Understanding Power over Ethernet (PoE): Standards, Operation, and Device Support

Power over Ethernet (PoE) is a technology that delivers electrical power along with data over standard Ethernet cables, enabling devices such as IP phones, wireless access points, cameras, and other IP endpoints to receive both power and network connectivity through a single cable.

PoE is needed because many endpoint devices are installed in locations without convenient power outlets, such as ceilings or outdoor sites, and centralized power simplifies management of large LAN deployments.

Key advantages of PoE include reliability (centralized power with backup), simplified cabling (single cable for power and data), and compliance with international standards using RJ45 interfaces.

PoE systems consist of two roles: Power‑sourcing Equipment (PSE), such as PoE switches, which provide power and perform detection and management; and Powered Devices (PD), such as wireless APs or cameras, which receive power. PDs are classified as standard or non‑standard based on IEEE compliance.

Two power delivery modes are defined by the IEEE: Alternative A (power on data pairs 1/2 and 3/6) and Alternative B (power on spare pairs 4/5 and 7/8). A PSE can implement only one mode, while a PD must be able to accept either.

The power negotiation process includes detection of PD, classification of PD power requirements, initiation of power, normal operation, and shutdown when the PD disconnects or overloads occur.

LLDP (Link Layer Discovery Protocol) can be used for power negotiation via the optional Power via MDI TLV defined in IEEE 802.1ab. The TLV format includes a type, length, and value field; Huawei adds a custom TLV to support IEEE 802.3bt negotiation.

Three IEEE PoE standards have been released: 802.3af (up to 15.4 W per port, 12.95 W usable), 802.3at (up to 25.5 W, 21 W usable), and 802.3bt (Type 3 up to 51 W, Type 4 up to 71.3 W) which also adds support for higher‑speed Ethernet (2.5GBASE‑T, 5GBASE‑T, 10GBASE‑T). The standards are commonly referred to as PoE, PoE+, and PoE++ (or 4PPoE).

Many devices support PoE, including Huawei S‑series campus switches, wireless access controllers, and AR routers. Whether a device can provide PoE depends on hardware design; a non‑PoE device cannot be upgraded to support PoE via software alone.