Advanced terminology for PoE switches

PoE switches use specialized terms such as PSE, PD, Mode A/B, 4PPoE, and classification events to describe power delivery, negotiation, and device management over Ethernet.

Key Terms and Concepts

Power Sourcing Equipment (PSE): The device that supplies power over Ethernet, typically a PoE switch or injector. PSEs detect connected devices, negotiate power levels, and manage the power budget for multiple devices simultaneously . Powered Device (PD): Any device that receives power via PoE, such as IP cameras, VoIP phones, or wireless access points. PDs are classified into power classes (Class 0–4 for standard PoE, Classes 5–8 for 4-pair PoE) to indicate their power requirements . PoE Standards:

• IEEE 802.3af (PoE): Provides up to 15.4W per port.
• IEEE 802.3at (PoE+): Provides up to 30W per port.
• IEEE 802.3bt (PoE++ or 4PPoE): Provides up to 60W or 100W per port using all four pairs of Ethernet cabling . Power Delivery Modes:
• Mode A (Alternative A): Power is delivered over the same pairs used for data transmission (pairs 1,2 and 3,6).
• Mode B (Alternative B): Power is delivered over the spare pairs (pairs 4,5 and 7,8).
• 4PPoE: Uses all four twisted pairs in parallel to increase power delivery capacity . Classification and Negotiation:
• Detection: PSE applies a small voltage to identify a PoE-compatible PD.
• Classification: PSE determines the PD's power class to allocate the correct power budget.
• Power-Up: Full power is delivered after negotiation, often fine-tuned using LLDP in 802.3at/802.3bt devices .
• Classification Events: Optional IEEE 802.3bt feature where the PD signals its maximum power mode, allowing the PSE to measure and allocate power efficiently, optimizing energy use and port capacity . Passive vs Active PoE:
• Passive PoE: Supplies a fixed voltage without negotiation; simpler but less flexible.
• Active PoE: Complies with IEEE standards and negotiates power with the PD to ensure safety and interoperability . Additional Advanced Terms:
• AutoClass: Allows PSE to dynamically adjust power allocation based on PD requirements.
• Endspan vs Midspan: Endspan refers to PoE integrated into a switch port, while midspan refers to an external injector between a non-PoE switch and the PD .
• PoE Pass-Through: A switch can receive PoE on one port and pass it to downstream devices, useful for daisy-chaining powered devices . Understanding these terms is essential for designing, deploying, and managing PoE networks efficiently, ensuring proper power allocation, device compatibility, and network scalability .