Power over Ethernet Technology Overview
Power over Ethernet (PoE) is a technology that enables low power (up to 25 watt) devices that are connected to 10BaseT, 100BaseT, and 1000BaseT networks to receive conditioned DC power through the network connection thereby eliminating the need for an AC or battery-based power. PoE enabled networks consist of Power Sourcing Equipment (PSE's) that provide DC power and Powered Devices (PD's) that are the consumer of DC power. End-Span PSE's provide both power and Ethernet switching/routing functions while Mid-Span PSE's add DC power to existing Ethernet links. Power is delivered in a common mode (e.g. via transformer center taps) using 2 of the 4 pairs within a Cat5 (or later version UTP) cable structure.
PoE was originally governed by the IEEE 802.3af standard that has since been replaced by 802.3at. Under 802.3af, all PSE's are required to furnish up to 15.4 Watts to PD's that may consume up to 12.95 Watts. Under 802.3at, PSE's and PD's are divided into power level categories and many PSE's are required to mutually communicate PD power requirements via classification protocols.
||MAC Layer Classification
||.44 - 13 Watts
||13 - 25.5 Watts
The IEEE Standard describes PSE's as if every PSE were a single port device, even though the vast majority PSE's are multi-port devices. Because of this, strict PSE Conformance Testing must be performed one port at a time. Multi-Port and power management behaviors are partially governed by the IEEE Standard, however, considerable lattitude and variation in PSE system behaviors is possible especially when total power available is sized smaller than the peak possible power draw, as is often done in Multi-Port PSE's.
Many PSE's combine IEEE standard behavior with proprietary ("legacy") powering modes and in doing this, they potentially add signaling and protocol behaviors that are not compliant with the IEEE standard. PSE Conformance Testing with the PowerSync Analyzer will typically expose these features.
An excerpt from Chapter 2 of the PowerSync Analyzer Reference Manual provide a more detailed technical overview of PoE Technology:
PoE Technology Overview from PSA Reference Manual