I’ll refer to this webpage for standard pinouts for ethernet cable containing 4 twisted pairs. TX is on pair NO.2 (Orange) Rx is on pair NO.3 (Green) Ground is on pair NO.1 (Blue). My question is why the pair NO.3 (Green) is far apart, separated by pair NO.1 (Blue)? From electrical interference point of view, I would think that I should not separate differential pair far apart because it will form a antenna loop, emitting and receiving interference.
It's a carry over from Telecom equipment.
Also, the pinout from that link is for that specific piece of equipment in regard to the ground and power on pair 1 and 4.
http://www.computerports.net/ethernet-port/
This shows a little more accurate representation for the pinout.
Still, my question is not answered. When those early engineer designing the pin-outs, what are the technical reasons for separating the pair NO.3 far apart to Pin3&6?
I mean there must be some problems to assign pin1&2 for Pair NO.1, pin3&4 for Pair NO.2, pin5&6 for Pair NO.3, and pin7&8 for Pair NO.4 (this would be a naive way of assigning pin-outs to 4 twisted pair), right?
I believe it's to reduce crosstalk on the connector, but I'm no expert.
Ok, I asked my friend who study electrical engineer. He suspects that the pin-out design is for legacy differential receiver, which needs common mode reference. Those receiver will have common mode reference pins in the middle between Rx pair pins.