Learn About Power over Ethernet (PoE)
Learn Power over Ethernet (PoE) Dual Signature Classifications
Learn Power over Ethernet (PoE) Single Signature Classifications | Introduction to Power over Ethernet (PoE) Inrush and Power Removal |
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 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 Current | Class Signature |
1 – 4mA | 0 |
9 – 12mA | 1 |
17 – 20mA | 2 |
26 – 30mA | 3 |
36 – 44mA | 4 |
The PD generates a specific combination of class signatures to indicate its requested class.
Table 2: DSPD Event Sequence for Each Requested Class
Event | ||||
Requested Class | 1 | 2 | 3 | 4 |
1 | 1 | 1 | 0 | /0/ |
2 | 2 | 2 | 0 | /0/ |
3 | 3 | 3 | 0 | /0/ |
4 | 4 | 4 | 0 | /0/ |
5 | 4 | 4 | 3 | 3 |
Legend: /n/ = Non-required event, if present, should be value ‘n’
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.
Table 3: Maximum Number of Class Events and Highest Assigned Class for Each PSE Type
PSE Type | Max Number of Class Events | Highest Assigned Class |
1* | 1 | 3 |
2* | 2 | 4 |
3 | 3 | 4 |
4 | 4 | 5 |
* 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.
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.
Table 4: PD Power Class from DSPD Requested Class and Number of PSE Class Events
Number of Class Events | ||||
Requested Class | 1 | 2 | 3 | 4 |
1 | Class 1 | Class 1 | Class 1 | X |
2 | Class 2 | Class 2 | Class 2 | X |
3 | Class 3 | Class 3 | Class 3 | X |
4 | Class 3 | Class 4 | Class 4 | X |
5 | Class 3 | Class 4 | Class 4 | Class 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
Table 5: Assigned Class and Number of Class Events Based on Available PSE Power and DSPD Requested Class
Requested DSPD Class | |||||
PSE Available Power Class | 1 | 2 | 3 | 4 | 5 |
1 | Class 1 1, 2 or 3 Events | No Power | No Power | No Power | No Power |
2 | Class 1 1, 2 or 3 Events | Class 2 1, 2 or 3 Events | No Power | No Power | No 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:
Mode A Class 5 Power Request:
Mode B Class 2 Power Request:
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 Class | PD Type |
1, 2, 3 or 4 | 3 |
5 | 4 |
Table 7: Possible PSE Type(s) Given Assigned Class
Assigned Class | PSE Type |
1 – 6 | 3 or 4 |
6 – 8 | 4 |
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.
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.
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