To properly design, deploy, and administer an 802.11 wireless network, in addition to understanding the OSI model and basic networking concepts, you must broaden your understanding of many other networking technologies.
For instance, when administering an Ethernet network, you typically need a comprehension of TCP/IP, bridging, switching, and routing. The skills to manage an Ethernet network will also aid you as a Wi-Fi administer because most 802.11 wireless networks act as “portals” into wired networks.
The IEEE only defines the 802.11 technologies at the Physical layer and the MAC sublayer of the Data-Link layer. In order to fully understand the 802.11 technology, it is necessary to have a clear concept of how wireless works at the first layer of the OSI model, and at the heart of the Physical layer is radio frequency (RF) communications.
In a wired LAN, the signal is confined neatly inside the wire and the resulting behaviors are anticipated. However, just the opposite is true for a wireless LAN. Although the laws of physics apply, RF signals move through the air in a sometimes very unpredictable manner.
Since RF signals are not saddled inside an Ethernet wire, you should always try to envision a wireless LAN as an “ever changing” network. Does this mean that you must be an RF engineer from Georgia Tech to perform an 802.11 site survey or monitor a Wi-Fi network?
Of course not, but if you have a good grasp of the RF characteristics and behaviors that we will define here, your skills as a wireless network administrator will be ahead of the curve.
Why does a wireless network perform differently in an auditorium full of people than it does inside an empty auditorium? Why does the performance of a wireless LAN seem to degrade in a storage area with metal racks?
Why does the range of a 5 GHz radio transmitter seem shorter than the range of a 2.4 GHz radio card? These are the type of questions that can be answered with some basic knowledge of how RF signals work and perform.
What Is an Radio Frequency Signal?
An RF signal starts out as an electrical alternating current (AC) signal that is originally generated by a transmitter. This AC signal is sent through a copper conductor (typically a coaxial cable) and radiated out of an antenna element in the form of an electromagnetic wireless signal.
Changes of electron flow in an antenna, otherwise known as current, produce changes in the electromagnetic fields around the antenna. An alternating current is an electrical current with a magnitude and direction that varies cyclically, as opposed to direct current, the direction of which stays in a constant form.
The shape and form of the AC signal—defined as the waveform —is what is known as a sine wave, as shown in Figure below. Sine wave patterns can also be seen in light, sound, and the ocean.
An RF signal radiates in a continuous pattern that is governed by certain properties such as wavelength, frequency, amplitude, phase, and polarity. Additionally, electromagnetic signals can travel through mediums of different materials or travel in a perfect vacuum.
When an RF signal travels through a vacuum, it moves at the speed of light, which is approximately 300,000,000 meters per second, or 186,000 miles per second. RF signals travel using a variety or combination of movement behaviors. These movement behaviors are referred to as propagation behaviors.
We will discuss some of these propagation behaviors, including absorption, reflection, scattering, refraction, diffraction, amplification, and attenuation.
Radio Frequency Characteristics
In every RF signal exists characteristic that are defined by the laws of physics:
We will look at each of these in more detail in the following sections.
When the movement of the electron flow changes direction in an antenna, electromagnetic waves that change and move away from the antenna are also produced. The waves consist of two component fields: the electrical (E-field) and the H-field, which is magnetic.
Think of a wave as a physical disturbance that transfers energy back and forth between these two fields. These fields are at right angles to each other, and the transfer of energy between these fields is known as oscillation.
Polarization is the vertical or horizontal positioning of an antenna. The orientation of the antenna affects the polarity of the signal. The electric field always resides parallel in the same orientation (plane) of the antenna element.
As shown in Figure 1, the parallel plane is called the E-plane and the plane that is perpendicular to the antenna element is known as the H-plane.
Wave polarity is defined as the position and direction of the electric field (E-field), as referenced to the surface of the earth. If an antenna element is positioned vertically, then the E-field is also vertical. Vertical polarization is when the E-field is perpendicular to the earth.
If an antenna element is positioned horizontally, then the electric field is also horizontal. Horizontal polarization is when the E-field is parallel to the earth. Antennas will often have polarity markings indicating which direction is vertical or horizontal.
As stated earlier, an RF signal is an alternating current (AC) that continuously changes between a positive and negative voltage. An oscillation, or cycle, of this alternating current is defined as a single change from up to down to up, or as a change from positive to negative to positive.
