Quick Answer: A straight through cable uses the same pinout at both ends, T568B on both plugs; a crossover cable puts T568A on one end and T568B on the other, which swaps the transmit pair onto the receive pair. Since auto MDI-X became standard around 1998, nearly every switch and network card corrects the pairing in hardware, so a straight through lead works for every normal connection. Buy crossover cable only for legacy hubs, direct host links on old equipment, or a fixed installation that specifies it.
The wiring difference is only two pairs, but the consequence depends on the link speed. At 100 Mbit/s the transmit pair sits on pins 1 and 2 and the receive pair on pins 3 and 6, so crossing them is what makes two transmitters talk. At 1 Gbit/s all four pairs carry data in both directions at once, which removes the reason for a crossover entirely.
The two pinouts in one view
T568B orders the pairs white-orange, orange, white-green, blue, white-blue, green, white-brown, brown. T568A swaps the orange and green pairs, so pins 1 and 2 become the green pair and pins 3 and 6 the orange pair. A straight through lead holds one standard at both ends; a crossover holds T568A at one end and T568B at the other.
| Connection | Cable | Pairs swapped | Result |
|---|
| PC to switch | straight through | none | link at any speed |
| PC to PC, 100 Mbit/s | crossover | 1-2 with 3-6 | link without auto MDI-X |
| Switch to switch, old | crossover | 1-2 with 3-6 | link, uplink port not used |
| PoE camera to injector | straight through | none | power and data |
| Two hosts, 1 Gbit/s | either | irrelevant | link, all four pairs used |
What auto MDI-X changed
Auto MDI-X sends a short training pattern at power up, listens for a reply and swaps the internal transmit and receive paths if nothing arrives. That negotiation runs before the link trains at speed, which is why a straight lead into an uplink port now works where it once needed a crossover. A port that fails to negotiate drops to 100 Mbit/s half duplex or shows no link at all.
When a crossover is still the right answer
Legacy switches and hubs
Industrial equipment built before about 2000, and some current DIN rail switches, still lack auto MDI-X. A crossover lead between two such switches restores the uplink without occupying a dedicated port.
Direct links in test setups
A bench link between two single board computers with fixed MDI ports needs the crossover, and it is easier to keep one labelled lead than to reconfigure a driver. Mark it at both ends, because a crossover in a patch panel is a fault waiting to be found.
Gigabit and the four pair rule
Why speed removes the problem
1000BASE-T transmits on all four pairs simultaneously and separates directions with hybrids, so every pair is bidirectional. A crossover therefore has nothing to correct, and the link comes up as long as the pairs are properly twisted.
The cost of a cheap lead
At gigabit speed the failure mode is no longer wiring order but pair quality. A flat slimline ethernet cable with 30 AWG conductors passes 10 metres and fails at 40; a stranded flex ethernet cable is the better choice for a moving link, and a solid ethernet cable belongs in the wall.
Testing a lead of unknown origin
A basic tester reports continuity only, so a crossover and a straight lead both look good. Use a tester that displays pin to pin mapping, or check with two known straight leads and a switch: if the link comes up, the port pair is self correcting and the cable is fine. For anything going into a wall, certify the run rather than trusting a lamp.
Practical scenarios
- Situation: a 2003 vintage hub in a workshop that will not link to a modern switch – one crossover lead between them, labelled, and a note in the panel schedule.
- Situation: two servers linked directly for a migration at 10 Gbit/s – straight through, because both ports auto negotiate and the crossover adds nothing.
- Situation: a patch panel where an installer used T568A at one end and T568B at the other – the run works at 100 Mbit/s and fails at 1 Gbit/s, so re terminate one end rather than replace the cable.
- Situation: a stage rig where leads are re plugged nightly – stranded flex cable with moulded plugs, and no crossover leads in the same box.
FAQ
Does a crossover cable damage a modern port?
No. Auto MDI-X detects the swap and corrects it, so the link comes up normally. The only risk is confusion later, when the lead is used somewhere that needs a straight through connection.
Can a crossover cable carry PoE?
Yes, on alternative B where power rides the same pairs as data, because the DC path is unaffected by which pair is which. Voltage appears on both pair sets in most designs, so the powered device still receives it.
Is T568A or T568B preferred?
T568B is the common commercial standard in the US and either is acceptable if the whole site is consistent. Mixing the two in one installation is what creates accidental crossovers.
