Being a sad "b**" I was reading the 802.11n std and noticed that if the no. of 802.15.1 channels falls below a given threshold then in the2.4GHz band then an AP may be set not to work in "40Mhz" channels.
"The rules that are applied when determining whether a 20/40 MHz BSS can be established are intended to avoid a full or partial overlap of the secondary channel of the 20/40 MHz BSS with an existing primary channel of either a 20 MHz BSS or a 20/40 MHz BSS. The lack of partially overlapping channels in the 5 GHz band allows these rules to be written as recommendations, while in the 2.4 GHz band, they are requirements. An additional constraint on establishing a 20/40 MHz BSS includes the allowance for any IEEE 802.11 device to explicitly prohibit the operation of the 20/40 BSS mode due to other considerations. For example, if an IEEE 802.15.1 WPAN device is operating in the area, that device is likely to be unable to communicate successfully with a paired receiver if the number of available IEEE 802.15.1 WPAN channels falls below a given threshold. Operation of a 20/40 MHz BSS in the 2.4 GHz band can contribute to the reduction of the number of available IEEE802.15.1 WPAN channels, possibly pushing the available channels below that threshold." page 501 Has anyone noticed this? The "other considerations" seem to indicate numerous possibilities and overall it seems pointless to have 40MHz channels available in 2.4GHz.