How Power over Ethernet (PoE) Detection Works

Introduction

Power over Ethernet (PoE) detection is a critical function within a PoE system. Its primary role is to determine whether the remote equipment connected to a Power Sourcing Equipment (PSE) is capable of receiving power and identifying it as a Powered Device (PD). This detection process is essential because it ensures that power is only delivered to devices that are designed to receive it, thereby preventing potential damage to existing Ethernet equipment that does not support PoE. By accurately identifying PDs, PoE detection safeguards the network infrastructure and maintains the integrity of non-PoE devices.

How it Works

The detection stage is comprised of two phases, pre-detection and detection. Pre-detection determines if any new load has been added to the network. Detection attempts to measure the effective resistance of the newly added device to determine if it is a valid PD. Some variation in effective resistance is permitted by the PSE to compensate for cable resistance, the likelihood of the PD detection resistor being behind a rectifier, etc. Very little variation of effective resistance is permitted by the PD as it is the device under detection.

Pre-Detection

PoE pre-detection is a process where the PSE sends a discovery signal approximately every 2 seconds to detect any new load that has recently been attached to the network. This discovery pulse is less than 500 milliseconds in duration and is sent on both active and inactive Ethernet ports. The PSE continues to send discovery pulses until it identifies that a pairset needs power, even if the device connected is not a PoE device. This ensures that the network can efficiently detect and manage new devices as they are connected.

Detection

PoE detection is a process where the PSE probes the pairset(s) for a PD and applies power only when a valid PD detection signature is found. For PSEs configured to apply 4-pair power, detection is performed on both pairsets. An effective resistance of 25 kΩ indicates that the remote device is a PD capable of receiving power.

During detection, the PSE measures the effective resistance of the PD using at least two voltage/current points, which must be at least 1V apart and within the range of 2.8V to 10V at the PSE Power Interface (PI). The PSE may source a voltage and measure current or source a current and measure voltage. It applies a series of low-voltage pulses, less than 10V, to measure the differential resistance and input capacitance of the PD device.

When the PD controller detects pulses in the range of 2.7 to 10V, it connects an external detection resistor to provide the detection signature. After detection, the PD disconnects the detection resistor to save power. The PSE cannot produce a voltage higher than 30V or source more than 5 mA of current during detection and it cannot power a pairset unless a valid detection signature is present.

Criteria

To ensure a valid detection signature in PoE systems, an Ethernet cable pairset must meet several criteria. Using differential resistance measurement allows static voltage offsets in the PD to be accounted for.

The effective resistance equation is:

Voltage offset can be calculated from the resistance measurement as follows:
\begin{equation} V_{OFFSET} = \frac{{V_{DETECT2}} * {I_{DETECT1}} - {V_{DETECT1}} * {I_{DETECT2}}}{{I_{DETECT1}} - {I_{DETECT2}}}\end{equation}

First, it must exhibit a resistive slope between 23.7 kΩ and 26.3 kΩ across the entire detection voltage range. Additionally, the Voltage offset (Voffset) should not exceed 2.0V, accounting for the voltage loss due to the rectifier in the PD circuitry, which allows the PSE to accept a resistive slope shifted right by up to 2.0V. The PD must not have any current offset, but the PSE should tolerate a current offset of up to 12 μA, which would shift the slope line upward.

Finally, the input capacitance must be between 50 nF and 120 nF and the input series inductance should be less than 100 μH. These criteria ensure proper detection and classification of PoE devices, facilitating reliable power delivery over Ethernet cables.

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Summary

The primary benefit of a PoE PD detection process is that it allows a PoE switch to automatically identify if a connected device is capable of receiving power over the Ethernet cable, preventing potential damage to devices that are not PoE-compatible, while also ensuring efficient power delivery to devices that are PoE-ready, leading to streamlined network installations and reduced potential issues. 

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Learn More

• Power over Ethernet (PoE), Power Delivery (PD) and DC-DC Design Considerations
• Designing a Type 1/2 802.3 or HDBaseT Type 3 Powered Device Front End Using PD702x0 and PD701x0 ICs
• Designing a Type 1/2 802.3 or HDBaseT Type 3 Powered Device Using PD702x1 and PD701x1 ICs
• Implementing Auxiliary Power in PoE
• Power over Ethernet (PoE) Solutions
• Power over Ethernet Terminology

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