Appendix A: Introduction to SAS ● 68Terminology Used in This ChapterFor convenience, SAS HBAs and SAS RAID controllers are referred to generically in thischapter as SAS cards. HBAs, RAID controllers, disk drives, and external disk drive enclosuresare referred to as end devices and expanders are referred to as expander devices.For convenience, this chapter refers to end devices and expander devices collectively as SASdevices.What is SAS?Legacy parallel SCSI is an interface that lets devices such as computers and disk drivescommunicate with each other. Parallel SCSI moves multiple bits of data in parallel (at the sametime), using the SCSI command set.SAS is an evolution of parallel SCSI to a point-to-point serial interface. SAS also uses the SCSIcommand set, but moves multiple bits of data one at a time. SAS links end devices throughdirect-attach connections, or through expander devices.SAS cards can typically support up to 128 end devices and can communicate with both SASand SATA devices. (You can add 128 end devices—or even more—with the use of SASexpanders. See page 72.)Note: Although you can use both SAS and SATA disk drives in the same SAS domain (see page72), Adaptec recommends that you not combine SAS and SATA disk drives within the samearray or logical drive. The difference in performance between the two types of disk drives mayadversely affect the performance of the array.Data can move in both directions simultaneously across a SAS connection (called a link—seepage 69). Link speed is 300 MB/sec in half-duplex mode. Therefore, a SAS card with eight linkshas a bandwidth of 2400 MB/sec.Although they share the SCSI command set, SAS is conceptually different from parallel SCSIphysically, and has its own types of connectors, cables, connection options, and terminology,as described in the rest of this chapter.To compare SAS to parallel SCSI, see How is SAS Different from Parallel SCSI? on page 73.
