To give you a little background, Augustin-Jean Fresnel was a French physicist and civil en- gineer who lived from 1788 to 1827. He correctly assumed that light moved in a wavelike motion transverse to the direction of propagation. His assumption, or claim, was correct. Because of his work, a method for determining where reflections will be in phase and out of phase between sender and receiver is based on his name. This method determines what is called the Fresnel zone.
Here is how Fresnel did it. First he divided the path into zones. The first zone should be at least 60 percent clear of obstructions. To visualize this, you can think of the shape of a football, which is wider in the middle. However, with the Fresnel zone calculation, you use an equation to determine what the size of the ball is at the middle. This helps to determine the width that a wave will be so you can make sure that no obstacles are in the path.
Figure 3-10 illustrates the height an antenna would need to be at different distances to overcome this. For example, for a 2.4-GHz system, at 7 miles you need to have the anten- nas mounted at 45 to 50 feeAlthough this is just an example, the numbers are pretty close, and at least you can get more of a visual of what you are up against in the real world. Again, do not spend too much time on this in preparation for the CCNA wireless exam, because it is not a concept you will be tested on.
Although this is just an example, the numbers are pretty close, and at least you can get more of a visual of what you are up against in the real world. Again, do not spend too much time on this in preparation for the CCNA wireless exam, because it is not a concept you will be tested on.
Received Signal Strength Indicator
The Received Signal Strength Indicator (RSSI) measurement uses vendor-specified values. Because of this, you cannot rely on it to compare different vendors. In the end, all this gives you is a grading of how much signal was received.
Keep in mind that the measurement is vendor specific, so the scale that is used might vary. For example, one vendor might use a scale of 0 to 100, whereas another might use a scale of 0 to 60. The scale is usually represented in dBm, so the two scales would not match up. It is also up to the vendor to determine what dBm is represented by 0 and what dBm is represented by 100.One tool that is used in wireless networks to give RSSI values is called Network Stumbler.
Signal-to-Noise Ratio
Signal-to-noise ratio (SNR) is the term used to describe how much stronger the signal is compared to the surrounding noise that corrupts the signal. To understand this, suppose you walk into a crowded park with many screaming kids and speak in a normal voice while on the phone. The odds are that the noise is going to be so loud that the person on the other end will not be able to distinguish your words from all the noise around you that is also being transmitted over the phone. This is how the wireless network operates. If the outside influences are causing too much noise, the receivers cannot understand the transmissions.
Link Budget
Link budget is a value that accounts for all the gains and losses between sender and re- ceiver, including attenuation, antenna gain, and other miscellaneous losses that might oc- cur. This can be useful in determining how much power is needed to transmit a signal that the receiving end can understand.
The following is a simple equation to factor link budget:
Received Power (dBm) = Transmitted Power (dBm) + Gains (dB) – Losses (dB)
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