Thirteen Years of Change -- Same Protocol For Transfer
When USB debuted in 1999, it offered maximum throughput of 12Mb/s. Today, USB 3.0 offers 4.8Gb/s. That's not surprising. What is surprising is that modern USB 3 controllers use the same Bulk-Only Transport (BOT) transfer protocol that first debuted in 1998. Before the advent of USB 3, relying on BOT made sense. Because modern hard drives were significantly faster than the USB 2 bus itself, it didn't really matter if the bus supported certain features -- the HDD was always going to be waiting on the host controller.
USB 3 changed that. With 4.8Gbits/s of throughput (600MB/s), only the highest-end hardware is capable of saturating the bus. That's exposed some of BOT's weaknesses, as detailed in the slide below.
UASP -- USB Attached SCSI Protocol -- is designed to fix all these problems, and bring USB 3 fully into the 21st century. It does this by implementing queue functions, reducing command latency, and allowing the device to transfer commands and data independently from each other.
That's where Asus comes in. You don't need a specific type of SSD or HDD to take advantage of UASP (more on that in a moment), but you do need a specific USB 3 controller. In Asus' case, that means an Asmedia (abbreviation: ASM) controller designed to support UASP, and an external enclosure that's also capable of taking advantage of the newer protocol. Asus sent us two different enclosures, both of which use the ASM 1051E. The motherboard the company provided for our testing, the P8Z77-V, has an onboard ASM 1042 controller as well.
Here's the really nifty part. While you'll see best performance with a UASP solution, those of you with Intel (and possibly NEC) controllers aren't out of luck. Asus is also offering what it calls "Turbo," or "Optimized BOT" mode. This operating mode conforms to the BOT standard (no multiple simultaneous data transfers, for example) but streams commands without waiting for constant acknowledgement. This significantly reduces latency and improves performance.