In a PoE system the Power Sourcing Equipment (PSE), sends supply voltage over the Ethernet cable to the Powered Device (PD). Since the cable voltage can be delivered in any polarity, the first task of a PD is to correct or rectify the polarity using diode bridges. In 2-pair PoE systems, the voltage can be delivered over either the data pairs or the spare pairs but not both, while in 4-pair PoE systems all pairs are powered. Therefore, two bridges are required inside the PD.