WI-FI (2.4GHz vs 5GHz bands)
The 2.4GHz band frequency ranges from 2.4GHz-2.4835GHz and is used by 802.11a, 802.11b, 802.11g. The band is divided into 14 fixed-frequency channels, each being 20MHz wide.
The gap between adjacent channels is 5MHz wide, and these channels have many overlaps. Thus, only 3 channels can be active at the same time. You can find non-overlapping channels like 1, 6, and 11. The 2.4GHz band is very crowded as this range is also used by devices like baby monitors, cordless phones, microwave ovens, Bluetooth, etc... Not many channels are available for use. And is susceptible to more interference.
But the advantage with the 2.4GHz band is that lower frequencies can go farther, as they have low attenuation.
Note: Channel 12, 13 are not legal in America, and channel 14 goes beyond 2.4GHz and is only used in Japan.
5GHz band is used by 802.11a, 802.11n, 802.11ac. The band is divided into UNII-1, UNII-2 Extended, UNII-3, and ISM. UNII stands for Unlicensed National Information Infrastructure. ISM stands for industrial, scientific, and medical. These are labels for different parts of the band.
5GHz band has 24 unlicensed bands, each 20 MHz wide. There is no overlapping among the 24 channels, thus making it less congested. It serves more data at higher speed rates.
To increase the bandwidth, two channels are combined into one, this is called channel bonding.