This chapter covers the following subjects:
Wireless Local-Area Networks: A brief history of wireless networking and some of the basic concepts.
How Bandwidth Is Achieved from RF Signals: The frequency spectrum used in RF transmissions.
Modulation Techniques and How They Work: How binary data is represented and transmitted using RF technology.
Perhaps this is the first time you have ever delved into the world of wireless networking. Or maybe you have been in networking for some time and are now beginning to see the vast possibilities that come with wireless networking. Either way, this chapter can help you understand topics that are not only tested on the CCNA Wireless exam but provide a good foundation for the chapters to come. If you are comfortable with the available frequency bands, the modulation techniques used in wireless LANs, and some of the standards and regulatory bodies that exist for wireless networking, you may want to skip to Chapter 2, "Standards Bodies."
This chapter provides a brief history of wireless networks and explores the basics of radio technology, the modulation techniques used, and some of the issues seen in wireless LANs.
You should do the "Do I Know This Already?" quiz first. If you score 80 percent or higher, you might want to skip to the section "Exam Preparation Tasks." If you score below 80 percent, you should spend the time reviewing the entire chapter. Refer to Appendix A, "Answers to the 'Do I Know This Already?' Quizzes" to confirm your answers.
"Do I Know This Already?" Quiz
The "Do I Know This Already?" quiz helps you determine your level of knowledge of this chapter's topics before you begin. Table 1-1 details the major topics discussed in this chapter and their corresponding quiz questions.
Table 1-1 "Do I Know This Already?" Section-to-Question Mapping
Foundation Topics Section
Wireless Local-Area Networks
How Bandwidth Is Achieved from RF Signals
Modulation Techniques and How They Work
Which of the following accurately describes the goal of RF technology?
To send as much data as far as possible and as fast as possible
To send secure data to remote terminals
To send small amounts of data periodically
To send data and voice short distances using encryption
Which of the following is a significant problem experienced with wireless networks?
Which two of the following are unlicensed frequency bands used in the United Stated? (Choose two.)
Each 2.4-GHz channel is how many megahertz wide?
How many nonoverlapping channels exist in the 2.4-GHz ISM range?
The 5.0-GHz range is used by which two of the following 802.11 standards? (Choose two.)
Which three of the following modulation techniques do WLANs today use? (Choose three.)
DSSS uses a chipping code to encode redundant data into the modulated signal. Which two of the following are examples of chipping codes that DSSS uses? (Choose two.)
Complementary code keying (CCK)
Cypher block chaining (CBC)
DSSS binary phase-shift keying uses what method of encoding at the 1-Mbps data rate?
11-chip Barker code
8-chip Barker code
With DRS, when a laptop operating at 11 Mbps moves farther away from an access point, what happens?
The laptop roams to another AP.
The laptop loses its connection.
The rate shifts dynamically to 5.5 Mbps.
The rate increases, providing more throughput.
Wireless Local-Area Networks
Although wireless networking began to penetrate the market in the 1990s, the technology has actually been around since the 1800s. A musician and astronomer, Sir William Herschel (1738 to 1822) made a discovery that infrared light existed and was beyond the visibility of the human eye. The discovery of infrared light led the way to the electromagnetic wave theory, which was explored in-depth by a man named James Maxwell (1831 to 1879). Much of his discoveries related to electromagnetism were based on research done by Michael Faraday (1791 to 1867) and Andre-Marie Ampere (1775 to 1836), who were researchers that came before him. Heinrich Hertz (1857 to 1894) built on the discoveries of Maxwell by proving that electromagnetic waves travel at the speed of light and that electricity can be carried on these waves.
Although these discoveries are interesting, you might be asking yourself how they relate to wireless local-area networks (WLANs). Here is the tie-in: In standard LANs, data is propagated over wires such as an Ethernet cable, in the form of electrical signals. The discovery that Hertz made opens the airways to transfer the same data, as electrical signals, without wires. Therefore, the simple answer to the relationship between WLANs and the other discoveries previously mentioned is that a WLAN is a LAN that does not need cables to transfer data between devices, and this technology exists because of the research and discoveries that Herschel, Maxwell, Ampere, and Hertz made. This is accomplished by way of Radio Frequencies (RF).
With RF, the goal is to send as much data as far as possible and as fast as possible. The problem is the numerous influences on radio frequencies that need to be either overcome or dealt with. One of these problems is interference, which is discussed at length in Chapter 5, "Antennae Communications." For now, just understand that the concept of wireless LANs is doable, but it is not always going to be easy. To begin to understand how to overcome the issues, and for that matter what the issues are, you need to understand how RF is used.
