This document discusses the requirements for different cabling systems. 100BASE-T and 100BASE-TX only require two wire pairs for transmission and reception, while 1000BASE-T requires all four pairs. While it is possible but non-standard to run two connections over Category 5 cable using the extra pairs with 10BASE-T and 100BASE-TX, 1000BASE-T needs all four pairs to operate so this is not possible with that higher speed cabling system.