I was a real nerd in high school: I took three years of electronics. The electronics class at my school was right next door to the auto shop. Of course, all the cool kids took auto shop, and only nerds like me took electronics. We hung in there, though, and found out all about capacitors and diodes while the cool kids were learning how to raise their cars and install 2-gigawatt stereo systems.
It turns out that a little of that high school electronics information proves useful when it comes to wireless networking-not much, but a little. You'll understand wireless networking much better if you know the meanings of some basic radio terms.
For starters, radio consists of electromagnetic waves that are sent through the atmosphere. You can't see or hear them, but radio receivers can pick them up and convert them into sounds, images, or-in the case of wireless networks-data.
Radio waves are cyclical waves of electromagnetic energy that repeat at a particular rate, or frequency. Figure 9-1 shows two frequencies of radio waves: The first is one cycle per second; the second is two cycles per second. (Real radio doesn't operate at a frequency that low, but I figured one and two cycles per second is easier to draw than 680,000 cycles per second or 2.4 million cycles per second.)
Figure 9-1: Radio waves frequently have frequency.
Tip | The measure of a frequency is cycles per second, which indicates how many complete cycles the wave makes in one second. (Duh.) In honor of Heinrich Hertz, who did not invent catsup but rather was the first person to successfully send and receive radio waves (it happened in the 1880s), cycles per second is usually referred to as hertz, abbreviated Hz. Thus, 1 Hz is one cycle per second. Incidentally, when the prefix K (for kilo, or 1,000), M (for mega, 1 million), or G (for giga, 1 billion) is added to the front of Hz, the H is still capitalized. Thus, 2.4 MHz (not 2.4 Mhz) is correct. |
The beauty of radio frequencies is that transmitters can be tuned to broadcast radio waves at a precise frequency. Likewise, receivers can be tuned to receive radio waves at a precise frequency and ignore waves at other frequencies. That's why you can tune the radio in your car to listen to dozens of different radio stations: Each station broadcasts at its own frequency.
A term related to frequency is wavelength. Radio waves travel at the speed of light. The term wavelength refers to how far the radio signal travels with each cycle. For example, because the speed of light is roughly 300 million meters per second, the wavelength of a 1 Hz radio wave is about 300 million meters. The wavelength of a 2 Hz signal is about 150 million meters.
As you can see, the wavelength decreases as the frequency increases. The wavelength of a typical AM radio station broadcasting at 580 KHz is about 500 meters. For a TV station broadcasting at 100 MHz, it's about 3 meters. For a wireless network broadcasting at 2.4 GHz, the wavelength is about 12 centimeters.
It turns out that the shorter the wavelength, the smaller the antenna needs to be in order to adequately receive the signal. As a result, higher-frequency transmissions need smaller antennas. You may have noticed that AM radio stations usually have huge antennas mounted on top of tall towers, but cell- phone transmitters are much smaller and their towers aren't nearly as tall. That's because cellphones operate on a higher frequency than AM radio stations do. So who decides what type of radio gets to use specific frequencies? That's where spectrums and the FCC come in.
The term spectrum refers to a continuous range of frequencies on which radio can operate. In the United States, the Federal Communications Commission (FCC) regulates not only how much of Janet Jackson can be shown at the Super Bowl but also how various portions of the radio spectrum can be used. Essentially, the FCC has divided the radio spectrum into dozens of small ranges called bands and restricted certain uses to certain bands. For example, AM radio operates in the band from 535 KHz to 1,700 KHz.
Table 9-1 lists some of the most popular bands. Note that some of these bands are wide-for example, UHF television begins at 470 MHz and ends at 806 MHz, but other bands are restricted to a specific frequency. The difference between the lowest and highest frequency within a band is the bandwidth.
Band | What It's Used For |
---|---|
535 KHz–1,700 KHz | AM radio |
5.9 MHz–26.1 MHz | Short wave radio |
26.96 MHz–27.41 MHz | Citizens Band (CB) radio |
54 MHz–88 MHz | Television (VHF channels 2 through 6) |
88 MHz–108 MHz | FM radio |
174 MHz–220 MHz | Television (VHF channels 7 through 13) |
470 MHz–806 MHz | Television (UHF channels) |
806 MHz–890 MHz | Cellular networks |
900 MHz | Cordless phones |
1850 MHz–1990 MHz | PCS Cellular |
2.4 GHz–2.4835 GHz | Cordless phones and wireless networks (802.11b and 802.11g) |
4 GHz–5 GHz | Large-dish satellite TV |
5 GHz | Wireless networks (802.11a) |
11.7 GHz–12.7 GHz | Small-disk satellite TV |
I was an English literature major in college, so I like to use literary devices, such as irony. Of course, irony doesn't come up much in computer books. So, when it does, I like to jump on it like a hog out of the water.
Here's my juicy bit of irony for today: The first Ethernet system was a wireless network. Ethernet traces its roots back to a network developed at the University of Hawaii in 1970: the Alohanet. This network transmitted its data by using small radios. If two computers tried to broadcast data at the same time, the computers detected the collision and tried again after a short, random delay. This technique was the inspiration for the basic technique of Ethernet, now called carrier sense multiple access with collision detection (CSMA/CD). The wireless Alohanet network inspired Robert Metcalfe to develop his cabled network, Ethernet, as his doctoral thesis at Harvard in 1973.
For the next 20 years or so, Ethernet was pretty much a cable-only network. It wasn't until the mid-1990s that Ethernet finally returned to its wireless roots.
Two of the bands in the spectrum are allocated for use by wireless networks: 2.4 GHz and 5 GHz. Note that these bands aren't devoted exclusively to wireless networks. In particular, the 2.4 GHz band shares its space with cordless phones. As a result, cordless phones can sometimes interfere with wireless networks.