100BaseT4 or 100Base-T4
From Network Dictionary Wiki
100BaseT4, also known as 100Base-T4, is a 100-Mbps baseband Fast Ethernet specification using four pairs of Category 3, 4, or 5UTP wiring. To guarantee proper signal timing and quality, a 100BaseT4 segment cannot exceed 100 meters in length. 100BaseT4 is part of the IEEE 802.3 standard.
100BASE-T4 was an early implementation of Fast Ethernet. It requires 4 twisted copper pairs and uses them in a rather mind-boggling way. One pair is reserved for transmit, one for receive and two pairs will switch direction as negotiated. A very unusual 8B6T code is used to convert 8 data bits into 6 base-3 digits (the signal shaping is possible as there are three times more 6 digit base-3 numbers than 8 digit base-2 ones). The two resulting 3 digit base-3 symbols are sent in parallel over 3 pairs using 3-level pulse-amplitude modulation (PAM-3).