He then proceeds on telling me that this is only applicable if there are networks not controlled by us using same spectrum. Since we manage the whole building then only our networks have to use the spectrum. If we had other people’s networks using the spectrum, then 40 MHz would not work well.
Please tell me if I am wrong, but I do not see how two of my APs using the same frequencies in the same area is not a bad idea, but if one of the APs is someone else’s on the same frequency its a totally different thing.
Well it is not about the fact that there are “networks not controlled by us”, it’s about the fact that the overall performance of a wifi channel is defined by the SLOWEST transmitter on that channel, if all are contending for air-time with the CSMA/CA mechanism. The CSMA/CA game is rolling dices to find out who may transmit. As statistics learn each get the same number of winning deals if the numbers on the dices are equal. Point is that once you win the game you can transmit your payload. A fast interface rate (300Mbps on 40MHz/2S) will use a very short air-time compared to that slow (MCS0, 6Mbps/20MHz/1S) transmitter. The slow transmitter will be using the RF spectrum for a long time, and as such dominate the average transmission rate more. Interface rate depends on previous transmissions successes (CCQ based on retransmits, transmits fail by interference, signal noise, distortion, etc etc). So for our 300Mbps fast connection and our 6 Mbps slow connection the average interface rate is (3006)/(300+6) for BOTH. = 5.88 Mbps. The 40MHz didn’t help much, 20MHz/2S would have given : (1446)/(144+6)= 5,76 Mbps
Still must think if a 40 MHz channel, contending with 2 20 MHz channels is still getting the same fair share of airtime. The CSMA/CA game for the 40 MHz channel must fulfill twice the winning deal, both channels must win. (and that’s only 25% of the cases, not 50%)
So the challenge is to find and eliminate the slow transmitters. (Check “Registrations” for your own network. You would need to sniff all transmissions to find others)
Elimination can be adding AP for better connection, avoiding wall transitions for better CCQ, or even just refusing those low connections (access-list signal strength limit and removing low basic rate settings (that’s not removing supported rates!)). The students or teachers smartphone is the weakest transmitter. Some devices do not support 40MHz or even 2S.
I don’t know your setup in school. But also know that WMM is about using different dices for different kind of traffic (lower numbers give higher priority). RouterOS always rolls the bigger numbers with WMM, until some special firewall mangle rules are added to change the priority. And even then be careful as higher priority traffic in RouterOS comes with no aggregation (A-MPDU) by default. (When you eventually win the dice game, you send only a small amount of data).