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
Chapter 1: Introduction to Wireless Networking Concepts 15 Amplitude 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
Chapter 1: Introduction to Wireless Networking Concepts 17