a google nemáme?
The T568B standard matches the older ATA&T 258A color code and is/was(?) the most widely used wiring scheme. It is also permitted by the ANSI/TIA/EIA standard, but it provides only a single pair backward compatibility to the USOC wiring scheme. The U.S. Government requires the use of the preferred T568A standard for wiring done under federal contracts.
http://www.duxcw.com/faq/network/diff568ab.htm
http://cableorganizer.com/articles/difference-betw een-T568A-and-T568B.html