The state of I/O standards these days can be confusing, even for experienced engineers, and this article attempts to explain some of the intricacies involved in picking the right docking station for your needs.
I recently upgraded my five year old MacBook Pro to a newer model with Thunderbolt 3 ports, and I was left with the dilemma of figuring out the best docking station for all my legacy USB peripherals and my Thunderbolt/Displayport monitor. My personal choice, with a Thunderbolt 3 equipped laptop, is to go with a Thunderbolt 3 docking station for my desk, and a cheaper portable USB-C hub when traveling. Both options are discussed below, with some tables comparing the different products based on capabilities and ports, but first, I'll cover some background on the different standards in play.
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A significant source of confusion over USB terminology arises due to the hard-to-differentiate naming conventions. For example, USB-A and USB-C (or Type A and Type B) refer to the physical connector design (the 'plug'), and USB2 and USB3.1 gen 2 refer to the protocol running over these connectors. These naming conventions for standards are further confused by various marketing terms overlayed on top of these standards, such as HighSpeed, SuperSpeed USB, SuperSpeed USB+, and their associated logos. Many different technical publications such as Tom's and Ars have some excellent articles intended to clear up some of these differences, so I'll only quickly summarize the relevant terms for this article before diving into what each docking station supports.
The three main things to consider when connecting two devices (e.g. computer and peripheral) are:
- Connector type (what the plug/socket looks like)
- Protocol/communication standard (what language is spoken over the connection)
- Cable type (what wires are used and how they're certified)
Let's take a look at each of these factors in the sections below.
USB-A, USB-B, and USB-C refer to the physical connector or plug that a cable or peripheral supports. A really detailed description of every single connection type and sub-type, with pictures, can be seen at this C2G (Cables2Go) site.
USB-A and USB-B have been around since the original days of USB 1.1, and are relatively large plugs that need to be oriented properly when you plug it in (so it has an "up" and "down" side to it, and can only go into a socket in the proper orientation.) USB-C, which came into the market around 2015, is smaller than the earlier -A and -B connectors (about the same size as Apple's Lightning connector), and is symmetric, so it doesn't have a specific up or down orientation it needs to be plugged in with. The newer MacBooks, MacBook Airs, and MacBook Pros (since about 2016) have switched exclusively to using USB-C physical connectors for both power and connecting peripherals.
Now, let's discuss how devices talk to each other once they're plugged in with one of these different plug types. These different "languages", or protocols, are referred to by their USB version numbers, with the newer versions supporting faster communication speeds across the cables. These speeds are expressed as bits per second (bps); and with speeds getting as fast as they are these days, there is usually a Giga(1 billion), multiplier on the speed to make it easier to say, so the unit becomes Gbps:
- USB 1.1 - Up to 0.012Gbps
- USB 2.0 - Up to 0.48Gbps
- USB 3.1 gen 1 - Up to 5Gbps
- USB 3.1 gen 2 - Up to 10Gbps
Thunderbolt 3 - Up to 40Gbps
All the USB standards could theoretically use the original Type-A connectors. However, it's more common these days to find USB 3.1 cables with Type-C connectors. In fact, the 2015, 2016 and 2017 MacBooks have a single USB-C port running USB 3.1 gen 1.
Where it gets interesting is that Thunderbolt 3 also uses USB-C connectors, and can run USB 3.1 gen 2 (and all the other earlier USB protocols) over its links. In addition, Thunderbolt 3 supports the 40Gbps Thunderbolt protocol, as well as 8 lanes of DisplayPort 1.2, 4 lanes of PCIe, and is fully bidirectional.
Even with compatible connectors on each end of the cable, once connected, the device communication speed may end up being limited to less than the maximum data rates supported by the protocol because the actual wiring inside the cable cannot support those data rates. For example, even if you have have two Thunderbolt 3 devices that support 40Gbps communication speeds, if your cable is passive and longer than 0.5m, you'll only get 20Gbps max across that connection.
To ensure your cable can reach the maximum speeds supported by your devices, your cable needs to be certified to meet a certain standard's speed. If you want to connect two USB 3.1 devices with USB-C ports at 10Gbps, in addition to making sure both devices support the USB 3.1 Gen 2 protocol, you need to make sure the cable manufacturer certifies the cable for "USB3.1 Gen 2" applications. Otherwise, it's likely you'll be limited by a max of 5Gbps of the Gen 1 standard.
Whew, if you made it this far, here's a quick summary of common ways these three factors interact:
One thing I haven't mentioned yet is the power capabilities of each of these different types of interfaces. Each type of connection can provide a certain amount of power to your devices by default. While the early USB2.0 protocol over USB-A connectors could only provide 2.5W of power, the later USB3.1 standard bumped this up to 4.5W. When you add the USB-C connector to the USB3.1 protocol, this figure gets further increased to 15W. Every single cable out there will support these max power ratings, since these numbers are required by the respective standard.
In addition to the standard power ratings, there's a newer optional capability called Power Delivery that can augment the max power capabilities of the device port to higher levels depending on the cable type used. This is how, for example, laptops from Apple and other manufacturers can support charging in the 60W to 80W range, depending on the model.
