PoE Gets Plugged In

Power over Ethernet Has Its Pluses and Minuses. A Well-Grounded Implementation Requires Looking at Both

Gerard NadeauBy Gerard Nadeau

The power over Ethernet (PoE) technology standard has generated a great deal of enthusiasm and support since its introduction in 2005. It’s an excellent fit for various devices with a network connection. But the current standard has a number of limitations that mean it isn’t the right fit for every application.

PoE delivers electric power from computers to smaller devices over the standard RJ45-jack Ethernet cable that ordinarily just carries data to and from the Internet or local network. Specifying design mechanisms for both network and devices, PoE can supply all the needed juice to power a Voice over IP (VoIP) office phone or a Wi-Fi network’s wireless access point (WAP), eliminating the need for a separate 110/220 Vac or battery power supply. In fact, VoIP phones and WAPs were the main motivations for the standard’s development.

However, the engineers developing the POE standard discussed a much wider range of possible applications. Among the most likely candidates were security cameras and other types of remote-monitoring devices. Environmental control sensors for building HVAC systems, time clocks, lighting control systems, industrial control systems and fire alarms were mentioned. The possibilities are vast; anything that needs a small amount of power and requires a network connection for control or monitoring can be considered. At one of the early working group meetings, POE’s wide-ranging potential prompted someone to comment that it could become the first international standard for delivering power. Forgot to pack your laptop’s power supply before flying to London or Brazil? PoE could charge your laptop via the common Ethernet connection. However, while such a scenario is certainly possible in the foreseeable future, it isn’t a reality today for several reasons.

PoE Defined

PoE is defined in IEEE Standard 802.3-2005, which was originally IEEE 802.3af, but it has since been absorbed into the 2005 revision. The standard defines two pieces of equipment, the power sourcing equipment (PSE) and the powered device (PD). The PD is the VoIP phone or wireless access point, whereas the PSE is the unit that provides the power.

The PSE, often referred to as a PoE injector, is a separate device typically installed in the wiring closet near the Ethernet switch or hub. The PSE inserts DC Voltage onto the twisted pair cable and can be either an endpoint or a midspan device. An endpoint PSE is one that contains both the data communication capabilities—Ethernet—and the power delivery mechanism. A midspan only contains the power delivery mechanism and is to be inserted in the network between the legacy—non-PoE capable—Ethernet switch and the PD. The midspan provides the ability to add PoE to the network without having to replace the existing infrastructure.

Probably the biggest limitation to the first generation of PoE equipment is the limited power that can be delivered. A PD is limited to drawing only 12.95 Watts. However, a quick search of a large Wi-Fi supplier’s product line reveals access points requiring over 18 Watts. Likewise, VoIP phones with impressive video displays may also require more power than the maximum that the current PoE standard can deliver. One of the primary reasons for the power limitation is the decision to support older cabling infrastructure and patch panels. The necessity of not exceeding the ratings of the installed infrastructure meant power had to be limited.

Another of PoE’s flaws is that, by design, midspan equipment can’t support 1000BASE-T—Gigabit Ethernet on copper using four pairs of Cat. 5 cable to achieve the gigabit data rate. 1000BASE-T is used in data centers for server switching and to uplink desktop computer switches, and it is increasingly finding a home on the PC desktop for various broadband applications. And while some vendors making proprietary midspans do support 1000BASE-T, by definition their designs are not standardized and could therefore lead to the kinds of interoperability problems that Ethernet solved long ago. Although PoE uses standard twisted pair cable, it also supports Cat. 3 cable or better, provided the cable meets the Ethernet equipment’s physical layer requirements.

Another advantage, and one of the requirements of PoE, is its meeting of limited power source and SELV requirements as defined in ISO/IEC 60950. Nominally, PoE equipment will work at or around telephony voltages—48 V. A licensed electrician is not required to install the cabling. In fact, most installed infrastructures will more than likely support PoE without any problems.

On the downside, some engineers express hesitation about PoE attributable to added cost, since the technology requires a PoE-enabled switch, which in order to supply power to other devices draws a significant quantity of power itself.

PoE+ and Beyond

Currently IEEE has appointed a DTE Power Enhancements task force to develop a revision of the PoE standard commonly referred to as PoEPlus. Some of the objectives of PoEPlus is to be completely backward-compatible with existing PoE equipment, and yet PoEPlus equipment will be capable of delivering up to 24 W of power. PoEPlus is targeting Cat. 5 or better infrastructure and will pursue supporting midspans for 1000BASE-T. Currently the standard is at draft 3.0, and the task force will probably ask the editor to write a new draft after the July 2008 IEEE plenary meeting in Denver.

PoEPlus promises to solve many of the problems with the existing standard. The increase in power will mean more powerful wireless access points can be deployed and VoIP phones with larger display screens can be designed along with that yet-to-be-discovered application that solves your direst problem. Plenty of companies already have prototype silicon and designs on the drawing board and in the prototype stage. These designs will need polishing as the standard comes closer to adoption. It may be another year before IEEE accepts the standard, but as usual products are likely to be available before the standard is ratified. Almost certainly, PoEPlus will make the technology an even more interesting option for developers.

Gerard Nadeau is the PoE consortium manager at University of New Hampshire InterOperability Lab (UNH-IOL) in Durham, N.H. He can be reached at grn@iol.unh.edu.

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