HP X3800sb Serial Attached SCSI technology, 3rd edition - Page 6

SAS protocol evolution Compared to parallel SCSI, SAS provides a more robust, stable, and scalable connection for direct attach storage devices. SAS-1 The first-generation SAS (SAS-1) has a link speed of 3.0 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 limiting the size of its routing table. You can connect edge expanders together to form a device set that can address up to 128 devices. A fanout expander lets multiple edge expanders or edge expander sets communicate with each other. Fanout expanders have to maintain a more extensive routing table. SAS-2 The second-generation SAS (SAS-2) 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 includes zoning capabilities to improve resource deployment flexibility, security, and data traffic management. SAS-2 maintains backward compatibility with SAS-1. SAS-2 devices (initiator, target, or expander) may support more than one communication speed. If any two linked devices support multiple speeds, the devices use the highest supportable speed. The linked devices determine that speed at start up during a speed negotiation process. A sequential series of speed negotiation windows (SNW) characterize this process. In SNW-1 and SNW-2, linked devices test established combinations of SAS speed(s), transmission amplitude, slew rate, de-emphasis, and spread spectrum clocking (SSC). In SNW-3, the linked devices negotiate link speed and SSC settings. Unlike SAS-1, SAS-2 allows for training of the PHY and for exchange of parameters. After SNW-3 has negotiated the speed and settings, a training speed negotiation window (Train-SNW) tests the fastest commonly supported speed. SAS-2.1 The SAS-2.1 standard defines 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 equalizing filter. The equalizer removes inter-symbol interference (ISI), a form of signal distortion. The drivers and repeaters increase the signal-to-noise ratio by boosting the received signal and 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. Active cables include a low-power equalizing filter to compensate for the dielectric and conductor losses and reduce NEXT. 6