Autoclass (AUC) in IEEE 802.3bt PoE: Maximizing Power Efficiency and Cost Savings
Power over Ethernet (PoE) has revolutionized how networked devices are powered and connected, offering a major cost advantage by eliminating the need for separate power and data infrastructure. With just a single Ethernet cable, PoE delivers both power and data, reducing installation complexity and cost while enhancing flexibility in device placement.
Since PoE operates under 60V DC, it also eliminates the need for costly AC protection devices like circuit breakers and simplifies the relocation of network equipment. And because it runs on globally accepted, standardized protocols, PoE ensures reliability and safety across systems.
The Evolution of PoE Standards
The IEEE has played a crucial role in standardizing PoE technologies. In September 2018, IEEE ratified the 802.3bt standard, addressing the growing power demands of modern network devices such as:
High-resolution CCTV cameras
5G infrastructure devices
Every PoE system comprises two components:
PSE (Power Sourcing Equipment) — switches or injectors that provide power.
PD (Powered Device) — the end equipment receiving power.
Power levels are regulated through a system of classifications. As each new IEEE PoE standard has emerged, both the number of power classes and the power capacities have increased:
While 802.3bt allows up to 90W at the PSE, only 71.3W is assured at the PD due to cable resistance losses — especially over longer distances (e.g., 100 meters of Ethernet cable). This loss is typically calculated using a maximum pair-set DC loop resistance of 12.5 ohms.
The Autoclass Advantage: Smarter Power Allocation
Previous standards, like 802.3af and 802.3at, allocated power based on the PD’s class, regardless of how much power the device actually used. This often led to inefficient over-provisioning.
To address this, the 802.3bt standard introduced Autoclass (AUC) — an optional feature that allows the PD to communicate its actual power requirements to the PSE. This optimization allows the PSE to allocate just the right amount of power, preserving the overall power budget and enhancing system efficiency.
Autoclass operates during a specific classification window (typically after 81ms). A PD indicates support for Autoclass by adjusting its classification current between 75.5ms and 84.5ms. Only Type 3 or Type 4 IEEE 802.3bt-compliant PSEs can detect this.
Once detected, the PD draws its maximum operational power immediately after system startup, allowing the PSE to record and allocate just enough power for each port.
Real-World Power Savings with Autoclass
Let’s look at two practical examples that demonstrate the significant benefits of Autoclass:
Example 1: Using Pure Copper Cat5e Cable
A PD needs 65W of power (Class 8).
Without Autoclass: PSE allocates 90W.
With Autoclass: PSE allocates 68.82W.
➡ Savings per port: 21.18W
In an 8-port deployment, this results in a savings of over 160W in total power budget.
Example 2: Using Copper-Aluminium Ethernet Cable
A PD requires 70W using budget cable of varying lengths.
Without Autoclass: PSE still allocates 90W.
With Autoclass: PSE allocates 73.78W.
➡ Savings per port: 16.2W
Across 8 ports, that’s 134W saved — significant in high-density networks.
Infomart’s PoweredEthernet: Built for Efficiency
All of Infomart’s IEEE 802.3bt PoE PD modules — including:
PEB9300BTÂ (all-in-one PoE boards)
— come with optional Autoclass support. This allows system designers to create products that intelligently measure, budget, and allocate actual power requirements, ensuring high efficiency and lower costs.
Maintain Power Signature (MPS) for Better Standby Performance
Infomart’s BT modules also support the Maintain Power Signature (MPS) feature, enabling ultra-low standby current without risking PSE disconnection. With built-in wall-adapter support, they also offer redundant power backup — a key requirement for mission-critical applications.
Conclusion: AUC Makes PoE Smarter
The IEEE 802.3bt standard, and particularly its Autoclass feature, represents a leap forward in PoE efficiency. By enabling power-aware communication between PDs and PSEs, Autoclass reduces unnecessary power allocation and unlocks more capacity across networks.
For organizations looking to build scalable, energy-efficient, and future-ready networks, choosing devices that support Autoclass is a smart move. And with Infomart’s Powered Ethernet solutions, achieving those goals has never been easier.
