2008-10-13
INTRODUCTION TO WIRELESS NETWORKS
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 the technology that surrounds them. You shouldn’t have to worry about the technical specifications just to place a long-distance telephone call or heat your lunch in a microwave oven or connect your laptop computer to a network. In an ideal world (ha!), the wireless link would work as soon as you turn on the power switch.
But wireless networking today is about where broadcast radio was in the late 1920s. The technology was out there for everybody, but the people who understood what was happening behind that Bakelite-Dilecto panel (Figure 2-1) often got better performance than the ones who just expected to 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 make up the network). This chapter describes the standards and specifications that control wireless
networks and explains how data moves through the network from one computer to another.
When the network is working properly, you should be able to use it without thinking about all of that internal plumbing—just click a few icons and you’re connected. But when you’re designing and building a new network, or when you want to improve the performance of an existing network, it can be essential to understand 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.
How Wireless Networks Work
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 you must define all three Introduct ion to Wireless Networks 13 when you invent a new network. In terms of the OSI 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 wireless network interface adapters and base stations that send and receive the radio signals. In a wireless network, the network interface adapters in each computer and base station convert digital data to radio signals, which they transmit to other devices on the same network, and they receive and convert incoming radio signals from other network elements back to digital data.
Each of the broadband wireless data services use a different combination of radio signals, data formats, and network structure. We’ll describe each type of wireless data network in more detail later in this chapter, but first, it’s valuable to understand some general principles.
Radio
The basic physical laws that make radio possible are known as Maxwell’s equations, identified by James Clerk Maxwell in 1864. Without going into the math, Maxwell’s equations show that a changing magnetic field will produce an electric field, and a changing electric field will produce a magnetic field. When alternating current (AC) moves through a wire or other physical conductor, some of that energy escapes into the surrounding space as an alternating magnetic field. That magnetic field creates an alternating electric field in space, which in turn creates another magnetic field and so forth until the original current is interrupted.
This form of energy in transition between electricity and magnetic energy is called electromagnetic radiation, or radio waves. Radio is defined as the radiation of electromagnetic energy through space. A device that produces radio waves is called a transmitter, and a complementary device that detects radio waves in the air and converts them to some other form of energy is called a receiver. Both transmitters and receivers use specially shaped devices called antennas to focus the radio signal in a particular direction, or pattern, and to increase the amount of effective radiation (from a transmitter) or sensitivity (in a receiver).
By adjusting the rate at which alternating current flows from each transmitter through the antenna and out into space (the frequency), and by adjusting a receiver to operate only at that frequency, it’s possible to send and receive many different signals, each at a different frequency, that don’t interfere with one another. The overall range of frequencies is known as the radio
spectrum. A smaller segment of the radio spectrum is often called a band.
Radio frequencies and other AC signals are expressed as cycles per second, or hertz (Hz), named for Heinrich Hertz, the first experimenter to send and receive radio waves. One cycle is the distance from the peak of an AC signal to the peak of the next signal. Radio signals generally operate at frequencies in thousands, millions, or billions of hertz (kilohertz or KHz, megahertz or MHz, and gigahertz or GHz, respectively).
The simplest type of radio communication uses a continuous signal that the operator of the transmitter interrupts to divide the signal into accepted patterns of long and short signals (dots and dashes) that correspond to individual letters and other characters. The most widely used set of these patterns was Morse code, named for the inventor of the telegraph, Samuel F.B. Morse, where this code was first used.
In order to transmit speech, music, and other sounds via radio, the transmitter alters, or modulates, the AC signal (the carrier wave) by either mixing an audio signal with the carrier as shown in Figure 2-2 (this is called amplitude modulation, or AM) or by modulating the frequency within a narrow range as shown in Figure 2-3 (this is called frequency modulation, or FM). The AM or FM receiver includes a complementary circuit that separates the carrier from
the modulating signal.
Figure 2-2: In an AM signal, the audio modulates the carrier.
Figure 2-3: In an FM signal, the audio modulates the radio frequency.
Because two or more radio signals using the same frequency can often interfere with one another, government regulators and international agencies, such as the International Telecommunication Union (ITU), have reserved certain frequencies for specific types of modulation, and they issue exclusive licenses to individual users. For example, an FM radio station might be licensed to operate at 92.1 MHz at a certain geographical location. Nobody else is allowed to use that frequency in close enough proximity to interfere with that signal. On the other hand, some radio services don’t require a license. Most unlicensed services are either restricted to very short distances, to specific frequency bands, or both.
