Ultra320 SCSI and adaptive active filtering: the alternative to transmitter pre-compensation - High Availability

Computer Technology Review, July, 2002 by Jorge L. Fernandez, Mark Evans, Russ Brown, Ivan Chan

Customer demand for faster data transfer rates, improved system performance, higher bus bandwidth, and better system reliability has never been greater. Applications like multi-stream video and audio, data warehousing applications, Web servers, RAID applications, large file transfers, non-linear editing, high-end graphics, electronic cinema, scientific data processing, video servers, image processing, high-definition playback, super computers and 3-D animation require state-of-the-art hard-disk drive (HDD) technology to meet system performance requirements.

SCSI technology continues to evolve to successfully meet the increasing demand for input/output (110) bandwidth. The SCSI interface offers stability, ease of connectivity, a large installed base, and a 20-year history of full backward compatibility. Ultra320 SCSI is the latest evolution and provides the highest-ever data bus bandwidth. Ultra320 SCSI is designed to handle the most demanding enterprise server applications.

The ANSI INCITS T10 Technical Committee (T10), with input from the SCSI Trade Association (STA), develops standards that define the requirements for the parallel SCSI interface. Standardized definitions for new SCSI transfer rates have been devised to prevent the interface from becoming a performance bottleneck.

Ultra320 SCSI, with a maximum transfer rate of 320MB per second, is the latest development for the SCSI parallel interface. Ultra320 SCSI is designed to handle the most demanding enterprise server applications. New features to enhance performance, improve data reliability, and increase ease of use have been developed by T10. Adaptive Active Filtering (AAF) is one of these features and will be discussed in detail in this article. For specifics on all Ultra320 SCSI features, refer to the ANSI draft standard document SCSI Parallel Interface-4 (SPI-4). The latest revision of this draft is available for review at www.t10.org.

Compensating for Transmission Line Signal Losses

One of the primary benefits of the SCSI interface over the ATA interface is its ability to support multiple devices on a single bus while providing higher reliability and signal integrity. By supporting more devices on the bus, SCSI provides better throughput by spreading the load across multiple hard disk drives, aggregating these drives' data rates over the SCSI bus. This results in exceptional system performance for enterprise-class servers and RAID applications. A cable interconnect system is used to provide a communication link between these devices. With this comes inherent transmission line issues, making it more difficult to achieve a good signal-to-noise ratio (SNR) to send and receive data accurately. As digital data transfer speed increases, the inter-symbol interference (ISI) effect in electronic communication via a long cable becomes significant to a point where simple voltage level detection is not reliable to extract data from the incoming signal. Ultra320 SCSI provides two methods to resolve t his problem: transmitter pre-compensation and adaptive active filtering.

Transmitter pre-compensation with cutback: This is an open-loop method of compensating for some of the signal loss that is most severe on the first part of a signal's transition. The transmitting device boosts the amplitude of the first part of the transition or cuts back the signal for the remainder of the transition. This provides additional signal amplitude where it is most needed and then decreases the amplitude to decrease the negative effects of cross-talk and reflections. Transmitter pre-compensation is sub-optimal because it cannot monitor changing conditions in the cable plant (i.e. adding devices to the bus). The signal boost is static.

Adaptive active filtering (AAF): Also known as "receiver equalization with filtering," AAF is a closed-loop method of improving received signal quality by amplifying the fundamental frequency of the signal in the receiver while filtering noise and other undesirable components. Devices implementing AAF establish the gain of their amplifiers by setting the amplitude of the high frequency portion of the training pattern to be the same as the low frequency portion at the beginning of the training pattern. Using the training pattern to perform this adjustment of signal amplitude provides for an inherent closed-loop system that can adjust signal quality for different cable plants and changes in system conditions (e.g., when a new device is added to a system causing the electrical characteristics of the cable plant to change). AAF settings may be adjusted as often as necessary because either the initiator or target may initiate a training pattern sequence. A receiver may disable transmitter pre-compensation in a tra nsmitter as a configuration using AAF performs better than a configuration using pre-compensation.

The AM Advantage

AAF improves signal integrity and, as a consequence, maximizes system performance. Signals processed with AAF enabled are sharper and have a better peak-to-peak amplitude definition. AAF enhances electrical signal margins reducing the effect of signal losses due to back-planes and cables. AAF compensates for variations in temperature, voltage, and process.