Each insulated wire of a twisted-pair LAN cable is colored differently. The colors form a standard code, so that each wire may be easily found and terminated properly. The color code for 4-pair LAN cables is shown in Table 5.4. Each pair of the cable has complementary colors. For example, pair 1 wires are coded white-blue and blue-white. The white-blue wire is a white wire with a blue stripe at intervals along the insulation of the wire. (The stripe is sometimes called a tracer.) Conversely, the blue-white wire is a blue wire with a white stripe. In a 4-pair cable, white will be the common color of all the wires, and the white wire will always be numbered (or punched down) first. Each pair of a 4-pair wire may be referred to by its color that is unique from the other three pairs. Thus, the “blue” pair ontains the white-blue and the blue-white wires, while the “green” pair contains the whitegreen and green-white wires.
Color-coding and proper termination of each wire color is very important in LAN wiring because the signals are polarity sensitive; reversing a pair will cause a failure. The polarity of each pair is often referred to as tip and ring, which stems from the days of telephone plug-boards used to route calls. The switchboard plug consisted of three contact areas, referred to as the tip, ring, and sleeve, much like a modern stereo plug. The primary color was wired to the ring and the secondary to the tip. The sleeve was used for grounding.