What Are T568A and T568B?
T568A and T568B are RJ45 wiring standards defined by the TIA/EIA-568 specification. Each standard specifies which of the 8 color-coded wires inside an Ethernet cable connects to which pin inside an RJ45 connector. When you crimp an RJ45 connector, you arrange the individual wires in a specific color order before inserting them into the connector. That color order is what T568A and T568B define.
Both standards use the same 8 wires organized into 4 twisted pairs: orange, green, blue, and brown. Each pair consists of a solid-color wire and a white-striped wire. The only difference between the two standards is which pair goes to which pin positions.
Critically, both standards produce cables that deliver identical Ethernet performance. The signals do not care which color wire carries them. What matters is that both ends of the cable follow the same standard so the transmit and receive pairs connect correctly end to end.
T568B Wiring Standard (Pin-by-Pin)
T568B is the most widely used wiring standard in the United States. If you are terminating cable in a commercial building, data center, or home network, this is almost certainly the standard you should follow. The wire order from pin 1 to pin 8 is:
| Pin | Wire Color | Pair | Function (100BASE-TX) |
|---|---|---|---|
| 1 | White/Orange | 2 | TX Data + |
| 2 | Orange | 2 | TX Data - |
| 3 | White/Green | 3 | RX Data + |
| 4 | Blue | 1 | Unused (100M) / Bidirectional (1G) |
| 5 | White/Blue | 1 | Unused (100M) / Bidirectional (1G) |
| 6 | Green | 3 | RX Data - |
| 7 | White/Brown | 4 | Unused (100M) / Bidirectional (1G) |
| 8 | Brown | 4 | Unused (100M) / Bidirectional (1G) |
The mnemonic most technicians use for T568B is: "orange, green, blue, brown" for the solid colors, with each solid preceded by its white-striped partner, except that the green pair splits around the blue pair (pins 3 and 6 instead of pins 3 and 4). This split is not an error. The RJ45 pinout was designed to maintain backward compatibility with older telephone wiring standards, which placed the first pair on the center pins.
T568A Wiring Standard (Pin-by-Pin)
T568A swaps the orange and green pairs compared to T568B. The green pair moves to pins 1-2, and the orange pair moves to pins 3-6. The wire order from pin 1 to pin 8 is:
| Pin | Wire Color | Pair | Function (100BASE-TX) |
|---|---|---|---|
| 1 | White/Green | 3 | TX Data + |
| 2 | Green | 3 | TX Data - |
| 3 | White/Orange | 2 | RX Data + |
| 4 | Blue | 1 | Unused (100M) / Bidirectional (1G) |
| 5 | White/Blue | 1 | Unused (100M) / Bidirectional (1G) |
| 6 | Orange | 2 | RX Data - |
| 7 | White/Brown | 4 | Unused (100M) / Bidirectional (1G) |
| 8 | Brown | 4 | Unused (100M) / Bidirectional (1G) |
T568A is the standard specified by the U.S. federal government for new installations (per ANSI/TIA-568). It is also sometimes used in residential construction. The reason for the government preference is backward compatibility with the USOC (Universal Service Order Code) telephone wiring system. In T568A, the green pair on pins 1-2 aligns with USOC pair 2, meaning a single RJ45 jack wired to T568A can support both Ethernet and legacy telephone connections.
The Actual Difference: What Changes Between T568A and T568B
The difference between T568A and T568B is smaller than most people expect. Only 4 of the 8 wires change position. The blue pair (pins 4-5) and the brown pair (pins 7-8) are identical in both standards. The swap happens exclusively between the orange and green pairs:
| Pin | T568B | T568A | Changed? |
|---|---|---|---|
| 1 | White/Orange | White/Green | Yes |
| 2 | Orange | Green | Yes |
| 3 | White/Green | White/Orange | Yes |
| 4 | Blue | Blue | No |
| 5 | White/Blue | White/Blue | No |
| 6 | Green | Orange | Yes |
| 7 | White/Brown | White/Brown | No |
| 8 | Brown | Brown | No |
To summarize: pins 1 and 2 swap with pins 3 and 6. The orange pair and the green pair trade places. Everything else stays exactly where it is. This is the entire difference between the two standards.
Which Standard Should You Use?
The short answer for most people is T568B. But the real answer depends on your specific situation:
Use T568B if:
- You are starting a new installation in a commercial building, data center, or home network in the United States. T568B is the de facto standard and what the vast majority of pre-made patch cables, patch panels, and wall jacks ship with.
- You are extending an existing T568B network. Look at the existing patch panels, wall jacks, or the wiring diagram posted on the structured wiring panel. If they are T568B, continue with T568B.
- You are unsure and have no existing infrastructure to match. When in doubt, T568B is the safer default because it is what most technicians expect and what most documentation references.
Use T568A if:
- You are working on a U.S. federal government installation. ANSI/TIA-568 specifies T568A as the recommended standard, and most government building specs require it.
- Your existing infrastructure uses T568A. Consistency within a building is more important than which standard you use. If the building is wired T568A, every new cable run should be T568A.
- Your local building code or job spec calls for T568A. Some jurisdictions and contractors specify T568A for residential construction.
