Overview
editThe 10BASE-T and 100BASE-TX Ethernet standards use one wire pair for transmission in each direction. The Tx+ line from each device connects to the tip conductor and the Tx- line is connected to the ring. 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 latter. 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 terminal device 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. 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 must also be used to connect two internally crossed devices (e.g., two hubs) as the internal crossovers cancel each other out. This can also be accomplished by using a straight through cable in series with a modular crossover adapter.
Because the only difference between the TIA/EIA-568-B 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 connector following T568A and the other T568B. Such a cable will work for 10BASE-T or 100BASE-TX. 1000BASE-T4 which uses all four pairs requires the other two pairs (1 and 4) to be swapped and also requires the solid/striped within each of those two pairs to be swapped.
| Pin | connections | Pins on plug face (jack is reversed) | |
|---|---|---|---|
| 1 |  white/orange stripe |
 white/green stripe |
|
| 2 |  orange solid |
 green solid | |
| 3 | white/green stripe |
white/orange stripe | |
| 4 |  blue solid |
 white/brown stripe | |
| 5 |  white/blue stripe |
 brown solid | |
| 6 | green solid |
orange solid | |
| 7 | white/brown stripe |
blue solid | |
| 8 | brown solid |
white/blue stripe | |
Note: It really does not matter if your Ethernet cables are wired as T568-A or T568-B, just so long as both ends follow the same wiring rules. Typical commercially available "pre-made" cables can, and often do, follow either standard depending on who made them.
What this means in the "Real World" is that you may discover that one manufacturer's cables are wired one way, and another's the other way, yet both are "correct" and will work.
Other technologies use different pairs to transmit data, so crossover cables for them have different configurations to swap the transmit and receive pairs:
- Twisted pair Token ring uses T568B pairs 1 and 3 (the same as T568A pairs 1 and 2), so a crossover cable to connect two Token Ring interfaces must swap these pairs, connecting pins 4, 5, 3, and 6 to 3, 6, 4, and 5 respectively.
- A T1 cable uses T568B pairs 1 and 2, so to connect two T1 CSU/DSU devices back-to-back requires a crossover cable that swaps these pairs. Specifically, pins 1, 2, 4, and 5 are connected to 4, 5, 1, and 2 respectively.
- A 56K DDS cable uses T568B pairs 02 and 04, so a crossover cable for these devices swaps those pairs (pins 01, 02, 07, and 08 are connected to 07, 08, 01, and 02 respectively).