More power to you

Q: How does Power Over Ethernet (POE) work to power up an access point? – Bob, Chicago

A: POE is a general term for several proprietary (and now an international standard) for providing low voltage power over data cabling. The IEEE 802.3af standard for Data Terminal Equipment power via Media Dependent Interface is what specifies the means for supplying DC power to equipment connected to Ethernet cabling.

The power supply for POE is typically in an Ethernet switch or an inline power injector. Both types of power supplies will not apply power to the data cabling until they sense an 802.3af compliant load present. This ensures that there isn’t a significant DC voltage on the cable or connector until the device to be powered is connected. Once the power supply senses the correct load, DC power is applied to the cable and connected device. The power specified by 802.3af is 48 VDC with current up to 350mA.

Using POE can reduce the costs and time involved in installing equipment, including wireless LAN access points and voice over IP desk phones, since you don’t have to install a 120 VAC where each device is located. POE also provides for centralized control of the power to all network access points, which can ease the management of a WLAN.

Join the Network World communities on Facebook and LinkedIn to comment on topics that are top of mind.

Copyright © 2003 IDG Communications, Inc.