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Related Products For this reason, channels 1, 6, and 11 are most commonly used because they do not overlap. In fact, many consumer-grade wire- less devices are hard set so you can choose only one of the three channels. Figure shows the 11 channels, including overlap. Again, notice that channels 1, 6, and 11 do not overlap. With This allows three nonoverlapping, noninterfering channels to be used in the same area. In the The 5-GHz range is also subdivided into channels, each being MHz wide.
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A total of 23 nonoverlapping channels exist in the 5-GHz range. 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.
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Wireless networks use modulation as a carrier signal, which means that the modulated tones carry 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 MHz-wide channel range of 2. 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. 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 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 , and the chipping code for the bit value of 0 is Therefore, after the data bits are sent, creates the chip sequence.
You can decode this chip sequence back to the value of 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 Barker Code To achieve rates of 1 Mbps and 2 Mbps, This code defines the use of 11 chips when encoding the data. The chip Barker code used in 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 bit Barker code and then modulated with DSSS. DSSS uses a different method for higher data rates, which allows the 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. For example, amplitude modulation, frequency modulation, and phase-shift keying are modulations.
The encoding would be that a degree phase shift is a 1, and 0-degree phase shift is a 0. This is binary phase-shift keying. In 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. Because of external factors, the amplitude of a signal is likely changed, and this in turn modifies the information you are sending. However, other factors, such as frequency and phase, are not likely to change.
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Specifically, in To begin, look at Figure , which shows a waveform. This waveform, or motion, is happening over a pe- riod of time. Figure 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.
In BPSK, two phases are used that are separated by degrees. BPSK can modulate 1 bit per symbol. To simplify this, a phase shift of degrees is a 1, and a phase shift of 0 degrees is a 0, as illustrated in Figure This is okay for lower data rates. QPSK has the capa- bility to encode 2 bits per symbol.
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This doubles the data rates available in BPSK while stay- ing within the same bandwidth. At the 5. 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. 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 The way they are implemented is a little different because Although at press time, the 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 Mbps by multiplexing data streams simultaneously in one channel. In other words, if you want data rates higher than 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. 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.
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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.