The Wi-Fi Alliance is issuing brands for fast "Draft N" equipment, but it is not testing the most controversial part of the specifications - whether the new Wi-Fi kit will cripple existing 802.11g networks.

The Draft N brand, launched last month, tests basic functions, but specifically does not test features that can block today's Wi-Fi systems.

D-Link, Netgear and Linksys have all received branding, but users cannot be sure whether fast equipment will spoil the networks they - and their neighbours - already have, according to Tim Higgins of SmallNetBuilder.

The 802.11n standard from the IEEE standard group will improve Wi-Fi performance to 100 Mbit/s or even 200 Mbit/s , but part of this improvement comes from using 40 MHz radio channels twice as wide as today's 20 MHz channels. In the limited space of the 2.4GHz band, these wider channels can interfere with today's 802.11g Wi-Fi networks.

Vendors hope that 802.11n Wi-Fi will ultimately avoid this problem by moving to the larger, emptier 5GHz band, but so far Draft N equipment has mostly been in the 2.4GHz band, and the IEEE has specified various ways that 802.11n kit can fall back to narrower 20MHz channels.

The Alliance's brand doesn't test any of that, however, Higgins discovered, while testing early branded kit - it simply tests the 20MHz channels. There is too much debate in the IEEE on the details of the switch between 20MHz and 40MHz channels, so the Alliance decided to leave 40MHz channel testing out altogether, the Alliance’s technical director Greg Ennis, told Higgins, after Higgins spotted that some Draft-N branded D-Link kit was breaking the Draft-N specifications and not playing nicely with other Wi-Fi equipment.

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Users will be able to turn 40MHz channels off and on, says Higgins, but how a system will behave in that mode is anyone's guess. "Expect experimentation and a variety of 'good' and 'bad neighbour' behaviours from now until 11n is finally approved," he warns.

"If you're going to make the jump to Draft 11n product and plan to enable 40 MHz channels in 2.4GHz, realise that anything goes when it comes to cooperation with neighbouring wireless LANs," says Higgins. "You might find angry neighbours banging on your door accusing you of knocking their wireless off the air. Then again, you might find your throughput severely reduced, too."

There is some hope that the Alliance will rein in wide channels using its 'Extensions Policy, put in after controversy over Atheros' made 'Super-G' Wi-Fi chips which extended the speed of 802.11g by bonding channels together, much as 802.11n will do. An Alliance FAQ says "Any product with a feature extension (such as an early feature of the IEEE 802.11n draft standard) that causes severe performance degradation and interoperability issues with another Wi-Fi certified product can have its certification revoked."

To get a badge removed, other vendors would have to file grievances against the culprit.

"Devices should come configured to use only a single 20MHz channel in 2.4GHz, as the D-Link device that Higgins tested does," says Glenn Fleishman of Wi-Fi Net News. "Apple’s 802.11n base station is locked to 20MHz only in 2.4GHz."