What is modal bandwidth, and how does it effect what distances gigabit ethernet can travel over fiber?
Modal bandwidth is caused by the fact that light in multimode fiber travels in rays or "modes" that take different times to get to through the fiber, causing dispersion. The longer the fiber, the greater the effect. This is a major factor in the distance limitation of GBE and the incentive for fiber manufacturers to develop better multimode fiber.
While the worst case distance for 62.5/125 FDDI-spec fiber using a 850 nm VCSEL source is only 220 m, laser-optimized 50/125 fiber capable of 1 km is now available.
http://www.thefoa.org/tech/FAQS/FAQ-GENL.htm
