The Two Wiring Standards
TIA-568 defines two wiring arrangements for Ethernet: T568A and T568B. Both produce a fully functional Ethernet cable — the difference is only which color occupies which pin position in the RJ45 connector. T568B is more common in North American commercial installations; T568A is required by some government specifications and is more common internationally. The critical rule is consistency: both ends of a straight-through cable must use the same standard, and both jacks on a wall run must match the patch panel termination at the other end.
T568A vs T568B Pin Assignments
| Pin | T568A Color | T568B Color | Pair | Function (Gigabit) |
|---|---|---|---|---|
| 1 | White/Green | White/Orange | Pair 3 / Pair 2 | BI_DA+ (bidirectional) |
| 2 | Green | Orange | Pair 3 / Pair 2 | BI_DA- |
| 3 | White/Orange | White/Green | Pair 2 / Pair 3 | BI_DB+ |
| 4 | Blue | Blue | Pair 1 | BI_DC+ |
| 5 | White/Blue | White/Blue | Pair 1 | BI_DC- |
| 6 | Orange | Green | Pair 2 / Pair 3 | BI_DB- |
| 7 | White/Brown | White/Brown | Pair 4 | BI_DD+ |
| 8 | Brown | Brown | Pair 4 | BI_DD- |
Pins 4, 5, 7, and 8 (blue and brown pairs) are identical between T568A and T568B. The only difference is that pairs 2 and 3 (orange and green) are swapped. 100BASE-TX uses only pins 1, 2, 3, and 6 (two pairs). Gigabit 1000BASE-T and 10GBASE-T use all 8 pins bidirectionally.
Why Pair Integrity Matters
Ethernet uses twisted pairs specifically because twisting cancels out electromagnetic interference that affects both conductors equally (common-mode noise). The twist rate is carefully engineered for each pair. If the pairs are split incorrectly during termination — for example, pins 1 and 3 are used as a “pair” instead of pins 1 and 2 — the conductors are no longer twisted together and pick up noise differently. This is called a split pair and is one of the most common wiring errors. A split pair often still links (the electrical connection exists) but causes elevated near-end crosstalk (NEXT) that degrades performance or prevents gigabit negotiation. A wire map tester detects split pairs; a basic continuity tester does not.
Crossover Cables
A crossover cable has T568A on one end and T568B on the other. This swaps the transmit and receive pairs so that two devices of the same type (two computers, two switches) can connect directly without a switch or hub in between. Modern equipment with Auto-MDIX (Automatic Medium-Dependent Interface Crossover) detects the cable type and adjusts automatically, making crossover cables unnecessary for any current hardware. However, crossover cables still appear in older installations and are worth knowing how to identify: a wire map tester will show pins 1↔3 and 2↔6 swapped between ends.
Frequently Asked Questions
Does it matter whether I use T568A or T568B?
Only in the sense that you must be consistent. Both standards produce identical electrical performance for Ethernet. What matters is that both ends of a cable match, and that patch panel terminations match the keystone jacks at the wall. In practice, use whichever standard is already in use in the building — if patch panels are labeled T568B, use T568B throughout. Starting a new installation with no existing infrastructure, either standard is fine; T568B is the common default in North America.
My cable links at gigabit but sometimes drops packets — could the wiring be wrong?
Yes. A cable with a split pair or marginal termination can link at 1G and even pass most traffic, but elevated crosstalk causes bit errors that the MAC layer must retransmit. High retransmission rates show up as latency spikes and throughput lower than expected. Test with a proper cable certifier (not just a wire map tester) that measures NEXT, FEXT, and return loss. Alternatively, swap the cable with a known-good patch cord and observe whether the problem disappears — if it does, the cable is the issue.
What happens if I mix T568A and T568B on the same cable?
The result is a crossover cable (pairs 2 and 3 are swapped end-to-end). On modern equipment with Auto-MDIX, the link will still come up at the correct speed because the hardware detects and compensates for the pair swap. On older equipment without Auto-MDIX, the link will not come up at all. Either way, the cable is not wired to standard and should be re-terminated — accidentally having crossover cables in a structured cabling installation causes confusion during troubleshooting and may cause link failures as equipment is replaced with older hardware over time.