PoE Power over Ethernet
A technology that allows standard twisted-pair Ethernet cables (Cat5e/Cat6) to deliver both data connections and electrical DC power simultaneously. This eliminates the need for separate power supplies for endpoint devices like VoIP phones, IP cameras, and Wireless Access Points (WAPs).
Standards & Wattage
| Standard Name | IEEE Designation | Max Power (at Switch) |
|---|---|---|
| PoE | 802.3af | 15.4 W |
| PoE+ | 802.3at | 30.0 W |
| UPoE (Cisco) | Pre-Standard 802.3bt | 60.0 W |
| PoE++ | 802.3bt | 90.0 W |
CCNA Exam Gotchas
PSE vs PD
Know the terminology. The switch delivering the power is the Power Sourcing Equipment (PSE). The camera or phone receiving the power is the Powered Device (PD). The PSE will not send voltage down a wire until it safely negotiates a power class with the PD, preventing older non-PoE laptops from being fried.
Power Budgets
Just because a 48-port switch supports PoE+ (30W per port) does not mean it has enough internal power supply wattage to provide 30W to all 48 ports simultaneously. Switches have an overall Power Budget (e.g., 370W). If you plug in too many heavy-draw devices, the switch will start denying power to newly connected ports, keeping the interfaces functionally down.