USB goes Full Circle with USB-C

USB-CUniversal Serial Bus (USB) was developed in the late 90s to replace a mess of slow PC connections with a high speed, one size fits all plug and data transfer protocol.  All newish devices had the plug, it was good, and there was no real decision or gotchas to look out for when buying new devices.

A decade or two later, things are again a mess.  Incremental changes to USB have offered progressive technical improvements, but at the cost of modified plugs and sometimes questionable backwards compatibility.  Mobile devices using the small variants of USB ports, or worse still proprietary plugs, have diversified the cables in use and ensured X won’t plug into Y.  New connection standards such as DVI, Firewire, HDMI, and Display Port emerged to meet specific needs for what, at a basic level, is a demand for local bandwidth that could in theory be carried by one cable.

An effort is again underway to resist the evil forces of cable proliferation and focus on the holy grail, one cable to carry them all, resulting in USB-C (also called USB Type C).  This new type of connector is intended to replace the mishmash of USB plugs, and potentially other existing data plugs to, become the standard connector to attach any peripheral to a computer or mobile device and to provide enough electrical power to run many of them.

USB-C is now available.

Physical Characteristics of USB-C

USB-C on the left compared to Type A on the right

USB-C on the left compared to Type A on the right

The original USB connector was a spade shaped plug on your PC end (Type-A plug), and a squarish plug on the peripheral end (Type-B Plug).  Smaller variants are available for mobile devices.

They would only plug in one way round.  When taking random stabs at plugging a USB cable into the back of a PC, under zero visibility, and while fighting off the tech eating spiders and poisonous dust clouds, it would be impossible to get the cable around the right way.

Problem solved.  The new USB Type-C is a small, reversible connector about the same size as a micro-USB plug but a little thicker, and pretty easy to get the cable into.

The USB-C Plug was developed by the same industry group who define and control the USB data transfer protocols, but should not be confused those protocols.  USB-C can carry USB 3.1 and other USB standards but is not limited to those standards.

USB Protocols and how USB-C fits in

Superspeed_USB_10GbpsThe last major USB standard release was USB 3.1 in 2013, a relatively minor upgrade from USB 3.0 that itself dates back to 2008.  USB 3.1s main call to fame was doubling theoretical transfer speeds from 5Gb/s to 10Gb/s.

For reasons that I fail to understand, the body that handles USB recently decided to make a naming change:

  • USB 3.0 changed to USB 3.1 Gen 1 (5Gb/s) also known as SuperSpeed USB
  • USB 3.1 changed to USB 3.1 Gen 2 (10Gb/s) also known as SuperSpeed USB 10Gbps

For comparison sake, the older and still very common USB standard, USB 2.0, supports 480Mb/s of raw data.

The new USB-C plug is able to carry data complying with either USB 3.1 protocol or USB 2 (with adapters), but is not limited to USB transfer protocols.  USB-C can handle other useful high bandwidth data streams including video and network traffic, and will be able to handle future high speed protocols. This is done through the use of alternate modes, where the system can hand over control of certain pins within the connector to carry traffic defined by protocols unrelated to USB.

Where faster Data Transfer Speeds Matter

USB transfer protocols indicate a theoretical maximum throughput in bits per second of raw data.  There are 8 bits to a byte, so for USB 2.0 – 480Mbits/s = 60MBytes/s, but then we need to reduce this figure further to take into account “overhead”, essentially the part of the data stream that is used to get the information we need transferred across.  To test this, try to transfer a large file from a USB 2 HDD to your PC and look at the speed you get.  It will be no higher than 35MB/s, not 60, being limited by the USB 2 standard less significant overhead.


USB 2 HDD transfer speed is capped by the USB Protocol

Your mouse, keyboard, and many other devices are just fine with USB 2.0, but HDDs and the occasional other USB device will work better with the faster protocols.

Modern Hard disk drives can transfer files at a much faster rate than 35MB/s so the upgrade to USB 3.0 can significantly increase transfer speeds.


USB 3 HDD transfer speed is capped by the maximum speed of the physical disk

Marketing people confuse the issue by often quoting the USB 3.0 or 3.1 maximum speed on the box of many supported products, particularly HDDs.  This is not even close to the transfer rates you will get with the usual mechanical drives, as they will be limited by the transfer rate of the drive, not the transfer protocol.

