The wiring standards for RJ45 data jacks and plugs, according to ANSI, TIA & EIA, includes two variations: T568A and T568B. These two standards are very similar. In fact, the only visible difference between the two is that pairs 2 and 3 (green & orange) are swapped. More so, in many cases, choosing to use one or the other is only a matter of preference. Still, they are not the same, and it is important to know and understand the differences before building or expanding a network.
Both T568A and T568B provide wiring schemes for terminating twisted-pair copper network cable (CAT cables) to 8-position RJ45 jacks and connectors. The pairs in these cables consist of four colors (blue, orange, green and brown), with each pair includes a solid-colored wire twisted with a wire of the same color, with white stripes. When untwisted, the 4 pairs result in 8 individual wires: one for each pin of the jack or plug.
When looking closely at the two wiring diagrams below, the only visual difference between T568A and T568B is that the pin positions for the green and orange pairs are flipped. Aside from the color placement variances, there are a couple of compatibility factors that can affect the choice of an RJ45 wiring scheme.
T568B is a more up to date scheme and also the most widely chosen wiring schematic because it matches AT&T’s old 258A color code. At the same time T568B accommodates for current and future needs. In addition, T568B offers backward compatibility with USOC, though for only one pair.
Schemes T568A and T568B:
So which one should one choose?
When building a new network, one may technically pick any one of the wiring schemes. No one scheme is better than the other, or is better suited for specific things. Both schemes are perfectly fitted for any installation type.
But when an existing network is being expanded, it is crucial to use the scheme in place. If the schemes do not match, data signals will not transfer. As a rule of thumb, T568A and T568B should not be combined or interchanged. In other words, use only one scheme in any one network. Do not use both.