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Learn Power over Ethernet (PoE) Dual Signature Classifications

Last modified by Microchip on 2025/06/26 12:31

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Introduction

Power-over-Ethernet (PoE) dual signature classification is a process designed to manage the power requirements of Dual-Signature Powered Devices (DSPDs). These devices feature two separate Powered Device (PD) controllers, each operating on a distinct pairset or Mode. Within a DSPD, two internal, isolated loads are combined into a single PD entity, connected by one Power Interface (PI). To ensure proper power allocation, Power Sourcing Equipment (PSE) must perform detection and classification on both pairsets independently. Each pairset can be assigned its own power class, allowing for flexible power management. Notably, one pairset can be detected and classified while the other remains either powered or unpowered, providing a robust and efficient power delivery system for complex networked devices.

Power over Ethernet Classification Waveform

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

Physical Layer Classification

PoE Physical Layer Classification for DSPDs follows a process similar to that of Single-Signature Powered Devices (SSPDs), utilizing class and mark events to request and receive the appropriate power class. In this process, DSPDs employ resistors on both pairsets to generate event currents, which are then read by the PSE as it applies voltage pulses. These current levels are translated into a data sequence that the PSE uses to determine the PD’s requested classes for both pairsets.

Table 1: Classification currents and Corresponding Class Signature

Classification CurrentClass Signature
1 – 4mA0
9 – 12mA1
17 – 20mA2
26 – 30mA3
36 – 44mA4

The PD generates a specific combination of class signatures to indicate its requested class.

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Table 2: DSPD Event Sequence for Each Requested Class

 Event
Requested Class1234
1110/0/
2220/0/
3330/0/
4440/0/
54433

Legend: /n/ = Non-required event, if present, should be value ‘n’

Information

Note: The number of classes and the event sequence for each class differ from those used by SSPDs.

The maximum number of class events and the highest assigned class are contingent upon the PSE’s type. Importantly, the PSE is designed not to produce more class events than necessary to fulfill the power request, ensuring efficient power allocation and management.

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Table 3: Maximum Number of Class Events and Highest Assigned Class for Each PSE Type

PSE TypeMax Number of Class EventsHighest Assigned Class
1*13
2*24
334
445

* Type 1 and 2 PSEs are not DSPS capable as they can only power one pairset.

This classification mechanism is crucial for the proper functioning and power distribution in PoE systems, enabling devices to communicate their power needs accurately and receive the appropriate power levels from the PSE.

Information

Question Mark in IC Package Question: What is the highest power class a Type 3 PSE can grant to a Class 5 DSPD?

Answer: Because a Type 3 PSE can only generate a maximum of three class events, the Class 5 DSPD can only be granted power Class 4 (see Table 3).

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Power Class Negotiation

In PoE power negotiation for DSPDs, the PD generates a specific combination of class signatures to indicate its requested class. The PSE then communicates the amount of power it will provide to the PD by producing a corresponding number of class events. The PSE is limited by its available power and cannot produce more class events than it can support, nor can it grant a PD a higher class than requested. If the PSE assigns a lower power class than requested, a process known as Power Demotion occurs, allowing the PD to operate in a feature-limited mode.

For DSPDs, power demotion can result in assignment to Class 3 if the PSE produces one class event, or to Class 4 if the PSE produces two or three class events (see Table 4). Unlike single-signature classification, each pairset in a DSPD must be independently classified, with the highest class assignable per pairset being Class 5. Consequently, fewer class events are required for DSPDs (four) compared to SSPDs (five). Additionally, only Type 3 and Type 4 PSEs support DSPDs. Type 3 and Type 4 DSPDs also modify their class signature response between the second and third events to indicate their capability to support 4-pair power.

