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.
Way back in 1947, the International Telecommunication Union (ITU) reserved a band of frequencies for what are commonly referred to as Industrial, Scientific, and Medical (ISM) uses besides telecommunications. Those devices include things like microwaves which can create interference for radio based telecommunications. Basically these types of devices were so noisy that the ITU gave them their own frequency band. Since in the ISM band must tolerate interference from ISM devices, it was an easy spot of spectrum to be used for unlicensed use. Originally proposed in 1980 and finally authorized in 1985, the FCC began to allow the use of the unlicensed ISM bands for communications purposes.
Two of the ranges in the ISM band commonly used for wireless networks include 2.400 – 2.500 GHz and 5.725 – 5.875 GHz. In addition to the ISM bands there are also the UNII bands in America. The FCC refers to the 5 GHz band as the Unlicensed National Information Infrastructure (U-NII) band. The UNII bands are further broken down into 4 ranges:
UNII-1 from 5.150 GHz to 5.250 GHz, aka Lower Band
UNII-2 from 5.250 GHz to 5.35 GHz, aka Middle Band
UNII-2 Extended from 5.470 GHz to 5.725 GHz, aka H Band
UNII-3 from 5.725 GHz to 5.825 GHz, aka Upper Band
In the US, the 2.4 GHz ISM band is broken down into 22 MHz wide channels separated by 5 MHz. With those requirements we get 11 available channels in the US, however a majority of those are overlapping. To avoid interference we should stick to the non-overlapping channels. This allows the use of channels 1, 6, and 11 in the US.
That seems kind of strange doesn’t it? With so many channels available why are we “limited” to only 3? If two access points use different channels which are too close together they will get overlapping channel interference. Basically what happens when devices are on the same channel they will know how long each other are transmitting for and will remain silent during that time to limit collisions. When devices are only separated by say 2 or 3 channels then they can still have collisions, but they will not hear the messages stating how long the other devices are transmitting. Therefore, this overlapping or adjacent channel interference is much worse than the co-channel interference because we can better handle the co-channel interference.
The 5 GHz spectrum is broken down into different bands listed earlier. The channels are spaced out 30 MHz wide in the lower band, and 20 MHz wide in the other bands. These channels are considered non-interfering though there is some slight overlap in the RF spectrum. This gives us a default of 25 channels available which makes the 5 GHz much more attractive for wireless networks.
This article was meant to be a brief introduction to the frequencies, bands, and channels which are used in 802.11 wireless networks. I will dig into some of these topics in greater detail, possibly touching on encoding mechanisms and the various 802.11 standards. If you found any of this information useful, please drop me a comment!