Before you harness the power, make sure you understand its limits

One of the latest rages in our industry is the concept of sending direct current (DC) down the wires of a twisted-pair data cable, and having that current power a networking device...

Feb 1st, 2005

One of the latest rages in our industry is the concept of sending direct current (DC) down the wires of a twisted-pair data cable, and having that current power a networking device, such as a camera, wireless access point, or IP phone. The concept, known as Power over Ethernet (PoE), has been and will continue to be covered in this and other cabling and networking publications.

The first time I heard of such a thing was at a BICSI conference, when a liaison between the Telecommunications Industry Association (TIA) and the Institute of Electrical and Electronics Engineers (IEEE) told the assembly of the IEEE's intention to enable such power distribution.

A few members of the audience were skeptical and vocal. After being told for years to physically separate communications cables from power cables through a building's pathways, they now were being told that someday those very communications cables would carry power within their sheaths.

Over time, the technology was fully developed and, to put it simply, it worked. Some of the world's most progressive network-engineering minds showed that a cable could indeed carry both data and power safely and effectively. Once that hurdle was cleared, the technology's ultimate benefit became clearer to many. It eliminated the need to run power wiring to every PoE-capable network device. It seemed to many, including me, like a panacea. I could envision PoE technology everywhere, once it got some traction among users and proved itself in the field for just a little bit longer.

Then I smartened up, paid attention, and gained a more realistic perspective not only on where PoE is most appropriate, but also on where it's even feasible. And I chose to devote this space to the topic now-not because I'm trying to stick a pin in the inflating balloon of PoE acceptance, but because I looked around quite recently and saw that just about every connectivity manufacturer (i.e., the companies whose patch panels you use all the time) now offers PoE equipment.

That being the case, I figure you're going to hear all about PoE from them sooner rather than later. So, before you fall into my trap and start viewing it as the answer for everything, let me share a little of what I recently have learned about PoE.

First, to ensure constant power to your powered devices, even when that cabling system hiccups (as it sometimes does) you will need either battery or uninterruptible power-supply backup. Remember to put that on the bottom line. Also, midspan PoE devices, such as PoE-enabled patch panels and standalone power-sourcing equipment, use the spare pairs of your Ethernet cabling. If you are running 1000Base-T over all four pairs of your cabling system, then you have no spare pairs and, therefore, cannot use midspan PoE technology. Kind of makes me wish the IEEE had gotten a little farther in its aborted quest to specify a Gig-over-two-pairs-of-Cat-6 standard.

And finally, approximately 13 watts of power are available to each powered device. That's not enough power to support equipment such as laptops or printers. And now the FUD (fear, uncertainty, and doubt) campaigns have begun about whether or not that's enough power to allow full functionality of some of the devices commonly associated with PoE-particularly point/tilt/zoom cameras.

Before we all get ahead of ourselves, as I'm afraid I did for a little while, let's remember that the IEEE's 802.3af Power over Ethernet specifications are in their first generation. We reported recently that interested parties are petitioning the IEEE to create specs that allow for more power distribution over each network link. As that is happening behind the scenes, you probably will be seeing more and more PoE equipment from more and more of your most common vendors.

Remember that there's also endspan PoE, in which the technology is built into the network switch. It's a more expensive proposition than midspan power injecting, but your circumstances may prove it to be your best option.

As always, we'll aim to keep you updated from both the standards scene and the field on how this technology might make an impact on you and your day-to-day work.

PATRICK McLAUGHLIN
Chief Editor
patrick@pennwell.com

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