100BASE-TX Crossover Wiring

When connecting two stations together over a segment, the transmit data pins of one MDI must be wired to the receive data pins of the other MDI, and vice versa. For a single segment connecting only two computers, you can do this by building a special crossover cable, with the transmit pins on the eight-pin plug at one end of the cable wired to the receive data pins on the eight-pin plug at the other end of the crossover cable.

FIGURE 10.2 100BASE-TX crossover cable

However, when you are wiring multiple segments in a building, it's much easier to wire the cable connectors "straight through" and not worry about whether the wires in the jumper cables or other twisted-pair cables in your building have been correctly crossed over. The way to accomplish this is to do the crossover wiring inside the repeater hub. The 100BASE-TX standard recommends this approach, and states that each port of the hub that is crossed over internally should be marked with an "X."

Quick Reference Guide to 100BASE-TX - 09 SEP 95

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