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Seagate Cheetah 10k.6 Review Get to know Ultra320 SCSI: There are a host of benefits that come with Ultra320 SCSI. First and foremost, Ultra320 SCSI has doubled the transfer speed across the SCSI bus from a burst rate of 160 MB/sec to a staggering 320 MB/sec. The benefit is increased performance due to sustained transfers and interfaces that share multiple devices (as SCSI interfaces can support up to 15 devices). Second, Ultra320 SCSI is now "packetized", which means that is supports packet protocol. Packetized devices use a dual transition data phase as opposed to an asynchronous data phase. This means that multiple commands can be sent over one connection, reducing overhead and maximizing bus throughput. A third improvement with Ultra320 SCSI is the new Quick Arbitration and Selection (QAS) feature. QAS helps reduce monopolization of the bus by one device by reducing the overhead of the control release - the bus can change from using one device to another more efficiently and helps maximize the bus speeds. Aside from mere improvements in speed, Ultra320 SCSI also improves reliability. First, miniscule differences in wire lengths along SCSI cables and internal traces cause aberrant data signals. While this would not matter at lower speeds, the increase in data transfer rates means that these signal delays can cause data integrity problems. Ultra320 SCSI compensates for these differences by utilizing a new feature called Training Pattern. During a training pattern, test data is sent out across all connections simultaneously. The device then compares the received pattern to the known actual transmission to discover possibly delays, and then 'remembers' them. During normal operation, the device compensates for the signal delays, which increases reliability at such high speeds. Second, the doubling of Ultra320 SCSI's operating frequency also means an increase in signal attenuation and noise propagation - similar to what large network clusters can experience. Additionally, Ultra320 SCSI experiences a detrimental effect known as Inter-Symbol Interference (ISI). ISI causes bit edges of signals to be skewed, and is caused by long series of either all 1s or 0s. This causes the cable to undergo charging in the manner of a capacitor, the result of which is anomalous effects on small signals that travel in the opposite direction due to magnetic fields and electronic resistance.
Pre-compensation reduces ISI problems by reducing voltage over long strings of 1s or 0s, thereby reducing charge q on the wire and thereby reducing the negative effects of ISI.
Finally, a feature called the Adjustable Active Filter (AAF) helps compensate for signal noise. The AAF helps reduce signal noise by filtering out the frequencies that are higher than the maximum Ultra320 SCSI frequency range. This is almost imperative when working at such high frequencies.
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