Carrier Waves – WLAN FoundationsBy CWNP On 08/16/2012 - 6 Comments
While the exact phrase carrier wave may not be referenced on the CWNP exams and training materials (though carrier signal is), it is an important concept in wireless communications. Wireless engineers and technicians must deal with many different wireless technologies. In the 802.11 standard (as amended) alone, you are dealing with multiple modulation techniques, which are methods used to impose information onto carrier waves to create a carrier signal. Therefore, a brief summary of carrier waves and why they are important is in order.
In Tom Standage’s exceptional book, The Victorian Internet, he documents the many signaling methods we humans have used throughout the recent centuries. The book documents how Claude Chappe and his brother communicated over great distances using time-bound audio signals. The signal was unary in nature in that there was only one signal – clanking a pot. However, the brothers had synchronized their clocks so that a clank was linked to a second on the clock and each number was linked to a letter to that a message could be sent. If the transmitting brother clanged the pot when the second hand was pointing to 12, the listening (receiving) brother new to translate the number 12 into the appropriate message.
As you can imagine, this system would not allow for rapid communications, but it did allow for communications over a short distance. Eventually, the brothers realized that sound waves were not good carriers of signals (since they attenuate so quickly and they take so long to arrive at the destination) so they developed a new system based on visual cues (light waves). Using a simple black and white two-sided panel (black on one side and white on the other) and a telescope, the brothers successfully communicated over a distance of about 10 miles.
What did both of these communications devices have in common? They both used waves to carry a signal. The first used sound waves and the second used light waves. Since light waves travel much faster than sound waves, the latter device worked much better and over greater distances.
However, a dilemma remained. Both of these early devices required a human interpreter on the other end at all times. The instrument of the human ear and the instrument of the human eye were used to interpret the data that was carried on the sound and light waves respectively. In order to send information without a human interpreter, scientists and engineers had to develop concepts and tools related to electricity.
Today’s carrier waves are almost always electromagnetic waves. Mechanical devices can be formed that transmit the waves and also receive the waves (called transmitters and receivers or combined as transceivers). This means that data can be sent and received by modulating the data onto the carrier waves by manipulating the waveform in some way. For example, the frequency can be modified to represent a binary 1 or a binary 0. The wave is generated, but it is manipulated in such a way so that it carries binary data and this makes it a carrier signal.
Frames are food, Tom Tagged with: WLAN, 802.11, Modulation, tom carpenter, carrier wave, carrier signal