Ethernet Crossover Cable - Overview

Overview

The 10BASE-T and 100BASE-TX Ethernet standards use one wire pair for transmission in each direction. By convention, one wire of the pair is designated "+" and the other "-". Following traditional telephone terminology, the + signal from each pair connects to the tip conductor, and the - signal is connected to the ring conductor. This requires that the transmit pair of each device be connected to the receive pair of the device on the other end. When a terminal device is connected to a switch or hub, this crossover is done internally in the switch or hub. A standard straight through cable is used for this purpose where each pin of the connector on one end is connected to the corresponding pin on the other connector.

One network interface controller may be connected directly to another without the use of a switch or hub, but in that case the crossover must be done externally in the cable or modular crossover adapter in a manner similar to how the null modem was used to directly connect two teleprinters. Since 10BASE-T and 100BASE-TX use pairs 2 and 3, these two pairs must be swapped in the cable. This is a crossover cable. A crossover cable was also used to connect two hubs or two switches on their upstream ports .

Because the only difference between the T568A and T568B pin/pair assignments are that pairs 2 and 3 are swapped, a crossover cable may be envisioned as a cable with one modular connector following T568A and the other T568B (see Jack crossover wiring). Such a cable will work for 10BASE-T or 100BASE-TX. Gigabit Ethernet (and an early Fast Ethernet variant, 100BASE-T4) use all four pairs and also requires the other two pairs (1 and 4) to be swapped. This meant common crossover cables available in the retail market were usually not compatible with the Gigabit Ethernet convention, but newer crossover cables could be made that worked for all speeds. The polarity of each pair is not swapped, but the pairs crossed as a unit: the two wires within each pair are not crossed.