When we connect devices which need a power supply to the switch (such IP phones or APs), if the additional power lines are used, things would be so messy and sometimes the power line could not be used. In such secnario, the PoE could help us.
There are about 4 procedures during the power supply in PoE.
1. PD detection
The PSE device transmits a lower detection every 2 seconds; if the PSE detects a specify resistance, it begins to negotiate the power supply capability.
2. Power supply capability negotiation
The negotiation contains two modes: the analysis of detected resistance and the LLDP power capability negotiation.
3. Power supply
The PSE start to supply power to the PD. At the begining of the power supply, the power voltage increases from a low voltage to 48V in a short period. After that, the power voltage stabilizes at 48v.
During the power supply, if more PDs are connected to the switch, the PSE may no longer provide enough power voltage to all of the PDs, so the PSE needs to manage the power supply, or administrator could adjust this manually.
4. Stop the supply
When the PD is removed or the power consumption lowers to the minimum value, PSE stops the power supply for the PD and turns to the phase one, beggining to detect the PD.