How Bandwidth Is Achieved from RF Signals
To send data over the airwaves, the IEEE has developed the 802.11 specification, which defines half-duplex operations using the same frequency for send and receive operations on a WLAN. No licensing is required to use the 802.11 standards; however, you must follow the rules that the FCC has set forth. The IEEE defines standards that help to operate within the FCC rules. The FCC governs not only the frequencies that can be used without licenses but the power levels at which WLAN devices can operate, the transmission technologies that can be used, and the locations where certain WLAN devices can be deployed.
Note - The FCC is the regulatory body that exists in the United States. The European Telecommunications Standards Institute (ETSI) is the European equivalent to the FCC. Other countries have different regulatory bodies.
To achieve bandwidth from RF signals, you need to send data as electrical signals using some type of emission method. One such emission method is known as Spread Spectrum. In 1986, the FCC agreed to allow the use of spread spectrum in the commercial market using what is known as the industry, scientific, and medical (ISM) frequency bands. To place data on the RF signals, you use a modulation technique. Modulation is the addition of data to a carrier signal. You are probably familiar with this already. To send music, news, or speech over the airwaves, you use frequency modulation (FM) or amplitude modulation (AM). The last time you were sitting in traffic listening to the radio, you were using this technology.
Unlicensed Frequency Bands Used in WLANs
As you place more information on a signal, you use more frequency spectrum, or bandwidth. You may be familiar with using terms like bits, kilobits, megabits, and gigabits when you refer to bandwidth. In wireless networking, the word bandwidth can mean two different things. In one sense of the word, it can refer to data rates. In another sense of the word, it can refer to the width of an RF channel.
Note - This book uses the term bandwidth to refer to the width of the RF channel and not to data rates.
When referring to bandwidth in a wireless network, the standard unit of measure is the Hertz (Hz). A Hertz measures the number of cycles per second. One Hertz is one cycle per second. In radio technology, a Citizens' Band (CB) radio is pretty low quality. It uses about 3 kHz of bandwidth. FM radio is generally a higher quality, using about 175 kHz of bandwidth. Compare that to a television signal, which sends both voice and video over the air. The TV signal you receive uses almost 4500 kHz of bandwidth.
Figure 1-1 shows the entire electromagnetic spectrum. Notice that the frequency ranges used in CB radio, FM radio, and TV broadcasts are only a fraction of the entire spectrum. Most of the spectrum is governed by folks like the FCC. This means that you cannot use the same frequencies that FM radio uses in your wireless networks.
As Figure 1-1 illustrates, the electromagnetic spectrum spans from Extremely Low Frequency (ELF) at 3 to 30 Hz to Extremely High Frequency (EHF) at 30 GHz to 300 GHz. The data you send is not done so in either of these ranges. In fact, the data you send using WLANs is either in the 900-MHz, 2.4-GHz, or 5-GHz frequency ranges. This places you in the Ultra High Frequency (UHF) or Super High Frequency (SHF) ranges. Again, this is just a fraction of the available spectrum, but remember that the FCC controls it. You are locked into the frequency ranges you can use. Table 1-2 lists the ranges that can be used in the United States, along with the frequency ranges allowed in Japan and Europe.
Table 1-2 Usable Frequency Bands in Europe, the United States, and Japan
2.4 GHz ISM
Table 1-2 clearly shows that not all things are equal, depending on which country you are in. In Europe, the 2.4-GHz range and the 5.0-GHz range are used. The 5.0-GHz frequency ranges that are used in Europe are called the Conference of European Post and Telecommunication (CEPT) A, CEPT B, CEPT C, and CEPT C bands. In the United States, the 900-MHz, 2.4-GHz ISM, and 5.0-GHz Unlicensed National Information Infrastructure (UNII) bands are used. Japan has its own ranges in the 2.4- and 5.0-GHz range. The following sections explain the U.S. frequency bands in more detail.
The 900-MHz band starts at 902 MHz and goes to 928 MHz. This frequency range is likely the most familiar to you because you probably had a cordless phone that operated in this range. This is a good way to understand what wireless channels are. You might have picked up your cordless phone only to hear a lot of static or even a neighbor on his cordless phone. If this happened, you could press the Channel button to switch to a channel that did not have as much interference. When you found a clear channel, you could make your call. The channel you were changing to was simply a different range of frequencies. This way, even though both your phone and your neighbor's were operating in the 900-MHz range, you could select a channel in that range and have more than one device operating at the same time.
