100BASE-T is a subset of Fast Ethernet that refers collectively to a number of Ethernet wiring standards that carry signals at 100Mbps over twisted pair cables. Its IEEE-designated identifier is derived from its characteristics. "100" refers to its rate of transmission of Ethernet signals at 100Mbps. "BASE" is shorthand for "baseband transmission," meaning that the medium exclusively transmits Ethernet signals. "T" denotes its use of twisted pair cables as its medium.

There are three 100BASE-T standards: 100BASE-TX, 100BASE-T4, and 100BASE-T2. For their network topology, all three variations make use of a star configuration, which involves a central hub or switch to which a number of devices are connected, in essence forming a star.

100BASE-TX, the most commonly used among the Fast Ethernet standards, requires two pairs of high-quality cables (CAT-5 or better) for a maximum network segment distance of 100 meters. It provides a point-to-point connection between two devices, so a hub or switch would be required to connect more devices.

Data is transmitted 4 bits a time through a procedure from the ANSI X3.263 FDDI specifications. 4B5B binary encoding converts the bits into 0 and 1 symbols. Through NRZI encoding, the symbols are transferred to the physical medium attachment layer. MLT-3 then encodes the data stream for the data to transfer to the medium, dependent sub-layer. The data has a maximum fundamental frequency of 31.25 MHz.

100BASE-T4 requires four pairs of average-quality cables (CAT-3, which carries signals up to 10Mbps). The "4" at the end of its identifier refers to the number of cables it requires. Of these four cables, one pair is for transmission, one is for reception, and the other two will go in whichever direction is negotiated by the devices they connect.

Data is transmitted 8 bits at a time using an 8B6T code that converts the 8 data bits into 6 base-3 digits, which are sent using 3-level pulse amplitude modulation (PAM-3). PAM-3 converts the 3-digit data bits into their corresponding voltage levels.

100BASE-T2, like 100BASE-T4, makes use of CAT-3 twisted pair cables, but it only requires two pairs rather than four, hence the "2" in its identifier. Data is transmitted 4 bits at a time.

A non-trivial scrambling procedure based on a linear feedback shift register expands the 4 bits into two 3-bit symbols. 5-level pulse amplitude modulation (PAM-5) converts the 3-bit symbols into their corresponding voltage levels, i.e. "000" is converted to a voltage level of 0, "001" is converted to a voltage level of +1, "010" is converted to a voltage level of -1, "011" is converted to a voltage level of -2, and 100 converted to a voltage level of +2.