HP DL360 Serial Attached SCSI storage technology, 2nd Edition - Page 5

Differential signaling All SAS devices have connection points called ports. One or more transceiver mechanisms, called phys, are located in the port of each SAS device. A physical link, consisting of two wire pairs, connects the transmitter of each phy in one device's port to the receiver of a phy in another device's port. The SAS interface allows the combination of multiple physical links to create two (2x), 3x, 4x, or 8x connections per port for scalable bandwidth. A port that has one phy is described as ―narrow‖ while a port with two to four phys is described as ―wide.‖ SAS uses differential signaling to transfer data over a physical link (Figure 2), which reduces the effects of capacitance, inductance, and noise experienced by parallel SCSI at higher speeds. SAS communication is full duplex, which means that each phy can send and receive information simultaneously over the two wire pairs. Figure 2. In differential signaling, positive minus negative equals 1500-900 = 600mV or 900-1500 = -600mV. The physical link rates for SAS and SATA technologies are listed in Table 2. Table 2. Physical link rates per direction Physical link rate 1.5 Gbps 3 Gbps 6 Gbps Generation Bandwidth SAS-1, SAS-1.1, SATA Revision 150 MB/s 1.0 SAS-1, SAS-1.1, SATA Revision 300 MB/s 2.0 SAS-2, SAS-2.1, SATA Revision 600 MB/s 3.0 4x bandwidth 600 MB/s 1200 MB/s 2400 MB/s 5