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A first look at USB 3.1 performance

With bonus Type-C connector glamor shots
— 11:30 AM on March 6, 2015

A new generation of USB devices is upon us.

A year and a half after the completion of the USB 3.1 spec, motherboards supporting the new standard are turning up in e-tail stocks. Also known SuperSpeed Plus, USB 3.1 promises a sizable performance increase over the previous generation.

It doubles the signaling speed from 5Gbps to 10Gbps, and it also changes the line encoding scheme from 8b/10b to 128/132b, meaning that only four bits out of every 132 (as opposed to two out of every 10) are sacrificed to encoding overhead. The result is a jump in maximum effective transfer rates from 500MB/s to 1.21GB/s—at least in theory.

USB 3.1 also introduces Type-C connectors and cables. Type-C connectors are reversible, so you can plug them in upside down without worrying about that whole USB quantum superposition mess. Type-C cables support Alternate Mode, as well, which allows them to carry DisplayPort audio and video alongside USB 3.1 data and as much as 100W of power. Some have called USB 3.1 Type-C a Thunderbolt killer, and it's not hard to see why.

Today, we're going to take our very first look at some USB 3.1 gear. Asus has supplied us with a drive enclosure and a matching motherboard, which will help us gauge the kinds of performance gains users can expect.

The hardware
According to Asus, our USB 3.1 enclosure is a "prototype for testing," not a retail product. It doesn't look half bad, though:

The enclosure features one of the new Type-C ports along with an old-school Micro-USB connector. The former is for data, while the latter supplies power to the enclosure. If this were a retail device, a single Type-C port would theoretically be enough for both power and data.

There's not much to say about the Type-C connector itself, except that it's predictably painless to use. Apple's Lightning connector still feels sturdier and smoother to insert, but the Type-C plug remains a big improvement over, well, pretty much all standard USB connector designs.

I bet some of you were expecting the enclosure to conceal a 2.5" SSD. Guess again! This puppy actually houses two mSATA drives running in RAID-0 mode. That arrangement should enable peak transfer rates greater than the mSATA interface's 6Gbps maximum.

Asus sent us the enclosure with a couple of Samsung's 840 EVO 250GB solid-state drives. The RAID-0 config results in a 500GB volume that should be faster than either drive on its own.

The drive enclosure wouldn't do us much good without a motherboard capable of harnessing USB 3.1's higher speeds. Asus' Z97-A/USB 3.1 fills that role. As its name suggests, this board is a USB 3.1-infused successor to the Z97-A we reviewed last year. It has a similar layout and feature set, but its rear cluster sports a couple of teal-colored SuperSpeed Plus Type-A ports. (The original Z97-A had two USB 2.0 ports in the same spot.)

Because they're Type-A, the new ports are backward-compatible with older USB devices. However, that advantage comes with a tradeoff: no support for Type-C's Alternate Mode or 100W power delivery. According to Asus, these USB 3.1 Type-A ports are limited to 4.5W.

Since Intel's Z97 chipset lacks built-in support for USB 3.1, the new ports are backed by an ASMedia ASM1142 controller. The ASM1142 can be fed by either one PCIe Gen3 lane or two PCIe Gen2 lanes. Asus uses the latter arrangement on the Z97-A/USB3.1.

The two PCIe Gen2 lanes provide the ASMedia controller with 10Gbps of bandwidth, but there's a catch. Actually, two catches. First, that 10Gbps is shared between the two USB 3.1 ports. Second, because PCIe Gen2 uses 8b/10b encoding, maximum effective transfer rates are limited to 1GB/s, somewhat shy of SuperSpeed Plus' 1.21GB/s theoretical maximum. Using PCIe Gen3 wouldn't address the second catch, since a single Gen3 lane would deliver 8Gbps with 128/130b encoding, for an effective 985MB/s.