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The definitions I've read say that crossover cable is cabling which the receive and transmit pairs are swapped around. My question is based on the fact that beyond this, I've seen some diagrams which swap the blue and brown pairs of wires as well as the orange and green wires.

What's the purpose of that - I thought those pairs are unused? Is it just for Gigabit Ethernet?

Do all types of ethernet cable classify as crossover as long as at least the transmit and receive pairs are swapped, and following that, would a cable with only the brown and blue pairs switched be called straight-through or crossover?

Edit: referring to T568A and T568B

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  • 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 can provide and accept your own answer.
    – Ron Maupin ♦
    Feb 19, 2018 at 20:00

3 Answers 3

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"Local custom terminology" in many places I've worked is:

Straight
1  2  3  4  5  6  7  8
1  2  3  4  5  6  7  8

4-pair Crossover
1  2  3  4  5  6  7  8
3  6  1  7  8  2  4  5

2-pair Crossover
1  2  3  4  5  6  7  8
3  6  1  4  5  2  7  8

Flat Swap or Rollover
1  2  3  4  5  6  7  8
8  7  6  5  4  3  2  1

2-pair crossover was for ethernet but also allowing other use of the cable, such as phone usage. 4-pair crossover is just for ethernet, but is needed if you have 4-pair ethernet.

We liked straight white for voice, yellow straight for ether, red for crossover. 2-pair crossovers went in the bin.

Ethernet crossover cables are not much used now that auto-crossover ethernet ports are so common.

Flat swap cables are mostly common in Cisco-type RS-232 console use and are normally "flat" cable, not twisted pairs. It is, of course, a crossover cable, but not for ethernet.

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Ethernet can run on different media, e.g. fiber-optic cabling (supposed to be installed in a crossover fashion, while UTP cabling is supposed to be installed straight-through). So when you refer to an ethernet cable, you really need to explain to which medium you refer.

A crossover cable is where the transmit line(s) on one and go the the receive lines on the other end, assuming both devices are the same type (both DTE or both DCE). Remember that UTP cabling is used for more than just ethernet, so don't limit yourself to believing that.

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  • My bad: thanks. I've updated my question.
    – VortixDev
    Nov 4, 2017 at 17:06
  • @VortixDev, remember that UTP cabling should be installed in a structured manner (see ANSI/TIA/EIA 568, Commercial Building Telecommunications Cabling Standard), which is agnostic to the protocol used. That means that the cabling is installed without regard to it being used for ethernet, POTS, token ring, etc. What works for 100BASE-TX (only crossing two pairs) may not work for a different LAN standard. LAN standards change, which is the reason for the structured cabling standard: you don't rewire when you change the LAN protocol you are using, and you may have multiple in the same building.
    – Ron Maupin ♦
    Nov 5, 2017 at 17:27
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we just ran some Ethernet in the home and I pinned out some connectors myself. You can find wiring guides just about anywhere, do a relevant Duck or Google search. Reading many of these, the "standard" default patch wiring appears to be T-568B both ends. Crossover is simply introducing T-568A at one end or the other.

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  • Typically, home wiring is always T568A, while businesses normally use T568B. In any case, home networking is explicitly off-topic here.
    – Ron Maupin ♦
    Jan 11, 2020 at 17:32

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