Serial Attached SCSI technologies and architectures, 4th edition

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to reduce NEXT. Low-power, low-latency, active cables are economical and enable high data transfer
rates using thinner wire gauges over longer cable lengths.
Storage power management
SAS-2.1 devices can turn off SAS physical links when they are idle. Any initiator can target and use
power management functions. Each SAS transceiver consumes about 200 mW. Turning off all SAS
physical links saves a little less than 1 W for a dual-ported drive with two transceivers as well as a
controller (or attached SAS expander) with two transceivers. Table 1 shows more examples of power
savings for a 9 W small form factor (SFF) SAS drive.
Table 1: Example of power management states for a 9 W SFF SAS drive
State
Description
Commands processed
Power savings
Recovery time
Active
Fully active
Yes
None
None
Stopped
Same as Standby
No
~7 W
15 ─20 s
Differential signaling
Every SAS device includes one or more PHYs in its ports. A physical link 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
(Figure 2). The SAS interface lets vendors combine multiple physical links to create 2x, 3x, 4x, or 8x
connections per port for scalable bandwidth.
Figure 2: Examples of differential signaling
Differential signaling reduces the effects of capacitance, inductance, and noise experienced by
parallel SCSI at higher speeds. In differential signaling, the positive signal minus the negative signal
equals 1500 – 900 = 600mV or 900 –1500 = – 600mV (Figure 2). SAS communication operates in
full duplex mode, which means that each PHY can send and receive information simultaneously over
the two wire pairs.
Table 2 lists the physical link rates for SAS and SATA.