powering devices three modes, a, b, , 4-pair available. mode delivers power on data pairs of 100base-tx or 10base-t. mode b delivers power on spare pairs. 4-pair delivers power on 4 pairs. poe can used on 1000base-t ethernet, in case there no spare pairs , power delivered using phantom technique. mode has 2 alternate configurations (mdi , mdi-x), using same pairs different polarities. in mode a, pins 1 , 2 (pair #2 in t568b wiring) form 1 side of 48 v dc, , pins 3 , 6 (pair #3 in t568b) form other side. these same 2 pairs used data transmission in 10base-t , 100base-tx, allowing provision of both power , data on 2 pairs in such networks. free polarity allows poe accommodate crossover cables, patch cables , auto mdi-x. in mode b, pins 4–5 (pair #1 in both t568a , t568b) form 1 side of dc supply , pins 7–8 (pair #4 in both t568a , t568b) provide return; these spare pairs in 10base-t , 100base-tx. mode b, therefore, requires 4-pair cable. the pse (power sourcing equipment), not pd (...