Posted by Eric Huang on October 17, 2011
All USB Storage products, Flash Drives, Thumb Drives, Hard Drives, and SSDs use a transfer protocol called “Bulk Only Transfer” or “BOT” protocol. This works reliably in Windows and Linux and other operating systems.
In USB, a Bulk Transfers refers to a transfer of data that must be 100% accurate when it arrives. No errors can be allowed. For example, if you are copying pictures from your camera to your computer, you want every color pixel to be 100% accurate. The same is true for printing a picture.
It also means the data does not need to arrive at a certain time. If an error occurs in a Bulk transmission, the system retries until the data accurately moves to the destination.
BOT is highly reliable, but the hard drive companies knew that with USB 3.0 a new method would be needed. BOT basically sends a single packet at a time. This works well for USB 2.0.
USB 3.0 lets you send packets along multiple USB 3.0 “streams.” To take advantage of this, Storage companies created a new USB Driver Class, a purely software feature, to enable faster USB transfers on USB 3.0. called “USB Attached SCSI Protocol” or “UASP.” UASP allows you to send packets along multiple streams in parallel, and even burst the data faster.
ASUS published a web page that does a fantastic job illustrating how UASP works with an animation sequence. I recommend you click on the image ASUS website below and take a look at it.
I should point out you need a hard drive or SSD that supports UASP inside the hard drive’s firmware to support this.
In the USB community, we’ve actually debated the usefulness of UASP. In our lab, we performance increases of 6-10% for early applications.
In a test of the ASUS motherboard with a UASP enabled ASMedia Hard Drive, shows a speed bump of about 33-37Megabytes per second going from about 261 to 293 Megabytes per second, or about a 13% increase in speed.
Today, mass market hard drives today won’t deliver data fast enough because they use SATA 3 Gb/s instead of the faster 6Gb/s. Most hard drives bridge from SATA to USB 3.0. Since USB 3.0 has a maximum effective throughput of about 4Gb/s, this is faster than existing SATA 3/Gb/s drives. SATA can be limiting. Within 2 years, this will change because hard drives will have either SATA 6Gb/s support, or native USB 3.0 support so SATA will not be the bottleneck.
3 Gigabits/second = 3Gb/s = 300 MB/s = 300 Megabytes/second
4 Gb/s = 400 MB/s
6 Gb/s = 600 MB/s
If you are building a USB 3.0 product today, it means you will deploy in about 12-18 months which means you should plan for UASP support or at least investigate UASP for your application.
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