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This chapter covers the following subjects:
Wireless Local-Area Networks: A brief history of wireless networking and some of the basic concepts.
How Bandwidth Is Achieved from RF Signals: The frequency spectrum used in RF transmissions.
Modulation Techniques and How They Work: How binary data is represented and transmitted using RF technology.
Perhaps this is the first time you have ever delved into the world of wireless networking. Or maybe you have been in networking for some time and are now beginning to see the vast possibilities that come with wireless networking. Either way, this chapter can help you understand topics that are not only tested on the CCNA Wireless exam but provide a good foundation for the chapters to come. If you are comfortable with the available frequency bands, the modulation techniques used in wireless LANs, and some of the standards and regulatory bodies that exist for wireless networking, you may want to skip to Chapter 2, "Standards Bodies."
This chapter provides a brief history of wireless networks and explores the basics of radio technology, the modulation techniques used, and some of the issues seen in wireless LANs.
You should do the "Do I Know This Already?" quiz first. If you score 80 percent or higher, you might want to skip to the section "Exam Preparation Tasks." If you score below 80 percent, you should spend the time reviewing the entire chapter. Refer to Appendix A, "Answers to the 'Do I Know This Already?' Quizzes" to confirm your answers.
The "Do I Know This Already?" quiz helps you determine your level of knowledge of this chapter's topics before you begin. Table 1-1 details the major topics discussed in this chapter and their corresponding quiz questions.
Foundation Topics Section
Wireless Local-Area Networks
How Bandwidth Is Achieved from RF Signals
Modulation Techniques and How They Work
Which of the following accurately describes the goal of RF technology?
To send as much data as far as possible and as fast as possible
To send secure data to remote terminals
To send small amounts of data periodically
To send data and voice short distances using encryption
Which of the following is a significant problem experienced with wireless networks?
Which two of the following are unlicensed frequency bands used in the United Stated? (Choose two.)
Each 2.4-GHz channel is how many megahertz wide?
How many nonoverlapping channels exist in the 2.4-GHz ISM range?
The 5.0-GHz range is used by which two of the following 802.11 standards? (Choose two.)
Which three of the following modulation techniques do WLANs today use? (Choose three.)
DSSS uses a chipping code to encode redundant data into the modulated signal. Which two of the following are examples of chipping codes that DSSS uses? (Choose two.)
Complementary code keying (CCK)
Cypher block chaining (CBC)
DSSS binary phase-shift keying uses what method of encoding at the 1-Mbps data rate?
11-chip Barker code
8-chip Barker code
With DRS, when a laptop operating at 11 Mbps moves farther away from an access point, what happens?
The laptop roams to another AP.
The laptop loses its connection.
The rate shifts dynamically to 5.5 Mbps.
The rate increases, providing more throughput.
Although wireless networking began to penetrate the market in the 1990s, the technology has actually been around since the 1800s. A musician and astronomer, Sir William Herschel (1738 to 1822) made a discovery that infrared light existed and was beyond the visibility of the human eye. The discovery of infrared light led the way to the electromagnetic wave theory, which was explored in-depth by a man named James Maxwell (1831 to 1879). Much of his discoveries related to electromagnetism were based on research done by Michael Faraday (1791 to 1867) and Andre-Marie Ampere (1775 to 1836), who were researchers that came before him. Heinrich Hertz (1857 to 1894) built on the discoveries of Maxwell by proving that electromagnetic waves travel at the speed of light and that electricity can be carried on these waves.
Although these discoveries are interesting, you might be asking yourself how they relate to wireless local-area networks (WLANs). Here is the tie-in: In standard LANs, data is propagated over wires such as an Ethernet cable, in the form of electrical signals. The discovery that Hertz made opens the airways to transfer the same data, as electrical signals, without wires. Therefore, the simple answer to the relationship between WLANs and the other discoveries previously mentioned is that a WLAN is a LAN that does not need cables to transfer data between devices, and this technology exists because of the research and discoveries that Herschel, Maxwell, Ampere, and Hertz made. This is accomplished by way of Radio Frequencies (RF).
With RF, the goal is to send as much data as far as possible and as fast as possible. The problem is the numerous influences on radio frequencies that need to be either overcome or dealt with. One of these problems is interference, which is discussed at length in Chapter 5, "Antennae Communications." For now, just understand that the concept of wireless LANs is doable, but it is not always going to be easy. To begin to understand how to overcome the issues, and for that matter what the issues are, you need to understand how RF is used.
To send data over the airwaves, the IEEE has developed the 802.11 specification, which defines half-duplex operations using the same frequency for send and receive operations on a WLAN. No licensing is required to use the 802.11 standards; however, you must follow the rules that the FCC has set forth. The IEEE defines standards that help to operate within the FCC rules. The FCC governs not only the frequencies that can be used without licenses but the power levels at which WLAN devices can operate, the transmission technologies that can be used, and the locations where certain WLAN devices can be deployed.
Note - The FCC is the regulatory body that exists in the United States. The European Telecommunications Standards Institute (ETSI) is the European equivalent to the FCC. Other countries have different regulatory bodies.
To achieve bandwidth from RF signals, you need to send data as electrical signals using some type of emission method. One such emission method is known as Spread Spectrum. In 1986, the FCC agreed to allow the use of spread spectrum in the commercial market using what is known as the industry, scientific, and medical (ISM) frequency bands. To place data on the RF signals, you use a modulation technique. Modulation is the addition of data to a carrier signal. You are probably familiar with this already. To send music, news, or speech over the airwaves, you use frequency modulation (FM) or amplitude modulation (AM). The last time you were sitting in traffic listening to the radio, you were using this technology.