Beware the 802.11n 'power' play

* What's the truth about 802.11n power consumption?

Next-generation 802.11n systems promise to considerably improve WLAN performance. But the processing required for the boost sucks up more power than the older 802.11a/b/g networks. Still, many enterprise-class Wi-Fi vendors claim to deliver full 802.11n capabilities without enterprise customers having to touch their power infrastructures. So what gives?

Power is just one issue for which you’ll have to don your detective’s hat when exploring the comparative performance, range, and upgrade issues associated with a move to 802.11n. But Siemens’ announcement last week that it had “solved the power problem for 802.11n” with its HiPath Wireless 802.11n product set, due to ship in March, set me on an investigative path to find out what the various suppliers are offering and what their recommended approaches to powering 802.11n networks are.

First, some background: Today’s widely installed 802.3af power-over-Ethernet (PoE) switches and power injectors supply about 15 watts of power at the switch port. To comply with Ethernet standards, 12.95 watts of the initial 15 watts must be sustainable over 100 meters. The 802.3af “power budget,” then, is 12.95 watts; however, some 802.11n Draft 2.0-based access points consume up to 18 watts.

I’m no math whiz. But it seems a leap in the laws of physics for these systems to squeeze the required power out of their existing infrastructures without some tradeoffs.

Indeed, there are ways to get around the problem. For instance, you can install APs with a single 802.11n radio in them, as single-radio devices are likely to operate within the power budget. If you seek a dual-radio implementation, with 802.11n capabilities in both the 2.4GHz and 5GHz bands, look under the hood at the vendors who say they can “do it all” with an 802.3af power infrastructure. Some may automatically disable some AP services, require two ports on your PoE switch and/or two cables, or sacrifice range to stay within the power budget and maintain performance. That may be OK with you, but you’ll want to know the score.

Another option is to use a pre-standard 802.3at power injector becoming available by Cisco and others. The 802.3at standard is in draft form and will offer 30 to 56 watts of power, depending on how it is ultimately implemented. Local powering of an 802.11n AP is an option, too.

We’ve barely scratched the surface of the PoE/802.11n landscape. So please stay tuned.

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