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: PoE Standard Class Power Output (Injector) Power Available (Device) Device Type 802.3af (PoE) 0 15.4W 0.44W to 12.95W Basic PoE Devices 802.3af (PoE) 1 4W 0.44W to 3.84W Low-Power Sensors 802.3af (PoE) 2 7W 3.84W to 6.49W IP Phones 802.3at (PoE+) 3 15.4W 6.49W to 12.95W Security Cameras 802.3at (PoE+) 4 30W 12.95W to 25.5W Wireless Access Points 802.3bt (PoE++) 5 45W 25.5W to 40W High-Power LED Lights 802.3bt (PoE++) 6 60W 40W to 51W PTZ Cameras 802.3bt (PoE++) 7 75W 51W to 62W Video Conferencing Systems 802.3bt (PoE++) 8 100W 62W to 71W High-Power Monitors   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.