2.2 Fast Ethernet
2.2.4 100BASE-T4
Not all building infrastructures use Category 5 cable; some use Category 3. Category 3 cable was installed in many locations to support voice transmission, and it is frequently referred to as voice-grade cable. It is tested for voice and low-speed data applications up to 16 megahertz (MHz). Category 5 cable, on the other hand, is intended for data applications, and is tested up to 100 MHz. Because Category 3 cable exists in so many installations, and because many 10BASE-T installations are on Category 3 cable, the IEEE 802.3u committee included this as an option.

As with 10BASE-T, 100BASE-T4 links work up to 100 meters. To support the higher data rates, 100BASE-T4 uses more cable pairs. Three pairs support transmission and one pair supports collision detection. Another technology aspect to support the high data rates over a lower bandwidth cable comes from the encoding technique used for 100BASE-T4. 100BASE-T4 uses an encoding method of 8B/6T (8 bits/6 ternary signals), thus significantly lowering the signaling frequency and making it suitable for voice-grade wire.