ethernet cables - why change positions of the wires?

[Icepick]

What is the reason that the Ethernet cable standard specifies switching the placement of the pairs to pin outs 3,6 and 4,5. I'm guessing that the answer has something to do with the physics of electricity. But why?

 

[drebo]

It has to do with the physical properties of twisted pair wiring when passing electrical current over it.

Also, Fast Ethernet only uses pins 1,2,3,6.

 

[PandaBear]

It has to do with 4 and 5 being a twist pair in the RJ11 if plugged in, and RJ11 is the standard phone cable.

 

[Icepick]

It has to do with 4 and 5 being a twist pair in the RJ11 if plugged in, and RJ11 is the standard phone cable.

Oh OK. I thought there might be some performance benefit to splitting the second pair between 3 and 6. Reducing inductance from causing some interference on one of the wires.

Thanks!

 

[MtnMan]

Wires are twisted in pairs, and must be used in pairs or you cannot transmit data more than a few feet. Twisting the pair greatly reduces the crosstalk between pairs, extending the distance data can be transmitted. Learn more here http://en.wikipedia.org/wiki/Twisted_pair

Cat 3,5,5e,6,6a uses pairs 1-2, 3-6, 4-5, 7-8. This scheme is not just for Ethernet, but will support phone line, both analog and digital, Token Ring, ATM and others.
10, 100 ethernet only uses 1-2 and 3-6, Token Ring uses 3-6 and 4-5, ATM 1-2 and 7-8.