Appendix A: Introduction to SAS ● 62You can connect up to 128 edge expanders to a fanout expander.You can use only one fanout expander in any single SAS domain (a topology of SAS—andpossibly SATA—end devices and expander devices). A single SAS domain can thereforecomprise up to 16,384 SAS ports (and therefore up to 16,384 SAS addresses).The following figure illustrates (in very basic terms) a SAS domain and shows how SAScontrollers, SAS and SATA disk drives, and expander devices can fit together in a large datastorage topology.How is SAS Different from Parallel SCSI?In summary, although SAS and parallel SCSI use the SCSI command set, how they move datafrom one place to another is very different. To support point-to-point serial data transport,SAS introduces new types of connectors, cables, connection options, and terminology.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 SCSI channel More than 128 disk drives per SAScontroller, using an expander (see page61)Supports single-port devices only Supports single- and dual-port devicesUses SCSI IDs to differentiatebetween devices connected to thesame adapterUses unique SAS addresses todifferentiate between devices
