How does PoE work and what are IEEE 802.3af, 802.3at, and 802.3bt standards?
Power over Ethernet (PoE) delivers electrical power and data over a single ethernet cable, eliminating separate power infrastructure for network devices.
How PoE Works
PoE uses twisted pairs in Cat5e/Cat6 cables to transmit DC power (typically 44–57V). The Power Sourcing Equipment (PSE) negotiates with the Powered Device (PD) to determine required wattage before delivering power.
PoE Standards Comparison
| Standard | Name | PSE Power | PD Power | Use |
|---|---|---|---|---|
| 802.3af | PoE | 15.4W | 12.95W | IP phones, basic cameras |
| 802.3at | PoE+ | 30W | 25.5W | PTZ cameras, WiFi APs |
| 802.3bt Type 3 | PoE++ | 60W | 51W | Video conferencing |
| 802.3bt Type 4 | 4PPoE | 90W | 71.3W | LED lighting |
Power Budget
A PoE switch has a total power budget shared across ports. Plan for 60–70% utilization to allow safety margins. A 24-port switch with 240W budget cannot power all ports at 30W.
Cable Distance
Standard PoE operates up to 100m over Cat5e or better. Extended PoE can reach 250m using proprietary technology, useful for surveillance deployments.
2 Other Answers
Very clear breakdown. I would note that 802.3bt Type 4 requires all 4 pairs.
PoE budget calculations saved my project last month.
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