100BASE-T

In 100 Mbps (megabits per second) Ethernet (known as Fast Ethernet), there are three types of physical wiring that can carry signals:

100BASE-T4 (four pairs of telephone twisted pair wire)
100BASE-TX (two pairs of data grade twisted-pair wire)
100BASE-FX (a two-strand optical fiber cable)
This designation is an Institute of Electrical and Electronics Engineers shorthand identifier. The "100" in the media type designation refers to the transmission speed of 100 Mbps. The "BASE" refers to baseband signalling, which means that only Ethernet signals are carried on the medium. The "T4," "TX," and "FX" refer to the physical medium that carries the signal. (Through repeaters, media segments of different physical types can be used in the same system.)

The TX and FX types together are sometimes referred to as "100BASE-X." (The designation for "100BASE-T" is also sometimes seen as "100BaseT.")