There should be 4 pairs of wires in the standard Category 5 cable. When a twisted pair is used as the transmission medium, only two pairs (4-core) of wires are required to complete the transmission and reception of signals. There are three standards in the fast network using twisted pair as the transmission medium, 100Base-TX, 100Base-T2 and 100Base-T4. Among them, the 100Base-T4 standard requires the use of all four pairs of lines for signal transmission, and the other two standards only require two pairs of lines. The 100Base-TX standard is the most popular in the fast network, so when we buy the twisted pair used in the 100M network, do not use a twisted pair with only 2 pairs. In the American Cable Standard (AWG), Category 3, Category 4, Category 5, and Category 5 twisted pairs are defined as 4 pairs. In Gigabit Ethernet, all 4 pairs of lines are required to communicate.