In computing, serial-attached SCSI (SAS) has replaced the older Parallel SCSI (Parallel Small Computer System Interface, usually pronounced “scuzzy”) bus technology that first appeared in the mid-1980s. SAS is an evolution of the parallel SCSI device interface into a serial point-to-point interface, and it is used for devices like hard drives and tape drives. INCITS 554-2023: Information Technology – SAS Protocol Layer – 5 (SPL-5) specifies the next generation of the protocol portion of Serial Attached SCSI (SAS).
Understanding SAS
The term serial-attached SCSI (SAS) refers to a set of serial device interconnect and transport protocols. It is a method used to access computer peripheral devices that employs a serial means of digital data transfer over thin cables. Moreover, SAS is defined in two specifications: the SAS standard itself and the SAS Protocol Layer (SPL) standard.
- SAS standard is comprised mainly of electrical and analog concerns as well as pictures of the enormous number of different physical connector types supported by SAS.
- SPL standard describes the actual SAS protocol itself which runs over the physical interconnect.
In the business enterprise, SAS is especially of interest for access to mass storage devices, particularly external hard disk drives and magnetic tape drives.
What Is INCITS 554?
The SCSI family of standards provides for many different transport protocols that define the rules for exchanging information between different SCSI devices. INCITS 554-2023 defines the rules for exchanging information between SCSI devices using a serial interconnect. Other SCSI transport protocol standards define the rules for exchanging information between SCSI devices using other interconnects.
What Is the SAS Protocol Layer?
The SAS Protocol Layer is the layer of the SAS interconnect that comprises the Serial SCSI Protocol (SSP), the Serial ATA Tunneled Protocol (STP), and the Serial Management Protocol (SMP). The SAS protocol is implemented on computers to use a dedicated link among the computer and disk drives, tape drives, and any other SCSI storage devices that are connected to the computer’s host bus adapters (HBAs) over a serial interface.
How Does SAS work?
SAS is a protocol for point-to-point serial transmissions between storage devices and computers.
- Point-to-point: all data transfers across SAS are sent directly between the two communicating entities: storage device and computer. Both these entities are connected by a physical cable.
- Serial: all data sent using SAS is transmitted a single bit at a time, in sequence.
What Uses SAS?
SAS is designed for businesses with substantial storage, backup, and archiving demands. It is widely considered to be the prevalent interface for direct-attached storage and is used to support hard drive controllers in enterprise-grade server farms.
What Is the SAS Layer Concerned With?
The SAS layer is concerned with:
- SAS Phy/Port/HA event management (LLDD generates, SAS Layer processes)
- SAS Port management (creation/destruction)
- SAS Domain discovery and revalidation
- SAS Domain device management
- SCSI Host registration/un-registration
- Device registration with SCSI Core (SAS) or libata (SATA)
- Expander management and exporting expander control to user space.
INCITS 554-2023: Information Technology – SAS Protocol Layer – 5 (SPL-5) is available on the ANSI Webstore.