POE Potential

By Cameron Sturdevant  |  Posted 2003-11-24 Print this article Print

POE Potential

Based on eWEEK labs work with currently available 10/ 100M-bps Ethernet-based devices—including VOIP handsets, network cameras and WLAN APs—we recommend that IT managers work POE into their two- to five-year plans for enhancing productivity.

During tests, we used WLAN APs from Hewlett-Packard Co., IP telephone handsets from Alcatel and network cameras from Axis Communications to test interoperability and to get hands-on experience using 802.3af-standard-compliant equipment.

We easily installed the devices by connecting a standard Ethernet patch cord to each. We installed an HP ProCurve 420 WLAN AP in just minutes, for example.

The cost advantages of POE became clear when we were able to place the HP AP in the most effective broadcast location—near the ceiling. We couldnt have done that if the AP had required AC power; we would have had to make a facility request to have an electrician rewire parts of our building to run power to the AP—a costly and time-consuming process.

Worries about the safety of POE appear unwarranted. We experienced no problems in our tests, and power has been running over Ethernet for some time (albeit not within the 802.3af standard).

One issue that has been brought up is the effect of POE on equipment that was never designed to accept electric power over the Ethernet interface. However, our tests showed that if a cable connected to the PSE is connected to a device that does not accept power, then the higher voltage associated with POE is not sent to the port. POE devices do send very low-voltage power to sense PDs, but this power is specifically set too low to harm or interfere with legacy devices.

Further regarding safety and equipment protection, the 802.3af spec calls for the voltage on the line to return to the sensing (or much lower-voltage) level within 400 milliseconds of a PD being disconnected from the PSE. After a disconnect, even if the RJ-45 connector is immediately replaced in the PD, the discovery and power classification negotiation must take place before power is resupplied to the port.

When a PD is connected to the PSE, a discovery and power classification negotiation occurs to provision power. This process, which takes 1 second or less, first discovers if the device is POE-capable. If a subsequent, optional negotiation takes place, the device can indicate one of four power classifications. Power classifications range from 0 (default, full 15.4 watts of power) to 4 (reserved for future use).

Next page: POE Planning

Cameron Sturdevant Cameron Sturdevant has been with the Labs since 1997, and before that paid his IT management dues at a software publishing firm working with several Fortune 100 companies. Cameron also spent two years with a database development firm, integrating applications with mainframe legacy programs. Cameron's areas of expertise include virtual and physical IT infrastructure, cloud computing, enterprise networking and mobility, with a focus on Android in the enterprise. In addition to reviews, Cameron has covered monolithic enterprise management systems throughout their lifecycles, providing the eWEEK reader with all-important history and context. Cameron takes special care in cultivating his IT manager contacts, to ensure that his reviews and analysis are grounded in real-world concern. Cameron is a regular speaker at Ziff-Davis Enterprise online and face-to-face events. Follow Cameron on Twitter at csturdevant, or reach him by email at csturdevant@eweek.com.

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