Q: What problems do you perceive now or in the future for such high-speed applications as 10Base-T and ATM switching, using low-bandwidth fiber-optic cable; for example, 160/200 micron? It will be especially significant for system suppliers offering 15- to 20-year guarantees to determine whether their systems will handle the future bandwidths and speeds. Also, how important is it that fiber-optic cables connecting to each other have matching bandwidths?
A: I am not aware that "system sup- pliers" in the United States are promising that their systems will accommodate future bandwidth requirements. I did not know if this was a common practice outside of the United States, so I contacted Steve Swanson, with Corning Inc. (Corning, NY), who represents Corning in various international standards forums. "We are not aware of anyone who has guaranteed anything about future performance--bandwidth included," says Swanson. "What is typically promised is maintaining current performance levels."
While optical fiber provides a robust migration path to support higher-speed applications, there are limits--an issue that is now being discussed in several standards bodies.
The Fiber Optic LAN Section (fols) of the Telecommunications Industry Association (TIA--Arlington, VA) has developed premise fiber technology recommendations for horizontal cabling (less than100 meters), building cabling (less than 300 meters) and campus backbone (less than 2000 meters) for several applications. Bit rate, source characteristics, distance and fiber characteristics are all factors in each application-specific equation. For example, a 62.5-micron multimode fiber with 160 MHz/km bandwidth will support gigabit applications at distances to 300 meters, while a 50-micron multimode fiber with 500-MHz/km bandwidth will support gigabit applications at distances to 1000 meters.
Joining two fibers of different bandwidth is not a problem. However, the channel performance will be dominated by the lower-bandwidth fiber. Steve explained that "When many different-bandwidth fibers are concatenated [daisy-chained together], the overall bandwidth is usually better than would be expected on the basis of the linear calculation."
For channel design calculations, I would use the fiber characteristics of the lowest-bandwidth fiber to be used in the channel.
Donna Ballast is a communications analyst at the University of Texas
at Austin and a bicsi registered communications distribution designer (rcdd). Questions can be sent to her at Cabling Installation & Maintenance or at PO Drawer 7580, University of Texas, Austin, TX 78713; tel: (512) 471-0112, fax: (512) 471-8883, e-mail: firstname.lastname@example.org.