Going Wi-Fi: A Practical Guide to Planning and Building an 802.11 Network

Spectrum is a conceptual tool used to organize and map a set of physical properties to delineate electromagnetic waves, which are produced by electric and magnetic fields, and which move through space at different frequencies. These frequencies are measured in Hertz (Hz), which is equivalent to the number of waves or cycles per second. (Megahertz (MHz) refers to 1000 kilohertz and gigahertz GHz refers to one million kilohertz.)

Figure 4.1: The electromagnetic spectrum. While this book deals with wireless data transmission, and this section discusses spectrum within that connotation, the reader should understand that "spectrum" is a general term that is used to encompass both the spatial and temporal properties of any medium, including your telco's copper wiring, fiber optic cable, coaxial cable, and ambient air.

The set of all possible frequencies is called the electromagnetic spectrum, which spans a wide range of physical energies such as radio, light and x-rays.

The subset of frequencies between 3 kilohertz (kHz) and 30 gigahertz (GHz) is known as the "radio spectrum." This spectrum is used by radio waves for communication, or to accomplish work such as a microwave oven heating food or radar detecting a storm. "Bands" refer to the ranges of frequencies within the radio spectrum.

Radio services (i.e. categories of radio use) utilize the "radio frequencies" within the electromagnetic spectrum. Examples of such radio services include the Cellular Radiotelephone Service, the Television Broadcast Service, and the Aviation Radio Service. But there are dozens of radio services, e.g. "governmental" (such as defense and space exploration), "non-governmental" (police radio or home satellite TV), and even flexible radio services that are generic and without a prescribed purpose.

Figure 4.2: The "Radio Frequencies" within the Electromagnetic Spectrum.