A summary can be seen in the following table:
It's important to note that if you have a laptop like the MacBook Pro that can support greater than 60W of power, then you need to have the higher power EMCA cable along with a dock that can deliver higher power to charge it at the fastest rate. If not, your dock will still work but may either not have enough power to charge the battery or charge at a slower rate.
Because of the backwards-compatibility of Thunderbolt 3 ports with USB 3.1 standards, a MacBook Air or Pro with Thunderbolt 3 can use either a cheaper USB-C docking station, or a more expensive Thunderbolt 3 docking station. If you have a MacBook, which as of early 2019 only offers USB3.1 gen 1 speeds on its single USB-C port, then there's no point in getting the faster Thunderbolt 3 docking stations and you should stick with the USB-C ones.
Either of these types of docks on the market these days can provide Ethernet, USB, SD card, headphone/mic, and video connectivity in addition to Power Delivery. The main difference is the max speeds each docking station will support, which becomes important if you're trying to connect to very high resolution monitors at high refresh rates, which we'll discuss in more detail below.
These are relatively inexpensive, ranging in price from $50 to $200. The cheaper portable hubs have a smaller number of ports, and limited power capabilities, if any. Unlike real USB power delivery that can route the docking station's own power supply onto a USB-C cable, these hubs will just pass through the power from your existing laptop power brick. If you want more ports or want to minimize the number of power adapters you have sitting on your desk, you'll have to upgrade to the >$100 docks listed below.
Further, if you're planning to connect high-resolution displays over HDMI or DisplayPort, then you'll need to be aware of the following:
- If all you need is 1080p HD displays (or even 2560x1600), then you are guaranteed 60Hz refresh rate support.
- If you need 4K (or even ultra-wide displays like 3840x1440), then a single USB-C cable can only support a 30Hz refresh rate while still maintaining enough bandwidth to supply USB 3.1 speeds on the hub's USB ports.
- If you absolutely need 4K at 60Hz using a USB-C dock, then you will need a dock that supports a special 4K/60Hz mode, with the caveat that now your USB ports will be limited to USB 2.0 speeds [*]. In addition, the computer needs to support this particular DisplayPort Alternate mode over USB-C, which the new Apple laptops do.
A summary of some of the docks available is shown below:
Some important notes:
- While there are DisplayLink (unrelated to any USB-C DisplayPort Alternate modes) capable docks out there that enable a higher number of display, support for that is very driver and manufacturer dependent.
- Very few docks out there will support the DisplayPort Alternate modes for 4K at 60Hz. The Cable Matters station is unique in that it has a switch to enable this mode.
- All of these docks use the DisplayPort Alternate standard to get the video data to the hub, at which point, depending on the hub, there may be some active circuitry implemented that allows the hub to convert the DisplayPort data to HDMI or other video out types. Although the USB-C standard discusses a HDMI Alternate mode for transmitting native HDMI signals over the cable directly, I have not seen any hubs that implement this.
- The SD card reader speeds can vary; if you want higher than UHS-I (aka SD 3.0) which runs at ~100MB/s, then you'll need to check the specs. As of now, there are a few that support UHS-II, but not UHS-III.
For future-proofing, if your computer supports it, I recommend just getting a Thunderbolt 3 docking station, which will get you higher speeds for your peripherals and better graphics, as well as the ability to daisy-chain up to 6 devices via the docking station.
Thunderbolt 3 can be thought of as the "super"-USB interface: It supports USB3.1 standards, and DisplayPort 1.2 modes (including 2x 4K 30bpp at 60Hz), but also adds in support for PCI Express Gen 3 and THunderbolt 10Gb virtual-Ethernet, all with the same USB-C connector.
My summarized survey of Thunderbolt 3 docks is below:
- You can get HDMI with an DP->HDMI adapter, so you don't necessarily need a hub with a built-in HDMI port. Just make sure to get an active adapter regardless of the target video connector type
- If you need high-speed SD card readers, then the OWC 14-port or CalDigit stations are your only options.
- The Diamond is a generic design that can be found under other brand names. The Monoprice rebrand sells for about $10 less but does not include a TB3 cable.
- I myself got the OWC 14-port, which can be found on sale in the open-box section of OWC's web store every now and then for a discount. I use Apple's Thunderbolt to Thunderbolt 2 adapter to connect to my Thunderbolt 2 display.
|[*]||A USB-C cable contains 4 lanes of wires (each lane is a pair of differential wires), plus a USB2.0 pair consisting of a TX and RX wire. Normally, USB 3.1 uses 2 lanes (one for TX and one for RX), leaving 2 lanes unused. DisplayPort Alternate mode (if supported by the peripheral and host), can utilize the unused pair of lanes for transmitting DisplayPort video a USB-C hub, which can in turn be relayed over a DisplayPort cable to the monitor. These two lanes can be used for DisplayPort 1.2a and supports enough bandwidth for a single 2560x1600 or 2 each of 1080p displays, or 4K at 30Hz. DisplayPort Alternate mode also supports using all 4 lanes in the USB-C connector, enabling enough bandwidth to drive 4K at 60Hz. However, this mode is not supported by most hubs, and it also pushes all USB connectivity to the USB2.0 dedicated pair, meaining you lose any high-speed USB 3.1 capability.|
Here's a set of links to cables and adapters for the different standards.