What is USB Attached SCSI (UAS)  : Complete Knowledge

Technology Overview, December 8, 2017

UAS is an abbreviated form for “USB Attached SCSI“ , which is mainly computer protocol that is used for moving data either to or from system using different types of USB storage devices like hard disk drives or HDDs, the solid-state drives or the SSDs, and also the thumb drives.

Initially UAS came into existence as section of USB 3.0 standard, but even at those initial stages. It could be also used with devices that comply with sluggish USB 2.0 standard, presuming the utilization of firmware, hardware, and drivers that are compatible with it. The UAS running drivers usually give faster data transfers in comparison to earlier USB Mass Storage device BOT or Bulk-Only Transport protocol drivers. Though UAS has been added in new USB 3.0 protocol standard, but it can be used also at USB 2.0 protocol speeds with the assumption of compatible firmware, hardware etc. When it is being used with solid state drive or SSD then, this USB attached SCSI (UAS) gets significantly faster than Bulk-Only Transport protocol (BOT) for random read as well as write; but even still, the speed would be considerably lower than that of native SATA 3 interface.

Useful Features of USB Attached SCSI

  • The USB attached SCSI or UAS usually depends upon USB protocol.
  • UAS makes use of standard command set of SCSI.
  • Using UAS, largely gives faster data transfers in comparison to earlier used USB devices that were Mass Storage device BOT (Bulk-Only Transport) drivers.

Difference between UAS and BOT

  • UAS is actually a new USB 3.0 protocol, which has somehow managed to replace the earlier used USB 2.0 BOT protocol.
  • Proposed for fully exploiting potential of USB 3.0 protocol, developers of UAS have introduced 2 main changes in the architecture of it.
    • One of them is the Four-Pipe Model. Earlier, the query and command signals used to share same pipe along with the data being transferred. However the UAS one does not mix the different types of data transfers, and in this, every each data type has a different pipe and not the shared one.
    • UAS is multi-threaded, whereas BOT is more or less a serial type of process.
    • The other one being Command queuing. Unlike BOT, the USB attached SCSI or UAS usually carries out the processing of data transfers in parallel, thereby, ruling out the requirement for waiting for the next-in-line signal.

Requirements to Run UAS

Understand this, simple thing that USB attached SCSI or UAS support is not any simple switch, which you can easily turn on. It needs 4 platform-oriented elements for its functioning, which are as follows:

  • The device hardware that supports USB attached SCSI
  • The device firmware that supports USB attached SCSI
  • USB attached SCSI support inside the driver stack of the USB
  • System controller which is compatible with the USB attached SCSI or UAS

If any of these four elements are missing then, UAS will not function and USB 2.0 protocol of BOT would become the fallback mode to preserve the compatibility.

Supported Operating System

  • Windows: MS has added in its OS the native support for USB attached SCSI (UAS) to new Windows 10. The drives that support UAS usually load “Uaspstor.sys” in place of earlier one, “Usbstor.sys.” The Window 10 version of MS Windows supports USB attached SCSI by default over the USB 2.0 protocol too.
  • Linux: From the year 2012, Linux kernel had also given native support for UAS, but had some sort of compatibility problems with the chipsets of Texas Instruments.
  • Mac: Apple has given native S support to the Operating System X 10.8 Mountain Lion. Here, in this, the drives that make use of the UAS depict as using the “IOUSBAttachedSCSI” rather than the earlier one, “IOUSBMassStorageClass kext”.

Wrapping Up

In the above- discussion, we have described the complete about the UAS. It makes easy for you to understand the complete concept over the same. If still you have any issue then, feel free to contact our technical support service.