Attenuation is the reduction in the amplitude (or strength) of a signal and is caused by many factors: obstacles in the path of the RF wave, the natural resistance of the atmosphere through which the RF wave travels, link-joint imperfections, and so on. Even if an obstacle does not block a signal completely, a signal may get attenuated by means of reflection, refraction, or absorption when it meets the obstacle. If the signal is reflected, it bounces off at an angle equal to the angle of incidence. If the signal is refracted, it passes through the obstacle but its path is altered. If the signal is absorbed, it is dissipated as thermal energy within the obstacle. In many cases, all three effects may occur to the same stream of RF energy hitting an obstacle. Figure 8-2 shows the differences between reflection, refraction, and absorption. Also, as a signal goes through the air medium, it suffers free-space loss, which is the natural attenuation that the air as a transmission medium imposes on any RF signal traveling through it.
There is a form of attenuation called multipath fading. When an RF wave takes multiple routes or paths to arrive at a receiver, it often arrives out-of-phase (out of sync). This means the given signal arrives at the receiver at different times and causes the signal to weaken when received by the receiver. Multipath fading is especially common in heavily built-up urban environments like office floors that have corridors and cubicles.
When all the various types of attenuation are combined, they form an aggregate attenuation value called path loss. This is the total amount of signal degradation, measured in decibels (dB), that is imposed on an RF signal by its transmission medium and the
Figure 8-2 Reflection, refraction, and absorption environment it goes through to get to the receiver, even including the link-joint loss between the receiver's antenna and the receiver's signal-processing hardware.
From a hacker's perspective, the attacker would try to reduce the amount of attenuation caused by the various factors in an attempt to get better RF signal reception from the target. The attacker is also helped by the fact that RF diffracts. Diffraction is the ability of an RF signal to bend around obstacles in its path to the receiver. Whether a given signal is able to do this depends on its wavelength vis-à-vis the size or diameter of the obstacle encountered. The longer the wavelength relative to the diameter of the obstacle, the easier it is to propagate around the obstacle. This is why you can often receive an AP's signal from behind a sign-post or small tree that is positioned between the signal receiver and the AP. Understanding the causes of attenuation allows the attacker to reposition himself or herself accordingly—to move closer to a signal source or reposition in such a manner that he or she has line-of-sight to the signal source so that the only attenuation encountered is free-space loss.
As you can see, many factors complicate whether an attacker can get a signal strong enough to extract the encoded data successfully. Ultimately, at the point of reception, an attacker is looking for a strong signal-to-noise ratio (SNR). This is a measurement of how good the quality of any received signal is at the point of reception, given the environmental noise and attenuation it suffers. Attackers would also try to augment their reception and transmission capabilities by employing hardware aids, such as external antennas.
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