Only the fastest of the modern SSD drives are now surpassing USB 3.0 speeds though given external drives are still mainly mechanical drives owing to cost, USB 3.0 is fine for now (or USB 3.1 Gen 1, if you prefer the new terminology!).

So what is the advantage of USB-C when transferring files using USB?  For time taken to transfer a file, not much (well, till you go with a SSD external), but have you ever been annoyed by that bulky AC Adapter powering your external HDD?  Well, USB-C can do something about that.

USB-C Power Delivery

The original USB specification allowed for up to 150mA of power at 5V, just 0.75W of power.  That was fine to power a mouse or keyboard but was never going to be adequate to power external HDDs or other more demanding devices.

USB 2 gave us a useful increase up to 2.5W and then USB 3.0 to 4.5W, and a power charging option was introduced at 7.5W but not simultaneous with data transfer.  Power supply was only at 5V.  Small steps, but it allowed for 2.5” HDDs to be powered off USB and mobile devices such as phones to be charged off USB ports, if over a long period of time.

With USB-C connectors we are finally seeing a major increase in overall power, and this time, at varying voltages.  The new plugs can support up to 5A at 5, 12, and 20V, potentially giving 20V x 5A = 100W of power.  Simultaneous data transfer is supported even at maximum power output.


This sort of power delivery will allow for substantial devices to be powered from the same cable carrying data.  A desktop sized monitor needing just one cable from PC to screen is a good start, getting rid of AC adapters from 3.5” external HDDs is another use.

The new power spec allows for power to run in either direction. Get home and plug your notebook into your desktop monitor by USB-C and let you monitor charge the notebook.  Goodbye power brick.

The management of power between devices has also seen major improvement.  Where power is limited and needs to be shared across multiple devices, the protocol allows for a varying of power supplied as demands vary.  For example, if a laptop is pulling a lot of power off a desktop USB port to charge, and then the monitor, also powered off USB is turned on, the power available to the laptop can be reduced to allow the monitor to run.  How well devices play together will be interesting to see!

Alternate Mode to support DisplayPort, Thunderbolt, and more

So getting back to that file transfer speed, and more generally, moving data fast, perhaps faster than USB 3.1 allows, does USB-C give us any other options beyond the 10Gb/s of USB 3.1?

Well, yes.  The USB-C specification allows data protocols outside the USB specification to be supported through an alternate mode system where the device can negotiate control of certain pins to be reconfigured to support data streams outside the USB data transfer specifications.

When no other demands are placed on the cable, USB 3.1 can use 4 lanes for USB Superspeed data transfer, but when using alternate mode some or all of these lanes can be used for other types of data signals.  These signals are not encapsulated within a USB signal, rather the signal is supported directly on the wires with the alternate mode protocol managing the signals allowed to be sent.  Depending on the configuration, various levels of USB transfer can coexist with these alternate protocols.

The most popular protocol supported by current USB-C implementations is Display Port, allowing a 4k screen to be driven at 60Hz with 2 lanes while still allowing USB Superspeed simultaneously.  Adapters are also available to attach an older monitor with DisplayPort or HDMI to a USB-C connector.  Not all USB-C plugs will support Display Port Alt Mode, so take care to check the specs of devices where you might want to use this feature.

Thunderbolt 3 (40Gb/s) is supported in some implementations of USB-C, providing a wide range of data types including high resolution video, audio, and high speed, 10Gb/s Peer to Peer Ethernet.  That last one is particularly interesting as a modern version of a laplink cable by cutting out the middle man and providing a high bandwidth device to device connection ten times faster than available across most LANs.

USB-C Availability

At time of writing, USB-C is available across a modest range of products, and many newly announced products are supporting the technology as they hit the market.

We are seeing niche products such as portable monitors powered through USB-C, but do not expect to see widespread peripheral support until the installed base of USB-C reaches critical mass, probably less than a year away.

To help build that installed base, we have motherboards (used for desktop computers), notebooks, and tablets with USB-C support, as well as expansion cards that enable USB-C on legacy desktops.

If buying now, USB-C is not a essential purchase, but for a machine you expect to have in use for at least the next few years, its a desirable feature and worth considering during the hunt for your next computer.

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