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Learn About Power over Ethernet (PoE)

Power over Ethernet (PoE) Connection Check and Powered Device (PD) Signature Configurations

Last modified by Microchip on 2025/06/26 12:33
   How Power over Ethernet (PoE) Detection Works   Learn Power over Ethernet (PoE) Single Signature Classifications   

Introduction

Power over Ethernet (PoE) technology has evolved significantly with the introduction of the IEEE® 802.3bt standard. This standard brought several advancements, including the Connection Check process and the classification of Powered Devices (PDs) into two main types: Single-Signature PDs (SSPD) and Dual-Signature PDs (DSPD).

The Connection Check was introduced with the IEEE 802.3bt standard to enhance the PoE start-up sequence. This process is crucial for determining the type of PD connected to the Power Sourcing Equipment (PSE) and ensuring proper power delivery.

The Connection Check serves several purposes in the PoE start-up sequence:

  • PD Type Identification: It allows a 4-pair capable PSE to determine whether the connected PD is an SSPD, DSPD, or an invalid PD
  • Power Delivery Determination: It assesses whether the PD can be powered over all four pairs and decides how the PSE will handle the PD

Unlike the Detection phase, which simply determines if a PD wants to receive power, the Connection Check provides detailed information about the PD's configuration and power requirements.

The IEEE 802.3 specifications do not provide a direct method for classifying PDs as SSPD or DSPD. Instead, the PSE must measure specific properties of each PD to make this determination. This involves analyzing the PD's power signatures and other electrical characteristics to accurately classify and manage the power delivery.

Power-over-Ethernet Waveform Example

Power Sourcing Equipment voltage (VPSE) and Powered Device current (IPORT)

Single-Signature PD Configuration

For both Modes, the power rail is shared through rectifiers. This means that the power delivered to the PD is managed through a common rectifier setup, ensuring consistent power delivery regardless of the Mode. The detection and classification properties remain the same whether the PSE inquires using Alternative A or Alternative B. This ensures that the PD can be correctly identified and classified by the PSE regardless of the wiring configuration used.

PoE SSPD Schematic Example

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Requirements

The PD must provide a valid detection signature on any pairset when the other pairset is not energized. This is crucial for the PSE to correctly identify and classify the PD. The active pairset must provide an effective resistance slope between 23.7 KOhm and 26.3 KOhm. This specific resistance range is necessary for the PSE to detect the PD accurately.

Information

Note: For more detailed criteria, refer to the "How Power over Ethernet (PoE) Detection Works" page.

The PD should not provide a valid detection signature when the other pairset is powered with a voltage between 3.7V to 57V or a current > 124uA. A PD architecture that uses a rectifier with both positive outputs sharing a rail and both negative outputs sharing a rail should naturally meet these criteria. This is due to the inherent characteristics of the rectifier, which ensures proper functioning under these conditions.

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Fail Modes

Excessive Voltage

If an excessive voltage is applied to the other pair set, the effective resistance response will become non-linear. This happens because the voltage on the other pairset prevents current from flowing on the active pairset until the voltage on the active pairset exceeds the voltage of the other pairset.

PoE SSPD Overvoltage Failure Signature

Excessive Current

If an excessive current is applied to the other pairset, the effective resistance slope will shift beyond the maximum permitted offset of 2.0V. This shift can disrupt the proper detection and classification process.

PoE SSPD Overcurrent Failure Signature

Natural Enforcement by Rectifier

In both cases, the properties of the PD’s rectifier naturally enforce these criteria by pushing the effective resistance beyond the acceptable slope and/or offset limits. This ensures that the PD does not provide a valid detection signature under inappropriate conditions, maintaining the integrity of the PoE system.

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Advantages

  • SSPD architecture is simpler to implement and manage because it involves a single detection and classification phase for each port which reduces the complexity of the design and can lead to lower costs
  • Due to its simplicity, SSPD can be more cost-effective, both in terms of initial setup and ongoing maintenance
  • SSPD is widely compatible with a broad range of PoE devices, making it a versatile choice for many applications
  • The straightforward nature of SSPD makes it easier to deploy and troubleshoot, which can be beneficial in large-scale installations

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Dual-Signature PD Configuration

In a Dual-Signature Powered Device (DSPD), each mode (or pair of wire pairs) connects to its own individual PD controller. This setup allows each mode to perform separate detection and classification processes. Therefore, each pair of wire pairs can independently communicate with the PSE to determine the power requirements and capabilities. This is particularly useful in applications where different parts of a device, such as a surveillance camera and its heater, need different power levels and can operate independently.

PoE DSPD Schematic Example

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Requirements

A key feature of DSPD is that it must provide a valid detection signature regardless of the voltage applied to the other mode. This means that even if one mode has a voltage between 0V and 57V, the other one must still be able to present a valid detection signature to the PSE. This ensures that the PSE can always detect and classify the PD correctly, regardless of the power state of the other mode.

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Advantages

  • DSPD architecture can deliver higher power levels to devices that require more energy, such as advanced IP cameras or high-performance wireless access points because DSPD can manage power delivery more efficiently across two signatures
  • DSPD allows for more granular control over power distribution, which can lead to better power management and optimization which is useful in environments with diverse power requirements
  • By using two signatures, DSPD can improve the efficiency of power delivery, reducing power loss and enhancing overall system performance
  • DSPD is better suited for advanced PoE devices that have higher power demands or require more sophisticated power management features

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4-Pair Power Requirements

In PoE systems, power is delivered over Ethernet cables to network devices. For 4-pair power delivery, both pairsets (the four pairs of wires in the Ethernet cable) must present a valid detection signature before power can be supplied. This detection signature ensures that the device connected to the Ethernet cable is PoE-compatible and can safely receive power.

Information

Note: For more information, see the “How Power over Ethernet Detection Works” page.

PSE determines whether the PD requires 4-pair power using a process shown in this flow diagram:

PoE Decision diagram to enable 4-pair power

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Summary

SSPD is advantageous for its simplicity, cost-effectiveness, and ease of deployment, making it suitable for standard PoE applications. DSPD offers higher power delivery, enhanced power management, and improved efficiency, making it ideal for advanced PoE applications with higher power requirements. Choosing between SSPD and DSPD architectures depends on the specific needs of the PoE deployment, including the types of devices being powered and the overall power requirements of the system.

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