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To forge new I/O architectures that will be unruffled by today's data-intensive applications, engineers have worked to overcome some of the inherent limitations of SCSI and bolster performance, data security and networking. And attention hasn't been limited to I/O alone; new developments in data storage topologies and subsystems that combine existing and new I/O standards add a layer of choices to the storage menu.

The Dating Pool The most successful of the new technologies are two serial SCSI standards that incorporate the existing SCSI command set: FC-AL (Fibre Channel-Arbitrated Loop) and IBM's SSA (Serial Storage Architecture).

FC-AL is based on fiber optics and operates as a truly arbitrated loop architecture with 126 nodes per loop. FC-AL can accommodate 100 MB per second per wire, for a total of 200 MB per second in a fully duplexed setup. The buzz is that FC-AL will hit 400 MB per second within the next year.

SSA has not been quite so popular. It, too, is a fiber-based standard that operates in a loop topology. But unlike FC-AL, SSA can accommodate 128 devices per loop and operates by simultaneously reading and writing in opposite directions around a given loop. SSA reads at 20 MB per second in each direction, and you can add multiple adapters--the most common configuration is two adapters, which can deliver 80 MB per second. This actually improves upon FC-AL, which, because of its stricter adherence to SCSI, can read in only one direction at a time. Upcoming SSA products reportedly will reach up to 160 MB per second.

Not only are FC-AL and SSA fast, but they serve up a tantalizing stew of enhancements. Out of the box, both can serve up to 30 meters (about 33 yards) with copper cables; use fiber optical cabling, and they can reach 10 kilometers (6.2 miles). Probably the most overlooked yet most important feature of both FC-AL and SSA is their ability to create fault-tolerant data storage networks instantly. Rather than using both wires in a full-duplex configuration, you can create two redundant data channels to each member of the FC-AL or SSA chain. This means you can create instant mirroring, or use firmware to create on-the-spot data redundancy checks.

Parallel SCSI's future may be limited, but parallel SCSI engineers haven't thrown in the towel just yet. Recent standards such as SCSI-3, Ultra2 SCSI and differential SCSI have bolstered SCSI's bandwidth and distance. In fact, Wide Ultra3 SCSI, which was still under development at press time, promises a data transfer rate of 160 MB per second, with 16 devices strung out over a 12-meter chain.

The top of the line for SCSI is Ultra2, with 40 MB per second and 8 devices, and Wide Ultra2, with 80 MB per second and 16 devices. Wide Ultra3 ups the ante to 160 MB per second with 16 devices. These are all available as LVD (Low Voltage Differential) SCSI, allowing the SCSI bus to extend to 12 meters in length. Still, SCSI doesn't address the distance and redundancy features delivered by SSA and FC-AL.