Serial Attached SCSI uses SCSI command
Comparison with parallel SCSI
The SAS "bus" operates point-to-point while the SCSI bus is multidrop.  Each SAS device is connected by a dedicated link to the initiator, unless an expander is used. If one initiator is connected to one target, there is no opportunity for contention; with parallel SCSI,  even this situation could cause contention.
SAS has no termination issues and does not require terminator packs like parallel SCSI.
SAS eliminates clock skew.
SAS allows up to 65,535 devices through the use of expanders, while Parallel SCSI has a limit of 8 or 16 devices on a single channel.
SAS allows a higher transfer speed (3, 6 or 12 Gbit/s) than most parallel SCSI standards. SAS achieves these speeds on each initiator-target connection, hence getting higher throughput, whereas parallel SCSI shares the speed across the entire multidrop bus.
SAS devices feature dual ports, allowing for redundant backplanes or multipath I/O; this feature is usually referred to as the dual-domain SAS.
SAS controllers may connect to SATA devices, either directly connected using native SATA protocol or through SAS expanders using Serial ATA Tunneling Protocol (STP).
Both SAS and parallel SCSI use the SCSI command set.Comparison with SATA
There is little physical difference between SAS and SATA.
SAS protocol provides for multiple initiators in a SAS domain, while SATA has no analogous provision.
Most SAS drives provide tagged command queuing, while most newer SATA drives provide native command queuing.
SATA uses a command set that is based on the parallel ATA command set and then extended beyond that set to include features like native command queuing, hot-plugging, and TRIM. SAS uses the SCSI command set, which includes a wider range of features like error recovery, reservations and block reclamation. Basic ATA has commands only for direct-access storage. However SCSI commands may be tunneled through ATAPI for devices such as CD/DVD drives.
SAS hardware allows multipath I/O to devices while SATA (prior to SATA 2.0) does not. Per specification, SATA 2.0 makes use of port multipliers to achieve port expansion, and some port multiplier manufacturers have implemented multipath I/O using port multiplier hardware.
SATA is marketed as a general-purpose successor to parallel ATA and has become common in the consumer market, whereas the more-expensive SAS targets critical server applications.
SAS error-recovery and error-reporting uses SCSI commands, which have more functionality than the ATA SMART commands used by SATA drives.
SAS uses higher signaling voltages (800–1,600 mV for transmit, and 275–1,600 mV for receive) than SATA (400–600 mV for transmit, and 325–600 mV for receive). The higher voltage offers (among other features) the ability to use SAS in server backplanes.
Because of its higher signaling voltages, SAS can use cables up to 10 m (33 ft) long, whereas SATA has a cable-length limit of 1 m (3.3 ft) or 2 m (6.6 ft) for eSATA.
SAS is full duplex, whereas SATA is half duplex.  The SAS transport layer can transmit data at the full speed of the link in both directions at once, so a SCSI command executing over the link can transfer data to and from the device simultaneously.  However, because SCSI commands that can do that are rare, and a SAS link must be dedicated to an individual command at a time, this is generally not an advantage with a single device.Serial SCSI Protocol (SSP) –  for command-level communication with SCSI devices.
Serial ATA Tunneling Protocol (STP) –  for command-level communication with SATA devices.
Serial Management Protocol (SMP) –  for managing the SAS fabric.For the Link and PHY layers, SAS defines its own unique protocol.
At the physical layer, the SAS standard defines connectors and voltage levels. The physical characteristics of the SAS wiring and signaling are compatible with and have loosely tracked that of SATA up to the 6 Gbit/s rate, although SAS defines more rigorous physical signaling specifications as well as a wider allowable differential voltage swing intended to allow longer cabling. While SAS-1.0 and SAS-1.1 adopted the physical signaling characteristics of SATA at the 3 Gbit/s rate with 8b/10b encoding, SAS-2.0 development of a 6 Gbit/s physical rate led the development of an equivalent SATA speed. In 2013, 12 Gbit/s followed in the SAS-3 specification. SAS-4 is slated to introduce 22.5 Gbit/s signaling with a more efficient 128b/150b encoding scheme to realize a usable data rate of 2,400 MB/s while retaining compatibility with 6 and 12 Gbit/s.Additionally, SCSI Express takes advantage of the PCI Express infrastructure to directly connect SCSI devices over a more universal interface.Nearline SAS
Nearline SAS (abbreviated to NL-SAS, and sometimes called midline SAS) drives have a SAS interface, but head, media, and rotational speed of traditional enterprise-class SATA drives, so they cost less than other SAS drives.  When compared to SATA, NL-SAS drives have the following benefits:: 20 
Dual ports allowing redundant paths
Ability to connect a device to multiple computers
Full SCSI command set
No need for using Serial ATA Tunneling Protocol (STP), which is necessary for SATA HDDs to be connected to a SAS HBA.: 16 
No need for SATA interposer cards, which are needed for pseudo–dual-port high availability of SATA HDDs.: 17 
Larger depth of command queues
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