SCSI Takes On USB At High-End

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April 08, 1999 (5:22 PM EDT)

By Mark Hachman,

Facing public challenges from other I/O technologies, the SCSI Trade Association released its technology road map at the Windows Hardware Engineering Conference in Los Angeles Thursday

Currently, SCSI exists in three generations: Ultra1 SCSI to Ultra3 SCSI, at bandwidths currently up to 160 megabytes per second. The trade association has charted a road map to Ultra5 SCSI by 2003, scaling up bandwidth to 640 MBps.

The road map was released as newer technologies threaten to infringe upon SCSI's traditional domain: high-end PCs, workstations, and servers. At the low end of the market, SCSI faces challenges from the second-generation version of the USB. At Intel's Developer Forum in March and again at WinHEC, Intel executives said they intend that USB 2.0 will replace SCSI in PCs.

In response, the trade association has scaled its road map to reflect theneeds of higher-end systems. According to Harold E. Mason, president of the SCSI Trade Association director of host marketing for LSI Logic, in Colorado Springs, the SCSI road map is designed for the amount of data four hard disk drives can transfer over a single bus simultaneously, the typical average.

The technology lets 16 devices be connected at once, while the USB 2.0 standard is designed with a single hard disk drive in mind, Mason said. In 1999, for example, a disk drive is expected to transfer about 20 MBps, scaling up to 88 MBps in 2003. At that time, four drives should transfer 353 MBps, leaving headroom for 16-drive arrays.

"From a mid-range to high-end users demanding external connectivity, for these users SCSI's the only choice," said Zulfi Qazilbash, product manager for Adaptec, in San Jose, Calif.

For enterprise applications, where IT managers are leery of adding new and unproved solutions, SCSI's installed base is its most potent argument. Disk drives with a SCSI interface represent 95 percent of high-end shipments, according to Dataquest, and 18.3 million drives shipped in 1998.

At the same time, SCSI has been challenged by Fibre Channel Arbitrated Loop and IEEE 1394 on the high end, Qazilbash said. But according to Qazilbash, Ultra3 SCSI outperforms Fibre Channel in most tests. "There's a place in the market for both; they don't necessarily overlap," he said.

Additional quality and management features will also be added in future revisions, including double-transition clocking, plus domain-validation margining and arbitration features to negotiate error-free data transfers between different generations of SCSI devices. A significant challenge will be to overcome SCSI's 12-meter cable length restriction; although "expanders" can serve as intermediary amplifiers of SCSI signals, they add additional cost, Mason said.