The Impact of Frequency and Wavelength on Offense and Defense

The frequency of a RF signal is simply how often the signal repeats or "cycles" in a given time period, often measured as one second. Frequency is inversely related to the length of the RF waveform, i.e., the distance an RF wave travels over a given time period (its wavelength). The higher the frequency of a given RF signal, the shorter the wavelength and vice versa. Figure 8-1 shows the pattern of an RF waveform as well as the concept of wavelength.

Thus, an AP emitting an RF signal at a frequency of 2.412 GHz will produce an RF wavelength that, in one second, repeats itself 2,412,000,000 times. To be able to repeat itself within the distance traveled in 1 second (electromagnetic signals are emitted at the speed of light, which is 3 x 108 meters per second), its wavelength must be relatively short. In this example, it is 12.437 cm. You can derive the wavelength for any given frequency via the following formula:

Wavelength = Speed of Light * (1/Frequency)

Thus, the wavelength of an AP configured to use channel 1 operating at 2.412 GHz = (3 x 108) * (1/(2412000000)) = 0.12437 m = 12.437 cm.

Now why is this important? Suppose you know the wavelength of a given signal (e.g., 2.4 GHz). With that information, you can design and build a cantenna (more on this later) that allows you to detect and sniff wireless traffic at ranges far in excess of the so-called 100-meter-bubble, which most people assume is the maximum coverage of a wireless access point. This means that the attacker can stay out of visual range and outside of your physical perimeter and still be able to hack away at your wireless infrastructure.

Or consider a wireless network administrator who wishes to restrict the RF from extending beyond a certain physical boundary. Apart from lowering the transmit power of the wireless device, he or she can also surround the boundary with a good-contact wire mesh that has a spacing between the mesh wire, which is less than half the wavelength of the frequency to attenuate, or weaken, the signal, the degree of which will be dependent among other things on the spacing.

Wavelength 4-►

/ \


\ V

Increasing time and increasing distance


Figure 8-1 RF waveform pattern showing wavelength and amplitude

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