PoE has different standards that define different power levels for PSEs and PDs. The original standard, IEEE
802.3af, defines two classes of PSEs: Class 3 (15.4 W) and Class 4 (30 W). The newer standard, IEEE 802.3at, also known as PoE+, defines four classes of PSEs: Class 0 (15.4 W), Class 1 (4 W), Class 2 (7 W), and Class 3 (12.95 W). The power level received at the PD is always lower than the power level provided by the PSE, due to cable resistance and power dissipation. The IEEE standards specify the minimum power level that must be received at the PD for each class of PSE. For a Class 4 PSE, the minimum power level received at the PD is
25.5 W910. References: CWNA-109 Study Guide, Chapter 7: Power over Ethernet (PoE), page
295; CWNA-109Study Guide, Chapter 7: Power over Ethernet (PoE), page 289.