2.2 Fast Ethernet
2.2.3 100BASE-TX

Many existing 10-Mbps twisted-pair systems use a cabling infrastructure based upon Category 5 unshielded twisted-pair (UTP) and shielded twisted-pair (STP). The devices use two wire pairs within the cable: one pair on pins 1 and 2 for transmit and one pair on pins 3 and 6 for receive and collision detection. The 100BASE-TX Ethernet format also uses this infrastructure. Your existing Category 5 cabling for 10BASE-T should support 100BASE-TX, also implying that 100BASE-TX works up to 100 meters, the same as 10BASE-T. 100BASE-TX Ethernet format uses an encoding scheme like FDDI of 4B/5B. This encoding scheme adds a fifth bit for every four bits of user data. That means there is a 25-percent overhead in the transmission required to support the encoding.