Learned this the hard way over the past few days after I switched my ASUS RT-N66U‘s channel bandwidth setting from 20 MHz to 20/40 MHz. For what it’s worth, here’s what ASUS’ official documentation says on the setting (emphasis mine):
802.11n can combine two 20 MHz channels to form an effective bandwidth of 40 MHz. 40 MHz enables higher data transmission rates to be achieved as compared to 20 MHz. When you select 20/40 MHz mode, the router decide to use 20 or 40 MHz based on the interference/contention the router detected. Care should be taken when using 40MHz mode, the legacy client may not be connected to the router.
However, when using a wider channel bandwidth, there are fewer channels available for other devices, making more interference/contention with neighboring WLANs due to increasing overlap. In order to avoid excessive interference, the Wi-Fi Alliance develops an advice: setting 20 MHz in 2.4GHz as default. 40MHz is still appropriate for some situations, e.g. in a warehouse, but we do not recommend that using 40MHz in the 2.4GHz band for dense residential areas.
I enabled it anyway, and noticed my Galaxy Nexus frequently being unable to access the internet despite being connected to the router. Switching back to 20 MHz solved the problem. Given the Galaxy Nexus’ release date, apparently “legacy” for ASUS means anything from late 2011 or preceding. For the record, the phone’s Wi-Fi radio is the Broadcom BCM4330XB2KFFBG. Broadcom radios have been suspect recently, so this isn’t surprising.
And yes, I know ASUS recommends 20/40 MHz not be used for residential settings, but my Dell Precision M4600, Amazon Kindle Paperwhite, and Sony Vaio Fit 15 E all handled it without complaint.
UPDATE: Also, disable “b/g protection” for 2.4 GHz frequency. Then, forget the network on the Galaxy Nexus and then reconnect to it.