The 2.4-GHz range is probably the most widely used frequency range in WLANs. It is used by the 802.11, 802.11b, 802.11g, and 802.11n IEEE standards. The 2.4-GHz frequency range that can be used by WLANs is subdivided into channels that range from 2.4000 to 2.4835 GHz. The United States has 11 channels, and each channel is 22-MHz wide. Some channels overlap with others and cause interference. For this reason, channels 1, 6, and 11 are most commonly used because they do not overlap. In fact, many consumer-grade wireless devices are hard set so you can choose only one of the three channels. Figure 1-2 shows the 11 channels, including overlap. Again, notice that channels 1, 6, and 11 do not overlap.
With 802.11b and 802.11g, the energy is spread out over a wide area of the band. With 802.11b or 802.11g products, the channels have a bandwidth of 22 MHz. This allows three nonoverlapping, noninterfering channels to be used in the same area.
The 2.4-GHz range uses direct sequence spread spectrum (DSSS) modulation. DSSS is discussed later in this chapter in the section "DSSS." Data rates of 1 Mbps, 2 Mbps, 5.5 Mbps, and 11 Mbps are defined for this range.
The 5-GHz range is used by the 802.11a standard and the new 802.11n draft standard. In the 802.11a standard, data rates can range from 6 Mbps to 54 Mbps. 802.11a devices were not seen in the market until 2001, so they do not have quite the market penetration as 2.4-GHz range 802.11 b devices. The 5-GHz range is also subdivided into channels, each being 20-MHz wide. A total of 23 nonoverlapping channels exist in the 5-GHz range.
The 5-GHz ranges use Orthogonal Frequency Division Multiplexing (OFDM). OFDM is discussed later in this chapter in the section "OFDM." Data rates of 6, 9, 12, 18, 24, 36, 48, and 54 Mbps are defined.
Modulation Techniques and How They Work
In short, the process of modulation is the varying in a signal or a tone called a carrier signal. Data is then added to this carrier signal in a process known as encoding.
Imagine that you are singing a song. Words are written on a sheet of music. If you just read the words, your tone is soft and does not travel far. To convey the words to a large group, you use your vocal chords and modulation to send the words farther. While you are singing the song, you encode the written words into a waveform and let your vocal cords modulate it. People hear you singing and decode the words to understand the meaning of the song.
Modulation is what wireless networks use to send data. It enables the sending of encoded data using radio signals. Wireless networks use modulation as a carrier signal, which means that the modulated tones carry data. A modulated waveform consists of three parts:
The volume of the signal
The timing of the signal between peaks
The pitch of the signal
Wireless networks use a few different modulation techniques, including these:
Multiple-Input Multiple-Output (MIMO)
The sections that follow cover these modulation techniques in further detail.
DSSS is the modulation technique that 802.11b devices use to send the data. In DSSS, the transmitted signal is spread across the entire frequency spectrum that is being used. For example, an access point that is transmitting on channel 1 spreads the carrier signal across the 22-MHz-wide channel range of 2.401 to 2.423 GHz.
To encode data using DSSS, you use a chip sequence. A chip and a bit are essentially the same thing, but a bit represents the data, and a chip is used for the carrier encoding. Encoding is the process of transforming information from one format to another. To understand how data is encoded in a wireless network and then modulated, you must first understand chipping codes.
Because of the possible noise interference with a wireless transmission, DSSS uses a sequence of chips. When DSSS spreads information across a frequency range, it sends a single data bit as a string of chips or a chip stream. With redundant data being sent, if some of the signal is lost to noise, the data can likely still be understood. The chipping code process takes each data bit and then expands it into a string of bits.
Figure 1-3 illustrates this process for better understanding.
As the laptop in the figure sends data over the wireless network, the data must be encoded using a chip sequence and then modulated over the airwaves. In the figure, the chipping code for the bit value of 1 is expanded to the chip sequence of 00110011011, and the chipping code for the bit value of 0 is 11001100100. Therefore, after the data bits are sent, 1001 creates the chip sequence.
You can decode this chip sequence back to the value of 1001 at the receiving access point. Remember, because of interference, it is still possible that some of the bits in the chip sequence will be lost or inverted. This means that a 1 could become a 0 and a 0 could become a 1. This is okay, because more than five bits need to be inverted to change the value between a 1 and a 0. Because of this, using a chipping sequence makes 802.11 networks more resilient against interference.
