The ANSI-approved Serial Attached SCSI (SAS) standard divides the SAS architecture into six layers and defines the function of each layer. This is Part 4 of a 5-part white paper series that describes in detail each of the layers in the SAS architecture as defined in the SAS standard. This paper discusses the transport layer, which constructs the frames used to transmit information between two SAS devices. It is the second-highest layer of the SAS architecture and the highest layer in the SAS port.