2.2 Fast Ethernet
2.2.5 100BASE-FX
The 802.3u specification identifies a variant for single-mode and multimode fiber-optic cables. The 100BASE-FX Ethernet format uses two strands (one pair) of fiber-optic cables, one for transmitting and one for receiving. Like 100BASE-TX, 100BASE-FX uses a 4B/5B encoding signaling at 125 MHz on the optical fiber. When should you use the fiber-optic version? In situations with extended distance requirements, electrical interference concerns or security concerns. One clear case is when you need to support distances greater than 100 meters. Multimode supports up to 2000 meters in full-duplex mode, and 412 meters in half-duplex mode. Single-mode works up to 10 kilometers (km)-a significant distance advantage. Other advantages of fiber include its electrical isolation properties.

For example, if you need to install the cable in areas where there are high levels of radiated electrical noise (near high-voltage power lines or transformers), fiber-optic cable is best. The immunity of the cable to electrical noise makes it ideal for this environment. If you are installing the system in an environment where lightning frequently damages equipment, or where ground loops exist between buildings on a campus, use fiber. Fiber-optic cable carries no electrical signals to damage your equipment.

In security conscious locations fiber offers a more secure solution over copper, because it is more difficult to tap and does not give off radio frequency RF.

Note that the multimode fiber form of 100BASE-FX specifies two distances. If you run the equipment in half-duplex mode, you can transmit only 412 meters. Full-duplex mode reaches up to 2 kilometers (km).