If we don’t know how RF works, we can’t make RF work the way we want it to.

A wavelength is the distance between two successive peaks. In simpler words, a wavelength is the distance that a single cycle of an RF signal travels.  It is important to understand that there is an inverse relationship between wavelength and frequency (λ = c/f). The formula demonstrates that the higher the frequency of an RF signal, the smaller the wavelength of that signal.

Higher frequency waves have shorter wavelengths and lower frequency waves have longer wavelengths. This is important for a wireless engineer to know for two reasons. First, the coverage distance is dependent on the attenuation through the air. Second, the higher the frequency the less the signal will penetrate obstacles such as walls, windows, and doors. 

Frequency is the number of times a specified event occurs within a specified time interval. It can also be defined as “how often an RF wave cycles per second,” as pictured in Figure 1. So, when we are talking about 2.4GHz WLAN radio cards, the RF signal is oscillating 2.4 billion times per second. Although wavelength and frequency do not cause attenuation, the perception is that higher frequency signals with smaller wavelengths attenuate faster than signals with a larger wavelength.

Figure 1

frequency

Access points using 5GHz radio cards will have shorter range and coverage area than access points operating at 2.4GHz. This is important for a wireless engineer to know because more 5GHz access points may have to be installed to provide the same coverage that can be achieved by a lesser number of 2.4Ghz access points.

Amplitude is often referenced as how loud or strong the signal is. In figure 2, we can see that the wavelength (λ) and amplitude of the RF signal. When an access point transmit at 100 mW, that is referred as transmit amplitude. When a radio receives an RF signal, the received signal strength is most often referred to as received amplitude.

Figure 2

Amplitude

Phase is another important property of an RF signal. When two waves are in phase, they strengthen the signal received. When two waves are 180 degrees out of phase, they can completely cancel the signal. In figure 3, we can see two waves in phase.

Figure 3

Phases

RF Behaviors

Absorption is the most common RF behavior. If a RF signal that does not pass through an object, then 100% absorption has occurred.

Reflection is when a wave hits a smooth object that is larger than the wave itself. In an enterprise environment, waves can reflect out of smooth surfaces such as doors, walls, and file cabinets. Anything made of metal, glass, and concrete may cause reflection as well.

Scattering is often referred as multiple reflections. This behavior occurs when an RF signal encounters some type of uneven surface and is reflected into multiple directions.

Refraction is when a RF signal bends as it passes through a medium, thus causing the direction of the wave to change. RF refraction most commonly occurs because of atmospheric conditions.

Diffraction is the bending of an RF signal around an object. Typically, diffraction is caused by partial blockage of the RF signal. These blockage areas can become dead zones or blind spots.

The five phenomena (Reflection, Refraction, Diffraction, Scattering, and Absorption) cause multiple copies of the signal to arrive at the receiver at slightly at different times. This is important to consider and it is one thing that we take advantage of in our modern WLAN with 802.11n and 802.11ac using what is called MIMO (Multiple-Input Multiple-Output). We’re taking advantage of these phase variations and using them to transmit multiple space streams.

When troubleshooting wired networks, engineers usually start at the layer 1, the Physical layer. The same best practice applies to WLAN troubleshooting. Learning the RF fundamentals is an essential step to properly troubleshoot, design, and manage wireless networks. 

Note: The terms definitions were obtained from the CWNA official study guide book.

 

 

Leave a Reply

Fill in your details below or click an icon to log in:

WordPress.com Logo

You are commenting using your WordPress.com account. Log Out /  Change )

Twitter picture

You are commenting using your Twitter account. Log Out /  Change )

Facebook photo

You are commenting using your Facebook account. Log Out /  Change )

Connecting to %s