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FC-AL: A First Step

The Fibre Channel–Arbitrated Loop (FC-AL) transport protocol supports 200MBps throughput, dual-port channels and hot-pluggable drives. Storage vendors are positioning FC-AL as the leading interface for SANs.

Think of FC-AL as SCSI on steroids. While SCSI can connect up to only 15 devices, FC-AL lets you connect as many as 125 devices to a single host adapter. And while SCSI requires hundreds of wires to attach devices, FC-AL's loop architecture lets you use one cable to create a loop among the attached devices.

For example, Paramount Pictures in Hollywood, California, recently implemented the FC-AL transport protocol as the company's first step toward a SAN. The company has eight high-end servers and scores of workstations on an FC-AL loop that's used to transfer video for the TV show Entertainment Tonight. The ample throughput allows editors to send video to the production crew for live news feeds and makes the editing process much faster. It used to take a few minutes to pull up video; now it happens in real time.

Paramount went with Fibre Channel because it had a 17-year-old video-editing system in place that was totally inefficient and obsolete. When Paramount researched the options, FC-AL was easiest to implement and integrate with the existing editing software and production systems.

FC-AL is attractive because it's a dual-port technology. This means that a server can simultaneously send and retrieve documents to the same tape drive with no waiting.

FC-AL is also built for distance, which is useful in an environment such as Unisys's, in which tape libraries are scattered throughout the building. While SCSI allows only a 25-meter stretch between machines, FC-AL supports spans of up to 10km.

Moreover, FC-AL is less expensive to implement than SCSI. FC-AL host adapters cost between $900 and $1,400; SCSI adapters cost only about $170, but you need to buy one SCSI adapter for every 7 to 15 drives. And because FC-AL requires fewer adapters, there's less maintenance and upkeep.

One of the most striking differences between it and SCSI is network availability during daily backups. Backups used to push the network to its limit and usually took the entire night. During a backup, many telecommuters were unable to log on because the network was so bogged down.

Now that the SAN routes back up traffic for the best performance, it takes about one hour, and even during that hour, accessing the network is no problem. Telecommuters can be a lot more productive.

The implementation of FC-AL itself is usually a no-brainer. Everything gets wired into the backplane of the circuit board with very few software changes.

However, interoperability must be considered when choosing a SAN. Pay attention to the details and compatibility with existing systems, and you won't have much trouble.

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