Fibre Channel, SCSI and You

Today's environment demands very fast transfer of large volumes of information. No wonder the Fibre Channel (FC) and Fibre Channel Arbitrated Loop (FC-AL) storage interfaces have generated so much interest. If you compare SCSI's current high-end 40MBps data transfer rate with FC's high-end 106MBps, you might think you can realize a 265 percent improvement in performance. But is FC that much faster? The Windows NT Magazine Lab compared the two interfaces.

ANSI developed the FC Standard in 1988 as a practical, inexpensive, and expandable method of using fiber optic cabling to transfer data among desktop computers, workstations, mainframes, supercomputers, storage devices, and display devices. ANSI changed the standard to support copper cabling; today, some kinds of FC use two-pair copper wire to connect the outer four pins of a nine-pin type connector, as you see in the photo. So despite the name, most current implementations of FC don't use fiber optic cabling.

FC-AL cable and connectors

SCSI cable and connectors

Copper wire connections work for up to 30 meters; beyond that distance, you must use fiber optic cabling and connectors. With fiber optic cables, you plug in an optic converter on both ends of the FC connection, and you can reach up to 10km. This distance is clearly superior to the current limitation of 25 meters for the differential SCSI technology. (For more information about SCSI, see Sean Daily, "SCSI and IDE: Defining the Differences," June 1997.)

FC's design is similar to the Open Systems Interconnect (OSI) network layers. FC supports several data communication protocols, including Fiber Distributed Data Interface (FDDI), High-Performance Parallel Interface (HiPPI), Intelligent Peripheral Interface (IPI)-3, Internet Protocol (IP), SCSI-3, Ethernet, Token Ring, and asynchronous transfer mode (ATM).

You can conFigure FC for data rates of 13.3MBps, 26.6MBps, 53.2MBps, and 106.4MBps; and it can achieve these transfer rates in both directions simultaneously (duplex). Thus, FC can transfer data at more than 200MBps, if usage is balanced in both directions--quite an improvement over the simplex and half-duplex interfaces such as SCSI. Work is under way for FC specifications of 400MBps (again, in both directions simultaneously; this technology could attain a data rate of more than 800MBps). Also note that an Ultra2 SCSI version is under development that will offer speeds up to 80MBps.

FC Configurations
You can conFigure FC ports three ways: in an FC-AL, in point-to-point links, or in a switch. The most common configuration (and the one you can buy now for Windows NT) is FC-AL, which ANSI developed to connect peripherals. In FC-AL, you usually connect the output of one FC device to the input of another FC device, and you connect the last device back to the first device, creating a loop. An FC-AL natively uses the SCSI-3 (SCSI FCP) protocols and can address 127 FC devices or nodes within the limitations of the 30-meter copper cable or the 10km optic cable. (A SCSI connection can handle only seven devices, excluding the computer.)

FC-AL is a simple closed serial loop. An FC-AL device has two connectors (one in and one out) an arrangement that makes connecting devices a breeze. You run a cable from the card in the computer to the first FC device and then connect each FC device in the chain to the next device. On the last device in the chain, you plug a loop-back connector into the out connector, which runs the data from the connector's send side to its receive side to complete the loop. Each connection uses four wires: two for transmitting and two for receiving. Using an electrical differential technique, FC (like differential SCSI) uses a balanced negative and positive wire pair to improve data integrity and to let you spread the network over greater distances.

FC and NT
Several vendors support FC in the NT market. Adaptec and Emulex have FC cards with drivers for NT, and Raidtec has a complete FC hard drive setup: Raidtec hard drive enclosure, an Emulex LightPulse PCI FC Host Adapter card, and Seagate FC-AL hard drives. Some manufacturers are producing hard drives for use in FC arrays. Unfortunately, no hardware-based FC RAID solutions are currently available. If you want RAID, you must use the slower, software-based RAID that is included with NT Server.

The Lab compared Raidtec's FC offering with its SCSI-3 RAID system, using an Amdahl 200MHz quad-processor Pentium Pro system with 512MB of RAM. Because hardware-based RAID isn't available for FC, we tested both systems using a four-drive stripe set, which writes the data evenly across all the drives without fault tolerance. To get a general idea of the difference in transfer speed, we timed copying NT's Service Pack 3 from one directory to another directory on the same disk. The FC system was about 5 seconds faster.