Serial Attached SCSI
Serial Attached SCSI (SAS) is a new generation serial communication protocol for devices designed to allow for much higher speed data transfers and is compatible with SATA. SAS uses serial communication instead of the parallel method found in traditional SCSI devices but still uses SCSI commands for interacting with SAS devices.
SAS supports up to 16,384 addressable devices in a SAS domain and point to point data transfer speeds up to 3 Gbit/s, but is expected to reach 10 Gbit/s by the year 2010. The SAS connector is much smaller than traditional parallel SCSI connectors allowing for small 2.5 inch drives.
The physical SAS connector is available in 3 different variants:
SFF 8482 - which is form factor compatible with SATA
SFF 8484 - hi-density internal connector for connecting up to 4 devices
SFF 8470 - hi-density external connector (aka Infiniband connector) for connecting up to 4 devices
Form factor compatibility with SATA allows for much cheaper SATA drives to connect to a SAS backplane. SAS drives are not compatible on a SATA bus and have their physical connector keyed to prevent any plugging into a SATA backplane.
Serial Attached SCSI supports three transport protocols:
- Serial SCSI Protocol (SSP) — Supporting SAS disk drives
- Serial ATA Tunneling Protocol (STP) — Supporting SATA disks
- Serial Management Protocol (SMP) — for managing SAS Expanders
A SAS domain is a set of SAS ports communicating with each other. A SAS domain contains one or more SAS devices and a service delivery subsystem. A SAS domain may be a SCSI domain. Each SAS device is assigned a World Wide Name (aka SAS address) assigned by IEEE for the particular vendor. The WWN uniquely identifies the device in a SAS domain just as a SCSI Id identifies a device in a parallel SCSI bus.