Some time ago I started to have a feeling that the time required to move files using Wi-Fi between my NAS and PCs has increased. About this, I noticed that the reported wireless link speed of my systems was 145 Mbit/s but I was reasonably sure that in the past that value was 300 Mbit/s.
My access point is a relatively old TP-Link model that runs a somewhat recent version of OpenWRT that I compiled and flashed, so yes, to the aficionado reader this may resemble my typical tinkering scenario, only this time I didn't touched the thing for a year or more.
According to the OpenWRT's Atheros chip-set documentation, for 802.11n the /etc/config/wireless configuration file looks correct:
config wifi-device 'radio0'
option type 'mac80211'
option hwmode '11g'
option path 'pci0000:00/0000:00:00.0/0000:01:00.0'
option channel 'auto'
option htmode 'HT40'
option txpower '15'
option country 'IT'
So 802.11n is basically 802.11g plus HT 1 mode with 40 MHz channel width, but strangely all the NICs in my home were connecting using only a width of 20 MHz.
Until yesterday I was unaware that there is a part of the 802.11 protocol that it's called "20/40 coexistence" or "good neighbor policy", so in few words, when a different AC is found, to avoid contemption of the Wi-Fi media, the channel's width is kept at 20 instead of 40 MHz.
I suppose, in the past few months, one or more of my neighbors has added or changed a Wi-Fi router and now my AC is correctly occupying less bandwidth.
I wanted to confirm it was really the case, and yes, using the OpenWRT's noscan option and a throughput test with iperf3 confirmed it's the presence of nearby ACs that reduces my wireless link speed.
Unfortunately the noscan is not a reasonable option2 as I can force the larger channel size but I still can't avoid contemption3 with the other ACs, so it looks like I need to move to the 5 GHz band and, with the occasion upgrade to 802.11.ac, with all the considerations that this imply.