High-Power and Long-Distance PoE Power Supply

Various types of terminals are connected on a campus network, and managing their power supply is a major challenge. Terminals such as IP phones, cameras, and data collectors all require DC power supply, and such devices are usually installed in corridors or on high ceilings where power sockets are unavailable. On many large-scale LANs, administrators must deal with the complicated task of managing multiple terminals and access devices that require unified power supply and management.

PoE, a wired Ethernet power supply technology that is widely applied on campus networks, can address this problem. It provides DC power for IP-based terminals while transmitting data signals and, when compared with traditional power supply modes, offers the following advantages:

  • • Low-cost: The power cabling cost is significantly reduced, and it is easy to lay out power cables.
  • • Reliable: Multiple powered devices (PDs) are powered in a unified manner, facilitating power backup. [1]

• Standard-compliant: PoE complies with IEEE 802.3af and IEEE 802.3at, and all PoE devices use uniform power interfaces and can be connected to PDs of different vendors.

Figure 4.25 shows a typical PoE system, which consists of power sourcing equipment (PSE), PDs, and PoE module (built in a PoE switch).

  • • PSE: refers to a PoE device that supplies power to PDs through Ethernet. The PSE also provides functions such as detection, analysis, and intelligent power management.
  • • PD: refers to a powered device, such as AP, portable device charger, card reader, and IP camera. PDs are classified into standard and nonstandard PDs, depending on whether they conform to IEEE standards.
  • • PoE module: provides power to a PoE system. The number of PDs connected to a PSE is limited by the power output of a PoE power module. PoE power modules are classified into built-in and external power modules, depending on whether they are pluggable.
  • 1. PoE power supply principle

The following uses Huawei S series switches as an example to describe the PoE power supply principle:

Step 1 PD detection: A PSE periodically transmits a low voltage with limited current through its ports to detect PDs (at 2.7-10.1 V,

Components in a PoE system

FIGURE 4.25 Components in a PoE system.

a detection period of 2 seconds). If a resistor with a specific resistance (19-26.5 kQ) is detected, the PSE considers the cable terminal to be connected to a PD.

Step 2 Power supply capability negotiation: The PSE classifies PDs and negotiates the power supply capability by resolving detected resistors or using the Link Layer Discovery Protocol (LLDP).

Step 3 Power supply startup: During the startup period (generally within 15 |is), the PSE gradually increases the voltage to 48 V DC in order to supply power to PDs.

Step 4 Normal power supply: When the voltage reaches 48 V, the PSE provides stable and reliable 48 V DC power for PDs. The power of a PD cannot exceed the maximum output power of the PSE.

Step 5 Power supply disconnection: The PSE constantly detects the input current of PDs while providing power to them. If the current of a PD falls below the minimum value or increases sharply, the PSE stops providing power to this PD and repeats PD detection. This situation occurs when a PD is disconnected from the PSE, encounters a power overload or short circuit, or its power consumption exceeds the power supply capacity of the PSE.

2. PoE standards compliance

IEEE 802.3af was the earliest standard for PoE power supply and effectively provides centralized power supply for terminals such as IP phones, APs, portable chargers, card readers, and cameras. Subsequently, the IEEE 802.3at standard further proposes PoE+, which can supply power to devices installed with high-power applications such as dual-band access, videotelephony, and pan-tilt-zoom video surveillance.

Today, more advanced service types and terminals continue to emerge, requiring even higher PoE input power. To satisfy such demands, Huawei has actively engaged in the formulation of the IEEE 802.3bt standard (also known as PoE-H-). In compliance with IEEE 802.3bt (draft), Huawei has developed and launched many PoE-H- switches capable of providing up to 60 W power. In addition, Huawei has unveiled the next-generation Universal Power Over Ethernet Plus (UPoE+) switches based on the new IEEE 802.3bt standard. The

UPoE+ function provides up to 90 W power, meeting the needs of more terminals. Table 4.9 provides detailed performance parameters of PoE, PoE+, PoE-H-, and UPoE+.

3. Perpetual PoE and fast PoE

Perpetual PoE technology delivers an uninterruptible power supply to PDs when a PoE device is rebooted or its software is upgraded. This technology ensures that PDs are not powered off during the reboot of the PoE device, eliminating any interruptions that may be triggered by a power failure.

When a PoE device reboots due to a power failure, it continues to supply power to the PDs immediately after being powered on without waiting for the reboot to complete. Compared with common PoE switches that typically require up to 3 minutes to begin supplying power to PDs, Huawei switches are capable of supplying power within 10 seconds of being rebooted, greatly reducing the service interruption time caused by power supply interruption.

4. Long-distance PoE power supply

Typically, the distance between a PoE device and a PD can be anywhere up to 100 m. Following the proliferation of wireless terminals, APs are now deployed across various scenarios, including outdoor spaces (such as campus playgrounds) that are inconvenient for

TABLE 4.9 Performance Parameters of PoE, PoE+, PoE++, and UPoE+







IEEE 802.3af

IEEE 802.3at

IEEE 802.3bt (draft)

IEEE 802.3bt (draft)

Power supply distance (m)





Maximum current (mA)





PSE output voltage (V DC)





PSE output power (W)





PD input voltage (V DC)





Maximum input power available at PDs (W)





cabling, and where power supply problems become more apparent. PoE switches that provide PoE power supply are generally deployed in extra-low voltage rooms. As a result, in wireless scenarios, the PoE power supply distance must be increased without compromising the AP uplink bandwidth.

The signal-to-noise ratio (SNR) is the most important indicator in electrical interface transmission. To support a longer transmission distance, we must reduce the SNR loss of the entire link. Multiple types of components and media exist on the transmission link of electrical interfaces, including physical chips, cards, interface connectors, and network cables on both ends. Based on these components and media, SNR parameters can be optimized to enable multi-GE interfaces of Huawei PoE devices in order to support a maximum transmission distance of 200 m when connected to specific APs. Optimization methods include:

a. Huawei switches and APs have built-in customized physical chips that support a maximum transmission distance of 200 m. In addition, Huawei-developed algorithms are used to improve the driver software, ensuring that SNR parameters can be optimized for long-distance transmission.

b. Huawei uses high-quality connectors and STPs to reduce SNR loss.

  • [1] Easy to deploy: Network terminals can be powered over Ethernetcables, without the need of external power sources.
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