Wlan

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CCNA Wireless Official Exam Certification Guide

Foundation Topics

Wireless Local-Area Networks
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. Thediscovery 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 ofMaxwell 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. Thediscovery 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, andHertz 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 theconcept 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.

How Bandwidth Is Achieved from RF Signals
To send data over the airwaves, the IEEE has developed the 802.11 specification, which defines half-duplex operations using the same frequency forsend 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 beused, and the locations where certain WLAN devices can be deployed.

Chapter 1: Introduction to Wireless Networking Concepts 9 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 tosend 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 acarrier 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.

Unlicensed Frequency Bands Used in WLANs
As you place more information on a signal, you use more frequency spectrum, or...