It is true that there are an infinite number of different frequencies of EM
waves. However, practically speaking, any creation of EM waves actually takes
up more than an infinitesimal amount of frequency space. Therefore, frequency
bands have a limited number of useable different frequencies, or communications
channels. Many parts of the EM spectrum are not useable for communications and
many parts of the spectrum are already used extensively for this purpose. The
electromagnetic spectrum is a finite resource.
One way to allocate this
limited, shared resource is to have international and national institutions
that set standards and laws as to how the spectrum can be used. In the US, it
is the FCC that regulates spectrum use. In Europe, the European
Telecommunications Standards Institute (ETSI) regulates the spectrum usage.
Frequency bands that require a license to operate within are called the
licensed spectrum. Examples include amplitude modulation (AM) and frequency
modulation (FM) radio, ham or short wave radio, cell phones, broadcast
television, aviation bands, and many others. In order to operate a device in a
licensed band, the user must first apply for and be granted the appropriate
license.
Some areas of the spectrum have been left unlicensed. This is
favorable for certain applications, such as WLANs. An important area of the
unlicensed spectrum is known as the industrial, scientific, and medical (ISM)
bands, which are shown in Figure
. These bands are
unlicensed in most countries of the world. The following are some examples of
the regulated items that are related to WLANs:
- The FCC has defined eleven 802.11b DSSS channels and their corresponding
center frequencies. ETSI has defined 13.
- The FCC requires that all antennas that are sold by a spread spectrum
vendor be certified with the radio with which it is sold.