There are two types of pinouts for terminating twisted pair cable with a 8P8C (RJ45) connector:
But why must one not simply align the wires serially, e.g:
Green pair – Orange pair – Blue pair – Brown pair
Why do the current standards offer a nontrivial solution?
Best Answer
UTP cables used in the 586 standard inherited their color code from the 25 pair color code developed by AT&T for cabling used in telecommunication purposes.
The following table shows the combination of colors to be used for each pair
This gives us this set of combinations:
UTP cabling simply used the first four combinations.
The order used in 586a and 586b is compatible with 1-pair and 2-pair Universal Service Order Codes (USOC) pinouts that are a requirement in federal contracts by the U.S. government.
Pair 1 connects to the center pins (4 and 5) of the connector. It gives compatibility with the first line of RJ11, RJ14, RJ25, and RJ61 connectors that all have the first pair in the center pins of these connectors.
The position of the other wires is because signal shielding would be optimized by alternating the "live" and "earthy" pins of each pair.
However it isn't possible because the outermost pair would be too far to meet the electrical echo requirements of high-speed LAN protocols.
That's why only one pair is "untwisted" and used as a shielding for the central pair.