Both AM and FM are analog methods because the signal that comes out of the receiver is a replica of the signal that went into the transmitter. When we send computer data through a radio link, it’s digital because the content has been converted from text, computer code, sounds, images or other information into ones and zeroes before it is transmitted, and it is converted back to its original form after it is received. Digital radio can use any of several different modulation methods: The ones and zeroes can be two different audio tones, two different radio frequencies, timed interruptions to the carrier, or some combination of those and other techniques.
Wireless Data Networks
Each type of wireless data network operates on a specific set of radio frequencies. For example, most Wi-Fi 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. Other Wi-Fi systems use a different unlicensed band around 5 GHz.
Unlicensed Radio Services
Unlicensed means that anybody using equipment that complies with the technical requirements can send and receive radio signals on these frequencies without a radio station license. Unlike most radio services (including other broadband wireless services), which require licenses that grant exclusive use of that frequency to a specific type of service and to one or more specific users, an unlicensed service is a free-for-all where everybody has an equal claim to the same airwaves. In theory, the technology of spread spectrum radio makes it possible for many users to co-exist (up to a point) without significant interference.
Point-to-Point
A point-to-point radio service operates a communication channel that carries information from a transmitter to a single receiver. The opposite of point-topoint 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
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 several different spread spectrum radio transmission systems, which are called frequency-hopping spread spectrum (FHSS), direct-sequence spread spectrum (DSSS), and orthogonal frequency division multiplexing (OFDM). Some older data networks use the slower FHSS system, but the first Wi-Fi networks used DSSS, and more recent systems use OFDM. Table 2-1 lists each of the Wi-Fi standards and the type of spread spectrum modulation they use.
Table 2-1: Wi-Fi Standards and Modulation Type
Wi-Fi Type Frequency Modulation
802.11a 5 GHz OFDM
802.11b 2.4 GHz DSSS
802.11g 2.4 GHz OFDM
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 the signals operate on a relatively wide band of frequencies, they are less sensitive to interference from other radio signals and electrical noise, which means they can often get through in environments where a conventional narrow-band signal would be impossible to receive and understand. And because a frequency-hopping spread spectrum signal shifts among more than one channel, 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 vulnerable 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, shortly before the United States entered World War II, she came up with the concept of changing radio frequencies to cut through interference. The New York Times reported in 1941 that her “red hot” invention (Figure 2-4) was vital to the national defense, but the government would not reveal any details.
Figure 2-4: Hedy Lamarr and George Antheil received this patent in 1942 for the
invention that became the foundation of spread spectrum radio communication.
She is credited here under her married name, H.K. Markey.
Antheil turned out to be the ideal person to make this idea work. His most famous composition was an extravaganza called Ballet Mechanique, which was scored for sixteen player pianos, two airplane propellers, four xylophones, four bass drums, and a siren. His design used the same kind of mechanism that he had previously used to synchronize the player pianos to change radio frequencies in a spread spectrum transmission. The original slotted paper tape system had 88 different radio channels—one for each of the 88 keys on a piano.
In theory, the same method could be used for voice and data communication as well as guiding torpedoes, but in the days of vacuum tubes, paper tape, and mechanical synchronization, the whole process was too complicated to actually build and use. By 1962, solid-state electronics had replaced the vacuum tubes and piano rolls, and the technology was used aboard US Navy ships for secure communication during the Cuban Missile Crisis. Today, spread spectrum radios are used in the US Air Force Space Command’s Milstar Satellite Communications System, in digital cellular telephones, and in wireless data networks.
Frequency-Hopping Spread Spectrum
Lamarr and Antheil’s original design for spread spectrum radio used a frequency-hopping system (FHSS). As the name suggests, FHSS technology divides a radio signal into small segments and “hops” from one frequency to another many times per second as it transmits those segments. The transmitter and the receiver establish a synchronized hopping pattern that sets the sequence in which they will use different subchannels.
FHSS systems overcome interference from other users by using a narrow carrier signal that changes frequency many times per second. Additional transmitter and receiver pairs can use different hopping patterns on the same set of subchannels at the same time. At any point in time, each transmission is probably using a different subchannel, so there’s no interference between signals. When a conflict does occur, the system resends the same packet until the receiver gets a clean copy and sends a confirmation back to the transmitting station.
