From Phantom Power to 90W High Efficiency

PoE operates on the principle of Phantom Power. By applying a DC voltage across the center taps of the Ethernet pulse transformers, we can send electricity down the same copper wires used for data without interfering with the differential signals.

The PoE Handshake (Detection & Classification)

A PoE switch (the Power Sourcing Equipment, or PSE) does not just blindly send 50V down a wire—that would fry non-PoE devices. Instead, a sophisticated four-stage handshake occurs:

1. Detection: The switch sends a low voltage looking for a specific 25kΩ resistance signature on the line.
2. Classification: The Powered Device (PD) indicates how much power it needs (Class 1-8).
3. Power Up: If the switch has enough power budget, it raises the voltage to ~48V-57V.
4. Monitoring: The switch continuously monitors current draw. If the device is unplugged, power is cut immediately.

The Infrastructure Constraint: Heat

The biggest challenge with 90W PoE is not the switch—it is the Cable Bundle. When you bundle 48 Cat6 cables together, all carrying 90W, the center of the bundle can reach dangerous temperatures.

Conclusion

PoE++ is maturing into the dominant electrical delivery system for the smart building. By combining data and power into a single, low-voltage cable, we reduce installation costs and enable centralized power management for the entire network infrastructure.