RF Channels and Bandwidth

In my previous post I outlined the frequency bands used for wireless LANs and briefly touched on the channels within those bands. I’m going to delve into those channels a little more in this post.

We’ve already established that a frequency band is a range of frequencies which are further divided into specific channels. Those channels are defined by standards bodies such as the FCC in the USA or the ITU internationally.

Channels within a band are defined based on a center frequency. Since real life is messy, we can’t limit an RF signal to just the center frequency. It spills over on each side of the center frequency to some degree, we call this sideband. The range of the spillage above and below the center frequency is what we call the signal bandwidth. Technically the bandwidth is the difference between the upper and lower cutoff frequencies. Another way to say it is that the signal bandwidth is the full range required to transmit on a specific RF frequency. These are defined in the 802.11 standards as the channel width.

As we said earlier, life is messy; but we do our best to clean it up. One of the tools we use to clean up a channel is a spectral mask. Spectral masks are overlays of what the channel should look like. Anything frequencies outside of the spectral mask are attenuated to help limit channel interference.

The problem that we sometimes run into and especially with the 2.4 GHz band is that the RF channel increments were defined prior to its adoption for communication use. In the 2.4 GHz (ISM) band the channels, or the center frequency, increment every 5 MHz. However in the 802.11 standards we have defined 11 MHz on each side of the center frequency which combined give the channel width of 22 MHz. Cramming something 22 MHz wide into something spaced out every 5 MHz quite obviously leads to channel overlap.

Hopefully this gives you some visualization on RF signal bandwidth and helps explain the discrepancy between the total number of channels in the 2.4 GHz band vs number of usable channels.

All images are my own and were crudely made using MS Visio.


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