Also, because more bits are sent for chipping (carrier) than there is actual data, the chipping rate is higher than the data rate.
To achieve rates of 1 Mbps and 2 Mbps, 802.11 uses a Barker code. This code defines the use of 11 chips when encoding the data. The 11-chip Barker code used in 802.11 is 10110111000. Certain mathematical details beyond the scope of this book make the Barker code ideal for modulating radio waves. In the end, and for the exam, each bit of data sent is encoded into an 11-bit Barker code and then modulated with DSSS.
Complementary Code Keying
When you are using DSSS, the Barker code works well for lower data rates such as 1-Mbps, 2-Mbps, 5.5-Mbps, and 11-Mbps. DSSS uses a different method for higher data rates, which allows the 802.11 standard to achieve rates of 5.5 and 11 Mbps. Complementary code keying (CCK) uses a series of codes called complementary sequences. There are 64 unique code words. Up to 6 bits can be represented by a code word, as opposed to the 1 bit represented by a Barker code.
DSSS Modulation Techniques and Encoding
Now that the data has been encoded using Barker code or CCK, it needs to be transmitted or modulated out of the radio antennas. You can think of it this way:
Encoding is how the changes in RF signal translate to the 1s and 0s.
Modulation is the characteristic of the RF signal that is manipulated.
For example, amplitude modulation, frequency modulation, and phase-shift keying are modulations. The encoding would be that a 180-degree phase shift is a 1, and 0-degree phase shift is a 0. This is binary phase-shift keying. In 802.11b, the data is modulated on a carrier wave, and that carrier wave is spread across the frequency range using DSSS. 802.11b can modulate and encode the data using the methods seen in Table 1-3.
Table 1-3 DSSS Encoding Methods
11 chip Barker coding
DSSS Binary Phase Shift Keying
11 chip Barker coding
DSSS Quadrature Phase Shift Keying
|8 chip encoding|
8 bits CCK coding
DSSS Quadrature Phase Shift Keying
|8 chip encoding|
4 bits CCK coding
DSSS Quadrature Phase Shift Keying
One method of modulation that is simple to understand is amplitude modulation. With amplitude modulation, the information sent is based on the amplitude of the signal. For example, +5 volts is a 1, and –5 volts is a 0. Because of external factors, the amplitude of a signal is likely changed, and this in turn modifies the information you are sending. This makes AM a "not-so-good" solution for sending important data. However, other factors, such as frequency and phase, are not likely to change. 802.11b uses phase to modulate the data. Specifically, in 802.11b, BPSK and QPSK are used.
Remember that phase is timing between peaks in the signal. Actually, that needs to be expanded further so you can really grasp the concept of BPSK and QPSK. To begin, look at Figure 1-4, which shows a waveform. This waveform, or motion, is happening over a period of time.
Figure 1-4 illustrates the next step in determining phase. The phase is the difference between the two waveforms at the same frequency. If the waveforms peak at the same time, they are said to be in-phase, or 0 degrees. If the two waves peak at different times, they are said to be out-of-phase. Phase-shift keying (PSK) represents information by changing the phase of the signal.
BPSK is the simplest method of PSK. In BPSK, two phases are used that are separated by 180 degrees. BPSK can modulate 1 bit per symbol. To simplify this, a phase shift of 180 degrees is a 1, and a phase shift of 0 degrees is a 0, as illustrated in Figure 1-5.
Encoding with Phase Shifting
802.11 also uses quadrature phase-shift keying (QPSK), which is discussed in the following section.
In BPSK, 1 bit per symbol is encoded. This is okay for lower data rates. QPSK has the capability to encode 2 bits per symbol. This doubles the data rates available in BPSK while staying within the same bandwidth. At the 2-Mbps data rate, QPSK is used with Barker encoding. At the 5.5-Mbps data rate, QPSK is also used, but the encoding is CCK-16. At the 11-Mbps data rate, QPSK is also used, but the encoding is CCK-128.
OFDM is not considered a spread spectrum technology, but it is used for modulation in wireless networks. Using OFDM, you can achieve the highest data rates with the maximum resistance to corruption of the data caused by interference. OFDM defines a number of channels in a frequency range. These channels are further divided into a larger number of small-bandwidth subcarriers. The channels are 20 MHz, and the subcarriers are 300 kHz wide. You end up with 52 subcarriers per channel. Each of the subcarriers has a low data rate, but the data is sent simultaneously over the subcarriers in parallel. This is how you can achieve higher data rates.
