Storage Management Strategies for the CIO
Storage distance extension refers to any of several different technologies that allow data communication in Fibre Channel storage area networks (SANs) over long spans of fiber optic cable. Unless some form of storage distance extension is employed, cable loss, latency (the time required for an impulse to make a round trip), and wavelength "blurring" impose a distance limitation of a few kilometers between the source (transmitter) and the destination (receiver) in a SAN.
Storage distance extension can be accomplished by any of the following methods: In metropolitan area networks (MANs), wavelength division multiplexing (WDM) can extend the distance between the storage source and destination to approximately 600 kilometers (km). In wide area networks (WANs), a technique called Generic Framing Procedure (GFP) can extend distances to about 4000 km. Using GFP, buffer credits limit the number of unacknowledged frames that accumulate, reducing problems that occur because of latency. Optical repeaters receive signals from an optical fiber, convert the signals to electronic form, amplify them, convert them back to optical form, and then send them along another length of optical fiber. (A device called an erbium amplifier acts as an optical repeater that amplifies optical signals directly.) An extended-wavelength gigabit interface converter (GBIC) or extended-wavelength small form-factor pluggable (SFPFEC) can be used to reduce the error rate at the cost of some throughput. These methods can be useful in systems where the spans of cable are near the maximum limits obtainable with other technologies.
Storage area networks that employ storage distance extension are expected to become increasingly common as the demand for long-distance, high-volume, high-speed offsite storage increases.