The 2.4 GHz ISM (Industrial, Scientific, Medical) Band

The 2.4 GHz ISM (Industrial, Scientific, & Medical) Band

Wi-Fi is a microwave technology, but that obscures the fact that it's a radio technology, and it has its place in the radio portion of the electromagnetic spectrum. Microwaves are just radio waves with very short wavelengths. The wavelength of your typical AM radio signal is a thousand feet. FM radio operates at a wavelength of a little under ten feet. Wi-Fi, by contrast, works at a wavelength of 12 centimeters- less than five inches. The frequency used by Wi-Fi gear is at a frequency of about 2.45 Gigahertz (GHz), which is 2.45 billion cycles per second. The key point to remember is that Wi-Fi isn't the only kid on that particular radio block.

The radio spectrum between 2.4 GHz and 2.4835 GHz was a pretty busy place prior to 1997, when the 802.11 standard was first published, and it's gotten a lot busier with the ascendance of Wi-Fi. The band itself goes back some years before 1997, and it was originally set aside by the Federal Communications Commission (FCC) for the use of industrial machinery like induction heaters (including microwave ovens), cordless phones, and various arcane medical and scientific gadgetry gathered under the description 'industrial, scientific, and medical.' Licenses are not required for individuals to operate ISM equipment designed for use within the band. However, manufacturers must apply for and receive type acceptance from the FCC before they can manufacture products for the band, which must adhere to fairly strict FCC requirements for power output, band use, and purity of emissions.

Wi-Fi gear is, like ISM equipment, unlicensed, and it also must undergo FCC type acceptance. However, Wi-Fi gear is not considered ISM equipment and does not have any kind of priority on the frequencies it uses over ISM equipment. In fact, if a medical or industrial device causes interference to a Wi-Fi network, the network's owner has no legal recourse other than to move to a different Wi-Fi channel, of which (for American users) there are only 11.

Two other significant wireless networking technologies, Bluetooth and HomeRF, share space on this band. As with Wi-Fi, both are secondary uses to virtually all other uses of the band, parts of which are used by public safety (police and fire) communications, news gathering services, seaborne and airborne radar, and numerous other things. (More on this in Appendix B, where I speak in detail of Wi-Fi legalities.)

The allocation of the band to Wi-Fi equipment is complex. Figure 1.3 shows diagrammatically how the band is laid out. Perhaps the most important thing to notice is that other regions of the world have a larger Wi-Fi frequency allocation than American users do. The Japanese, in particular, have the use of a full 100 MHz between 2.4 and 2.5 GHz for 802.11b operation. American users have only the first 83.5 MHz of that space, with Europe falling somewhere in the middle. Parts of Europe have access only to limited subsets of the band, France being the most notable example.

Figure 1.3: The 2.4: GHz Wi-Fi Channels, National Allocations, and Overlap.

Wi-Fi channels are spaced 5 MHz apart. However, the bandwidth of a Wi-Fi channel is 25 MHz. This means that individual channels overlap with two adjacent channels in either direction. Channel 6, for example, overlaps channels 4, 5, 7, and 8. For American users, at least, this means that only the three channels 1, 6, and 11 may be used simultaneously in the same general space without any channel interference. Channel 14 is something of an outlier and is set apart slightly from the first 13 channels. Channel 14, however, is used mostly in Japan and (along with channels 12 and 13) is not sanctioned by the FCC.