thunderbolt jack, of course. If you lack free ports on your computer and aren’t ready to add an external dock, that can constrain your choice, too.
For people who need truly massive amounts of power and can’t afford to spend vast sums for SSDs, arrays of hard drives organized into a single container—a RAID—can make a lot of sense.
As an example, Other World Computing lets you configure its Thunderbay mini storage system with 16 TB of hard drives across four drive bays and can achieve about 1.6 MB/s (12.5 Gbps) over a Thunderbolt 3 interface. That configuration costs $879.
For comparison, the company’s Envoy Express Thunderbolt 3 drive is $1,499 for the same throughput with a single SSD at half the capacity—8 TB. (On the other hand, the SSD drive makes not a peep compared to the fans required for the RAID, and is less prone to failure.)
USB and Thunderbolt Power
USB and Thunderbolt both have a long history of delivering power through their jacks. For the first many years of USB and the first several of Thunderbolt, that power was just a trickle. It was enough to run some bus-powered drives, though many needed an AC connection to deliver a consistent amount of juice. As USB matured, it rose to offer enough power to recharge an iPod and later an iPhone.
To reiterate earlier sections of this book, USB and Thunderbolt’s early formats offered these maximum wattages:
USB: 2.0, 2.5 W; 3.0, 4.5 W; 3.2, 7.5 W, or 100 W over USB-C
Thunderbolt: 1 and 2, 10 W; 3, 15 W; 4, 15 W plus one port up to 100 W on a host computer
Jacks have to meet certain wattage minimums, while for cables, it’s optional unless they purport to have certifications that allow them to pass higher amounts of power. It’s vitally important when purchasing cables to confirm their specs on promised power capability. The difference is typically among 7.5 W (USB), 15 W (Thunderbolt), 60 W (USB and Thunderbolt), 100 W (USB and Thunderbolt), and 240 W (USB). (The 60 W flavor of Thunderbolt cable shouldn’t exist under any spec I can find, but some cables are sold with that label.)
The USB Power Delivery (PD) specification governs how power flows over USB standards and for Thunderbolt 3 and 4.
USB PD has matured from version 1.0 to 3.1 between 2012 and 2022. Even in its 1.0 release, USB PD allowed for up to 100 W over appropriate cables and between capable devices, including bidirectional power—a host could charge a peripheral and vice versa. But little hardware supported it initially.
Version 2.0 of USB PD appeared alongside the release of USB-C and marked the first time that a USB cable was put into broad use to provide high-wattage charging for mainstream laptops. (A later 3.0 update provide more flexibility in amperage and voltage levels, but no other user-facing features.)
USB-C–based charging quickly found its way into modern power packs, which grew in capacity, allowing a portable battery to charge mobile devices and a laptop.
When Thunderbolt 3 appeared, it adopted the USB PD specification, a sort of perfect pairing of high-throughput data, flexibility over what was carried across a single USB-C port, and high-wattage power. Using Thunderbolt 3 to provide power was a natural fit for Apple, which from 2015 to early 2021 only released laptops with USB-C–based charging.
Finally, in 2021, the USB-IF released USB PD 3.1 (and USB-C 2.1), raising the maximum power from a compliant port over a certified cable to 240 W. But they also added an intermediate level. Instead of 7.5 W and 100 W, the new tiers are 7.5 W, 60 W, and 240 W—a formerly 100 W device would now need to support up to 240 W.
In case that’s not crystal clear, you can just consult this simple iconography chart developed by the trade group to cover USB4 and USB PD 3.1 (Figure 31). While it’s feels like too many different elements smashed together, each logo in isolation makes sense at a glance: 20 Gbps or 40 Gbps, 60 W or 240 W. (7.5 W ports are unmarked.)
Related to higher-wattage charging is “fast charging,” a sort of loosey-goosey term that lacks a technical definition. Generally, it means delivering a lot of power to a battery in a device in such a way that it avoids overheating or overcharging lithium-ion battery cells. Li-ion batteries can be damaged if they’re charged too fast and too close to a theoretical 100%. That’s why when you’re charging a mobile device, tablet, or laptop, you might notice that it rapidly recharges up to the 60 to 80% range, at which point it slows way down.
Fast-charging methods from many different companies require the presence of a particular chip, firmware, port, and AC adapter or battery pack, and sometimes the right cable. The best known of these is Qualcomm’s Quick Charge, mostly used in smartphones. Apple added a fast-charging mode to their 2021 14-inch and 16-inch MacBook Pros and 2022 MacBook Air.
In the right configuration, fast charging can replenish a battery in a fraction of the time of “standard” charging. It might add 50% charge to the battery in tens of minutes compared to hours.
There’s a big limit, however: fast charging generally operates only when the battery is about half full because it’s safest to push more watts into a nearly “empty” battery than one that’s still got half a charge. Fast charging appears to stop at roughly 50% to 60%, based on Apple’s description, at which an Apple laptop drops to “normal” charging: moderate speed to 80%, and slower up to 100%.
The USB-IF approved the use of the term “fast charging” in 2018 for devices that met certain specifications that allowed changing up the voltage delivered in small increments, confirming to an optional part of USB PD 3.0. It even created logos to differentiate “Charger” from “Fast Charger.” However, it seems to have dropped the symbol and the branding program for now, even though some USB-C devices provide fast charging.