Fibre Channel and SCSI: Building Bridges

Posted on June 01, 1998

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Fibre Channel and SCSI: Building Bridges

By Ahmad Zamer

The advent of Fibre Channel appears to have divided the storage world into two camps: serial Fibre Channel vs. parallel SCSI. In reality, relatively short field experience with Fibre Channel products has given rise to a third camp that calls for the coexistence of the two interfaces. The fact is SCSI and Fibre Channel will coexist for the foreseeable future.

There is no doubt that Fibre Channel specifications represent a significant step ahead of Ultra SCSI. There is also no doubt that Ultra SCSI suffers from basic limitations that complicate its use. However, the situation is not that simple. Users do not buy technology for technology`s sake. They purchase solutions to solve their problems. They look at SCSI and Fibre Channel products as the pieces to their storage puzzle.

The continuous evolution of SCSI makes it certain that serial storage technologies such as Fibre Channel will have to exist in an interface market that is dominated by SCSI for at least the next few years, according to industry analysts. With every SCSI improvement, the case for coexistence becomes that much stronger.

This is not new. New technology always coexists with old technology while the old is being phased out. In many cases, established technologies find ways to continue to exist alongside newer and presumed "killer" technologies. For example, many observers were quick to write off the floppy disk the moment they heard of CD-ROM--only to retract their denouncements when Iomega came out with the Zip drive. Today, the floppy disk is used by millions of people alongside Zip drives and CD-ROMs.

In this sense, networking technology bears a close resemblance to storage technology. In the local-area networking market, for example, Ethernet was and still is king despite rumors to the contrary when ATM entered the scene. Ethernet re-invented itself into a switched technology that rivals alternatives.

Why Coexistence?

Users need compelling reasons to switch from existing and familiar technologies to newer, unknown ones. In the case of ATM and Ethernet, for example, speed and distance were supposed to compel users to switch to ATM. But while ATM was taking shape, Ethernet evolved and addressed users` concerns. Ethernet suppliers were rewarded with customer loyalty and a boom in business.

Fibre Channel is riding the wave created by the promise of speeds of up to 100MBps, operating distances of up to 10 kilometers, and the ability to connect up to 126 devices on one Fibre Channel-Arbitrated Loop.

Is this reason enough for users to switch technologies? Yes and no. For users with little or no need for long operating distances, shared storage, or for bandwidths that exceed 80MBps, Fibre Channel proponents will be hard pressed to provide a compelling reason for them to switch. And for users with a small or moderate number of storage devices, SCSI is more than adequate. Finally, for users who need to support diverse devices such as optical disks, tape drives, and scanners, SCSI is still the only game in town.

Presenting new technology as a complement to more familiar and widely accepted predecessors ensures less resistance from users and a smoother transition for those who may eventually need the new technology. From a technology standpoint, replacing a well-established standard such as SCSI is a daunting task, regardless of Fibre Channel`s technical merits. The SCSI standard is widely adopted by vendors and users and it is the interface of choice for computer storage. In addition, SCSI has a widespread, well-established support infrastructure. There are many suppliers and alternative sources for SCSI components, from silicon and cables to host adapters and subsystems. Fibre Channel, on the other hand, does not yet enjoy such widespread adoption, nor is its support infrastructure anywhere near comparable to that of SCSI. Until such fundamental support is in place, Fibre Channel will face a steep climb, during which time coexistence will be wiser than a dogfight.

SCSI, unlike Fibre Channel, supports all types of storage devices, including hard drives, optical storage devices, solid-state disks, printers, scanners, tape drives, and removable storage devices. For now, Fibre Channel only supports hard drives.

And then, there is the issue of backward compatibility--or the lack thereof. The growth and dominance of SCSI is largely due to the SCSI community`s strict adherence to backward compatibility. So far, backward compatibility has not been Fibre Channel`s strong suit. Users who invested in quarter-speed Fibre Channel are still wondering about the ability of these products to work with newer generations of Fibre Channel products. This is yet another reason why the Fibre Channel community needs to think coexistence. Backward compatibility protects investments for both users and suppliers.

How to Coexist

Building bridges is a good starting point for a strategy of coexistence. For example, some vendors have developed Fibre Channel to Ultra-SCSI converters, or bridges, that enable users to attach their SCSI storage devices to Fibre Channel loops. These converters open the door to many possibilities. They protect users` investments in SCSI storage, while enabling users to benefit from the Fibre Channel technology features that are not available with SCSI.

While Fibre Channel needs to accommodate SCSI devices, SCSI needs to evolve and meet the challenges posed by newer technologies. The SCSI community recognizes this need and is moving on many fronts to ensure SCSI`s viability.

For example, in keeping with the tradition of doubling the bandwidth with each generation of the SCSI protocol, Ultra2 SCSI has a data transfer rate of 80MBps. In addition, Ultra2 SCSI is a dual-mode protocol, which supports low-voltage differential (LVD) and single-ended signals. This capability reduces the difficulties associated with the incompatible signals used in differential and single-ended SCSI. Finally, Ultra2 SCSI increases SCSI`s operating distances from 3 meters for four devices to 12 meters for 16 devices.

Connectivity has also been a limiting factor. Logical unit number (LUN) bridges, however, significantly increase the number of devices that can be connected to a single SCSI bus. These bridges enable users to connect up to seven devices to one SCSI ID, allowing users to address 105 devices with a single 16-bit SCSI adapter. With the right number of LUN bridges, a quad-channel host adapter can address up to 420 devices using one PCI slot.

Operating distance has been another constraint. However, the use of signal repeaters and bus extenders addresses this problem. These devices segment the bus into two domains and reproduce the SCSI signal, which enables users to double the operating distance. And multiple SCSI bus extenders can be cascaded to achieve even greater operating distances.

SCSI and Fibre Channel will continue to exist as complementary I/O interfaces. Both technologies will improve in ways that enable them to serve the storage market in unique ways.

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A LUN bridge connects up to seven on-line devices to one SCSI ID.

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Specialized converters extend the SCSI bus, converting single-ended and low-voltage differential signals.

Ahmad Zamer is product manager at ATTO Technology, Inc., in Amherst, New York. He can be reached at mail to:azamer@attotech.com


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