CCNA Wireless Official Exam Certification Guide part 5 potx

Modulation is what wireless networks use to send data.. Wireless networks use modulation as a carrier signal, which means that the modulated tones carry data.. A modulated waveform consi

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 5-2.4-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 useOrthogonal 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 ofmodulation 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 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

Data

“1001”

Spreading Using the Chipping Code and Sending:

Chipping Code Converted Back to Data Bits:

LWAPP

“00110011011 11001100100 11001100100 00110011011”

Equals

“1001”

“00110011011 11001100100 11001100100 00110011011”

Figure 1-3 Chipping Sequence

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

Key Topic

Table 1-3 DSSS Encoding Methods

1 11 chip Barker coding DSSS Binary Phase Shift Keying

2 11 chip Barker coding DSSS Quadrature Phase Shift Keying

8 bits CCK coding

DSSS Quadrature Phase Shift Keying

4 bits CCK coding

DSSS Quadrature Phase Shift Keying

Also, because more bits are sent for chipping (carrier) than there is actual data, the chip-ping rate is higher than the data rate

Barker Code

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, 5.5-2-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

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

Topic

Period

Phase

Figure 1-4 Waveform

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

802.11 also uses quadrature phase-shift keying (QPSK), which is discussed in the follow-ing section

QPSK

In BPSK, 1 bit per symbol is encoded This is okay for lower data rates QPSK has the capa-bility to encode 2 bits per symbol This doubles the data rates available in BPSK while stay-ing 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 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 maxi-mum resistance to corruption of the data caused by interference OFDM defines a num-ber 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

0-Degree Phase Shift

180-Degree Phase Shift

Figure 1-5 Encoding with Phase Shifting

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, “Stan-dards Bodies,” covers the differences in the OFDM implementations

MIMO 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 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

Topic

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 transmistransmission-by-transmis-sion 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 mulmul-tiple 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 de-vices 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 anten-nas, 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, colli-sion 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

Table 1-4 Key Topics for Chapter 1

Table 1-2 The usable frequency bands for WLANs in the

United States, Europe, and Japan

10

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 taTa-bles 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

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

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 airwaves

Take the “Do I Know This Already?” quiz first If you do well on the quiz, you may want

to skim through this chapter and continue to the next If you score low on the quiz, you should spend some time reading through the chapter These standards are important be-cause they are something you will deal with on a day-to-day basis in wireless networking

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 2-1 details the major topics discussed in this chap-ter and their corresponding quiz questions

1. The FCC regulates wireless usage in which of the following countries?

a. United States of America

b. United Arab Emirates

c. United Kingdom

d. Europe, Asia, and Asia

Table 2-1 “Do I Know This Already?” Section-to-Question Mapping