PoE (Power over Ethernet) Explained

Ethernet cables connect devices in a computer network. Originally, they were designed to carry data signals. The devices they connect require a separate power supply. Power over Ethernet (PoE) is a technology that enables them to carry DC power along with data. This technology removes the requirement for a separate power supply source for the connected devices.

How PoE works

Ethernet cables contain eight wires in four pairs. From these, only four wires are used for data transmission. These wires connect to pins 1, 2, 3, and 6. These wires create two circuits: one for transmitting and another for receiving.

An Ethernet cable requires and uses only four wires for data transmission, even though it contains eight wires. The remaining four wires (pins 4, 5, 7, and 8) remain unused. Initially, PoE used these wires to carry power. It uses two terms: PSE (Power Source Equipment) and PD (Powered Devices) to define the devices connected to both ends of the cable. PSE is the device that provides power. PD is the device that receives power.

Not all devices are PD-enabled. If a device is not PD-enabled, it does not accept power from the Ethernet cable. It uses its own power source. If you connect it to PSE via an Ethernet cable, it will receive power from two sources. This situation can damage the device receiving power from both sources. To handle this situation, PoE adds a 25-kW resistance between the PD device and power pairs. It protects the PD device from receiving power from both sources. It will receive power from the PSE only if it is not receiving power from another power source, avoiding any risk of damage. PoE can transmit power over Ethernet cables (Cat5, Cat5e, and above) up to 100 meters.

Security cameras, phones, wireless access points, speakers, and web cameras are examples of PD devices. PoE switches and routers are examples of PoE devices.

PoE standards

Two standards define specifications for PoE implementations: IEEE 802.3af and IEEE 802.3at.

IEEE 802.3af was defined in 2003. It is called the PoE standard. It provides up to 15.4 W of power. However, the PD device at the other end of the cable does not receive the full 15.4 W. Due to power dissipated in the cable, the device at the other end receives only 12.95 W.

IEEE 802.3at was released in 2009. It is called PoE+. It provides up to 30 W DC power. The device connected at the other end will receive up to 25.5 W guaranteed.

Cisco redefined IEEE standards to provide interoperability between data and power. It defined Universal Power Over Ethernet (UPoE) for its PoE devices. It is called the 4PPoE Type 3 standard. It supports up to 60W of power. In 2018, Cisco updated it with 4PPoE Type 4. It is called UPoE+. It supports up to 90W of DC power. Both UPoE and UPoE+ utilize all four pairs for power transmission.

Identifying PoE devices

Cisco includes the letter P in the switch model name to indicate it supports PoE. In other words, you can easily identify which Cisco switch is PoE or which is not. If it has the letter P in its model name, it is PoE-capable. For example, the switch WS-C3850-24P-L is a PoE device while WS-C3850-24T-L is not.

Advantages of PoE

• It reduces network cost.
• It removes the requirement for electrical cables.
• It provides more granular control over connected devices.
• It makes power management easier.
• It eliminates AC-DC adapters and connectors required to connect PD devices.
• It offers centralized control over all supported and connected PD devices.
• It enables flexibility. You can reposition any connected PD device.
• It intelligently protects PD devices from overpowering and underpowering.
• It provides reliability by controlling the power supply from the centralized location.
• It provides scalability. You can install a new PD in an available port of the PoE device.

Conclusion

Using the same Ethernet cable for data and power transmission opens new opportunities. You can use the same single cable to connect and power lamps, monitors, computers, laptops, security cameras, LED lighting, alarms, and a lot more devices. In this tutorial, I explained what PoE technology is, how it works, and its standards.