OFDM is not used in 802.11b because 802.11b devices use DSSS. 802.11g and 802.11a both used OFDM. The way they are implemented is a little different because 802.11g is designed to operate in the 2.4-MHz range along with 802.11b devices. Chapter 2, "Standards Bodies," covers the differences in the OFDM implementations.
MIMO is a technology that is used in the new 802.11n specification. Although at press time, the 802.11n specification had not yet been ratified by the IEEE, many vendors are already releasing products into the market that claim support for it. Here is what you need to know about it, though. A device that uses MIMO technology uses multiple antennas for receiving signals (usually two or three) in addition to multiple antennas for sending signals. MIMO technology can offer data rates higher than 100 Mbps by multiplexing data streams simultaneously in one channel. In other words, if you want data rates higher than 100-Mbps, then multiple streams are sent over a bonded channel, not just one. Using advanced signal processing, the data can be recovered after being sent on two or more spatial streams.
With the use of MIMO technology, an access point (AP) can talk to non-MIMO-capable devices and still offer about a 30 percent increase in performance of standard 802.11a/b/g networks.
Dynamic Rate Shifting
Now that you have an idea of how data is encoded and modulated, things will start to get a little easier. Another important aspect to understand, not only for the exam but for actual wireless deployments, is that the farther away you get from the access point, the lower the data rates are that you can achieve. This is true regardless of the technology. Although you can achieve higher data rates with different standards, you still have this to deal with.
All Cisco wireless products can perform a function called dynamic rate shifting (DRS). In 802.11 networks, operating in the 2.4-GHz range, the devices can rate-shift from 11 Mbps to 5.5 Mbps, and further to 2 and 1 Mbps depending on the circumstances. It even happens without dropping your connection. Also, it is done on a transmission-by-transmission basis, so if you shift from 11 Mbps to 5.5 Mbps for one transmission and then move closer to the AP, it can shift back up to 11 Mbps for the next transmission.
This process also occurs with 802.11g and 802.11a. In all deployments, DRS supports multiple clients operating at multiple rates.
Sending Data Using CSMA/CA
Wireless networks have to deal with the possibility of collisions. This is because, in a wireless topology, the behavior of the AP is similar to that of a hub. Multiple client devices can send at the same time. When this happens, just like in a wired network where a hub exists, a collision can occur. The problem with wireless networks is that they cannot tell when a collision has occurred. If you are in a wired network, a jam signal is heard by listening to the wire. To listen for a jam signal, wireless devices need two antennas. They can send using one antenna while listening for a jam signal with the other. Although this sounds feasible, especially because MIMO technology defines the use of multiple antennas, the transmitting signal from one antenna would drown out the received signal on the other, so the jam signal would not be heard.
To avoid collisions on a wireless network, carrier sense multiple access collision avoidance (CSMA/CA) is used. You are probably familiar with carrier sense multiple access collision detect (CSMA/CD), which is used on wired networks. Although the two are similar, collision avoidance means that when a device wishes to send, it must listen first. If the channel is considered idle, the device sends a signal informing others that it is going to send data and that they should not send. It then listens again for a period before sending. Another way to supplement this is using request to send (RTS) and clear to send (CTS) packets. With the RTS/CTS method, the sending device uses an RTS packet, and the intended receiver uses a CTS packet. This alerts other devices that they should not send for a period.
Exam Preparation Tasks
Review All Key Concepts
Review the most important topics from this chapter, noted with the Key Topics icon in the outer margin of the page. Table 1-4 lists a reference of these key topics and the page number where you can find each one.
Table 1-4 Key Topics for Chapter 1
The electromagnetic spectrum
The usable frequency bands for WLANs in the United States, Europe, and Japan
The 2.4-GHz channels
Understanding chipping sequences
DSSS encoding methods
Phase-shift encoding and how it works
Complete the Tables and Lists from Memory
Print a copy of Appendix B, "Memory Tables," (found on the CD) or at least the section for this chapter, and complete the tables and lists from memory. Appendix C, "Memory Tables Answer Key," also on the CD, includes completed tables and lists to check your work.
Definition of Key Terms
Define the following key terms from this chapter, and check your answers in the Glossary:
FCC, IEEE, ETSI, bandwidth, Hz, ISM, UNII, channels, DSSS, OFDM, amplitude, phase, frequency, chipping code, Barker code, CCK, BPSK, QPSK, MIMO, DRS, CSMA/CA, RTS, CTS
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