100BASE-FX is another variation of the 100Base-T standard with the exception that it is Fast Ethernet that is transmitting over optical fiber. Like the other 100Base standards, it uses 2 wires or in this case, strands of optical fiber in order to transmit data: one is for receiving (RX) while the other is for transmitting (TX).

The optical fiber utilizes 1300 nm near-infrared (NIR) light wavelengths that are transmitted at the rate of 100 Mbps. The 100Base-FX can be used in a maximum length of 412 meters if being used in half-duplex connections or as 2 kilometer lengths in the case of full-duplex transmissions over optical fiber.

100BASE-FX utilizes the same encoding line codes as 100BASE-TX does and is compatible with MX Series,M Series and T Series routers. 100BaseFX standard is based on IEEE 802.3u standard which in itself is a progression of the IEEE 802.3 specifications.

Similarly, 100BaseFX is also related to the 100BaseTX standard which is why when referred to collectively, they are referred to as 100BaseX. Similarly to the other 100Base standards, the 100BaseFX can be wired in a star topology that uses fiber-optic cabling in either Ethernet switches or fiber-optic hubs.

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