HP DL360 Serial Attached SCSI storage technology, 2nd Edition - Page 6

SAS protocol evolution SAS-1 The speed of the first-generation SAS (SAS-1) link is 3.0 gigabits per second (Gb/s). The original SAS standard defined two classes of expanders: edge expanders and fanout expanders. An edge expander attaches directly to targets, or to another edge expander, which reduces its complexity (and cost) by constraining the size of its routing table. Edge expanders can also be connected together to form an edge expander device set, allowing the set to address up to 128 devices. A fanout expander allows multiple edge expanders or edge expander sets to communicate with each other; therefore, it has to maintain a more extensive routing table. SAS-2 The second-generation SAS (SAS-2) link speed doubles the physical link rate to 6.0 Gb/s. SAS-2 eliminates the distinction between fanout and edge expanders by replacing them with self-configuring expanders. SAS-2 also enables zoning for enhanced resource deployment flexibility, security, and data traffic management. SAS-2 is backward compatible with SAS-1. Each SAS device (initiator, target, or expander) may support one or more SAS communication speeds. If any two linked devices support multiple speeds, the highest speed will be used if performance is critical or a slower speed may be used if reliability is more important. During this speed negotiation process, linked SAS devices determine their mutually supported SAS speed(s), and other speed-related options, such as transmission amplitude, slew rate, de-emphasis, and spread spectrum clocking. After the speed negotiation process is complete, the SAS-2 standard provides a time interval, or training window, during which the linked devices exchange predefined signals to test the link using the selected speed(s) and speed-related options. SAS-2.1 The enhancement to the SAS-2 standard, SAS-2.1, will define active cables, storage power management, and additional connectors (see ―Cabling and connectors‖). Also, SAS-2.1 splits out the protocol layer into a separate standard, SAS Protocol Layer (SPL). Active cables To help reduce cable weight, improve cable management, and improve airflow, the SAS 2.1 standard implements thinner cables with active circuitry. Active circuitry includes built-in drivers and repeaters, along with an equalizer. The equalizer removes inter-symbol interference (ISI), a form of signal distortion. The drivers and repeaters reduce the signal-to-noise ratio (SNR) by boosting the received signal and by reducing near-end crosstalk (NEXT). NEXT occurs when two wires are close enough for the signal traveling in one wire to interfere with the signal traveling in the other. Cable designers reduced this phenomenon by placing a low-power equalizing filter inside the cable to compensate for the dielectric and conductor losses. Economical, low-power, low-latency, active cables enable high data transfer rates using thinner wire gauges over longer cable runs (Figure 3). 6