As you probably know, any signal that is sent without wires is called a radio transmission. A common example is that the radio in your car receives transmissions. Similarly, a standard cell phone works by receiving and transmitting radio signals.
Every device that broadcasts a radio transmission does so at a particular frequency, which is the oscillations, or movement from peak to trough, of the electromagnetic wave created by the transmission.
The entire set of radio frequencies is known as the radio spectrum. Contiguous portions of the radio spectrum are called bands, as in "the FM band."
Radio frequencies describe the oscillations of a radio wave. For example, if you are tuned to an FM radio station at 92.5, it means that the radio transmission is oscillating at 92.5 megahertz per second. 92.5 megahertz (pronounced "may-ga-hurts" and abbreviated MHz) means that the radio transmission wave oscillates, or moves from its valley to its peak, at a rate of 92,500,000 times per second. If you think of this as listening from a distance to a really rapidly vibrating tuning fork, you have the right picture.
The AM radio spectrum was developed before the FM spectrum, so it is lower down the spectrum, ranging from 535 kilohertz to 1.7 megahertz, or 535,000 to 1,700,000 oscillations per second. For example, 720 on the AM dial means that your radio receiver is tuned to a frequency of 720,000 oscillations per second.
There are frequencies above, or higher than, the FM frequency as well as below it. In fact, as I'll explain in a moment, Wi-Fi transmissions run at some of these higher frequencies.
One thousand megahertz is equal to one gigahertz (pronounced "gig-a-hurts" and abbreviated GHz). So when you refer to the 2.4GHz frequency, you are actually talking about 2,400,000,000 (2.4 billion) oscillations per second.
There are only so many frequencies in the radio spectrum that can be used for transmissions. This has inevitably led to the potential for conflicts about usage, as well as attempts to dominate particular frequencies.
Although it is commonly referred to as the 2.4GHz band, the actual spectrum is 2.39GHz 2.417GHz. In the case of the 5GHz spectrum band, the band actually runs from 5.47GHz to 5.725GHz.
As a partial answer to frequency conflicts, the government has regulated the usage of most of these frequencies. In the United States, government regulation of radio frequencies is controlled by the Federal Communications Commission (FCC).
Some frequencies are reserved for particular usages, such as the military. Others, such as the AM and FM bands, are licensed. This means that only the licensees can use the frequency for the purpose it was licensed. In addition, some areas of the spectrum have been set aside for unlicensed uses. These set-aside areas include the 2.4GHz and 5GHz spectrums, which is what Wi-Fi uses.
The uses of some of the frequencies in the radio spectrum are shown in Figure 2.1.
Figure 2.1. Selected uses for the radio spectrum.
The fact that the 2.4GHz and 5GHz frequencies have been set aside for unlicensed usages does have an extremely important implication: They are cheap to use. This gives these "free" spectrums an unfair competitive advantage compared to using a spectrum that someone has paid for. But there are some legal restrictions on what you can do within the free spectrums (see Chapter 17, "Adding Wi-Fi Antennas to Your Network," for more information).
There are conflicts within the 5GHz band as well as the 2.4GHz band, but 5GHz band conflicts primarily concern competing usages such as radar and satellite radio, which are being ironed out by the FCC.
Also, the 2.4GHz spectrum has become like a shanty town in which it is cheap to live. All kinds of transmission devices have crowded into the neighborhood, from microwaves to cordless telephones. These devices can interfere with your Wi-Fi transmissions and reception. In Part IV, "Creating a Wi-Fi Network," I'll show you how to best avoid problems with competing 2.4GHz devices when setting up a Wi-Fi network. (Interference is a two-way street. You also don't want your Wi-Fi network wreaking havoc on your cordless phones and other devices that use the spectrum.)