Chapter 1: Overview
PoE
PoE+
Enhanced PoE
Power is injected on the Ethernet cabling along with data by Power Sourcing Equipment (PSE), like an Ethernet LAN switch or router. The IEEE 802.3af, Power over Ethernet specification can provide up to 15.4 watts of power at the PSE. A PD under the IEEE 802.3af specification can use no more than 12.95 watts. The difference in maximum power levels provided by the PSE and available at the PD is in accounting for worst case power loss in the cabling between the PSE and PD, which can be influenced by cable length, quality, and other factors. This amount of wattage is sufficient to power the majority of current generation PoE Devices.
PoE+ supplies the higher power required from a new generation of network attached devices. These new devices, such as, multiple radio IEEE 802.11n wireless access points, powered pan tilt and zoom IP security cameras, thin clients, door locks, touch screen displays, and video phones frequently require more than the 12.95 watts available with IEEE 802.3af.
The IEEE 802.3at specification can provide up to 30 watts of power at the PSE. A PD under the IEEE 802.3at specification can draw up to 25.5 watts of power, which is sufficient to power a new generation of higher powered PDs.
The IEEE 802.3at specification requires that PDs support a flexible Layer 2 power classification method using Link Layer Discovery Protocol - Media Endpoint Discovery
The IEEE 802.3at specification is backwards compatible with the IEEE 802.3af specification. Powered Devices complying with IEEE 802.3af are compatible with the IEEE 802.3at Power Sourcing Equipment.
Devices that support the IEEE 802.3at specification are optimized to operate with IEEE 802.3at PSE to support dynamic power management. PSEs that support the IEEE 802.3af specification interoperate with IEEE 802.3at compliant PDs, as long as the PD can operate using 12.95 watts of power (but without dynamic power allocation and management).
Enhanced PoE delivers between 15.4W and 20W per port for PDs that require more than 15.4W, for applications such as building security and video surveillance. Enhanced PoE supplies between 15.4W and 20W per port at 48VDC. Note that IEEE 802.3at standard PoE+ PDs requiring 56VDC cannot use Enhanced PoE PSEs instead of IEEE 802.3at standard PoE+ PSEs. Enhanced PoE PSEs cannot replace IEEE 802.3at standard PSEs when using any PoE+ PDs.
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