wifi 802.11ac speed vs bandwidth and channel planning

Question

Using a dpc3941t cable modem with Xfinity internet, and it provides dual band wifi.

From my pc connected to the modem via cat6 cable i currently get a download speed test of just over 100 mbit/sec consistently.

Using the 2.4ghz wifi in mode 802.11 g/n my samsung phone over wifi will do a download speed test at ~46 mbit/sec {makes sense since 802.11g maxes out at 54 mbit/sec). But if I connect using the 5 GHz wifi in mode 802.11ac my phone will do the equivalent download speed of my pc at just over 100 mbit/sec.

I have my wifi configured manually and set to 802.11ac on a single channel which is 149. Previous modem (with different firmware) allowed for channel bandwidth selection of 20, 40, or 80 MHz; my new modem does not allow for channel bandwidth selection.

by manually setting to channel 149 am I restricting myself to a 20mhz channel bandwidth?

More importantly, what is the maximum data rate of a 20mhz channel in 802.11ac? Does QAM factor in and if so how? I am trying to find max data rates of 802.11ac based on channel selection and bandwidth but cannot... not as simple as 802.11g = always 54 mbit/sec max ?

I ask because my internet plan is supposed to be 25 mbit/sec, since i just signed up I assume that's why i'm getting the 100 mbit download speed. Obviously this wifi connection will never be the weak link since it's capable of 100 mbit/sec now, and I chose channel 149 because a wifi scan of my area shows that channel to be the most free and furthest away from others that are in use in the area. Am I correct in doing what I am doing to have reliable wifi? Not concerned about configuring wifi for max speed- if it were maxed at 54 mb/sec that would even be fine considering what i use it for.

Answer 1

By manually setting to channel 149, am I restricting myself to a 20mhz channel bandwidth?

Setting your 802.11ac AP in channel 149 doesn't necessarily limit your AP to less than 80MHz as this band (called UNII-3) can be used in the U.S. without bandwidth limitation.

Channel 149 itself is not the 80MHz channel (it is ch155*) but usually, in commercial APs, you only set the primary 20MHz channel of the 80MHz one.

What is the maximum data rate of a 20mhz channel in 802.11ac? Does QAM factor in and if so how?

The modulation does factor in, in the same way it does in 11n. The higher the modulation, the higher the data rate. The max data rate depends on the number of spatial streams between the transmitter & the receiver. You can check this link for the exact data rates. Also, the channel won't directly ponder in the data rate, but more in the general performances as all channels do not allow the same max emission power.

Though, please note that expressively using 20MHz BW in 802.11ac has no use in a residential use case. It cancels any improvement brought by 11ac technology. If you're afraid of a performance loss because there's multiple 80MHz APs in the area, don't be as the most efficient way is to set your AP in 80MHz too, in the same primary channel as the other APs.

I ask because my internet plan is supposed to be 25 mbit/sec, since i just signed up I assume that's why i'm getting the 100 mbit download speed.

I have no answer to that, but might as well profit from it. It is possible though that the speed test works in a way that is not subject to the plan limitation.

Am I correct in doing what I am doing to have reliable wifi?

If you want to be sure, you can download Wi-Fi Scanner on your computer, which will show you the different APs present in your area in all the channels that can be covered by your computer Wi-Fi interface.

Additional note:

Using the 2.4ghz wifi in mode 802.11 g/n my samsung phone over wifi will do a download speed test at ~46 mbit/sec {makes sense since 802.11g maxes out at 54 mbit/sec)

As you said, you're in 11n, so you're not limited by 11g data rates. In 11n you are limited by the capacity of the lesser device. If your AP is 2x2:2 and your station 3x3:3, you will be limited to the data rates allowed by the usage of 2 spatial streams. This difference is due to the fact that back when 11n didn't exist, there was no more than one radio chain on a device, be it a station or an AP.

You need to note that the data rates are all theoretical values, that cannot be achieved in a standard setup. For example, the maximum you can get from 11g data rates in faraday cage is approximately 35Mbps.