USB 3.0: The new speed limit

We check out the first devices equipped with USB 3.0's SuperSpeed spec

Have you spent too much time waiting for large files to crawl between a computer and an external hard drive? Don't fret -- USB 3.0 has arrived. Not only can it move data faster and provide more power, but it's compatible with USB 2.0 devices.

Developed by the USB Implementers Forum (USB-IF), USB 3.0's SuperSpeed specification promises a theoretical top speed of 5Gbit/sec. versus USB 2.0's 480Mbit/sec.

The key to blending old and new is NEC's μPD720200 controller chip. It has the circuitry for USB 2.0 and 3.0 transfers inside and can use either, depending on what's plugged in. Right now, it's the only game in town, but look for other companies, including Symwave, Fujitsu and Via, to introduce their own USB 3.0 chips in the coming months.

The first round of USB 3.0 cards and devices works with Windows Vista and Windows 7; Apple hasn't decided whether to support the new standard. The basic software for USB 3.0 has been in the Linux kernel since last fall, and the needed drivers are slowly coming out.

There are already a few USB 3.0 devices available. To test them, I used a Lenovo ThinkPad W510 with USB 3.0 built in. I tried out a variety of new devices, including the Buffalo DriveStation USB 3.0 HD-HXU3 external hard drive; a StarTech SuperSpeed USB 3.0 to SATA Hard Drive Docking Station in combination with my current Western Digital WD Caviar Blue external drive; and a Seagate BlackArmor PS 110 USB 3.0 Performance Kit, a portable hard drive that includes its software on a USB 3.0 ExpressCard.

What's new in USB 3.0?

Unlike the change from USB 1.0 to USB 2.0, USB 3.0 brings actual physical differences to the connectors. The flat USB Type A plug (that goes into the computer) looks the same, but inside is an extra set of connectors; the edge of the plug is colored blue to indicate that it's USB 3.0.

On the other end of the cable, the Type B plug (that goes into the USB device) actually looks different -- it has an extra set of connectors, so it looks a bit like a USB plug that's been crimped a little ways down one end. There's also a new Micro Type B plug that has all its connectors laid out horizontally.

USB 3.0 plug
The USB 3.0 plug has an extra set of connectors.

As a result, you won't be able to fit a USB 3.0 cable into a USB 2.0 device. However, you will be able to plug USB 3.0 devices -- and cables -- into your current computer; you just won't get the speed advantage. (Note: To get the most out of USB 3.0, the cable needs to be less than about 9 feet long, down from the USB 2.0 16-foot limit.)

The reason for the new connector is that the USB 3.0 cable contains nine wires (four more than a USB 2.0 cable); eight carry data and one is used as a ground. Despite the increase in wires, however, the cables should be no thicker than those used by USB 2.0. There will be a big difference in performance, however. USB 2.0 is like a single-lane country road that needs to handle the morning-commute traffic in and out of L.A. There are jams and slowdowns when too much data is going back and forth. With nine wires available, USB 3.0 has an additional two lanes of traffic in each direction to smooth the flow between the computer and the device.

Unlike USB 2.0, which requires synchronous transfers, where the data is asked for and then sent, the 3.0 host controller doesn't have to poll the USB device every time it wants to send data. This streamlines the flow with high-speed asynchronous transfers.

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