20 vs. 40MHz.
Wider the channel with (in aggregate), the more natural noise is in the system. Laws of physics. Add to that the undesired nearby signals across 40MHz of WiFi systems nearby, and .. as is always the case, a much better signal-to-noise ratio is needed for 40MHz. A higher received signal strength, in both directions, is needed in 40MHz mode.
IMO: 40MHz should not be used in 2.4GHz WiFi, except if you are either rural, or have antennas on both ends that are 30 degrees or less, on the horiz. plane.
Whole heartedly agree. Back in townhouse living with 7 nearby networks visible and 3 of them at 60dB or higher strength, 40MHz on 2.4GHz produced noticably faster Wifi throughput, but ONLY if I was in the same room with the router. Walk to the otherside of my 3 floor, 1920sq-ft townhouse (so about 25ft away) with one wall in between and it was about 10% slower than 20MHz mode. Go upstairs where 2 nearby networks were actually at HIGHER signal strength to my own, and speed was roughly half what 20MHz mode was (mine was coming in at -65dB, the neighbors around -50 and 60dB). Either was crap, but 20MHz mode was at least faster (which meant around 800KB/sec instead of 300-400KB/sec).
In my current house, which is merly borderline rural (1 acre, a few sorrounding properties at 1-1.5 acres, a bunch of properties at 3-20 acres) my closest neighbor is -85dB or so. I have two routers, one set to 40MHz 6+11 and the other one in the basement on the otherside of the house set to 20MHz and channel 1. No where in my house is signal strength below -75dB for either router (a big masonry fireplace is between my 40MHz AP and the rest of the house), but in terms of what it is connecte to, nothing is below -60dB. Worst through put is when connected to the 20MHz router at the edge of transistioning to the 40MHz AP I can get about 7MB/sec.
With the 20MHz router off laying in bed, the 40MHz router is crap signal strength and slow, but 20MHz is only slightly faster. About 1.2MB/sec versus 1MB/sec. Outside at the edge of receive range, 20 versus 40MHz makes a bit more of a difference with generally no drops and around 600-1,000KB/sec on 20MHz and occasional drops and 200-700KB/sec on 40MHz.
However, close in, the 40MHz router provides 21MB/sec down and up and the 20MHz router up close only manages about 12MB/sec down and up. Most places I benefit from having the main router set to 40MHz and leave the 20MHz router for areas the 40MHz router can't reach (and the 20MHz router also covers part of my yard that I care about getting wifi in, and the attenuation is high, so 20MHz is beneficial there).
TL;DR, yes, 20MHz mode is good for A) a number of nearby networks present and B) high attenuation situations. That goes for 5GHz to, its just you are a lot less likely to get picking up 5GHz networks and/or they be high enough signal strength to matter...but since 5GHz is attenuated by walls more so than 2.4GHz is, you are more likely to be getting a weak 5GHz signal. So playing with settings in 5GHz is also a good idea.
A lot depends on where you need signal and where you care about having a fast connection.
