HOW WI-FI WORKS
HOW WI-FI WORKS. Up to a point, it’s quite possible to treat your wireless network as a set of black boxes that you can turn on and use without knowing …
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HOW WI-FI WORKS
Up to a point, it\’s quite possible to treat your wireless network as a set of black boxes that you can turn on and use without knowing much about the way they work. That\’s the way most people relate to all of the technology that surrounds them — you shouldn\’t have to worry about the 802.11b specification to connect your laptop computer to a network. In an ideal world (ha!), it would work just as soon as you turn on the power switch.
But wireless Ethernet today is about where broadcast radio was in 1923. The technology was out there, but people spent a lot of time tweaking their equipment. And the people who understood what was happening behind that Bakelite-Dilecto panel were able to get better performance from their radios than the ones who expected to just turn on the power switch and listen. In order to make the most effective use of wireless networking technology, it\’s still important to understand what\’s going on inside the box (or in this case, inside each of the boxes that makes up the network). This chapter describes the standards and specifications that control wireless networks, and it explains how data moves through the network from one computer to another.
Figure 1.1: Every new technology goes through the tweak-and-fiddle stage
When the network is working properly, you should be able to use it without thinking about all of this internal plumbing — just click a few icons on your computer\’s screen, and you\’re connected. But when you\’re designing and building a new network, or when you want to tweak the performance of an existing network, it can be essential to know how all that data is supposed to move from one place to another. And when the network does something you aren\’t expecting it to do, you will need a basic knowledge of the technology to do any kind of useful troubleshooting. Moving data through a wireless network involves three separate elements: the radio signals, the data format, and the network structure. Each of these elements is independent of the other two, so it\’s necessary to define all three when you invent a new network. In terms of the familiar OSI (Open Systems Interconnection) reference model, the radio signal operates at the Physical layer, and the data format controls several of the higher layers. The network structure includes the interface adapters and base stations that send and receive the radio signals. In a wireless network, the network adapters in each computer convert digital data to radio signals, which they transmit to other devices on the network, and they convert incoming radio signals from other network elements back to digital data. The IEEE (Institute of Electrical and Electronics Engineers) has produced a set of standards and specifications for wireless networks under the title \”IEEE 802.11\” that defines the format and structure of those signals.
The original 802.11 standard (without the \”b\” at the end) was released in 1997. It covers several different types of wireless media: two kinds of radio transmissions (which we\’ll explain later in this chapter) and networks that use infrared light. The more recent 802.11b standard provides additional specifications for wireless Ethernet networks. A related document, IEEE 802.11a, describes wireless networks that operate at higher speeds on different radio frequencies. Still other 802.11 radio networking standards with other letters are also moving toward public release. The specification in widest use today is 802.11b. That\’s the de facto standard used by just about every wireless Ethernet LAN that you are likely to encounter in offices and public spaces and in most home networks. It\’s worth the trouble to keep an eye on the progress of those other standards, but for the moment, 802.11b is the one to use, especially if you\’re expecting to connect to networks where you don\’t control all the hardware yourself.
The wireless networks described in this book follow the 802.11b standard, but much of the same information also applies to other kinds of 802.11 networks. You ought to know about two more names in the alphabet soup of wireless LAN standards: WECA and Wi-Fi. WECA (Wireless Ethernet Compatibility Alliance) is an industry group that includes all of the major manufacturers of 802.11b equipment. Their twin missions are to test and certify that wireless network devices from all of their member companies can operate together in the same network and to promote 802.11 networks as the worldwide standard for wireless LANs. WECA\’s marketing geniuses adopted the more \”friendly\” name of Wi-Fi (short for Wireless Fidelity) for the 802.11 specifications and changed their own name to the Wi-Fi Alliance. Once or twice a year, the Alliance conducts an \”interoperability bake-off\” where engineers from many hardware manufacturers confirm that their hardware will communicate correctly with equipment from other suppliers. Network equipment that carries the Wi-Fi logo has been certified to meet the relevant standards, and to pass those interoperability tests. Figure 1.2 shows the Wi-Fi logos on network adapters from two different manufacturers.
802.11b networks operate in a special band of radio frequencies around 2.4 GHz that have been reserved in most parts of the world for unlicensed point-to-point spread-spectrum radio services. The unlicensed part means that anybody using equipment that complies with the technical requirements can send and receive radio signals on these frequencies, without the need for a radio station license. Unlike most radio services, which require licenses that grant exclusive use of a frequency to a single user or group of users, and which restrict the use of that frequency to a specific type of service, an unlicensed service is a free-for-all, where everybody has an equal claim on the same piece of the spectrum. In theory, the technology of spread-spectrum radio makes it possible to coexist with other users (up to a point) without significant interference. A point-to-point radio service operates a communication channel that carries information from a transmitter to a single receiver. The opposite of point-to-point is a broadcast service (such as a radio or television station) that sends the same signal to many receivers at the same time. Spread spectrum is a family of methods for transmitting a single radio signal using a relatively wide segment of the radio spectrum. Wireless Ethernet networks use two different spread-spectrum radio transmission systems, called FHSS (frequency-hopping spread spectrum) and DSSS (direct-sequence spread spectrum). Some older 802.11 networks use the slower FHSS system, but the current generation of 802.11b and 802.11a wireless Ethernet networks use DSSS. Spread-spectrum radio offers some important advantages over other types of radio signals that use a single narrow channel. Spread spectrum is extremely efficient, so the radio transmitters can operate with very low power. Because they operate on a relatively wide band of frequencies, they are less sensitive to interference from other radio signals and electrical noise, which means that the signals are often able to get through in environments where a conventional narrow-band signal would be impossible to receive and understand, and because a frequencyhopping spread-spectrum signal shifts among multiple channels, it can be extremely difficult for an unauthorized listener to intercept and decode the contents of a signal. Spread-spectrum technology has an interesting history. It was invented by the actress Hedy Lamarr and the American avant-garde composer George Antheil as a \”Secret Communication System\” for directing radio-controlled torpedoes that would not be subject to enemy jamming. Before she came to Hollywood, Lamarr had been married to an arms merchant in Austria, where she learned about the problems of torpedo guidance at dinner parties with her husband\’s customers. Years later, during World War II, she came up with the concept of changing radio frequencies to cut through interference. Antheil turned out to be the ideal person to make this idea work. His most famous composition was something called Ballet Mechanique, which was scored for 16 player pianos, two airplane propellers, four xylophones, four bass drums, and a siren. He used the same kind of mechanism that he had previously used to synchronize the player pianos to change radio frequencies in a spread-spectrum
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