The power of Power-over-Ethernet
The latest Power over Ethernet (PoE) standards, ratified last year, boost power delivery to support a broader range of devices, from 802.11n Wi-Fi access points and pan-tilt-zoom (PTZ) cameras to WiMAX subscriber stations and even laptop computers. By doubling DC power delivery to as much as 60W watts (W) using all four Ethernet cabling pairs, high-power PoE even supports outdoor IP cameras with integrated motors and/or heaters, which can require up to 51 W.
PoE technology delivers power alongside data over standard Ethernet cabling, enabling IP-based devices to be installed where there are no available power outlets. PoE also is a smart power technology, which means that power sourcing equipment (PSE) applies power only to compliant powered devices (PDs), and then actively monitors and limits the maximum power each PD consumes. This intelligence also can be used to remotely monitor and control PDs. Additionally, PoE protects the network, by electrically disabling the PDs in case of a physical disconnection or a malfunction such as a short or overload.
Deployment
PoE can be deployed either as an integral feature of the network switch, or via peripheral equipment called midspans that are installed between the existing switch and the network’s PDs. Midspans are ideal when there is no need to upgrade the network switch. They can be specified in a wide range of port densities, and offer important remote-management and energy-saving capabilities that are not available with PoE-enabled switches.
For maximum power delivery, the latest PoE standard’s four-pair powering option delivers electricity with a low, 600mA current level rather than the 1.2A level that would have been required by 60W two-pair midspans. In other words, for the same 60 W at the source, 51 W can be delivered over CAT5 cable via four-pair solution, as compared to 42 W for two-pair solutions. Alternatively, four-pair powering can be used to power two-pair devices with 30 W of power. This approach dissipates up to half the power and consumes almost 15 percent less energy than two-pair solutions, equating to $25 in annual savings per powered device (assuming energy costs of $0.10 per kilowatt hour).
To enable four-pair powering, IEEE802.3at-2009 standard considers the PD the power interface, as opposed to the whole device being powered. The two power interfaces, which can be connected, each take 25.5 W inside the same box. One power interface operates over the two pairs using lines 1, 2, 3 and 6, and the other operates over the 2 pairs that use lines 4, 5, 7 and 8.
