Learn About New IEEE 802.3bt Features

Introduction

IEEE^® 802.3bt is the latest standard for Power over Ethernet (PoE), introducing several advanced features to enhance power delivery and device compatibility. Among its key innovations are:

• AutoClass, which allows powered devices to communicate their actual power needs for optimized power allocation.
• Extended power, enabling delivery of up to 90W per port to support more demanding devices.
• Power demotions ensure devices can gracefully reduce their power consumption if the available power is limited.
• Maintain Power Signature (MPS), which improves the reliability of power delivery by ensuring that power sourcing equipment can accurately detect and maintain connections with powered devices.

Let’s explore each of these new features and the benefits they offer.

AutoClass

The AutoClass feature in IEEE 802.3bt is designed to optimize the allocation of Power Sourcing Equipment (PSE) power supply budgets to Powered Devices (PDs) far more efficiently than traditional physical layer or even data link layer classification methods. With AutoClass, the PSE can allocate power based on the actual consumption needs of the PD, rather than relying solely on the worst-case scenario for the assigned class. Importantly, AutoClass can only be used to return unused power to the PSE; a PD cannot request more power than its assigned class limit. This feature is supported exclusively by Type 3 and Type 4 PSEs and PDs and is defined only for single-signature PDs, not for dual-signature PDs.

The core concept of AutoClass is that a PD can request an AutoClass measurement during the physical layer classification process. This is achieved by the PD transitioning its given (non-zero) class signature to a class signature of zero after approximately 81 ms during the first classification event (1). Other aspects of classification remain as previously defined. After power-up, AutoClass allows the PD to draw its maximum expected power for a short period, enabling the PSE to measure the actual power consumption (2). The PSE then adjusts its power allocation based on this real measurement, which accounts for the PD’s true requirements, actual cable losses, and the PSE’s measurement uncertainty.

AutoClass is an optional extension of the physical layer. If a PSE does not support AutoClass but the PD requests it, the PD will still follow the AutoClass procedure, but the PSE will not perform the measurement or adjust its power budget; instead, it will allocate power based on the assigned class. For example, a Class 8 PD requiring 65W, connected via a 25m AWG 23 patch cord, would typically be allocated 90W by the PSE (the worst-case scenario). With AutoClass, the PSE measures the actual draw (e.g., 66.5W), adds the required margin (1.25W for Class 8), and allocates 67.8W—saving 22.2W, or nearly 25 percent, compared to the worst-case allocation.

Additionally, AutoClass measurements can be requested via the Link Layer Discovery Protocol (LLDP), allowing PDs to utilize AutoClass even if they cannot meet the strict physical layer timing requirements for maximum power draw immediately after power-up. This flexibility further enhances the efficiency and adaptability of power allocation in modern PoE networks.

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Extended Power

The extended power feature in PoE, often referred to as extend mode or long-range PoE, significantly increases the maximum distance over which both data and power can be transmitted. Traditionally, standard PoE supports cable runs up to 328 feet (100 meters), but with extend mode, this distance can be stretched up to 820 feet (250 meters). This capability is particularly valuable for applications where devices such as IP cameras, wireless access points, or remote sensors need to be installed far from the main network switch or power source.

The primary tradeoff with extend mode is a reduction in data transmission speed. When activated, the Ethernet connection speed drops from the standard 100 Mbps or 1G bps to 10 Mbps. However, this speed is still sufficient for many common applications, such as video surveillance or basic network connectivity, where high bandwidth is not critical.

Extend mode is typically enabled via a manual toggle switch on the PoE switch, allowing users to activate the feature for all ports or select specific ports as needed. This flexibility makes it easy to adapt the network to various deployment scenarios without requiring additional equipment or complex configuration. By enabling longer cable runs, the extended power feature reduces the need for additional network infrastructure, lowers installation costs, and increases deployment flexibility for a wide range of PoE-powered devices.

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Power Demotions

The power demotion feature in PoE is a mechanism that allows PSE, such as Ethernet switches, to provide less power to a PD than the PD originally requested. This is particularly useful because many PoE-enabled switches are not designed to deliver full power on every port simultaneously, as this would significantly increase costs. Instead, the PoE standard permits power demotion, where the PSE assigns the PD a lower type than requested during the classification handshake. When a PD is demoted, it is automatically assigned the highest power class within the lower type. For example, if a PD requests Type 3 power but the PSE can only provide Type 2, the PD will be assigned the highest class within Type 2.

