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Table Of Contents  CertiGuide to A+ (A+ 4 Real)
 9  Chapter 15: Wireless LANs

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Wireless Technologies

To understand why a wireless network works, or more importantly does not, you must have a basic knowledge of Radio Frequency (RF). One of the most important obstacles you may cover when working with a wireless network is proper coverage. All access points (APs), or the transmitting end of the wireless network, come with an antenna. Many consumer APs even have the antennas hidden inside, out of site, but still working. Whenever you position your antenna, you must remember that very important to antenna placement is Line of Sight (LOS). Line of Sight means you are physically able to see one antenna from the other. This is especially important when connecting networks over long distances, since the actual bulge of the earth, in addition to trees, buildings, etc. must be taken into consideration. Now, this does not mean that in a simple, small network that an AP (“Access Point”) must be in the same room as the end user’s equipment. However, you should always keep placement in mind beit because anything impeding the LOS will influence your throughput, whether it is a desk, a wall, or a steady stream of people.

Also important to understanding and preparing a WLAN is the conversion of watts or milliwatts (mW) to decibels (dB). Anytime you are considering extending a WLAN or increasing its range, you are going to need an antenna. To know what kind of antenna, you must do the math. This is important because a mismatch of the output can crash your network, and seriously damage your equipment. The short lesson is to remember the following basic reference numbers:

  • -3 dB = half the power in mW

  • +3 dB = double the power in mW

  • -10 dB = 1/10 the power in mW

  • +10 dB = ten times the power in mW

This is essential to calculating the amount of gain or loss, since using this formula you can always figure the amount by 10 or 3. For example, you use this formula to calculate for a network with an AP at 20 dB, your external antenna is 6dB, and your cable causes loss at 3dB (see Figure 481).


Figure 481: Cable Loss

 


This is a very simple equation. You simply add it up like this:

20 – 3 + 6 = 23

You have just calculated your Equivalent Isotropic Radiated Power, or EIRP. This is crucial to setting up a WLAN with any real power, because you must adhere to your country’s rules. In the United States, the FCC sets maximum EIRP output at 4 W. Moreover, since 4 Watts = 36 dBm (decibels referenced to an isotropic radiator), 23 dBm is well within the limits.

For a better understanding of the math involved in figuring your antenna output and necessary power, I suggest you visit sites like http://www.zytrax.com/tech/wireless/calc.htm or http://www.swisswireless.org/wlan_calc_en.html.


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Chapter 15: Wireless LANs
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