Poised for power [power over Ethernet]

This article details the power-over-Ethernet specification (802.3af) which defines a method to deliver power to devices such as IP phones and security systems from networking equipment such as routers and switches. This takes place over the same twisted-pair cable used to carry data over Ethernet networks, and removes the need to install separate power wiring or rely on sources derived from a wall-mounted power supply. Accounting for typical losses in the cabling, this system permits each powered device (PD) to draw up to a maximum of 12.95 W. Each PD must include at least one DC/DC converter that transforms the input voltage to levels that suit its construction and also provides a 1500 V isolation barrier. To stay active, the device under power must draw a minimum current of 10 mA. This allows detection logic within the power source to remove power to a port when equipment is disconnected. The article also discusses revisions to the specification which would enable higher power levels to be supplied.