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?


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.

  • 3
    +1 for hysterical raisins... and source – Nixphoe Jul 18 '11 at 21:40
  • 3
    "...T568B standard is dead & deprecated"??? Source? – dbasnett Jul 18 '11 at 22:17
  • 1
    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
  • 3
    @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 0:20

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.

  • 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 0:25

Your Answer

By clicking "Post Your Answer", you acknowledge that you have read our updated terms of service, privacy policy and cookie policy, and that your continued use of the website is subject to these policies.

Not the answer you're looking for? Browse other questions tagged or ask your own question.