If you dipped your toes into WiFi hacking, you probably encountered a channel setting here and there. But what are these channels, and how do they work?

2.4 GHz WiFi Channels & Basics

2.4 GHz Wi-Fi channels (802.11b,g WLAN)
Source: https://en.wikipedia.org/wiki/List_of_WLAN_channels (CC BY-SA 3.0)

WiFi is a communication protocol that can run on different frequencies, the most common band is 2.4 GHz, which is divided into 14 channels.

Channel 14, however, is only used in rare cases. And North America uses only channels 1 to 11.

Each 2.4 GHz channel is 20 MHz wide plus a 2 MHz gap as Guard band.
But as you can see in the diagram above, neighboring channels overlap. That's why channels 1,6, and 11 are usually the first choice. They don't overlap each which leads to less interference.

This overlapping is also why in some applications, when you can scan a specific WiFi channel, you still receive packets from neighboring channels.

Your home router will automatically select the best-performing channel based on other existing WiFi networks nearby. But this automatic selection is not always the greatest. To get a picture of the networks and channels nearby, you can use a WiFi scanner app.

With the 802.11n WiFi standard, it's also possible to use two 20 MHz channels to form a 40 MHz wide channel (for example, channel 1-5 or channel 9-13). This, however, is not used a lot at the 2.4 GHz range since it overlaps with a lot of other channels. It would provide the advantage of higher bandwidth, but it's is often crippled by the added interference.

5 GHz Channels

As you probably know, 5 GHz is the more modern option when using WiFi since it allows for much higher data rates. This is also partly due to the channels because, at 5 GHz, we have channels 36 to 165 available for WiFi. Each one is 20 MHz wide.

But that's not all. Channels can be combined to form 40, 80, or even 160 MHz channels to allow incredibly high bandwidths.

If you want to learn more about the 5 GHz channels, we recommend checking out this incredibly detailed table on Wikipedia.

Other Frequencies

Cell Tower
Photo by Vyacheslav Shatskiy / Unsplash

WiFi works on a variety of frequencies. 6 GHz seems to be the next big thing. But it can also run on 900 Mhz, 3.65 GHz, and even 60 GHz.

A lower frequency will generally be slower but allow for a higher range.
While frequencies like 60 GHz can allow for incredibly high speeds but will have problems going through a single wall.

To make use of these new WiFi frequencies, you'll need both an access point (i.e., WiFi router) and a client that supports that frequency.

Channel Hopping

Photo by Glenn Carstens-Peters / Unsplash

Having a lot of available channels is great because it means that more networks can coexist without interfering with each other.
It also means that anyone who wants to monitor the WiFi traffic, for good or bad reasons, will have to put in more effort.

With one WiFi interface, you can only monitor one channel at a time. So you either have to invest in a lot of hardware to surveil as many channels as you can, or you have to do channel hopping.

Channel hopping means 'hopping' every X milliseconds to another channel in a loop. Imagine it like quickly switching through all the TV channels.

That way, you can run through and receive data from many channels. But because you can only receive data from one channel at a time, the more you hop, the more information you might lose on any other channel you're currently not scanning on.

To further visualize this problem, we made a Project called WiFi Satellite some time ago that uses 14 ESP32 boards to monitor the traffic of all 14 2.4 GHz WiFi channels.

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