USB Attached SCSI

USB Attached SCSI (UAS) is a computer protocol used to move data to and from USB storage devices such as hard drives, solid-state drives, and thumb drives. UAS depends on the USB protocol, and uses the standard SCSI command set. UAS is defined across two standards, the T10 "USB Attached SCSI" (T10/2095-D) referred to as the "UAS" specification, and the USB "Universal Serial Bus Mass Storage Class - USB Attached SCSI Protocol (UASP)" specification. The T10 technical committee of the International Committee for Information Technology Standards (INCITS) develops and maintains the UAS specification; the SCSI Trade Association (SCSITA) promotes the UAS technology. The USB mass-storage device class (MSC) Working Group develops and maintains the UASP specification; the USB Implementers Forum, Inc. (USB-IF) promotes the UASP technology.

UAS Goals

• Designed to directly address the failings of the USB mass-storage device class bulk-only transports (BOT)
  • Enables command queuing and out-of-order completions for USB mass-storage devices
  • Eliminates software overhead for SCSI command phases
• Up to 64K commands may be queued
• SCSI SAM-4 compliant
• USB 3.0 SuperSpeed and USB 2.0 High-Speed versions defined
  • USB 3.0 SuperSpeed – host controller (xHCI) hardware support, no software overhead for out-of-order commands
  • USB 2.0 High-speed – Significantly enables command queuing in USB 2.0 drives
• Streams were added to the USB 3.0 SuperSpeed protocol for supporting UAS out-of-order completions
  • USB 3 host controller (xHCI) provides hardware support for streams

External links

• USB Attached SCSI Protocol (UASP) v1.0 and Adopters Agreement
• "USB Mass Storage Bulk-Only Transfer" is a link that often rots, lately found buried at "USB" -> Developers -> Documents -> Class Specs -> Approved -> Mass Storage -> "Mass Storage Bulk Only 1.0".