SAS overview

The term Serial-attached SCSI (SAS) refers to a set of serial device interconnect and transport protocols.

This set of protocols defines the rules for information exchange between devices. SAS is an evolution of the parallel SCSI device interface into a serial point-to-point interface.

SAS physical links are a set of four wires used as two differential signal pairs. One differential signal transmits in one direction while the other differential signal transmits in the opposite direction. Data can be transmitted in both directions simultaneously.

Physical links are contained in ports. A port contains one or more physical links. A port is a wide port if there are multiple physical links in the port. A port is a narrow port if there is only one physical link in the port. A port is identified by a unique SAS worldwide name (also called a SAS address). A SAS controller contains one or more SAS ports.

A path is a logical point-to-point link between a SAS initiator port in the controller and a SAS target port in the I/O device (for example, a disk).

A connection is a temporary association between a controller and an I/O device through a path. A connection enables communication to a device. The controller can communicate to the I/O device over this connection using either the SCSI command set or the ATA/ATAPI command set, depending on the device type.

An expander facilitates connections between a controller port and multiple I/O device ports. An expander routes connections between the expander ports. There is only a single connection through an expander at any given time. Using expanders creates more nodes in the path from the controller to the I/O device.

If an I/O device supports multiple ports, then it is possible to have more than one path to the device when there are expander devices on the path.

A SAS fabric refers to the summation of all paths between all controller ports and all I/O device ports in the SAS subsystem.