Draft 802.11n Requires Access Points To Use A Single Channel Only In Case Overlapping Networks Are Detected

I am having a good time these days browsing through the current draft D2.00 of the 802.11n standard to find out about the details of the compromise reached in the IEEE working group for the new 100+ MBit/s Wifi standard. Besides MIMO, one of the corner stones of reaching speeds beyond 100 MBit/s on the application layer is to combine two standard 20 MHz channels and transmit on them simultaneously.

This is pretty difficult to impossible in the 2.4 GHz band which only has space for 3 independent 20 MHz networks or a single 802.11n 40 MHz network together with one 20 MHz legacy network (for details see here). In my Paris flat, for example, there are already 13 networks operating in this band, many using the same channels.

In such an environment, a ‘draft n’ compliant access point has no chance to use a 40 MHz channel as according to chapter 9.20.4 of the draft standard, an access point detecting frames of another network on it’s primary or secondary 20 MHz channel has to immediately deactivate the 40 MHz channel mode. Further, it has to remain in 20 MHz channel mode for at least 30 minutes after the last frame from a different network has been detected.

I guess the standard allows the access point to switch to another channel to avoid the detected network but in the 2.4 GHz ISM band there is only one alternative. So I wonder if some vendors have put an option into their settings that allows locking the access point to a 40 MHz channel!? Not that this would be very polite, or not cause any problems to other networks and one’s own if traffic of other networks is higher than an occasional traffic burst.

So if you have a ‘draft n’ network at home, what kind of access point do you have and does it allow locking operation to 40 MHz?