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Table 4: PD Power Class from DSPD Requested Class and Number of PSE Class Events

 Number of Class Events
Requested Class1234
1Class 1Class 1Class 1X
2Class 2Class 2Class 2X
3Class 3Class 3Class 3X
4Class 3Class 4Class 4X
5Class 3Class 4Class 4Class 5

Legend:

  • Green cells: Requested class matches assigned class
  • Yellow cells: PD was power demoted
  • Red cells: PSE not permitted to produce this class event

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Table 5: Assigned Class and Number of Class Events Based on Available PSE Power and DSPD Requested Class

 Requested DSPD Class
PSE Available Power Class12345
1

Class 1

1, 2 or 3 Events

No PowerNo PowerNo PowerNo Power
2

Class 1

1, 2 or 3 Events

Class 2

1, 2 or 3 Events

No PowerNo PowerNo Power
3

Class 1

1, 2 or 3 Events

Class 2

1, 2 or 3 Events

Class 3

1, 2 or 3 Events

Class 3

1 Event

Class 3

1 Event

4

Class 1

1, 2 or 3 Events

Class 2

1, 2 or 3 Events

Class 3

1, 2 or 3 Events

Class 4

2 or 3 Events

Class 4

2 or 3 Events

5

Class 1

1, 2 or 3 Events

Class 2

1, 2 or 3 Events

Class 3

1, 2 or 3 Events

Class 4

2 or 3 Events

Class 5

4 Events

Legend:

  • Green cells: Requested class matches assigned class
  • Yellow cells: PD was power demoted
  • Red cells: PSE not permitted to produce this class event

Example

  • Type 4 PSE negotiating with a Type 4 DSPD requesting Class 5 over Mode A and Class 2 over Mode B:

Power over Ethernet Dual Signature Powered Device (DSPD) Block Diagram

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Mode A Class 5 Power Request:

Power over Ethernet Dual Signature Mode A Classification Waveform

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Mode B Class 2 Power Request:

Power over Ethernet Dual Signature Mode B Classification Waveform

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Mutual Identification

PoE mutual identification for DSPDs involves a process where both the PSE and the PD can "somewhat" discover each other's type. This identification process is facilitated in the classification phase, where the PSE can gain information about the PD's type based on its requested class. However, it is important to note that PDs requesting Class 4 or below cannot be uniquely identified, as these classes do not provide sufficient granularity for unique identification. Additionally, a PD can use the PSE assigned class and the duration of the PSE's first class event to help classify the PSE type. This mutual identification process ensures that both the PSE and PD can operate efficiently within their power capabilities and requirements, enhancing the overall functionality and compatibility of PoE systems.

Table 6: Dual-Signature PD Type Based on Requested Class

Requested ClassPD Type
1, 2, 3 or 43
54

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Table 7: Possible PSE Type(s) Given Assigned Class

Assigned ClassPSE Type
1 – 63 or 4
6 – 84

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Data Link Layer (DLL) Classification

PoE Data Link Layer (DLL) Classification for DSPDs is a process that closely mirrors the SSPD DLL Classification, with the key distinction that power negotiation occurs for each mode individually. In this classification system, the PSE has the capability to transition between supplying power over 2-pair and 4-pair configurations, adapting to the power requirements of the PD. This flexibility allows the PSE to efficiently manage power distribution based on the PD's needs. Additionally, PD power requests are comprehensive, encompassing both pairsets, which ensures that the PD can communicate its power requirements accurately for optimal performance. This dual signature approach enhances the efficiency and reliability of power delivery in PoE systems, accommodating a wider range of devices and applications.

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Summary

The PoE dual signature classification process involves several key steps to ensure efficient power delivery and device compatibility. Power class negotiation permits the PSE and PD to agree on the appropriate power class, ensuring that the PSE can supply the necessary power. Mutual Identification can be employed to allow the PSE and PD to confirm each other's identities and capabilities to prevent mismatches and ensure proper operation. The DLL classification is an optional additional layer of communication, using data packets to further refine and confirm the power requirements and capabilities, ensuring optimal power delivery. The classification process ensures that PoE systems can efficiently and safely power a wide range of devices over Ethernet connections.

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