12 CCNA Wireless Official Exam Certification Guide
5 GHz
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 mean-
ing 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:
Amplitude: The volume of the signal
Phase: The timing of the signal between peaks
Frequency: The pitch of the signal
Wireless networks use a few different modulation techniques, including these:
DSSS
OFDM
Multiple-Input Multiple-Output (MIMO)
The sections that follow cover these modulation techniques in further detail.
DSSS

understand how data is encoded in a wireless network and then modulated, you must first
understand chipping codes.
Chipping Codes
Because of the possible noise interference with a wireless transmission, DSSS uses a se-
quence of chips. When DSSS spreads information across a frequency range, it sends a sin-
gle 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 se-
quence will be lost or inverted. This means that a 1 could become a 0 and a 0 could be-
come 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.
[[]]00110011011 11001100100 11001100100 00110011011
1001
Key
Topi
c
02_1587202115_ch01.qxd 9/29/08 2:43 PM Page 13
14 CCNA Wireless Official Exam Certification Guide
Table 1-3 DSSS Encoding Methods
Data Rate Encoding Modulation
1 11 chip Barker coding DSSS Binary Phase Shift Keying

802.11b can modulate and encode the data using the methods seen in Table 1-3.
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.
Key
Topi
c
02_1587202115_ch01.qxd 9/29/08 2:43 PM Page 14
Chapter 1: Introduction to Wireless Networking Concepts 15
Amplitude
Period
Phase
Figure 1-4 Wa v e fo r m
BPSK
Remember that phase is timing between peaks in the signal. Actually, that needs to be ex-
panded 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 pe-
riod of time.
Figure 1-4 illustrates the next step in determining phase. The phase is the difference be-
tween 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.

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 sig-
nals. 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 ad-
vanced signal processing, the data can be recovered after being sent on two or more spa-
tial 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.
Key
Topi
c
02_1587202115_ch01.qxd 9/29/08 2:43 PM Page 16
Chapter 1: Introduction to Wireless Networking Concepts 17
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 ac-
tual 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. Al-
though 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 hap-
pens without dropping your connection. Also, it is done on a transmission-by-transmis-
sion 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 mul-
tiple clients operating at multiple rates.

Table 1-3 DSSS encoding methods 14
Figure 1-5 Phase-shift encoding and how it works 16
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 num-
ber where you can find each one.
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 Ta-
bles 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
02_1587202115_ch01.qxd 9/29/08 2:43 PM Page 18
02_1587202115_ch01.qxd 9/29/08 2:43 PM Page 19
This chapter covers the following subjects:
Wireless Standards and Regulatory Commit-
tees: Looks at the wireless regulatory committess
and some of their requirements.
Wi-Fi Certification: Discusses how Wi-Fi devices
are certified for interoperability.
03_1587202115_ch02.qxd 9/29/08 2:46 PM Page 20
CHAPTER 2
Standards Bodies
It took a long time for wireless to come together as we know it today. If it weren’t for the
standards bodies and committees, there’s no telling where the technology would be. In
this chapter, you will look at the standards bodies as well as the bodies that regulate the