Work experience-troubleshooting of twisted pair

The wireless AP is powered by a POE switch, and the fault is that it is unable to access the Internet through the AP. The corresponding Ethernet port on the POE switch flashes yellow several times after reboot, but the green light is not on; the AP light is always on. At first, I thought it was an AP fault, but after the replacement, the network cable was unplugged and plugged into an ordinary switch. the other end was connected to a laptop, found that there was no network connection, and determined that there was a physical problem with the twisted pair. After careful inspection, it was found that the first and second wires were in the wrong position, and the network returned to normal after re-doing the crystal head.

Note: POE power supply mode:

Mode A (idle line 45778 power supply): 45 (blue and white) is +, 78 (brown and white) is-

Mode B (data line 127.36): 36 (green, green) is +, 12 (orange, white, orange) is-

The supply voltage defined by the IEEE802.3af standard is DC 440057v and the supply voltage defined by IEEE 802.3at standard is 50V 57v, but data is usually transmitted using 1236 (orange-white, orange, green-white, green), 45 (blue-white, blue-white) power supply positive, 78 (brown-white, brown) power supply negative.