How does PoE technology work?

Power over Ethernet (PoE) technology allows Ethernet cables to carry both data and electrical power to network devices over a single cable. This eliminates the need for separate power supplies and reduces cable clutter, making the installation of devices like IP cameras, wireless access points, and VoIP phones more efficient. Here’s a breakdown of how PoE technology works:

1. Basic Components of PoE

Power Sourcing Equipment (PSE): This is the device that delivers power over the Ethernet cable. It could be a PoE-enabled switch, a PoE injector, or a router with PoE capabilities. The PSE determines how much power is needed and delivers it accordingly.

Powered Device (PD): The device that receives both power and data from the Ethernet cable. Examples include IP cameras, wireless access points, VoIP phones, and other networked devices. The PD communicates with the PSE to receive the appropriate amount of power.

Ethernet Cable: PoE typically uses standard Cat5e, Cat6, or higher Ethernet cables to transmit both power and data over the same cable. The cable is divided into pairs of wires, some of which are used for data transmission, while others are used for power delivery.

2. How Power is Delivered Over Ethernet

PoE technology works by sending low-voltage DC power over the same twisted-pair cables used for data transmission. There are two main methods of delivering power:

Spare-Pair Powering (Alternative B): In a standard Ethernet cable, only two of the four twisted pairs of wires are used for data transmission in 10BASE-T and 100BASE-T networks. The unused pairs (pins 4, 5, 7, and 8) can carry power without affecting data transmission.

Phantom Powering (Alternative A): In 1000BASE-T (Gigabit Ethernet) and faster networks, all four wire pairs are used for data. In this method, the PSE superimposes the power on the data pairs (pins 1, 2, 3, and 6) without affecting the data signal. This is done by using the DC component of the signal for power delivery while the AC component handles data.

3. PoE Negotiation and Power Allocation

The PSE and PD must communicate to ensure that the correct amount of power is delivered. This process is governed by the IEEE PoE standards:

Detection: The PSE checks whether the connected device is PoE-compatible by applying a low voltage to the cable. If the PD has a signature resistance of about 25 kΩ, the PSE detects that it is PoE-capable.

Classification: The PSE classifies the PD to determine its power requirements. PoE devices are divided into different power classes based on the amount of power they need, ranging from Class 0 (default) to Class 4 (high power). This allows the PSE to allocate the appropriate amount of power and optimize power distribution across multiple devices.

Power Delivery: After classification, the PSE begins supplying power to the PD. The voltage is typically between 44 and 57 V DC, with the current varying based on the device's power needs.

Monitoring: The PSE continues to monitor the power usage of the PD. If the device is disconnected, the PSE immediately stops providing power to avoid overloading the circuit.

4. PoE Standards

PoE technology is standardized under the IEEE 802.3 family of protocols, with different versions specifying varying power levels:

--- IEEE 802.3af (PoE): The original PoE standard provides up to 15.4 watts of power at the PSE and up to 12.95 watts at the PD, after accounting for power loss in the cable. This is suitable for low-power devices like VoIP phones and simple wireless access points.

--- IEEE 802.3at (PoE+): An enhanced version of PoE that provides up to 30 watts at the PSE and up to 25.5 watts at the PD. This is used for more power-hungry devices, such as IP cameras and high-performance wireless access points.

--- IEEE 802.3bt (PoE++ or 4-Pair PoE): The latest PoE standard, which supports higher power levels, offering up to 60 watts (Type 3) or 100 watts (Type 4) at the PSE. This is used for power-intensive devices such as PTZ (pan-tilt-zoom) cameras, LED lighting, and high-performance wireless devices.

5. PoE Advantages

Simplified Installation: PoE allows devices to receive both power and data over a single cable, reducing the need for additional power outlets and streamlining installation.

Cost Savings: By using PoE, businesses can save on installation costs, avoid the expense of running separate electrical wiring, and reduce the need for power adapters.

Flexibility: PoE enables the deployment of devices in locations where power outlets may not be available or convenient, such as ceilings, walls, or outdoor locations.

Centralized Power Management: PoE allows for centralized management of power, enabling network administrators to monitor and control the power supply to connected devices. This can improve energy efficiency and simplify troubleshooting.

6. PoE Limitations

Power Budget: The total power available from a PoE switch is limited by its power budget. This means that only a certain number of devices can be powered simultaneously, depending on their power requirements.

Cable Length: PoE is limited by the maximum Ethernet cable length, which is typically 100 meters (328 feet). BENCHU GROUP's long-distance transmission technology can transmit up to 250 meters without the relay devices. Beyond this distance, power delivery and data transmission become unreliable without using PoE extenders or repeaters.

Conclusion

PoE technology is a powerful and flexible solution for powering network devices without the need for separate power supplies. By delivering power and data over a single Ethernet cable, PoE simplifies installation, reduces costs, and provides centralized power management. It's widely used in modern networking environments for devices like wireless access points, IP cameras, and VoIP phones.