Power over Ethernet (PoE)
Technology for delivering power and data over a single Ethernet cable.
Simplifying Network Installations
Power over Ethernet (PoE) is a transformative technology that allows electrical power to be transmitted along with data over a standard . This innovation eliminates the need for separate power supplies and dedicated electrical wiring for a wide range of network devices, such as IP cameras, VoIP phones, wireless access points, and IoT sensors.
By delivering power directly through the network cabling, PoE greatly simplifies installation, increases placement flexibility for devices (as they are no longer tethered to a nearby power outlet), and reduces overall infrastructure costs.
How PoE Works: Key Components and Process
A PoE-enabled system consists of two primary types of devices:
- Power Sourcing Equipment (PSE): This is the device that supplies power over the Ethernet cable. PSEs can be integrated into a network device, such as a PoE switch, or can be a standalone device, known as a PoE injector, which adds power to an existing Ethernet line.
- Powered Device (PD): This is the end device that receives both power and data from the Ethernet cable, such as an IP camera, VoIP phone, or wireless access point. PDs are designed to safely accept this power and use it for their operation.
The PoE Handshake
To ensure safety and prevent damage to non-PoE devices, a negotiation process known as the "PoE handshake" occurs before any significant power is sent over the line:
- Detection: The PSE sends a low voltage signal to check if the connected device is PoE-compatible. The PD responds by presenting a specific resistance signature.
- Classification: Once a PD is detected, the PSE may perform an optional classification step to determine the PD's power requirements. The PD signals its power class by drawing a specific amount of current.
- Power-up: If detection and classification are successful, the PSE supplies full operational power to the PD.
Evolution of PoE Standards
The IEEE 802.3 working group has standardized several PoE versions to meet the growing power demands of modern network devices. Each standard increases the available power budget.
| Parameter | PoE (IEEE 802.3af) | PoE+ (IEEE 802.3at) | PoE++ (IEEE 802.3bt) |
|---|---|---|---|
| Power at PSE Output | 15.4 W | 30 W | Up to 90 W |
| Min. Power at PD Input | 12.95 W | 25.5 W | Up to 71.3 W |
| Voltage at PSE | 44 – 57 V | 50 – 57 V | 52 – 57 V |
| Max Current | 350 mA | 600 mA | ~960 mA per pair |
Note: The difference between PSE output power and PD input power is due to power losses in the copper cable over its length.
Advantages and Challenges of PoE
Advantages:
- Flexibility and Convenience: Devices can be installed in optimal locations, such as high on walls or ceilings for access points and cameras, without worrying about nearby power outlets.
- Cost and Time Savings: Eliminates the need for hiring an electrician to install new power outlets for network devices, significantly reducing installation time and costs.
- Reliability and Centralized Power Management: PoE devices can be connected to a central via the PoE switch, ensuring that the network remains operational even during a power outage.
Challenges:
- Power Budgeting: The total power required by all connected PDs must not exceed the total power budget of the PSE (PoE switch). Careful planning is needed for large installations.
- Distance Limitation: Standard Ethernet cabling is limited to 100 meters (328 feet). The power delivered to the PD decreases over this distance due to cable resistance.
- Heat Dissipation: When multiple high-power PoE cables are bundled together, the generated heat can degrade cable performance and poses a safety risk if not managed correctly. Proper cable selection (e.g., higher categories, shielded cables) and installation practices are crucial.