Power over Ethernet (PoE): The Complete Guide to Power and Data Over One Cable

Power over Ethernet (PoE) enables both power and data transmission over a single network cable, making it a standard solution for IP cameras, Wi-Fi access points, VoIP phones, and other devices where running separate electrical lines or adapters is inconvenient. With PoE, you only need one cable instead of two, and the device receives power directly via twisted pair Ethernet.

What Is PoE and Why Is It Needed?

PoE, or Power over Ethernet, is a technology that allows electrical power and network data to travel through a single Ethernet cable. This means devices can operate without a local power supply or outlet nearby, receiving everything they need through one wire.

The main goal of PoE is to simplify the installation of equipment in hard-to-reach locations-on ceilings, building facades, poles, or server racks. This is particularly relevant for:

• IP surveillance cameras,
• Wi-Fi access points,
• VoIP phones,
• mini servers, network sensors, and controllers.

PoE lowers installation costs, eliminates the need for separate power lines, and enhances system reliability: power is supplied centrally from a switch or injector, devices can be restarted remotely, and everything is manageable via the network.

How Power over Ethernet Works: The Basic Power Transmission Scheme

PoE delivers power over the same pairs that transmit network signals, without disrupting Ethernet operation. This works because twisted pair cabling transmits data differentially, while power is delivered as a DC voltage that doesn't interfere with packet transmission.

There are two types of devices in a PoE system:

• PSE (Power Sourcing Equipment): The power source, such as a PoE switch or injector
• PD (Powered Device): The powered device, like a camera, access point, or phone

When you connect a device, the PoE switch first checks whether it supports PoE by sending a low-level "probing" signal at a safe voltage. If the PD responds with the correct resistance, the switch determines its class and then supplies power-usually at 48V. This mechanism prevents power from being sent to non-PoE devices.

Depending on the standard, data and power may share the same pairs or use different ones. Modern PoE uses all pairs for power, reducing losses and increasing available wattage.

PoE Standards: 802.3af, 802.3at (PoE+), 802.3bt (PoE++)

PoE has evolved over the years, with each new standard increasing available power and improving device compatibility. All official standards use PD detection, ensuring safe and interchangeable operation.

802.3af (PoE)

• Power per device: up to 15.4 W (realistically about 12.95 W)
• Suitable for IP cameras, VoIP phones, low-power access points
• Voltage: 44-57 V

802.3at (PoE+)

• Power: up to 30 W (about 25.5 W available)
• Supports more powerful Wi-Fi access points and PTZ cameras
• Fully backward compatible with 802.3af

802.3bt (PoE++) / Type 3, Type 4

• Type 3: up to 60 W
• Type 4: up to 90-100 W per device
• Uses all four cable pairs, minimizing power loss
• Ideal for panoramic cameras, thin clients, panels, and high-power equipment

All bt and at standards always verify the device before supplying power, making PoE safe. This sets them apart from passive PoE, which lacks this verification step.

Active vs. Passive PoE: What's the Difference and Which Is Safer?

Active PoE complies with IEEE standards (802.3af/at/bt) and automatically detects what's connected. A switch or injector sends a test signal and only applies power if the correct resistance is detected, protecting devices from damage-no power is supplied to non-PoE equipment.

Key features of active PoE:

• Safe operation
• Compatible between different brands
• Uses 44-57 V
• Strictly follows power delivery standards

Passive PoE is a non-standard implementation, often found in MikroTik gear and budget IP cameras. It provides a fixed voltage (commonly 24V or 48V) without checking compatibility. Connecting a device not designed for this voltage can cause damage.

Key features of passive PoE:

• Lower cost
• Not IEEE-compliant
• Potentially hazardous to unsupported devices

In summary, active PoE is always preferable-safe, reliable, and universal. Passive PoE should only be used when you are certain of device compatibility.

PoE Injector: How It Works and When to Use One

A PoE injector adds power to an Ethernet cable when your switch doesn't support PoE. It has two inputs and one output: one port receives the network signal, the second connects to the power adapter, and the output delivers a combined stream of data and power.

1. Receives standard Ethernet from a switch or router
2. Adds power according to the required standard (usually 48V)
3. Transmits both data and power to the device over a single cable

Common use cases:

• For IP cameras when the switch lacks PoE
• For Wi-Fi access points mounted high up
• For individual devices in a distributed network
• When replacing the entire switch with a PoE model isn't practical

Injectors come in active (IEEE 802.3af/at/bt) and passive versions. Active injectors are safe and compatible with all PoE devices; passive ones are suitable for specific scenarios and may damage unsupported equipment.

