Beefchicken Industries

568A or 568B?

An argument that crops up over and over again is an argument of the relative merits of using 568A or 568B for ethernet cables. The argument is silly.

If you cut a piece of unshielded twisted pair cable, and you look at one end, the pairs in clockwise order will be blue, orange, green, brown. If you look at the other end of the same piece of cable, the clockwise order will be brown, green, orange, blue. This is an unavoidable constraint of the physical reality in which we and the cable co-exist.

Depending on which end of the cable you’re holding, the pairs will “lay” nicer in 568A order or 568B order when you’re trying to sort them out to stuff them in the RJ45 plug. If you’re trying to terminate the end that is more B friendly with an A colour code, you have to fight the pairs a little more.

Most equipment made this century supports “Auto MDI-X”, which means the equipment will automatically detect if it’s connected using a cross-over cable (568A to 568B) or straight through cable (A to A or B to B). So if a person really felt this was a problem, they could terminate one end 568A and the other 568B, and the equipment would figure it out on its own. No harm, no foul, but a bit silly solution to a non-problem.

But seriously, it doesn’t matter. The amount of twist lost to wrangling the pairs into the correct order is by no means a violation of the standard. It’s nothing to worry about at all.

In category 5 cable, each pair has a different rate of twist; the blue pair has the tightest twist at 1.83 turns per inch, followed by green at 1.65, orange at 1.42 and brown at 1.32, so it could be argued that certain pairs are better than others for high speed data, right?

Well, not really.

With 10Base-T/100Base-T ethernet, the ethernet signal is carried by pins 1,2, 3 and 6 of the ethernet jack, which corresponds to the green and orange pairs for 568A, or the orange… and… green pairs 568B. So it doesn’t matter which standard you use, you’re still using the same pair of pairs. There is absolutely no difference in signal integrity. And let’s be realistic, the standard for the last 20 years has been 1000Base-T, which uses all four pairs, and treats all four as equal.

And one final point, I’ve heard that X is better for data, Y is better for for voice (you choose which is A or B). Again, it doesn’t matter. Traditional voice (ye Plain Olde Telephone Service of yore) only uses the blue pair, which is always on pins 4 and 5 of the jack, which is the blue pair on both 568A and 568B.