There's been a lot of buzz on the internet in the past few days about the speed of USB 3.0. Some sites are reporting that recent tests of the new standard are producing slower-than-expected results, and many readers are confused about how realistic the touted theoretical bandwidth of 5000 megabits/sec really is. We spoke with Jeff Ravencraft, President of the USB Implementer's Forum, (who also gave us our first look at USB 3.0 back at last year's IDF conference) to set the matter straight and get a demo of the latest SuperSpeed hardware in action.
When most people reference the speed of USB 2.0 (or HighSpeed), they typically cite 480mbit/sec. If you do the math (8 bits to a byte), that translates to 60Mb/sec data transfers. Of course, you'll never find any existing USB device that's able to transfer files at that speed -- the most we've gotten out of portable hard drives and USB keys ranges from 25-35MB/sec. Indeed, that's because 480mbit/sec is only the theoretical maximum bandwidth of USB 2.0.
With USB 3.0, the same distinction between theoretical and usable speeds applies. The technical limitation of SuperSpeed is 5000mbit/sec, more than ten times that of Highspeed. But we'll most likely never see file transfers topping 600MB/sec, and it's unreasonable to expect that we ever will. So where does the limitation lie? Much of it comes at the driver and application level, where software overhead is at fault. The new xHCI drivers are still in their infancy (only finalized several months ago), and won't be optimized for a long time to come. Additionally, the existing Mass Storage driver that's built into your OS was developed for the HighSpeed spec, and will probably be updated as SuperSpeed matures.
But even at this stage (and keep in mind that most consumers won't see USB 3.0 until the end of the year at the earliest), USB 3.0's demo speeds are truly impressive. Fresco Logic, one of the forum's partners that has developed a PCI-E xHCI Host card, demoed their setup for us. Data transfer along a 2 meter cord (3 meters is the maximum length allowed by the spec) showed speeds of up to 231MB/s writes and 256MB/s reads (simultaneously) when moving 64k packets. And when the signaling rate is measured directly at the link, we really do see speeds approaching the theoretical max.
As of now, the USB Implementer's Forum is approaching their promise of "10 times the speed of USB 2.0". With optimized hardware and software in the coming years, they expect that consumers will see speeds of 400-450MB/sec. But when even the fastest existing solid state drives can't output more than 250MB/sec, we think that you'll be more than happy with the usable speeds at USB 3.0's initial release.