How does PSE (power sourcing equipment) know when a PD (powered device) needs POE?
In most networks, PSE will be connected to some devices that support PoE and some that do not. Consequently, in order to prevent damage to non-PoE devices, the 802.3af specification includes a negotiation mechanism between PSEs and the stations connected to them. Under the specification, PSEs apply a slight voltage on the power-delivery pairs. If there is a valid PD on the cable, it will present a specific resistance and a capacitance. Typically, this discovery process requires approximately 500 milliseconds. The PSE will apply the full wattage if it discovers a PD, but the flow is current-limited to prevent damage to the cable and equipment under fault conditions. After discovery, a PD must draw a minimum current for the power flow to remain active. If, for example, the PD is unplugged; the PSE will discontinue power supply over the cable. The discovery process will be repeated when the PD is returned to service.
