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(pronounced as separate letters) Short for Serial Attached SCSI, an evolution of parallel SCSI into a point-to-point serial peripheral interface in which controllers are linked directly to disk drives. SAS is a performance improvement over traditional SCSI because SAS enables multiple devices (up to 128) of different sizes and types to be connected simultaneously with thinner and longer cables; its full-duplex signal transmission supports 3.0Gb/s. In addition, SAS drives can be hot-plugged.
In topologies with more than two connected devices, SAS calls for the use of expander devices to allow for more than one host to connect to more than one peripheral.
SAS devices can communicate with both SATA and SCSI devices (the backplanes of SAS devices are identical to SATA devices). A key difference between SCSI and SAS devices is the addition in SAS devices of two data ports, each of which resides in a different SAS domain. This enables complete failover redundancy as if one path fails, there is still communication along a separate and independent path.

A typical Serial Attached SCSI system consists of the following basic components:
An Initiator is a device that originates device service and task management requests to be processed by a target device and receives responses for the same requests from other target devices. Initiators may be provided as an on-board component on the motherboard (as is the case with many server-oriented motherboards) or as an add-on host bus adapter.
A Target is a device containing logical units and target ports that receives device service and task management requests for processing and sends responses for the same requests to initiator devices. A target device could be a hard disk or a disk array system.
A Service Delivery Subsystem is the part of an I/O system that transmits information between an initiator and a target. Typically cables connecting an initiator and target with or without expanders and backplanes constitute a service delivery subsystem.
Expanders are devices that are part of a service delivery subsystem and facilitate communication between SAS devices. It facilitates connection of multiple SAS End devices to a single initiator port.

Serial Attached SCSI (SAS) supports both SCSI and ATA Three transport protocols
– Serial SCSI Protocol (SSP)
• Supports SAS (SCSI) disk drives, tape drives, etc.
– Serial ATA Tunneling Protocol (STP)
• Supports Serial ATA disk drives
– Serial Management Protocol (SMP)
• Supports SAS expanders

Connector types:

• SFF 8482 – SATA compatible, 29 Drive Connector
• SFF 8484 –32 Multi-Lane 4i, up to four devices
• SFF 8470 – External connector (InfiniBand connector), up to four devices
• SFF 8087 –Internal 36 position Mini SAS , up to four devices
• SFF 8088 –External 26 Mini SAS with Universal Key to fit 2, 4 & 6, up to four devices

A. SAS Internal Connector

• SAS Drive connectors
• SAS Drive plug connector (SFF 8482)
  
  
• SAS 4i connectors
• SAS 4i cable receptacle connector (SFF 8484)

• Mini SAS 4i cable plug connector (SFF 8087)

B. SAS External Connector

• SAS 4x connectors
• SAS 4x cable plug connector (SFF 8470)

• Mini SAS 4x cable plug connector (SFF 8088)

Serial Attached SCSI (SAS) is a data transfer technology designed to move data to and from computer storage devices such as hard drives and tape drives. It is a point-to-point serial protocol that replaces the parallel SCSI bus technology that first appeared in the mid 1980s in corporate data centers, and uses the standard SCSI command set. At present it is slightly slower than the final parallel SCSI implementation, but in 2009 it will double its present speed to 6 Gbit/s, allowing for much higher speed data transfers than previously available, and is "downwards"-compatible with second generation SATA drives. SATA 3.0 Gbit/s drives may be connected to SAS backplanes, but SAS drives may not be connected to SATA backplanes.

SAS HDD Speed 3Gb/s , SAS 2.0 Speed 6Gb/s

• 3.5" SAS HDD, Capacity 36/73/146/300 GB, Rotation speed 10000/15000 RPM
• 2.5" SAS HDD, Capacity 36/73/146 GB, Rotation speed 10000/15000 RPM
  

SAS and SATA disk-drive receptacles are standardized by SFF-8482. They are designed to reinforce the connectivity supported by their correlating standards. SAS host controllers (initiators) and expanders may support both SAS and SATA disk drives, but SATA controllers can only support SATA disk drives. SAS disk-drive receptacles are therefore keyed to prohibit them from mating with SATA cable or backplane plugs. SAS cable and backplane plugs will mate with either SAS or SATA drives.

SAS disk drives are generally ‘dual-port’, providing a redundant connection to the drive for more faulttolerant systems. The pins for the second port are located on the opposite side of the receptacle from the side that provides primary SAS/SATA connection (located where the keying feature would be on a SATA plug). A SAS or SATA disk-drive connector provides an array of 15 pins for power and seven pins for each high-speed serial data connection. Many drives also provide the four-pin legacy power connector for non-backplane applications, but power cable adapters (four-pin legacy to 15-pin SAS/SATA power plugs) are readily available for those drives that do not provide that connector. The ‘data’ portion of the connector provides two pins each for transmit and receive pairs, with each pair straddled by signal ground pins. The ground pin between pairs is shared (ground-signal-signal-ground-signal-signal-ground). below shows both SAS and SATA plugs with cables and receptacles.

SAS	SATA

SATA drives may be connected to SAS backplanes, but SAS drives may not be connected to SATA backplanes.

References:www.scsita.org