What's the Difference Between T568A and T568B?
When it comes to wiring RJ45 data jacks and plugs, ANSI, TIA and EIA agree on two wiring standards: T568A and T568B. While these standards are very similar and can oftentimes be chosen according to nothing more than the installer’s preference, there are a few significant differences between the two, and it’s very important to know about these before you begin to build – or expand – your network.
How are T568A and T568B similar?
What do the T568A and T568B wiring schemes look like?
What makes them different?
Even though backward compatible with both one-pair and two-pair USOC wiring schemes, T568A has been largely superseded by the more up-to-date T568B. T568B and has become – overall – the most widely chosen wiring schematic because it matches AT&T’s old 258A color code, but at the same time accommodates for current and future needs. In addition, T568B offers backward compatibility with USOC, though for only one pair.
Can T568A and T568B be combined or interchanged? How do I know which one to use?
Why is it so important that cabling updates and additions be made in accordance with the network’s original wiring standard? Continuity. It’s simple, really… if the wires don’t match up color-to-color and stripe-to-stripe when plugs and jacks are connected, data signals just won’t transfer.
There are rare instances when T568A-wired components need to be connected to T568B-wired components, and in these cases, a crossover cable (a patch cord that has an A-configured plug at one end, and a B-configured plug at the other) is used to smoothly transition between standards without compromising data.
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There is no Transmission differences between the two cabling schemes. The North american preference for T568B is a local preference and in the wider world, T568A is the preferred option. But as long as both ends match it makes no difference to the transmission characteristics of data.
Looking in the latest TIA-568C.1 it stats that T568B is "optional" not prefered. If you want true backward compatibility with the old USOC wiring scheme then T568A is the wiring scheme "preferred".
It is a shame this article's Google rank is higher than the Wikipedia article, which has a much better explanation.