How do PoE Switches Work?

PoE, also known as Power over Ethernet or Active Ethernet, is a technology that simultaneously transmits data and power over 10BASE-T, 100BASE-TX, and 1000BASE-T Ethernet networks, with a reliable power supply distance of up to 100 meters. This allows for remote centralized power supply to devices such as IP cameras, wireless access points (APs), and data acquisition terminals in industrial environments. For these devices requiring power, there's no need to consider their own power supply cabling; power is supplied simply by connecting an Ethernet cable.

PoE System Components

According to the IEEE 802.3af standard, a complete PoE system includes two types of devices: Power-sourcing Equipment (PSE) and Powered Device (PD).

PSEs are primarily used to supply power to other devices. They can be further divided into two types: Midspan (PoE functionality is external to the switch) and Endpoint (PoE functionality is integrated into the switch). Examples include Ethernet switches, routers, and other network switching devices. A powered device (PD) is a device that can be powered via a network cable, such as an IP camera or a wireless access point (AP).

The Working Principle of a PoE Switch

In addition to the data transmission capabilities of a standard switch, a PoE switch can also provide DC power to end devices via the same Ethernet cable.

PoE technology primarily relies on two key components:

Power Sourcing Equipment (PSE): In this case, this is the PoE switch.

Powered Device (PD): This is a device that requires power via a network cable, such as an IP phone, a wireless access point (AP), or an IP camera.

Its working principle can be divided into the following three main stages:

1. Detection

Before power supply begins, the PoE switch (PSE) must first determine whether the device connected to the port is a PoE powered device (PD). This process is crucial, protecting non-PoE devices or ordinary switch ports from damage caused by high voltage.

Detection: The switch applies a small detection voltage (typically between 2.8V and 10V) to the connected network cable port.

Judgment: If it detects the characteristic resistance signature (typically 25 kΩ) of a PoE-compliant PD device, the switch knows it's a device that can be powered. If it doesn't detect this resistance, the switch will only perform normal data transmission and will not provide power.

2. Classification

After confirming it's a PD device, the PSE needs to determine the power level required by the PD device. This ensures the switch only provides the power the device needs, thus optimizing energy use and managing the overall power budget.

Steps: The switch applies a slightly higher voltage and measures the current drawn by the PD device from that voltage.

Determination: Based on the current value, the PD device is "classified" into different power classes (e.g., Class 1 to Class 8). For example, a Class 4 IP camera might require up to 25.5W of power.

Version: Different PoE standards (e.g., PoE, PoE+, UPoE) correspond to different maximum power ratings.

3. Powering & Monitoring: After detection and classification, the PSE begins supplying the PD device with full voltage (44V to 57V DC for standard PoE) and corresponding power.

Powering: DC power is transmitted to the PD device through two or all four pairs of wires in the Ethernet cable.

  Spare Pair: Power is supplied using the wire pair not used for data transmission (applicable to 10/100BASE-T).

  Data Pair: Power is supplied using the wire pair used for data transmission (also called Phantom Power, applicable to all speeds).

Data: Meanwhile, data packets continue to be transmitted on the Ethernet cable at normal speed; the two processes do not interfere with each other.

Monitoring: The switch continuously monitors the power supply status. If a PD device disconnects, or experiences an overcurrent, short circuit, or other abnormal situation, the switch will immediately cut off power to protect the device and network.

Key Advantages of PoE:

• Simplified Cabling: No separate power cables are needed for terminal devices; a single network cable can handle both data and power supply.

• Flexible Deployment: Devices can be installed on walls, ceilings, or other locations without power outlets.

• Centralized Management: Power supply is centralized at the switch, facilitating power management and troubleshooting.