Trying figure out the key differences between T568A and T568B wiring, if any exist.

I found claims in a thread that T568A has advantages over T568B due to the fact that each twisted-pair in a cat5/cat6 varies slightly in the tightness of the winding. That would imply that, while nominal, selection of twisted-pairs for TX/RX does have a physical impact. (http://community.spiceworks.com/topic/422129-t568a-or-t568b)

Can someone explain the reason why one might claim 568A is better than 568B; or if this isn't true?

  • Unless you're a physicist trying to win a bet, there's no difference. Yes, there are different levels of twist per pair (something about the phase of any crosstalk, go ask the EE's.)
    – Ricky
    Commented Jan 21, 2015 at 2:45
  • en.wikipedia.org/wiki/TIA/EIA-568#T568A_and_T568B_termination should answer the question. Commented Jan 21, 2015 at 4:05
  • It's just about conceivable that which colour pairs you choose might affect the crosstalk marginally, so people who say it can't possibly make any difference are only 99% right. It's really hard to see how for something like 100BASE-TX where you use the green pair and the orange pair on one standard, and the orange pair and the green pair on the other it makes much difference. And as for 1000BASE-T, where you use all 4 pairs in both directions anyway...
    – richardb
    Commented Jan 22, 2015 at 23:52
  • Did any answer help you? if so, you should accept the answer so that the question doesn't keep popping up forever, looking for an answer. Alternatively, you could provide and accept your own answer.
    – Ron Maupin
    Commented Aug 11, 2017 at 3:34

1 Answer 1


Neither is "better." It's a matter of convention only. T568B has always appeared to me to be in much more widespread use.

It is true that bulk cat5/6 has variances in the tightness of the winding of each pair, but that wouldn't cause one spec to be more appropriate than the other. Only if one spec required more untwisting for termination or something like that could we say that it may be "worse" or degrade the transmission quality more.

  • 1
    The cable DOES vary the number of twists per inch from one pair to another in order to minimize crosstalk. You are correct that this has no real bearing on termination selection.
    – Ron Maupin
    Commented Jan 20, 2015 at 16:04
  • -A and -B specs were important when devices didn't auto-mdix and you wanted to connect between two like-pinned pieces of equipment. Now the difference is usually did you start work as a phone tech or a PC tech...
    – cpt_fink
    Commented Jan 23, 2015 at 5:50

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