What is a powered device?
In Power over Ethernet (PoE), a powered device (PD) is the equipment at the far end of the cable that receives both data and DC power over its Ethernet connection. The other side is the power sourcing equipment (PSE): a PoE switch (endspan) or a PoE injector (midspan) that supplies the power. Because one cable carries both, a PD needs no nearby outlet or power brick.
Common PDs include:
- VoIP desk phones
- Wireless access points
- IP security cameras
- Door access controllers, badge readers and intercoms
- Small IoT gear such as sensors
How a PD gets power
- Detection: the PSE applies a low voltage and looks for the PD's signature resistance (about 25 kilohms). A device without that signature, such as an ordinary laptop, gets no power, which is why standard PoE ports are safe for non-PoE gear.
- Classification: the PD signals its power class so the PSE knows how much to reserve from its budget.
- Power on: the PSE switches on the full voltage (about 44 to 57 V DC, depending on the standard) and keeps monitoring. If the PD is unplugged, the PSE removes power.
PoE power budgets
| Standard | Common name | Max at the PSE port | Available at the PD |
|---|---|---|---|
| IEEE 802.3af | PoE (Type 1) | 15.4 W | About 12.95 W |
| IEEE 802.3at | PoE+ (Type 2) | 30 W | About 25.5 W |
| IEEE 802.3bt Type 3 | PoE++ | 60 W | About 51 W |
| IEEE 802.3bt Type 4 | PoE++ | Up to 90 W | About 71 W |
The difference allows for power lost as heat in up to 100 meters of cable. A PD that needs PoE+ may not boot, or may run with features turned off, on a plain 802.3af port.
Troubleshooting PDs
- Device won't power on: confirm the switch port supports PoE and has it enabled, check the cable, and try a known-good PoE port or an injector.
- Devices drop when new ones are added: the switch has run out of total PoE budget. Managed PoE switches show the power drawn per port and let you set port priorities.
- Device reboots at random: look for a marginal cable or a PD drawing more than its class allows, and check the switch's PoE status.
PoE PD vs. USB PD
Don't confuse the two "PDs". In PoE, PD means powered device. In USB, USB Power Delivery (USB PD) is a charging protocol that negotiates higher power over USB-C for laptops and phones. They're unrelated technologies.
For the CompTIA A+ exam
Core 1 (220-1201) objective 2.5 covers PoE switches, injectors and the PoE standards. Expect scenarios where a phone, camera or access point won't start: know that the device is the PD, the switch or injector is the PSE, and that a non-PoE port or a switch out of PoE budget is a common cause.