The very short version is that Wi-Fi is a way for wireless devices to communicate.
Wi-Fi is the Wi-Fi Alliance's name for a wireless standard, or protocol, used for wireless communication. I'll tell you a bit more about this wireless standard and its variations, known collectively as IEEE 802.11, later in this chapter. (IEEE stands for the Institute of Electrical and Electronics Engineers, which defines the standard.)
Standards and protocols are mostly of interest to engineers (however, see the sidebar "What Is a Standard?" for more information if you are curious).
But Wi-Fi has garnered a huge amount of attention from people who would normally be unconcerned about engineering details: in other words, normal human beings such as you and me. Students, professionals, homemakers, Comparative Lit majors, and office workers are all talking about Wi-Fi.
The really big question is: Why is Wi-Fi getting all this attention? I'll get to that soon. I'll also show you how Wi-Fi can change your life. (For real!) But first, I'd like to tell you a little bit more about what Wi-Fi is.
For now, you need to know that Wi-Fi devices are certified interoperable (within certain limits that I'll discuss shortly) and run on some flavor of 802.11, a medium-range wireless networking standard. It's important to know that 802.11 runs at speeds fast enough to handle network traffic (perhaps a little slower than a wired network). I'll be telling you about transmission speeds in more detail later in this chapter, but the crux of the matter is that if you are connected via Wi-Fi to the Internet (either through a private network or a hotspot), the Wi-Fi connection will not be the bottleneck, if there is one.
All you might ever want or need to know about wireless spectrums is that they are used to send and receive signals by your mobile computer, cell phone, and garage door opener. If anything more than this makes your eyes glaze over, please skip this section. But otherwise, if you are curious, please read on.
As you probably know, any signal 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 receivingand transmittingradio 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 that Wi-Fi uses, you are actually talking about 2,400,000,000 (2.4 billion) oscillations per second.
You don't really need to know much about frequencies to understand Wi-Fi. But if you are curious, for historical reasons, lower frequencies have been developed and used sooner than higher frequencies. You might be interested to know that visible light oscillates on the electromagnetic frequency at a higher frequency than any used for radio transmissions.
Only so many frequencies in the radio spectrum can be used for transmissions. This has inevitably led to the potential for conflicts about usage, as well as attempts to dominate particular frequencies.
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, or "free," uses. These set-aside areas include the 2.4GHz and 5GHz spectrums, which is what Wi-Fi uses, as I'll explain later in this chapter in "The Free Spectrums."
The uses of some of the frequencies in the radio spectrum are shown in Figure 8.1.
Figure 8.1. Selected uses for the radio spectrum (not drawn to scale; source U.S. Department of Commerce).
If you find that your Wi-Fi device is getting interference from some other appliance such as a microwave or wireless telephone, one of the first things to try is moving either your Wi-Fi device, or the other device, to a new physical location.
Wi-Fi Standard Layers
The 802.11 (and Wi-Fi) standard includes what is called a physical layer. This physical layer uses something known as Direct Sequence Spread Spectrum technology (DSSS) to prevent collisions and avoid interference between devices operating on the same spectrum. You'll find much the same kind of technology in your wireless telephone handset. The idea here is that you don't want the signal coming out of your microwave unit to interfere with your email (or vice versa).
In addition to its physical layer, each 802.11 Wi-Fi device has an media access control (MAC) layer. The MAC layer specifies how a Wi-Fi device, such as a mobile computer, communicates with another Wi-Fi device, such as a wireless access point.
Together, the physical and MAC layers, along with extensions intended to implement extra features (such as security), make up the 802.11 Wi-Fi standard.
The Free Spectrums
Unlike many other wireless standards, 802.11 runs on "free" portions of the radio spectrum as I just explained. This means that (unlike cell telephone communications) no license is required to broadcast or communicate using 802.11 (or Wi-Fi).
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.
The free portions of the radio spectrum used by 802.11 (and Wi-Fi) are the 2.4GHz band, and, more recently, the 5GHz band. As you might know, many household appliances such as microwave ovens and (most significantly) wireless telephone handsets also use these free spectrums.
With a wireless telephone handset, a base station is connected to the telephone line, and the handset communicates with the base station over the "free" radio frequency, so that you can roam about your home or office while talking on the phone. Clearly, these wireless telephone handsets are not the same thing as cell phones, which do not connect to a telephone wire at all and use licensed portions of the spectrum.
In other words, 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, "Your Own Wireless Network," I'll show you how to best avoid problems with competing 2.4GHz devices when setting up a Wi-Fi network.
There are conflicts within the 5GHz band as well as the 2.4GHz band, but less so because the 5GHz band conflicts primarily concern competing usages such as radar and satellite radio, which are being ironed out by the FCC.
Interference, of course, is a two-way street. You also don't want your Wi-Fi network wrecking havoc on your cordless phones and other devices that use the spectrum.