A wavelength is the distance between the two successive crests (peaks) or two successive troughs (valleys) of a wave pattern. In simpler words, a wavelength is the distance that a single cycle of an RF signal actually travels.
It is very important to understand the following statement: The higher the frequency, the less distance the propagated wave will travel. AM radio stations operate at much lower frequencies than wireless LAN radios.
For instance, WSB-AM in Atlanta broadcasts at 750 KHz and has a wavelength of 1,312 feet, or 400 meters. That is quite a distance for one single cycle of an RF signal to travel.
In contrast, some radio navigation satellites operate at a very high frequency, near 252 GHz, and a single cycle of the satellite’s signal has a wavelength of less than .05 inches, or 1.2 millimeters. Figure 2.3 displays a comparison of these two extremely different types of RF signals.
The majority of wireless LAN (WLAN) radio cards operate in either the 2.4 GHz frequency range or the 5 GHz range. In Figure below, you see a comparison of a single cycle of the two different frequency WLAN radio cards.
As you can see by these illustrations, the wavelengths of the different frequency signals are different because, although each signal only cycles one time, the waves travel dissimilar distances. In Figure 4, you see the formulas for calculating wavelength distance in either inches or centimeters.
Inches: wavelength = 11.811/frequency (GHz)
Centimeters: wavelength = 30/frequency (GHz)
As previously mentioned, an RF signal cycles in an alternating current in the form of an electromagnetic wave. You also know that the distance traveled in one signal cycle is the wavelength. But what about how often an RF signal cycles?
Frequency is the number of times a specified event occurs within a specified time interval. A standard measurement of frequency is hertz (Hz) , which was named after the German physicist Heinrich Rudolf Hertz. An event that occurs once in 1 second is equal to 1 Hz.
An event that occurs 325 times in 1 second is measured as 325 Hz. The frequency at which electromagnetic waves cycle is also measured in hertz. Thus, the number of times an RF signal cycles in 1 second is the frequency of that signal.
Different metric prefixes can be applied to the hertz (Hz) measurement of radio frequencies:
1 hertz (Hz) = 1 cycle per second
1 kilohertz (KHz) = 1,000 cycles per second
1 megahertz (MHz) = 1,000,000 (million) cycles per second
1 gigahertz (GHz) = 1,000,000,000 (billion) cycles per second
So when we are talking about 2.4 GHz WLAN radio cards, the RF signal is oscillating 2.4 billion times per second!
Another very important property of an RF signal is the amplitude, which simply can be characterized as the signal’s strength or power. Amplitude can be defined as the maximum displacement of a continuous wave. With RF signals, the amplitude corresponds to the electrical field of the wave.
When you look at an RF signal in an oscilloscope, the amplitude is represented by the positive crests and negative troughs of the sine wave. In Figure 5, you can see that ( λ ) represents wavelength and (y) represents the amplitude.
The first signal’s crests and troughs have more magnitude, thus it has more amplitude. The second signal’s crests and troughs have decreased, and therefore the signal has less amplitude. Note that although the signal strength (amplitude) is different, the frequency of the signal remains constant.
A variety of factors can cause an RF signal to lose amplitude, otherwise known as attenuation. Different types of RF technologies require varying degrees of transmit power. AM radio stations may transmit narrow band signals with as much power as 50,000 watts.
The radio cards in most indoor 802.11 access points have a transmit power range between 1 milliwatt (mW) and 100 mW. You will learn later that Wi-Fi radio cards can actually receive signals with amplitudes as low as billionths of a milliwatt.
Phase is not a property of just one RF signal but instead involves the relationship between two or more signals that share the same frequency. The phase involves the relationship between the position of the amplitude crests and troughs of two waveforms.
Phase can be measured in distance, time, or degrees. If the peaks of two signals with the same frequency are in exact alignment at the same time, they are said to be in phase. Conversely, if the peaks of two signals with the same frequency are not in exact alignment at the same time, they are said to be out of phase .
Figure 6 illustrates this concept.
What is important to understand is the effect that phase has on amplitude when radio cards receive multiple signals. Signals that have 0 (zero) degrees phase separation (in phase) actually combine their amplitude, which results in a received signal of much greater signal strength, or twice the amplitude.
If two RF signals are 180 degrees out of phase (the peak of one signal is in exact alignment with the trough of the second signal), they cancel each other out and the effective received signal strength is null. Depending on the amount of phase separation of two signals, the received signal strength may be either cumulative or diminished.