PoE Switches: Operation and Advantages

A PoE switch combines network switching and power sourcing functionality. Each PoE port contains a PSE module that supplies and manages power. Unlike an injector, which powers just one port, a PoE switch can power multiple devices-4, 8, 16, 24, or even 48 ports at once.

1. The switch checks if the device supports PoE
2. It determines the load class and allowed power
3. Delivers power according to the standard and monitors it during operation
4. Provides protection against overload, short circuit, and overheating

Benefits of a PoE switch:

• Convenience: one cable for both data and power
• Safety: active PoE prevents power from reaching unsupported devices
• Power distribution: the switch manages the total load
• Remote management: restart PoE ports via software

When you have several PoE devices in a system, a PoE switch is much more convenient and reliable than using individual injectors.

Devices That Support PoE: Cameras, Access Points, Phones

PoE powers devices that need a constant supply but are difficult or impractical to connect to a local socket. This has made PoE a standard in residential and commercial network infrastructure.

Most Common PoE Devices:

• IP Surveillance Cameras: The main use case for PoE. Cameras are often installed on facades, ceilings, or poles-places without easy access to outlets. PoE simplifies installation and increases system reliability.
• Wi-Fi Access Points: Often installed on ceilings or high walls. PoE eliminates the need to run separate power lines. Most modern business-class access points use 802.3at (PoE+).
• VoIP Phones: Office phones get both power and connectivity from a single cable, speeding up workspace deployment.
• Network Sensors, Controllers, Mini Servers: Access control systems, climate sensors, and IoT devices are increasingly powered via PoE.
• Monitors, Panels, and Mini-PCs with PoE++: The 802.3bt (PoE++) standard allows powering devices up to 90-100 W, including small terminals and displays.

The list expands every year-PoE streamlines infrastructure and reduces network deployment costs.

PoE Power Output: How Many Watts Can Ethernet Deliver?

PoE's power output depends on the standard and the number of cable pairs used. Higher classes deliver more watts, and the latest versions use all four pairs to minimize losses.

• 802.3af (PoE): up to 15.4 W per port (about 12.95 W usable)
• 802.3at (PoE+): up to 30 W per port (about 25.5 W usable)
• 802.3bt (PoE++): Type 3: up to 60 W; Type 4: up to 90-100 W

Such power levels are sufficient for demanding devices-large PTZ cameras, powerful Wi-Fi 6 access points, information panels, and mini-PCs.

Ethernet cables can deliver power over significant distances-up to 100 meters-but losses increase with length, so the power available at the cable's end will be slightly lower. For high-power devices, it's best to use high-quality Cat 5e or better cable.

PoE Power Transmission: Pairs, Polarity, and Device Safety

PoE delivers power in two ways: over the same pairs used for data or over spare pairs. The method depends on the standard and Ethernet type.

• Mode A (data pairs): Power is supplied over pairs 1-2 and 3-6.
• Mode B (spare pairs): Power is supplied over pairs 4-5 and 7-8.

PoE standards support both methods; devices automatically accept either. In modern gigabit networks, all four pairs carry data, so PoE delivers power over all pairs to reduce losses.

Equipment Protection:

Active PoE (IEEE standards) includes several safety features:

• Resistance detection before power is applied
• Load class identification
• Short circuit protection
• Shutdown on overheating or overload
• Automatic port recovery after faults

As a result, it's practically impossible to connect active PoE incorrectly: power isn't supplied until the device confirms compatibility.

Conclusion

PoE is a convenient and safe way to deliver both power and data through a single Ethernet cable. Thanks to the 802.3af/at/bt standards, PoE has become foundational for IP cameras, access points, VoIP phones, and many IoT devices. It eliminates the need for separate wiring, simplifies installation, and makes maintenance more reliable.

Active PoE automatically detects device type, supplies safe voltage, and manages power in real time, eliminating risk of equipment damage. Modern PoE switches can power dozens of devices simultaneously, and PoE++ standards support high-power panels and mini-PCs.

Understanding how PoE works helps you choose the right equipment and build a reliable network where both power and data travel over a single cable-no extra adapters or power bricks needed.