802.11n and PoE: Who's promising what?

There is a bit of a disconnect between today's 802.3af power-over-Ethernet products and the juice required by many enterprise-class pre-standard 802.11n implementations shipping or poised to ship this quarter. What do the vendors say about this potential dilemma?

First, note that many vendors use the same 802.11n chipsets from Atheros. The chipset consumes the lion’s share of the access point’s (AP) power. Where vendors can innovate to reduce an AP’s power consumption is in the upstream AP Ethernet connection, the CPU power used to manage the AP and RF amplifier efficiency, say industry observers.

That said, Siemens, Bluesocket, Colubris, and Meru Networks all use the Atheros 3x3 XSPAN chipsets. The first-generation 11n XSPAN chips shipped two years ago and the lower-power, second generation XSPAN 11n chipsets shipped in fall 2007. The chipset runs 3x3 spatial streams, but picks the best two out of the three for signal delivery. All these vendors claim that you can use their pre-11n products with your existing 802.af infrastructure without giving up a darned thing in terms of connect rate, throughput, range and features.

Hmm. Tom Alexander, CTO at VeriWave, a company that makes 802.11 testing tools, says he would be “hard-pressed to believe there is no feature tradeoff. And many 802.11n features — such as enhanced QoS — are still to come. Some of the system vendors are likely to be caught short.”

Aruba and Trapeze also use the Atheros chipsets but are more conservative in their promises. For example, “we have power management in our APs to shut down streams as necessary” to remain within the power budget, says Mike Tennefoss, head of Aruba strategic marketing.

Trapeze director of product marketing David Cohen says his company’s forthcoming 802.11n products will work with 802.3af power infrastructures out of the box at full data rates delivered over both radios. “But the range is somewhat reduced,” he says.

He said Trapeze could claim the same no-compromise performance and range that the other Atheros vendors do, but that it would come with a warning. “It’s like driving a car 120 miles per hour on a sunny day on a clear track. It’s not likely you’d want to do the same thing on a rainy Monday morning in traffic. A difference of circumstance such as that would take 802.11n beyond the specs and could overtax the board and increase [mean time between failures].”

Trapeze suggests several options for optimizing the implementation:

* Use a third-party high-power PoE injector. Trapeze’s forthcoming 802.11n AP requires 17 watts, Cohen says, and the power injector Trapeze will resell (which can also power 802.3af-compatible devices) supports a maximum of 60 watts.

* Use two cables’ worth of 802.3af (doubling the wattage available to the AP).

* Deploy a device from PowerDsine that adds more power to 802.3af technology onto unused Ethernet cable pairs.

* Turn off one radio or run 802.11g on the second radio.

Cisco, which uses a different chip vendor, also offers several power options. Chris Kozup, senior manager of mobility solutions, notes that the Cisco Aironet 1250 802.11n AP uses 18 watts of power, and he also recommends running one 802.11g radio or a single radio module.

Joanie Wexler is an independent networking technology writer/editor in Silicon Valley.