100BASEFX

100BASE-FX is a Fast Ethernet standard for transmitting 100 megabits per second over fiber-optic cabling. It is defined in IEEE 802.3u, published in 1995, and describes a point-to-point, full-duplex link that uses two separate fiber strands—one for each direction.

The physical layer of 100BASE-FX typically uses multimode fiber and transceivers operating around 1310 nanometers. Common fiber types include 50/125 μm and 62.5/125 μm multimode fiber, with connectors such as ST, SC, or FC. The standard envisions two fibers per link, enabling simultaneous transmitting and receiving in opposite directions.

Encoding and signaling for 100BASE-FX employ 4B/5B encoding with NRZI signaling, providing robust transmission over fiber. The nominal data rate is 100 Mbps, and the media is suited for relatively long distances compared with copper Ethernet. The maximum practical link length on standard multimode fiber is on the order of about 2 kilometers, depending on fiber type and transceiver characteristics.

In network deployments, 100BASE-FX is used to connect devices such as switches, bridges, and network interface cards over fiber links, often serving as uplinks or inter-building connections where copper is impractical. It complements other Fast Ethernet variants like 100BASE-TX (copper) and is part of the broader family of Ethernet standards that preceded gigabit Ethernet.

Today, 100BASE-FX has largely been superseded by faster fiber and copper technologies (such as 1000BASE-SX/LX and beyond), but it remains in use in older installations and environments with existing fiber infrastructure where higher speeds are not required.