Where Each Standard Is Used in Practice
| Scenario | Standard | Why |
|---|---|---|
| Commercial office buildings (US) | T568B | Industry default; matches pre-made patch cables |
| Data centers | T568B | Consistency with vendor documentation and equipment |
| Residential networks (US) | T568B | Most common; matches consumer equipment |
| U.S. federal government buildings | T568A | Required by ANSI/TIA-568 for government installations |
| Some residential construction codes | T568A | Backward compatibility with telephone wiring (USOC) |
| International installations | Varies | Some countries prefer T568A; check local standards |
| Extending existing network | Match existing | Consistency within a building is the priority |
Crossover Cables: T568A on One End, T568B on the Other
A crossover cable is made by wiring one end to T568A and the other end to T568B. This swaps the transmit and receive pairs, which was historically required to connect two similar devices directly, like a switch to a switch or a PC to a PC, without a switch in between.
| Pin | End A (T568A) | End B (T568B) |
|---|---|---|
| 1 | White/Green | White/Orange |
| 2 | Green | Orange |
| 3 | White/Orange | White/Green |
| 4 | Blue | Blue |
| 5 | White/Blue | White/Blue |
| 6 | Orange | Green |
| 7 | White/Brown | White/Brown |
| 8 | Brown | Brown |
The only scenario where you might still need a crossover cable is connecting legacy equipment that predates Auto-MDI/MDI-X, or certain specialized industrial and embedded devices. Check the device documentation before making the cable.
Common Mistakes When Wiring T568A and T568B
Most T568A/T568B errors fall into a handful of categories. Knowing what to watch for will save you from unnecessary rework and troubleshooting.
Mixing standards accidentally
This is the most common mistake, especially for technicians who learned one standard and assume the other from memory. The orange and green pairs swap positions, but the blue and brown pairs stay put. If you remember T568B and try to "reverse" it to get T568A, you might accidentally swap the wrong pairs. Always reference a pinout diagram rather than working from memory. Keep a printed reference card in your tool bag.
Memorizing one standard and guessing the other
T568A and T568B differ only on 4 of 8 pins, but confusing which 4 pins swap is easy to do under time pressure. Some technicians memorize the T568B wire order by rote and then try to mentally rearrange it for T568A jobs. This is how split pairs happen. If a job calls for T568A, look at the T568A diagram directly.
Different standards on each end of a cable
Wiring one end T568A and the other T568B creates a crossover cable. On modern networks with Auto-MDI/MDI-X, the cable will likely still work, but it creates confusion for anyone who works on the cable later. A cable tester will flag this immediately. If both ends show a straight-through wiremap in the same standard, the cable is correct.
Different standards on the patch panel vs. the wall jack
This is a variant of the mixing problem. The patch panel in the closet might be punched down to T568B, but the wall jack in the office was punched down to T568A. The result is an inadvertent crossover on a permanent link. Modern Auto-MDI/MDI-X will compensate, but this should be corrected to prevent future headaches.
Split pairs from incorrect memorization
A split pair occurs when the correct pins are connected but using wires from different twisted pairs. For example, pin 1 uses white/orange (correct for T568B) but pin 2 uses white/green instead of orange. The wiremap shows continuity on all pins, but the two wires on pins 1-2 are not from the same physical twist, which destroys crosstalk cancellation. A basic wiremap tester may pass this cable. You need a tester with dedicated split pair detection to catch it.
How to Verify Your Wiring Standard
Visual inspection is a good first step, especially with pass-through connectors where you can see the wire color order through the front of the connector. But the only definitive way to confirm your pinout is with a cable tester.
Visual check (pass-through connectors)
If you are using pass-through connectors like the EZ-RJ45, the wires extend through the front of the connector before you crimp. You can visually confirm the color sequence against your reference diagram. This catches gross miswires but cannot detect split pairs.
Cable tester verification
A cable tester plugs into both ends of the cable and maps every pin connection. It will immediately show:
- Wiremap: Whether each pin on one end connects to the correct pin on the other end.
- Standard detection: Some testers (like the VDV MapMaster 3.0) identify whether the cable is wired T568A, T568B, or is a crossover.
- Split pair detection: Whether the wires on each pin pair come from the same physical twist.
- Opens and shorts: Any pins that are disconnected or bridged together.
For any installation beyond a single patch cable, testing every cable run with a proper tester is not optional. It is the difference between a clean install and callbacks.
Frequently Asked Questions
What is the difference between T568A and T568B?
The only difference is that the orange and green wire pairs swap positions. In T568B, the orange pair is on pins 1-2 and the green pair is on pins 3-6. In T568A, the green pair is on pins 1-2 and the orange pair is on pins 3-6. The blue pair (pins 4-5) and brown pair (pins 7-8) are identical in both standards. Both deliver the same Ethernet performance.
Should I use T568A or T568B?
Use T568B for any new installation in the United States unless a job specification or building code requires T568A. T568B is the dominant standard in commercial and residential networks. If you are extending an existing network, match whatever standard is already in place. The only common scenario where T568A is required is U.S. federal government installations per ANSI/TIA-568.
Can I mix T568A and T568B on the same network?
You should not. While individual cables will work regardless of which standard they use (as long as both ends match), mixing standards within a building creates confusion during maintenance and increases the chance of wiring errors. Pick one standard and use it consistently for every cable run, patch panel, and wall jack in the installation.
What happens if I wire one end T568A and the other T568B?
You have created a crossover cable. On modern networks with Auto-MDI/MDI-X (standard on equipment manufactured after roughly 2005), the cable will usually still work. However, it should be corrected to avoid confusion for anyone who works on the cable later. A cable tester will flag this as a non-standard wiring configuration.
Why does the U.S. government require T568A?
T568A is backward-compatible with the USOC (Universal Service Order Code) wiring used for telephone systems. The green pair on pins 1-2 in T568A aligns with USOC pair 2, which means a single RJ45 jack wired to T568A can support both Ethernet and legacy telephone connections without rewiring. This was a practical consideration when government buildings were transitioning from telephone to data networks.
Get the Pinout Right Every Time
Pass-through connectors let you see the wire color order before you crimp. Pair them with a cable tester and you will never second-guess a termination again.