Library » 2000

QAS Boosts SCSI Transmission Rates

by: Charles Gimar
Performance Analyst Storage Components Division
LSI Logic Corp. Colorado Springs, CO
Also appeared in EE Times, Nov. 3, 2000.

The growing demand for bandwidth to and from storage is putting pressure on protocols like the Small Computer Systems Interface (SCSI) point-to-point data-transmission standards. Such protocols must continually evolve to provide more data-transmission bandwidth, greater configuration options and improved management tools.

It is not sufficient to double or quadruple the bandwidth to increase performance. Additional changes must also be made to reduce nondata-overhead portions of the protocol. Quick Arbitrate and Select (QAS) is a technique, first included in the SPI-3 (Ultra160) SCSI standard, to reduce protocol overhead when devices arbitrate for and gain access to the SCSI bus. Even though QAS and Information Unit (packetized) data transfers are features of the SPI-3 standard, these features, with modification, are in the SPI-4 (Ultra320) SCSI draft standard. Systems containing Ultra320 SCSI production components implementing packetized data transfers using QAS can be expected in the latter half of 2001.

Simple Expanders Extend SCSI Bus Reach into the Future (PDF)
A white paper by David Rotman of Compaq Computer Corp. and Harry Mason of LSI Logic discussing how simple expanders (versus bridging expanders) allow for a significant increase in overall attainable SCSI bus length and SCSI domain diameter.

SCSI expanders are active interconnect components that logically connect and physically isolate segments of a SCSI domain. In addition to isolating segments, expanders allow for a significant increase in overall attainable SCSI bus length and SCSI domain diameter. SCSI expanders are available in two basic types, simple and bridging. For the purpose of this summary, we will largely concentrate on simple expanders. Simple expanders available today do not consume a SCSI ID and are capable of handling Single-ended (SE), and/or one of two modes of differential (Diff), Low Voltage Differential (LVD) or High Voltage Differential (HVD) SCSI transmission on either side.