What is POE?
PoE (Power over Ethernet) is a technology that transmits power over network cables. Utilizing existing Ethernet infrastructure, PoE allows data transmission and power supply to IP terminal devices, such as IP phones, access points (APs), and IP cameras, through the same network cables.
PoE is also known as Power over LAN (PoL) or Active Ethernet and is sometimes referred to as Ethernet power supply. To standardize and promote the development of PoE technology, and to address compatibility issues between power sourcing and powered devices from different manufacturers, the IEEE Standards Committee has successively released three PoE standards: IEEE 802.3af, IEEE 802.3at, and IEEE 802.3bt.
Why Need POE??
With the increasing prevalence of IP phones, network video surveillance, and wireless Ethernet devices in networks, the demand for power support through Ethernet itself is becoming more urgent. In most cases, terminal devices require DC power, and these devices are often installed on high ceilings or outdoors, where suitable power outlets are difficult to find. Even if outlets are available, there is usually no space for the AC-DC converters that terminal devices need. Additionally, in many large LAN applications, administrators need to manage multiple terminal devices that require a unified power supply and management. Due to the limitations of power supply locations, managing the power supply becomes very inconvenient. Power over Ethernet (PoE) effectively solves this problem.
PoE is a wired Ethernet power supply technology that allows network cables used for data transmission to also provide DC power. This effectively addresses the centralized power supply needs of terminal devices such as IP phones, wireless access points (APs), portable device chargers, card readers, cameras, and data collection devices. PoE power supply has the following advantages: reliability, simplified connections, and standardized implementation:
Reliability: A single PoE device can power multiple terminal devices, enabling a centralized power supply and providing power backup. Simplified connections: Terminal devices do not require an external power source; they only need one network cable. Standardized: Complies with international standards, using a globally unified RJ45 power interface, ensuring compatibility with devices from different manufacturers.
How Does Poe Work??
PoE Power Supply System Components
The PoE power supply system includes the following two device roles:
Power-sourcing Equipment (PSE): This PoE device supplies power to the powered devices over Ethernet. It provides functions such as detection, analysis, and intelligent power management. An example of PSE is a PoE switch.
Powered Device (PD): These are devices that receive power, such as wireless APs, portable device chargers, card readers, and cameras. PDs are classified as either standard PDs or non-standard PDs based on their compliance with IEEE standards.
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PoE Power Supply Device Roles
According to the IEEE standard definition, PSE devices are classified into two types:
MidSpan: The PoE functionality module is external to the device.
Endpoint: The PoE functionality module is integrated into the device.
Endpoint PSE devices are further divided into two power supply modes based on the power supply wire pairs they use:
Alternative A: This mode supplies power through the data pairs. PSE supplies power to the PD using the 1/2 and 3/6 wire pairs, with 1/2 forming the negative pole and 3/6 forming the positive pole. The 10BASE-T and 100BASE-TX interfaces use the 1/2 and 3/6 pairs for data transmission, while the 1000BASE-T interface uses all four pairs for data transmission. Since DC power and data frequencies do not interfere with each other, it is possible to transmit both power and data over the same pairs simultaneously.
Alternative B: This mode supplies power through the spare pairs. PSE supplies power to the PD using the 4/5 and 7/8 wire pairs, with 4/5 forming the positive pole and 7/8 forming the negative pole.
The IEEE standard does not allow both power supply modes to be used simultaneously. PSE devices can only provide one method of power supply, but PD devices must be able to adapt to both situations.
PoE Power Negotiation Process
When the PSE device is powered on and the PD device is connected to the PSE device through the network, the PSE and PD start the power negotiation process:
- PD Detection: The PSE periodically outputs a small, current-limited voltage at the port to detect the presence of a PD device. If a specific resistance value is detected, it indicates that the cable is connected to a PD device that supports the IEEE 802.3af or IEEE 802.3at standard (resistance value between 19kΩ and 26.5kΩ, usually with a small voltage of 2.7V to 10.1V, and a detection cycle of 2 seconds).
- Power Capability Negotiation (PD Classification): The PSE classifies the PD and negotiates the power supply. This can be achieved by analyzing the resistance values sent between the PSE and PD, or through the Link Layer Discovery Protocol (LLDP) for discovering and advertising power capabilities.
- Power Supply Initiation: During the startup phase (usually less than 15μs), the PSE device begins to supply power to the PD device, gradually increasing from a low voltage until it reaches 48V DC.
- Normal Power Supply: After the voltage reaches 48V, the PSE provides a stable and reliable 48V DC power supply to the PD device, with the PD's power consumption not exceeding the PSE's maximum output power.
- Power Shutdown: During the power supply process, the PSE continuously monitors the PD's current input. If the PD's current consumption drops below the minimum level, or there is a sudden current surge (such as when the device is unplugged or the PD experiences a power overload, short circuit, or exceeds the PSE's power supply capacity), the PSE will cut off the power and repeat the detection process.