Learning Objectives

• Understand how Power over Ethernet delivers both data and power over a single cable
• Identify the IEEE 802.3 PoE standards and their power budgets
• Calculate power budgets for multi-device PoE deployments
• Recognize the limitations and safety mechanisms of PoE

What is Power over Ethernet?

Power over Ethernet (PoE) delivers electrical power alongside data over standard twisted-pair Ethernet cabling. This eliminates the need for separate power supplies at each device, simplifying installation for devices in ceiling spaces, outdoors, or locations without nearby power outlets.

PoE uses a technique called phantom power: the power is carried on the same wire pairs as data, using a center-tap transformer at each end. The DC current is superimposed on the signal without interfering with it, because Ethernet signals are AC-coupled and the transformer isolates the DC path from the data path.

| Application | Typical Devices | |---|---| | VoIP phones | No wall wart needed, works during power outages via UPS | | Wireless access points | Ceiling-mounted without running electrical | | IP security cameras | Outdoor and ceiling locations without nearby outlets | | IoT sensors | Building automation, environmental monitoring | | Point-of-sale terminals | Clean desk installations |

IEEE 802.3 PoE Standards

| Standard | Name | Power at PSE | Power at PD | Pairs | Cable Required | |---|---|---|---|---|---| | IEEE 802.3af (2003) | PoE | 15.4 W | 12.95 W | 2 pairs | Cat 3 or better | | IEEE 802.3at (2009) | PoE+ | 30 W | 25.5 W | 2 pairs | Cat 5 or better | | IEEE 802.3bt Type 3 (2018) | PoE++ | 60 W | 51 W | 4 pairs | Cat 5e or better | | IEEE 802.3bt Type 4 (2018) | PoE++ | 90 W | 71.3 W | 4 pairs | Cat 5e or better |

The acronyms: PSE (Power Sourcing Equipment) is the switch or injector. PD (Powered Device) is the device consuming power.

The power difference between PSE and PD is lost as heat in the cable. For 802.3af at 100 m of Cat 5e cable, about 2.45 W is dissipated as heat, leaving 12.95 W available at the device.

Power Budget Calculation

When deploying PoE, you must calculate the total power draw against the switch's power budget.

Example: An office deployment with:

• 12 cameras (802.3af, 12.95 W each)
• 8 access points (802.3at, 25.5 W each)
• 24 VoIP phones (802.3af, 6.5 W each)

Calculation:

• Cameras: 12 × 12.95 W = 155.4 W
• Access points: 8 × 25.5 W = 204 W
• VoIP phones: 24 × 6.5 W = 156 W
• Total power required: 515.4 W

A 48-port PoE+ switch with a 740 W power budget would handle this (515.4 W < 740 W). Always leave at least 20% headroom for future devices or peak draw scenarios.

PoE Detection and Classification

PoE is not simply "always on." The PSE follows a careful sequence before delivering power:

1. Detection — PSE applies a low voltage (2.7–10.1 V) and measures the resistance. A valid PD presents a 23.75–26.25 kΩ signature resistor. If detected, proceed. If not, no power is delivered (prevents damage to non-PoE devices).

2. Classification — PSE raises voltage to 14.5–20.5 V and measures the current draw to determine the device class:

| Class | Power Available | Typical Device | |---|---|---| | 0 | 15.4 W | Default, unclassified | | 1 | 4.0 W | Low-power sensors | | 2 | 7.0 W | Legacy VoIP phones | | 3 | 15.4 W | Cameras, access points (802.3af) | | 4 | 30.0 W | PoE+ devices | | 5 | 45.0 W | PoE++ Type 3 | | 6 | 60.0 W | PoE++ Type 3 | | 7 | 75.0 W | PoE++ Type 4 | | 8 | 90.0 W | PoE++ Type 4 |

3. Power-up — PSE ramps voltage to 48–57 V DC and begins delivering power. The PSE continuously monitors current; if it exceeds the limit or drops too low (disconnect), power is cut.

Key Takeaways

• PoE delivers DC power alongside data using center-tap transformers (phantom power)
• IEEE 802.3af = 12.95 W, 802.3at = 25.5 W, 802.3bt Type 4 = 71.3 W at the PD
• Power budget must account for both cable loss and device consumption
• PoE uses a three-phase startup: detection, classification, power-up
• Leave at least 20% headroom in your PoE power budget