Power demotion is available for both single-signature and dual-signature PDs. The PSE must not produce more class events than necessary to satisfy the PD’s requested power (e.g., a PSE capable of Class 5 power cannot produce four class events if the PD only requests Class 3). However, the PSE may produce fewer class events than required, signaling a power demotion. This approach allows higher-power PDs to operate in a reduced-feature mode when connected to lower-power PSEs, rather than not operating at all. More information on power demotion is available for SSPD and DSPD devices in their respective classification lessons.

To ensure compatibility, PDs must be able to function at the various power levels that a PSE may grant. If a PD cannot adjust its operation to the available power, it risks drawing too much power and being shut down by the PSE. Most Type 2, 3, and 4 PDs on the market include mechanisms to communicate the received type back to their main system controller, enabling them to adjust their functionality accordingly. Additionally, dual-signature PDs that are demoted (e.g., a Class 5 PD assigned to Class 4) can use the Data Link Layer (DLL) to request their original power level again. If the PSE’s available power increases, it may reclassify the PD and allocate the full requested power. More information on DLL is available for SSPD and DSPD devices in their respective classification lessons.

Power demotion thus provides flexibility and resilience in PoE networks, ensuring that devices can still operate, with reduced functionality, when full power is not available, and allowing for dynamic adjustment as network power conditions change.

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Example 1 – A PSE with Class 6 Power Available Demotes a PD Requesting Class 8 to Type 3 and Grants PD Class 6 Power

A PD requests Class 8 power, and the PSE has Class 6 power available. In this situation, the PSE demotes the PD to Type 3, and the PD receives Class 6 power.

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Example 2 – A PSE with Class 5 Power Available Demotes a PD Requesting Class 8 to Type 2 and Grants PD Class 4 Power

A PD requests Class 8 power, and the PSE only has Class 5 power available. The PSE can only demote the PD by type, so the PD cannot be granted Class 5 power. If the PSE granted the PD Type 3 power, the PD would be granted Class 6 power. Instead, the PSE must grant the PD Type 2 power.

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Maintain Power Signature (MPS)

The MPS feature in PoE is essential for ensuring that a PD remains connected and powered by the PSE after initial power-up. Once power is applied, the PSE continuously monitors the current draw from the PD to confirm its presence. An IEEE-compliant PD must draw a minimum current, known as the MPS, to avoid being disconnected. The required MPS value is 10 mA for classes 1 to 4 and 16 mA for classes 5 to 8. If the PSE does not detect this minimum current for at least 400 ms, it will turn off the port, ensuring that voltage is not applied to a disconnected or faulty cable.

Many isolated DC-DC converters naturally consume the required minimum current even when there is no load. However, some converters, especially those with sleep modes or synchronous buck converters, may not meet the MPS requirement without additional circuitry. Similarly, PDs used in redundant power applications or those with auxiliary power sources (like wall adapters) need an automatic pre-load to keep the PSE port active when a particular channel is in standby. Without this, the PD could reset when the currently active power source is removed, disrupting operation.

There are two main methods for providing the MPS, constant current and pulsed current. The constant current method is straightforward and can be implemented using a 5 kΩ, 1W resistor in series with a 100V signal FET, which connects across the PoE line to draw the necessary current when needed. The pulsed current method is more efficient, requiring an additional timer to periodically drive the FET with a low duty cycle, thus significantly reducing the power wasted on maintaining the MPS. The timing requirements for pulsed MPS are specified in the IEEE standard, ensuring reliable detection while minimizing energy loss.

MPS is a critical feature for maintaining reliable PoE connections, especially in advanced or redundant power scenarios, and helps prevent unnecessary power loss or device resets.

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Summary

IEEE 802.3bt introduced a suite of advanced features that significantly enhance the capabilities and efficiency of PoE systems. The standard’s new features—such as AutoClass, extended power delivery up to 90W per port, power demotions, and MPS—enable more precise power management, support for a broader range of high-powered devices, and improved network reliability. These enhancements allow organizations to deploy more demanding devices like advanced Wireless Access Points (WAPs), security cameras, and digital signage without additional electrical infrastructure. Overall, 802.3bt’s innovations lead to greater flexibility, energy efficiency, and scalability in modern network environments.

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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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