How does a PoE injector negotiate power with a connected device?

A Power over Ethernet (PoE) injector is a device that adds power to an Ethernet cable, enabling non-PoE network switches or routers to deliver both power and data to a connected PoE-enabled device (PD), such as an IP camera, access point, or VoIP phone. The negotiation of power between a PoE injector and a connected device follows a standardized process defined by IEEE 802.3af, 802.3at (PoE+), and 802.3bt (PoE++) standards.

The power negotiation process involves three main phases:

1. Detection

2. Classification

3. Power Delivery & Maintenance

1. Detection Phase – Identifying a PoE Device

Before supplying power, the PoE injector checks whether the connected device is PoE-compatible.

--- The injector sends a low voltage (2V to 10V DC) on the Ethernet cable.

--- The connected device (if PoE-compatible) contains a signature resistance of 25 kΩ between specific wire pairs.

--- If the injector detects this resistance, it recognizes the device as a valid PoE-powered device (PD) and proceeds to the next step.

--- If no valid resistance is found, the injector does not provide power, preventing damage to non-PoE devices.

2. Classification Phase – Determining Power Requirements

Once the injector detects a PoE-compatible device, it determines how much power the device needs by following the IEEE PoE classification process.

The injector applies a 15V to 20V test voltage and measures how much current the device draws.

Based on the current drawn, the device is assigned to one of the PoE power classes:

If the powered device does not classify itself, the injector defaults to Class 0 (15.4W max).

3. Power Delivery & Maintenance Phase – Continuous Power Management

After determining the power requirements, the PoE injector starts delivering the required voltage (typically 48V DC) to the powered device.

--- The device only draws the power it needs within its classification.

--- The injector monitors power consumption continuously.

--- If the device disconnects or exceeds its power budget, the injector shuts off power to prevent damage.

Additionally, IEEE 802.3bt (PoE++) introduces Autoclass and LLDP (Link Layer Discovery Protocol) for more precise power negotiation, enabling dynamic power adjustments based on real-time needs.

Conclusion

A PoE injector follows a structured negotiation process to detect, classify, and supply power to a connected device safely and efficiently. By following IEEE PoE standards, the injector ensures that non-PoE devices are protected, appropriate power levels are delivered, and power efficiency is maintained. This makes PoE technology a reliable and scalable solution for powering networked devices in various applications.