CCNA Wireless Official Exam Certification Guide part 5 potx

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 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 02_1587202115_ch01.qxd 9/29/08 2:43 PM Page 12 Chapter 1: Introduction to Wireless Networking Concepts 13 Si 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 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 2 11 chip Barker coding DSSS Quadrature Phase Shift Keying 5.5 8 chip encoding 8 bits CCK coding DSSS Quadrature Phase Shift Keying 11 8 chip encoding 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-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 bitscanberepresentedbyacodeword,asopposedtothe1bitrepresentedbyaBarkercode. 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 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. 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 02_1587202115_ch01.qxd 9/29/08 2:43 PM Page 15 16 CCNA Wireless Official Exam Certification Guide 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 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. 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. 02_1587202115_ch01.qxd 9/29/08 2:43 PM Page 17 18 CCNA Wireless Official Exam Certification Guide Table 1-4 Key Topics for Chapter 1 Key Topic Item Description Page Number Figure 1-1 The electromagnetic spectrum 10 Table 1-2 The usable frequency bands for WLANs in the United States, Europe, and Japan 10 Figure 1-2 The 2.4-GHz channels 11 Figure 1-3 Understanding chipping sequences 13 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 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 Foundation Topics Section Questions Wireless Standards and Regulatory Committees 1–8 Wi-Fi Certification 9–10 03_1587202115_ch02.qxd 9/29/08 2:46 PM Page 21 . the subcarriers are 02_ 158 72021 15_ ch01.qxd 9/29/08 2:43 PM Page 15 16 CCNA Wireless Official Exam Certification Guide 0-Degree Phase Shift 180-Degree Phase Shift Figure 1 -5 Encoding with Phase. 11001100100 11001100100 00110011011 1001 Key Topi c 02_ 158 72021 15_ 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. 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