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Cable lengths for Fibre Channel SANs

by Rick Cook

A well-known rule of thumb for Fibre-Channel cable runs says they can stretch 10 km. But that rule is only for...

a best-case configuration. While this is the maximum possible operating distance, the 10-km specification may have to be reduced, depending on the installation you're planning.

The maximum distances in a typical Fibre Channel SAN depend on the kind of optical fiber used and its diameter. SAN manufacturers typically recommend maximum distances for various kinds of fiber. Cable manufacturers specify the highest frequency at which their cables will carry a signal of acceptable quality for one kilometer (MHz-km). This specification is called modal bandwidth.

The 10-km maximum for a Fibre Channel connection assumes the use of single-mode fiber, which is usually only used for long runs, such as to a remote mirror site. It is more expensive and requires more care in splicing and attaching connectors than multimode fibre, which is commonly used for installations within buildings, and has a much shorter range.

The usable length of the cable also depends on the size of the fiber, which is normally expressed as the diameter of the fiber/diameter of the cladding (i.e. a number like 50/125). Dell Computer recommends that 50/125 multimode fiber lengths be limited to 500 meters and the more common 62.5/125 fiber (used in most ATM and FDDI installations) be limited to 175 meters. 9/125 single-mode fibre has a recommended maximum length of 10 km.

The usual method of specifying fibre's distance capability is the modal bandwidth, defined in MHz-km. A 20 MHz-km fiber can carry a 20-MHz signal for one kilometer with acceptable quality, and a lower-frequency signal proportionally further.

Dell Computer has prepared a technical brief giving an overview of Fiber Channel media and connections for SANs, including the company's recommendations for its SAN products. It is available at:

Rick Cook has been writing about mass storage since the days when the term meant an 80K floppy disk. The computers he learned on used ferrite cores and magnetic drums. For the last twenty years he has been a freelance writer specializing in storage and other computer issues.

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