APs and clients transmit data from one location to another in form of electromagnetic waves. These waves operate at different frequencies. Frequency is measured in Hertz (Hz). When a wave travels, it takes a signal loss due to a phenomenon called “Free Space Path Loss”, or for short “FSPL”.
As a rule, lower frequencies have a longer wavelength and can travel further than higher frequencies. Of course, obstacles do further weaken the wireless signal. 2.4GHz travels further than a 5GHz wave and will get more easily through an obstacle.
2.4GHz APs have a longer history than 5GHz capable APs. The problem is, that in most places the 2.4GHz spectrum has become overcrowded with competing APs (neighbours AP, Bluetooth, Baby monitors, Wireless Headphones etc…)
All these devices using the same frequency range from 2400 MHz to 2483 MHz can cause saturation. "Channel saturation" happens when too many Wireless Access Points ("WAP") have a Wi-Fi Network SSID operating on the same channel and are too close together. This causes interference which in turn causes network slowness and network disconnects.
5GHz gives you much more flexibility. It offers more non-overlapping channels to choose from than the 2.4GHz spectrums. Mind that the number of available channels for 5GHz depends on the region you live in.
Because 5GHz provides shorter signal coverage than the 2.4GHz and has more available channels, will be the right choice to choose for a high-density situation for example shared offices, buildings and highly populated areas.
To avoid interference and maximizing the performance of Wi-Fi it is crucial to understand how channels work.
In radio communication, an AP receives a channel and bandwidth assignment over which it transmits and receives signals from clients. This channel assignment pertains to the centre frequency of the first 20MHz used by the station. Latest technologies use channel bonding, this permits to use larger channel bandwidth for higher throughput.
Unifi defines that with HT20, HT40 and HT80 (20MHz, 40MHz and 80MHz). Higher width means more throughput but, the drawback is that this uses more available radio spectrum and you'll have less clean channels to use for additional APs. The wider you run your AP, more power must be spread across a wider channel. This results in a shorter range.
For 2.4GHz it's recommended to use HT20 with channels 1, 6 and 11. HT40 and HT80 are best used with 5GHz.
It's important to know your environment, you should aim to use channels with low utilisation. To achieve this, you can do an RF scanning in advanced Wi-Fi equipment. For example, you can use the ability of the latest Unifi AC APs to do an RF-Environment scan. This scan will show you also radios that a "normal" Wi-Fi scanner won't see. It can scan the full spectrum and shows the utilisation of each available channel and channel width.
In a normal case, you would install your AP without making this scan resulting in a very low throughput and you wouldn’t know why!
Why is it important to use a clean channel?
There are two reasons. First is noise.
Imagine you're in a packed bar with lots of people and you're trying to talk to a friend just a few feet away, but you're not allowed to shout. You're only allowed to talk.
The more noise there is, the less you'll understand, and you will have to repeat your words until your friend understands you or in some case may need to come closer otherwise the message won’t reach him.
The second reason is a clearly defined communication from a neighbouring RF source talking place on your channel. To understand this one, you need to understand how a communication is regulated by the 802.11 standards.
It is based on CSMA/CA Carrier Sense Multiple Access/Collision Avoidance Protocol. An 802.11 station will listen to the wireless channel prior to transmitting the data. If the channel is available, then the station will transmit. If the channel is occupied the station will wait a random time until it listens again. This is done to avoid that two stations transmit at the same time. The receiver would experience a collision and data would have to be retransmitted. The more stations you have the more likely a collision is to occur.
Also, an optional protocol is the RTS/CTS Request to Send/Clear to Send. The station asks if it can send data at this moment, the AP tells all the clients to be quiet for a given amount of time and sends the requester a clearance as an approval to send for that specific chunk of time.
All the above is a so-called Collison Domain. A collision domain is a network segment connected by a shared medium or through repeaters where data packets may collide with one another while being sent. The collision domain applies particularly in wireless networks
Now since the carrier is an RF signal and has a defined channel, It could be that your AP hears on the same channel many other stations that are not part of your SSID. And this will make you Collision Domain much larger. More stations, more waiting and more collisions = slower throughput.
If you'd like to know more, please refer to:
https://dl.ubnt.com/guides/training/courses/UBWA_V2.1_Training_Guide_03-09-17.pdf