Is it true that computers only need three of the four pairs to transmit data? I have the third pair on a very long wire, goes through conduit, etc. Can I switch one of the pairs and make it work?


It was the bluewhite and green pair.

  • A cable analyzer would be helpful here, to see if it's a break in the cable (and at what distance) versus a simple termination issue. – Brian Knoblauch Nov 9 '09 at 20:44
  • Is that what is used to determine which pair is broken? I have been using a tool for that. I do not have the tool that determines where approximately the line is broken at, just that all four pair are not firing. And the third pair is the culprit. – johnny Nov 10 '09 at 19:16

Of the four pairs, you use only 2 for normal 100base operation. Some forms of Gigabit ethernet uses all four pairs. If one pair went bad, you should be able to substitute another pair, but it isn't an ideal thing to do since it becomes a non-standard color code (potentially) and it limits your future speed. Which pair died - what colors on the wires?

  • but which pair can I use? which pair are the two that are supposed to be used? – johnny Nov 9 '09 at 18:49
  • Depending on your cabling, it would normally be white/orange, orange, white/green, blue, white/blue, green, white/brown, brown from left to right. That or swap green for orange. The only lines used are #1, #2, #3, #6 so use the others - the blue or brown pairs. – Blackbeagle Nov 9 '09 at 19:09
  • I do not mean to be dumb. But, since the third pair are not firing and that blows up on the white/blue, green, which wires do I not need that I can switch with those two wires? Are you saying use the last two the brown and brown/white in place of the white/blue, green pairs? – johnny Nov 10 '09 at 19:08
  • You would try to use white/orange, orange, white/blue, green, white/green, blue, white/brown, brown. – Blackbeagle Nov 10 '09 at 23:44

As long as you switch both ends correctly, that should work just fine.

If not, devices such as these (which use the spare lines in a Cat5+ cable to run a second set of signals) would not work.


If the cable goes through conduit i would think that the termination on the ends of the cable are the problem - try re terminating both ends first.

  • I've done that three times. That is not it. Thanks though. – johnny Nov 9 '09 at 18:48

Your standing cabling goes:

1  2    3  4     5   6    7   8
O  O/W  G  Bl/W  Bl  G/W  Br  Br/W

The Orange and Green pairs are used — pins 1, 2, 3 and 6. You could replace the Green with Blue and the Orange with Brown if you wish.

But regardless, I recommend attaching a new cable to the end of the existing one, and pulling it through (with someone feeding). Non-standard cabling is just asking for trouble down the road.

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