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I've always wondered why there are the two different wiring standards for Cat5 UTP cabling. The only technical difference between the two is the green and orange pair are swapped, which accomplishes the exact same thing as far as wiring goes (as long as the terminations are using the correct pin-outs).

Is there some historical reason that we have both standards? Is there a technical reason? Should I require my wiring to be one standard or the other? Or it just one of those strange and annoying things you have to deal with for no (readily apparent) reason?

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    *["The nice thing about standards is that you have so many to choose from."](http://en.wikiquote.org/wiki/Andrew_S._Tanenbaum)* – jscott Jul 18 '11 at 23:08
  • @jscott: I was just thinking this. I'm glad I refreshed before posting it... – Adrien Jul 18 '11 at 23:13

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The answer is that both schemes are around (and will probably remain around for the foreseeable future of CAT5 cable) for hysterical raisins. (The Wikipedia article alludes to the history here)

Basically the T568B standard is dead & deprecated at this point, but my advice is that in the absence of any specific guidance (i.e. when you're not using a punchdown block that has color/stripe codes marked on it) use the standard that is the most prevalent in your region (or company).

When using marked equipment, follow the markings.
When making a new deployment, use T568A.

voretaq7
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    +1 for hysterical raisins... and source – Nixphoe Jul 18 '11 at 21:40
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    "...T568B standard is dead & deprecated"??? Source? – dbasnett Jul 18 '11 at 22:17
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    Actually, per EIA-568C, both A or B are acceptable. – Scrivener Jul 18 '11 at 22:42
  • @voretaq - downvote until cited or corrected. – dbasnett Jul 18 '11 at 22:51
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    @DBasnett From the wiki: "TIA/EIA-568-B specifies that horizontal cables should be terminated using the T568A pin/pair assignments", leaving T568B for purely, ahem, hysterical raisins. Feel free to pay for a copy of the full specification document at www.tiaonline.org if you feel the need. – Hyppy Jul 19 '11 at 00:20
  • My issue was with the d & d statement. The specification has recommended A for some time now. All I can speak of with any certainty is that US commercial has been, and likely will continue to be, 568B. Or are you suggesting that we mix 568A and 568B? Maybe rip out all that 568B and install new 568A? I started installing twisted pair cable when coax was king, and it has all been 568B. – dbasnett Jul 19 '11 at 11:25
  • @DBasnett - Hyppy has it - I am suggesting that you follow the recommendation in the specification, and I believe my answer is fairly clear that you **ABSOLUTELY SHOULD NOT** mix specs in a single installation (emphasis added here to be even more clear). 568B continues to exist because "AT&T did it that way". 568B will continue to exist because it is so prevalent. 568A should be rolled out in new deployments unless there is a reason to use 568B (per the spec). Hope that's clear(er) :-) – voretaq7 Jul 19 '11 at 15:01
  • @voretaq7 - Maybe I wasn't clear. You made a claim (dead & deprecated) and I asked for a source that says THAT. As long as the standard permits either, 568B will be the standard in the US I think. You did give some good advice. – dbasnett Jul 19 '11 at 15:16
  • @dbasnett - I believe that a standard permitting both and implicitly discouraging one (by explicitly recommending the other) qualifies as "deprecated". To me anything that is deprecated is dead, or at least should be allowed to die peacefully in its sleep wherever possible. This is just my interpretation of the standard - there are many others, but this one is mine :-) – voretaq7 Jul 19 '11 at 15:33
  • @dbasnett - also re 568B remaining the standard in the US - it will certainly be around for a while - quite possibly forever. This is why I say to use "what is most prevalent in your region (or company)" - Despite a personal preference for one standard we have two, so you live with the one that's there. My current facility is wired for 568B, and will stay that way absent a good reason to rip out all the cable and start over. – voretaq7 Jul 19 '11 at 15:37
  • @voretaq7 - Like I said, you did give good advice. My issue is you mistaking your opinion for fact. "I believe that a standard permitting both and implicitly discouraging one (by explicitly recommending the other) qualifies as "deprecated". They recommended one. How you interpret that is opinion. "To me anything that is deprecated..." and then you use your opinion as fact. All I can speak to with any knowledge is US Commercial installations. If that is your environment using anything but the deprecated(sic) 568B is an invitation for problems. – dbasnett Jul 19 '11 at 16:42
  • Don't believe 568B is anywhere near dead. I never see or use anything else (UK). – BlueCompute May 25 '16 at 14:38
  • If you ask me, it’s 568A that’s around for *hysterical raisins.* USOC (which 568A hearkens back to) pre-dates AT&T 258A (later re-christened as 568B), so 568A seems to be the real throwback here. It wouldn’t come as a surprise to me if the only sustaining reasons as to why this dichotomy exists in the first place (and will continue to exist…) are: a) crossover cables, and b) some poor sods with backgrounds in telephony espied the *new hotness* that was 258A, and, sensing an opportunity w/ the crossover use-case, decided to wedge themselves into various committees to “set things right.” ;-) – Mark G. Jun 22 '17 at 03:05
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The reason had to do with analog voice circuits, and in particular having two separate voice circuits come to your residence.

The most prevalent in the commercial installed base in the United States is 568B (B for Bell ;).

see Registered Jack aka RJ-xx for secondary tip and ring.

New cabling should not use CAT5 / 5E. CAT6 or higher should be the choice.

dbasnett
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  • Note that Cat6 is only good for 1GbE, like Cat5e. If you want cable good for 10GbE you have to use Cat6a or Cat7. If you want 40/100 GbE you need Cat7a. – Chris S Jul 20 '11 at 19:43
  • Doesn't the max distance decrease when the speed is in excess of 1 Gbps, like half? – dbasnett Jul 20 '11 at 19:52
  • Cat6 may work for 10GbE up to 37m; Cat6a is rated for at least 100m. Cat7a is rated for 55m at 40GbE and 15m at 100GbE (though some models predict it will actually work at 100m with future chipsets). – Chris S Jul 21 '11 at 00:25