Serial Attached SCSI (SAS) is a term referring to various technologies designed to implement data transfer between computer devices. The SAS protocol is a serial successor to the parallel Small Computer System Interface. In the SAS protocol, all SAS devices are either an initiator device, a target device, or an expander device. Initiator devices are devices that begin an SAS data transfer, while target devices are the devices to which initiator devices transfer data. Together, initiator devices and target devices are known as end devices.
SAS expanders are devices that facilitate data transfer between multiple initiator devices and multiple target devices. The SAS protocol utilizes a point-to-point bus topology. Therefore, if an initiator device is required to connect to multiple target devices, a direct connection must be made between the initiator device and each individual target device in order to facilitate each individual data transfer between the initiator device and each individual target device. SAS expanders manage the connections and data transfer between multiple initiator devices and multiple target devices. SAS expanders may contain SAS devices.