What are AP Bands?

Q1:

What is their use

These 2 bands, 2.4GHz and 5GHz, are extensively used in consumer technologies for the transmission of data wirelessly. They are both free to use, no need to apply for a license. On a phone, they are use for wireless connections to other access points, routers, or to create hotspots for other phones, tablets, etc.

There are other devices that use these bands such as cordeless phones, routers, automatic garage door openers, etc.

Q2:

what effects will be seen if we modify the =>WCNSS_qcom_cfg.ini<= file in order to support 40MHz

You should make sure that your WIFI chip supports those wide channel. If it does, then the bandwidth will increase (see the difference between range and bandwidth in the note section).

What Channel Bandwidths on 2.4GHz and 5GHz?

On The 2.4GHz Frequency Band

When using the 2.4GHz broadcasting radio you should generally use a channel bandwidth block 20MHz wide. This is because there are more non-overlapping channels available when using 20MHz (as opposed to 40MHz) which means there is less likelihood of congestion or clashing channels.

You can also use 40MHz on the 2.4GHz broadcasting radio. However it congests the Wi-FI in the area so if you live in a built up area it probably isn’t a great idea as it will interfere with other 2.4GHz users.

On The 5GHz Frequency Band

When using 5GHz, however, it is possible to use wider channel bandwidths for increased bandwidth. As such on the 5GHz channel you can use the 40MHz, 80MHz or even the 160MHz channel bandwidths (if your router or AP supports it) for increased bandwidth.

Note:

Difference between range and bandwith

Range : how far your data can travel. In most cases, the higher the frequency of a wireless signal, the shorter its range.

The biggest reason for this is that higher frequency signals cannot penetrate solid objects like walls and floors as well as lower frequency signals. Thus, the 2.4 GHz has a farther range than the 5 GHz frequency.

Bandwidth (speed): Higher bandwidth means that files will download and upload faster, and high-bandwidth applications such as streaming video will perform much smoother and faster.

Higher frequencies allow faster transmission of data, also known as bandwidth. Therefore, the 5GHz with its higher bandwidth will provide much faster data connections than 2.4 GHz.

Sources:

Using the 2.4 and 5GHz bands

What is the difference between 2.4 GHz and 5 GHz?

Why Your Router Has Two Wifi Bands and How They Work

20MHz vs 40MHz vs 80MHz vs 160MHz: What Channel Bandwidth Should I Use?