In uk.d-i-y, Lurch wrote:

The outlets I've bought mention T568B and T586A using what appears to be
the same wiring colours but in a different sequence. What's the difference?

ISTR it being something to do with interconnection with voice systems
and not using the same pairs.

No - the differences between 568A and 568B are only in the colour of wires
used, but both specify "straight-through" (when used at both ends of any
single run, natch) and keep the pairs just right for Ethernet.

There is a third wiring scheme which *is* incompatible with Ethernet over
distance, called USOC. That runs inner-to-outer across all 8 pins, so pairs
are used for 1-8, 2-7, 3-6, and 4-5. Since "normal" Ethernet uses 1-2 and
3-6, and for noise immunity over non-trivial cable lengths it really rather
does matter that these use a single twisted pair each, using USOC (which
was a voice "standard" which has almost-completely faded into history, and
is just about never seen on any new installs) knackers the reliability of
Ethernet over twisted pair... the wannabe balanced/out-of-phase
Tx+/Tx- signals which so very much wanted to be running over a single
twisted pair to-and-from pins 1-2 are now (sob, sob, feel the heartache)
split up (oh horror, horror, horror, tongue nor heart cannot conceive nor
name thee; a pair of star-crossed lovers; etc. etc. etc chiz chiz) across
(gasp) two different singles in two *different* pairs (never the twain
shall meet; die, die, die). Worst of all is that simple electrical
continuity tests as done by multimeter or "el cheapie" network testers
don't reveal split-pairs (it's the thousand-quid-and-upwards PentaScanner
and cousins which have the requisite smarts), and networking might still
"work", but be slow and unreliable because of all the retransmissions (as
higher-level network layers notice that there's Something Wrong and cause
resends). The original cable installer or "oh yes, this place is all wired
up" handwaver disappears, leaving a headscratching puzzle for whoever ends
up with network management responsibility....

Stefek