Appendix A: Introduction to SAS ● 72Some backplanes support daisy-chain expansion to other backplanes. For example, you candaisy-chain (connect one to the next) up to nine Adaptec S50 enclosures to a single SAS card ina host system.SAS Expander ConnectionsA SAS expander device literally expands the number of end devices that you can connecttogether. Expander devices, typically embedded into a system backplane (see page 71), supportlarge configurations of SAS end devices, including SAS cards and SAS and SATA disk drives.With expander devices, you can build large and complex storage topologies.There are two types of SAS expanders: fanout expanders and edge expanders. Each performs adifferent role in a storage system. (For more information about how SAS expanders work, referto the STA Web site at www.scsita.org.)You can connect up to 128 SAS ports to an edge expander. (A single edge expander cantherefore support up to 128 SAS addresses.)You 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 including thefanout expander).The next figure illustrates (in very basic terms) a SAS domain and shows how SAS cards, SASand SATA disk drives, and expander devices can fit together in a large data storage topology.SAS CardSAS CardDisk DrivesDisk DrivesDisk DrivesDisk DrivesDisk DrivesDisk DrivesFanout ExpanderDisk DrivesEdgeExpanderEdgeExpanderSATASATASATASATASASSASSASSATAEdgeExpanderSATASATASASSASEdgeExpanderEdgeExpanderSAS CardSAS DomainSAS DomainSATA SAS
