100BASE-T4 Multiplexing and Demultiplexing
In this image, we can see how data is transmitted by the Ethernet card and demultiplexed out onto three pairs of the Category 3 Cable. Bytes are multiplexed at the receiving end and placed back in the correct order. The fourth pair of the wire is used for collision detection. In this way, each wire only has to carry 33.3 Mbps, for an aggregate throughput of 100 Mbps. This is how 100 Mbps Ethernet can run on Category 3 unshielded twisted-pair cable.