One of the major differences between 802.11n and all previous WiFi standards is the fact that 802.11n can operate within a 20 MHz channel like 802.11a/b/g — or it can consume two consecutive channels for double the bandwidth! With twice the bandwidth, it seems like everyone should configure 802.11n to operate in a 40 MHz channel, right?
Not exactly. The problem is, when you consume two channels, you end up shrinking the number of non-overlapping channels available. If you enable 40 MHz channels in the 2.4 GHz spectrum, you only have two non-overlapping channels to choose from – as opposed to three when using 20 MHz channels. That means your likelihood of interference increases dramatically. In the 5 GHz spectrum, things are a bit better as you shrink your non-overlapping channels from 23 down to 11. In fact, the use of 40MHz channels at 5GHz will increase the coverage area at higher data speeds.
So in other words, you should almost never enable 40 MHz channels in the 2.4 GHz range — but should seriously consider designing and configuring your 802.11n 5 GHz space to use 40 MHz channels…just make sure you do a proper wireless site-survey first.