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In the T568 wiring, sometimes called the RJ-45, pin 3 and pin 6 have the same color pair. I've wondered why it was so decided. I think it would have been simpler to make the 3, 4 and 5, 6 pins the same color pair. Is there any reason?

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It's done that way because the 6p connector (standard telephone wiring, with either two wires:== RJ11 or four wires:==RJ14) puts TIP and RING on the center two positions, with TIP2 and RING2 (for a second phone line) flanking them.

The prime pair in the six-position plug is pins #3 of 6 and 4 of 6, and secondary pair is pins 2 and 5. Going to an eight-position connector for Ethernet, the scheme was kept for those four center wires, now renumbered 3-4-5-6 and new pairs were put on the 1-2 of 8 and 7-8 of 8 positions.

There was a long-ish period, with 10baseT and 100baseT wiring, which only used four wires, and some (annoying) cable systems only routed four wires for Ethernet; those weren't compatible with 1000baseT when gigabit Ethernet was introduced. The four-wire cables were using pins 1-2 and 3-6 of the eight-wire connector, so would allow a 4-5 pair to be added for telephones...

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    There was also the nice bonus during 10/100 days that if you needed an extra port in a location that already had one port and you didn't want to (or couldn't in some cases) put another switch in that location, you could split the cable into two parts, either by punching down appropriately to a pair of jacks or using an adapter on each end. Jun 14 at 14:15
  • I find it interesting that this was done purely for legacy compatibility. I have been laboring under the delusion it had something to do with stray capacitance in the wire.
    – Geo...
    Jun 14 at 16:03
  • It was more than just legacy compatibility. This actually allowed you to run a single cable and connect either a POTS phone (one or two lines) or even a typical phone system (e.g., Panasonic Hybrid) along with Ethernet. Sort of like you often now find a VOIP phone connected to one Ethernet jack with a jack on it to piggyback a computer. Jun 14 at 19:15

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