Appendix A: Introduction to SAS ● 56The following figure illustrates (in very basic terms) a SAS domain and shows how SAS cards,SAS and SATA disk drives, and expander devices can fit together in a large data storagetopology.How is SAS Different from Parallel SCSI?In summary, although SAS and parallel SCSI both use the SCSI command set, how they movedata from one place to another is very different. To support point-to-point serial datatransport, SAS introduces new types of connectors, cables, connection options, andterminology.Generally speaking, SAS is faster and more flexible than parallel SCSI, and provides moreoptions for building your storage space. SAS lets you mix SAS and SATA disk drives together,and lets you connect many, many more devices.The following table describes many of the main differences between the two interfaces.SAS CardSAS CardDisk DrivesDisk DrivesDisk DrivesDisk DrivesDisk DrivesDisk DrivesFanout ExpanderDisk DrivesEdgeExpanderEdgeExpanderSATASATASATASATASASSASSASSATAEdgeExpanderSATASATASASSASEdgeExpanderEdgeExpanderSAS CardSAS DomainSAS DomainSATA SASParallel SCSI Serial Attached SCSIParallel interface Serial interfaceMaximum speed 320 MB/secshared by all devices on the busMaximum speed 600 MB/sec per phywhen in full-duplex modeSupports SCSI devices only Supports SATA and SAS disk drivessimultaneouslyUp to 16 devices per SCSIchannelMore than 128 disk drives per SAScard, using an expander (see page 55)Supports single-port devices only Supports single- and dual-portdevicesUses SCSI IDs to differentiatebetween devices connected tothe same adapterUses unique SAS addresses todifferentiate between devicesUser intervention required to setSCSI IDsSAS addresses self-configured bySAS devicesRequires bus termination Requires no bus terminationStandard SCSI connectors SAS connectors (see page 21)
