Learn Power over Ethernet (PoE) Single-Signature Classifications

Introduction

The Power over Ethernet (PoE) single-signature classification process is a critical step in ensuring that Powered Devices (PDs) receive the appropriate amount of power from Power Sourcing Equipment (PSE). This process begins after the PSE successfully detects a valid PD. During classification, the PSE applies a specific voltage range to the PD, which then draws a constant current to signal its power class. This current is regulated by an external resistor connected to the PD's classification pin. The classification process helps the PSE determine the power requirements of the PD, ensuring efficient and safe power delivery. This method is essential for compliance with IEEE^® 802.3 standards, including IEEE 802.3af, IEEE 802.3at, and IEEE 802.3bt, which define the voltage levels and current draws for different power classes.

Physical Layer Classification

The PoE classification process for Single-Signature Powered Devices (SSPDs) involves a series of steps where the PD uses resistors to generate specific event currents. These currents are read by the PSE as it applies voltage pulses. The current levels generated by the PD are translated into a data sequence that the PSE uses to determine the PD’s requested power class.

Table 1: Classification Currents and Corresponding Class Signature

The PD indicates its requested class by generating a particular combination of class signatures.

Table 2: SSPD Event Sequence for Each Requested Class and Supported PSE Types

Legend:

/n/ = non-required event; if present will be value ‘n’

- = non-compliant event

The classification process varies depending on the type of PSE. Type 1 and Type 2 PSEs produce a short first class event, while Type 3 and Type 4 PSEs produce a long first class event.

The maximum number of class events and the highest assigned class are contingent on the PSE’s type. Importantly, the PSE is designed to produce no more class events than necessary to accommodate the PD’s power request, ensuring efficient power allocation and management.

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

Let’s walk through two classification examples. The first is a Type 2 PSE providing a PD with Class 4 power.

Example: Type 2 PSE Providing Type 2 PD with Class 4 Power

A: The PSE provides a voltage on the pairset to generate a short first class event indicating it is either a Type 1 or 2 PSE. The PD loads the pairset with a resistance that provides a Level 4 class event by drawing between 36 and 44 mA (refer to Table 1).

B: The PSE produces a second class event and again reads a Level 4 class event from the PD. Since Type 2 PSEs are limited to 2 cass events (see Table 3), the highest power class the PSE can provide is Class 4 even if the PD was seeking a higher power class (see Table 2).

C: PSE provides PD Class 4 power in accordance with Table 2.

In this classification example, a Type 4 PSE provides a PD with Class 7 power. Let’s see how it works.

Example: Type 4 PSE Providing PD with Class 7 Power

A: The PSE provides a voltage on the pairset to generate a long first class event indicating it is either a Type 3 or 4 PSE. The PD loads the pairset with a resistance that provides a Level 4 class event by drawing between 36 and 44 mA (refer to Table 1).

B: The PSE produces a second class event and again reads a Level 4 class event from the PD.

C: The PSE reads a Level 2 class event for its third pulse because it is Type 4 and capable of producing up to five class events (see Table 3). The change in current indicates that the PD complies with the IEEE 802.3bt standard.

D: The PSE reads a Level 2 class event for its fourth pulse.

E: Again, the PSE reads a Level 2 class event for its fifth and last pulse.

F: PSE provides PD Class 7 power in accordance with Table 2.

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Power Class Negotiation

The PoE power class negotiation process for SSPDs involves a series of interactions between the PD and the PSE to determine the appropriate power allocation. Initially, the PD generates a specific combination of Class Signatures to indicate its Requested Class, which signifies the amount of power it needs. In response, the PSE communicates the amount of power it will provide by producing a corresponding number of class events. The PSE is constrained by its available power and cannot produce more class events than it can supply. Additionally, the PSE is not permitted to grant a power class higher than what the PD has requested. If the PSE assigns a lower power class than the PD requested, a situation known as power demotion occurs, where the PD must operate with less power than initially indicated. This negotiation ensures that the power distribution is managed efficiently and within the capabilities of the PSE.

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

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

Examples

Type 4 PSE has Class 5 power available and PD requests Class 3 or lower power with one class event:

Type 4 PSE has Class 5 power available and PD requests Class 4 power with three class events:

Type 4 PSE has Class 5 power available and PD requests Class 5 power with 4 class events:

Type 4 PSE has Class 5 power available and PD requests greater than Class 5 power:

Updated Behaviors for IEEE 802.3bt Standard

The new PoE features for SSPDs based on the IEEE 802.3bt standard introduce several enhancements to improve efficiency and performance. Type 3 and Type 4 PSEs now produce a longer first class event, ranging from 88 to 105 milliseconds, compared to the 6 to 75 milliseconds for Type 1 and Type 2 PSEs. This adjustment helps in better power negotiation and delivery. Additionally, the maximum length of class events 2 through 5 has been reduced to 20 milliseconds to lower the thermal load on PDs, ensuring safer and more efficient operation.

To facilitate accurate power classification, both PSE and PD must generate a mark event after the final class event. This allows the PD to distinguish between a long class event and a single class event, ensuring proper power allocation. For Type 1 PDs, classification remains optional; they will be assigned a Class Signature 0, and the PSE will default to assigning Class 3 power limits in such cases.

Type 3 PDs, however, are required to support Physical Layer Classification, and Class 0 does not exist for them, ensuring more precise power management. When Type 3 and Type 4 PSEs are connected to a Type 1, Class 0 PD, they will assign the PD to Class 3, which uses the same power limit, maintaining compatibility and consistent power delivery across different device types. These enhancements in the IEEE 802.3bt standard ensure more efficient power management, better thermal performance, and improved compatibility across various PoE devices.

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Mutual Identification

PoE mutual identification for SSPDs involves a process where both the PSE and the PD can somewhat discover each other's type. This identification process is facilitated by 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: SSPD Type Based on Requested Class

Table 8: Possible PSE Type(s) Given Assigned Class and Length of First Class Event

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Data Link Layer (DLL) Classification

PoE Data Link Layer (DLL) classification for SSPDs is a process that occurs after the PD receives operating power. This classification is based on the Link Layer Discovery Protocol (LLDP), which allows for communication between the PD and the PSE. One key aspect of DLL classification is that the PD cannot request a higher power allocation using DLL than what was determined during the Physical Layer classification. All PDs that are greater than Class 3 are required to support DLL classification. This protocol enables the PSE to inform the PD about the available power before a power class is requested.

The advantages of DLL classification include the ability for PDs to reduce their power allocation, thereby returning any unused power back to the PSE. Additionally, PDs can request the full power allocation and a higher power class after experiencing a power demotion. This dynamic power management ensures efficient use of available power resources and enhances the overall flexibility and functionality of PoE systems.

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Summary

PoE single-signature classification is a process that ensures efficient power delivery and device compatibility over Ethernet cables. During classification, the PSE and PD communicate to determine the appropriate power class, ensuring that the PSE can supply the required power level. The IEEE 802.3bt standard introduces updated behaviors, including support for higher power levels and more efficient power delivery mechanisms. Mutual identification can be used to allow the PSE and PD to recognize each other’s capabilities and requirements, ensuring compatibility. Also, the DLL classification is an optional feature that can provide an additional layer of communication, allowing for more granular power management and enhanced features beyond the physical layer classification. The classification process ensures that PoE systems can efficiently and safely power a wide range of devices over Ethernet connections.

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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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