8.1 Introduction to PoE

Power-over-Ethernet (PoE) is a means of distributing power to Ethernet devices over the Ethernet cable using either data or spare pairs. PoE eliminates the need for power supplies at the Ethernet device. Common applications of PoE are security cameras, IP Phones and PDA chargers. The host or mid-span equipment that supplies power is the Power Source Equipment (PSE). The load at the Ethernet connector is the Powered device (PD). PoE protocol between PSE and PD controlling power to the load is specified by IEEE Std 802.3at-2009. Transformers are used at Ethernet host ports, mid-spans and hubs, to interface data to the cable. A DC voltage can be applied to the center tap of the transformer with no effect on the data signals. As in any power transmission line, a relatively high 48 V is used to keep current low, minimize the effect of IR drops in the line and preserve power to the load. Standard POE delivers approximately 13 W to a type 1 PD, and 25.5 W to a type 2 PD. Figure 46 shows the overview schematic of a POE port.