Reason for Ethernet wiring config?

[watercooled]

Is there a reason why Ethernet cables are wired they way they are i.e. the way one of the pairs is connected to pins 3 and 6 rather than using pins next to each other?

[synaesthesia]

I was always told it was to reduce crosstalk, as the pins kept furthest apart are rx- and rx+ - however I never understood why that would be the case for rx but not tx :/

[watercooled]

But why would it be only one pair if that was the case? I think there was an old standard wired like: [p1][p2][p3][p4][p4][p3][p2][p1] which had bad crosstalk problems because of how the pins were spaced, I thought using pins next to each other would be better in that respect.

[peterb]

I think it is historical. Pins 4, 5, 7, and 8 are unused for 10base100 in both T568A and 568B standards and the wiring looks similar to the 2 wire+ bell convention in UK telecomms and the US telecomms RJ11 wiring standard.

I found this article (pretty old) which gives some credence to that, but I can't see any electrical reason why the crossover should be there (and it makes wiring the plugs a pain!)

[Jay]

why is there and a and a b and why is b used more than a?

[watercooled]

I think it is historical. Pins 4, 5, 7, and 8 are unused for 10base100 in both T568A and 568B standards and the wiring looks similar to the 2 wire+ bell convention in UK telecomms and the US telecomms RJ11 wiring standard.

I found this article (pretty old) which gives some credence to that, but I can't see any electrical reason why the crossover should be there (and it makes wiring the plugs a pain!)

What I thought really, didn't think it could be an electrical reason.
@Jay: I think it's just down to tradition. A 10/100 crossover cable uses one standard on either end so maybe it has something to do with that.

[peterb]

I think B is UK/Europe, A is more USA - but why, I don't know, unless it is linked to the initial telecoms us (you can plug and RJ11 into an RJ45. (or strictly, a symmetrical 8/8 socket as there are "RJ45" plugs that have offset keyways)