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A quick first look at USB 3.0 performance

SuperSpeed comes to USB
— 8:45 PM on December 6, 2009

USB 2.0 is so last millennium. No, seriously. We've had the specification since the year 2000, which is technically the previous millennium. Given how quickly the PC industry moves, though, it might as well have been a full millennium ago. Back in 2000, Intel was pushing Pentium III CPUs, 3dfx was still selling graphics processors, and Windows XP was a year away. Things have certainly changed since the so-called Hi-Speed USB spec was released, and its 480Mbps peak data rate has been grossly inadequate for quite some time now.

In the real world, you're lucky to push more than about 37MB/s through a USB 2.0 port. To give you an idea of just how slow that is, the latest crop of 500GB, 5,400-RPM notebook drives (which just happen to be popping up in all sorts of USB-attached external enclosures), can sustain transfer rates in the 67MB/s range. The 3.5" desktop drives that typically populate bulkier USB storage devices are even faster still. Heck, just about every hard drive we've ever reviewed is capable of saturating a second-gen USB link.

Making the most of the copious bandwidth available in USB 3.0 is going to be a little more challenging, though. The gen-three USB spec boasts a peak data rate of 5Gbps—roundabout 600MB/s, without taking overhead into account. Interestingly, this latest leap in USB bandwidth is proportionally smaller than the previous one. The original USB spec topped out at 12Mbps, making the jump to 480Mbps for USB 2.0 a forty-fold increase in available bandwidth. USB 3.0 only amounts to about a 10X increase over the prior standard.

Nevertheless, this new "SuperSpeed" spec should easily handle next-gen storage devices, even when you factor in overhead. According to the final specification, taking into account SuperSpeed USB's 8b/10b encoding, flow control, packet framing, and protocol overhead reduces effective throughput to a "realistic" 400MB/s for actual applications. Mechanical hard drives still have a long way to go before they can sustain transfer rates that saturate an old-school 150MB/s Serial ATA link, and even the fastest solid-state drives on the market aren't pushing data at much more than 200MB/s.

A USB 3.0 cable: SS stands for SuperSpeed

Despite the fact that the SuperSpeed standard was made available in late 2008, industry heavyweights haven't flocked to USB 3.0. The new spec isn't supported by any core-logic chipsets on the market—not even Intel's P55 Express, which debuted just a few months ago. Rumor has it that Intel chipsets may not add USB 3.0 support until some time in 2011. AMD and Nvidia haven't announced plans for SuperSpeed-compliant core-logic products, either.

You don't have to wait on chipset providers to get in on SuperSpeed USB, though. NEC has crafted a USB 3.0 controller of its own: the D720200. The NEC chip offers a pair of third-generation USB ports, and it interfaces with the host system via a single gen-two PCI Express lane. Second-gen PCI Express lanes only offer 500MB/s of bandwidth in each direction, however, so the NEC controller won't be able to take full advantage of SuperSpeed's fat data pipe. The limited host interface bandwidth will have to be shared between both of the D720200's USB ports, as well.

Already, Gigabyte has revamped much of its enthusiast-oriented motherboard lineup to incorporate the NEC controller. Asus has added the chip to a number of its premium motherboards, too, and the firm has also whipped up a U3S6 add-in card that features the D720200 alongside Marvell's 88SE9123 6Gbps Serial ATA controller.

The Asus U3S6 situates is next-gen I/O chips behind a PLX bridge chip. The bridge splits the card's four-lane PCI Express interface evenly between the two I/O chips. Asus' decision to use a physical x4 interface means the card won't work in an x1 slot that isn't notched to accept longer cards. However, it does ensure the NEC and Marvell controllers will each get 500MB/s of bandwidth (the maximum each can use), even in a system that's saddled with first-gen PCIe or the P55 Express chipset's half-speed gen-two lanes.

Two USB 3.0 ports grace the U3S6's expansion slot cover, and a pair of 6Gbps SATA ports can be found on the card. We probed the Marvell controller's performance in great detail when we recently tested Seagate's SATA 6Gbps Barracuda XT hard drive. I encourage you to read that review for the skinny on this new chip. Today, our focus is on the U3S6's SuperSpeed USB performance.