Wi-Fi channel planning in 2.4 GHz

I recently taught a course in channel planning for Wi-Fi networks and thought it would be useful to put together a short primer on the topic. 

While Wi-Fi operates in two main frequency bands -- the 2.4 GHz band and the 5 GHz band -- this post will focus on channel planning in the 2.4 GHz frequency range. 

For starters, there are 14 channels in the 2.4 GHz spectrum, but only 11 of these (channels 1 -11) are authorized by the Federal Communications Commission for use in the United States.  Channels 12, 13, and 14 are available for use in other parts of the world. 

However, it's not a good idea to simply place access points on every channel between 1 and 11.  802.11 is a spread spectrum technology.  Each of the channels is 22 MHz "wide" yet the center frequency for each channel is only 5 MHz apart.  Therefore, the channels overlap.  Hence, there are only three non-overlapping channels -- channels 1, 6, 11. 

This is depicted in the graph below with the bolded red lines depicting the non-overlapping channels:

24GHzChannels.jpg

Use of overlapping channels causes errors, which results in packet re-transmissions and significantly decreased network performance. 

The same problem exists when using more than one access point on the same channel in the same physical area.  This is called co-channel interference.  Co-channel interference should be avoided as it can have a dramatic effect on network performance. 

One side note for enterprise deployments. Remember I said that channels 12, 13 and 14 are available for use in other parts of the world.  One potential attack method a hacker could use is to purchase an access point designed for one of these countries (or hack the firmware), and then use it as a rogue access point operating on an illegal channel.  Therefore, you should ensure your wireless audits include sweeps for rogue APs on channels 12, 13 and 14. 

Douglas J. Haider is a Principal Technologist with Xirrus. He hosts a personal blog at WiFiJedi.com, and micro-blogs on Twitter @wifijedi

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