I know that Ethernet over twisted pairs have two standards as the figure below:
The pairs are twisted to avoid cross-talk.
But why in both standards there is one pair that is not adjacent (pin 3 always pairs with pin 6)?
Once I have got a wrong long cable that was working intermittently. It was made with all rings adjacent to their corresponding tips. After correcting it worked fine.
But I am sure that I have already seen a short cable that worked fine too. Hence there must be an effect that this configuration avoids which I have googled and not found.
Best Answer
Data transmission use differential signalling, in pair two wires carry opposite polarity. It lower sensitivity to external interference and to cross interference (betwen different pairs in cable).
Pair 3,6 not adjacent only in jack. All other way it adjacent. In-adjacency in jack make low difference in interference, but make compatibility with phone, who use primary central pair (4,5)
As Tinti comment. If for transmission of differential signal used not twisted pair (it named split pair) cross talk raise dramaticaly and such link not work more that 10m. (UPD: not more that 10m in Full Duplex mode, that is auto detected in most cases)