For some older 802.11 wireless data services, the unlicensed 2.4 MHz band is split into 75 subchannels, each of them 1 MHz wide. Because each frequency hop adds overhead to the data stream, FHSS transmissions are relatively slow.
Direct-Sequence Spread Spectrum
The direct-sequence spread spectrum (DSSS) technology that controls 802.11b networks uses an 11-chip Barker Sequence to spread the radio signal through a single 22 MHz–wide channel without changing frequencies. Each DSSS link uses just one channel without any hopping between frequencies. As Figure 2-5 shows, a DSSS transmission uses more bandwidth, but less power than a conventional signal. The digital signal on the left is a conventional transmission in which the power is concentrated within a tight bandwidth. The DSSS signal on the right uses the same amount of power, but it spreads that power across a wider band of radio frequencies. Obviously, the 22 MHz DSSS channel is a lot wider than the 1 MHz channels used in FHSS systems.
A DSSS transmitter breaks each bit in the original data stream into a series of redundant bit patterns called chips, and it transmits them to a receiver that reassembles the chips back into a data stream that is identical to the original. Because most interference is likely to occupy a narrower bandwidth than a DSSS signal, and because each bit is divided into several chips, the receiver can usually identify noise and reject it before it decodes the signal.
Figure 2-5: A conventional signal (left) uses a narrow radio frequency bandwidth. A DSSS signal (right) uses a wider bandwidth but a less powerful signal.
Like other networking protocols, a DSSS wireless link exchanges handshaking messages within each data packet to confirm that the receiver can understand each packet. For example, the standard data transmission rate in an 802.11b DSSS WI-Fi network is 11Mbps, but when the signal quality won’t support that speed, the transmitter and receiver use a process called dynamic rate shifting to drop the speed down to 5.5Mbps. The speed might drop because a source of electrical noise near the receiver interferes with the signal or because the transmitter and receiver are too far apart to support full-speed operation. If 5.5Mbps is still too fast for the link to handle, it drops again, down to 2Mbps or even 1Mbps.
Orthogonal Frequency Division Multiplexing
Orthogonal frequency division multiplexing (OFDM) modulation, used in 802.11a Wi-Fi networks, is considerably more complicated than DSSS technology. The physical layer splits the data stream among 52 parallel bit streams that each use a different radio frequency called a subcarrier. Four of these subcarriers carry pilot data that provides reference information about the remaining 48 subcarriers, in order to reduce signal loss due to radio interference or phase shift. Because the data is divided into 48 separate streams that move through separate subcarriers in parallel, the total transmission speed is much greater than the speed of data through a single channel.
The subcarrier frequencies in an OFDM signal overlap with the peak of each subcarrier’s waveform matching the baseline of the overlapping signals as shown in Figure 2-6. This is called orthogonal frequency division. The 802.11a standard specifies a total of eight data channels that are 20 MHz wide. Each of these channels is divided into 52 300 kHz subcarriers.
Figure 2-6: In OFDM, the peaks of overlapping frequencies don’t interfere with one another.
When a Wi-Fi radio receiver detects an 802.11a signal, it assembles the parallel bit streams back into a single high-speed data stream and uses the pilot data to check its accuracy. Under ideal conditions, an 802.11a network can move data at 54Mbps, but like DSSS modulation, the OFDM transmitter and receiver automatically reduce the data speed when the maximum transmission rate is not possible due to interference, weak signals, or other lessthan perfect atmospheric conditions.
The more recent 802.11g specification was designed to combine the best features of both 802.11b (greater signal range) and 802.11a (higher speed). To accomplish this objective, it uses OFDM modulation on the 2.4 GHz frequency band.
Why This Matters
The great science fiction writer Arthur C. Clarke once observed that “Any sufficiently advanced technology is indistinguishable from magic.” For most of us, the technology that controls high-speed spread spectrum radio could just as easily be a form of magic, because we don’t need to understand the things that happen inside a transmitter and a receiver; they’re just about invisible when we connect a computer to the Internet. As mentioned earlier in this chapter, you don’t need to understand these technical details about how a Wi-Fi transmitter splits your data into tiny pieces and reassembles them into data unless you’re a radio circuit designer.
But when you know that there’s a well-defined set of rules and methods that make the connection work (even if you don’t know all the details), you are in control. You know that it’s not magic, and if you think about it, you might also know some of the right questions to ask when the system doesn’t work correctly. If knowledge is power, then knowledge about the technology
you use every day is the power to control that technology rather than just use it.
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