Wireless Local-Area Networks

Although wireless networking began to penetrate the market in the 1990s, the technology has actually been around since the 1800s. A musician and astronomer, Sir William Her- schel (1738 to 1822) made a discovery that infrared light existed and was beyond the visi- bility of the human eye. The discovery of infrared light led the way to the electromagnetic wave theory, which was explored in-depth by a man named James Maxwell (1831 to 1879). Much of his discoveries related to electromagnetism were based on research done by Michael Faraday (1791 to 1867) and Andre-Marie Ampere (1775 to 1836), who were researchers that came before him. Heinrich Hertz (1857 to 1894) built on the discoveries of Maxwell by proving that electromagnetic waves travel at the speed of light and that electricity can be carried on these waves.

Although these discoveries are interesting, you might be asking yourself how they relate to wireless local-area networks (WLANs). Here is the tie-in: In standard LANs, data is propagated over wires such as an Ethernet cable, in the form of electrical signals. The dis- covery that Hertz made opens the airways to transfer the same data, as electrical signals, without wires. Therefore, the simple answer to the relationship between WLANs and the other discoveries previously mentioned is that a WLAN is a LAN that does not need ca- bles to transfer data between devices, and this technology exists because of the research and discoveries that Herschel, Maxwell, Ampere, and Hertz made. This is accomplished by way of Radio Frequencies (RF).

With RF, the goal is to send as much data as far as possible and as fast as possible. The problem is the numerous influences on radio frequencies that need to be either overcome or dealt with. One of these problems is interference, which is discussed at length in Chapter 5, “Antennae Communications.” For now, just understand that the concept of wireless LANs is doable, but it is not always going to be easy. To begin to understand how to overcome the issues, and for that matter what the issues are, you need to understand how RF is used.

Wireless Networks

Many influences can act on a wireless transmission. For that reason, it is important to un- derstand what is actually involved in a wireless transmissions so you know exactly what is being affected. This section reviews what a wavelength is, how frequency it is used in wireless transmission, and what the purpose of amplitude is. In addition, it covers how Ef- fective Isotropic Radiated Power (EIRP) is calculated and what it defines.


Influences on Wireless Transmissions

Free Path Loss Model
To understand Free Path Loss, you can think of jumping smack into the middle of a pud- dle. This would cause a sort of wave effect to spread in all directions away from you. The closer to you that the wave is, the larger it is. Likewise, the farther away from you that wave travels, the smaller it gets. After a certain distance, the wave widens so much that it just disappears.

You might recall learning that an object that is in motion stays in motion until something stops it. But nothing stops the wave. It just disappears. This is where you get the term free. Take a look at Figure 3-1, and you can see that as the wave—or, in this case, the radiated wireless signal—travels away from the source, it thins out. This is represented by the bold dots becoming less and less bold.

You might also notice that the farther away the signal gets from the center, the sparser the dots are. Figure 3-1 has a single transmitting device (you could relate that to an access point) and many receiving devices. Not all the receiving stations get each one of the dots or signals that the transmitter sent. A device closer to the transmitter usually gets a more concentrated signal, and a receiver farther away might get only one dot.

Determining the range involves a determination of the energy loss and the distance. If you place receivers outside of that range, they cannot receive wireless signals from the access point and, in a nutshell, your network does not work.


Absorption
An effect of absorption is heat. When something absorbs a wave, it creates heat in what- ever absorbed the wave. This is seen in microwaves. They create waves that are absorbed by your food. The result is hot food. A problem you can encounter is that if a wave is en- tirely absorbed, it stops. While this effect reduces the distance the wave can travel, it does not change the wavelength or the frequency of the wave. These two values do not change as a wave is absorbed.

You might be asking what some possible sources of absorption are. Walls, bodies, and carpet can absorb signals. Relate it to sound. If you had really loud neighbors who were barbecuing outside your bedroom window, how could you deaden the sound? You could hang a blanket on the window or board up the window. Things that absorb sound waves also absorb data waves.

How can this affect your wireless deployment? Looking at Figure 3-2, you can see an of- fice that has just been leased and ready to move in. After a quick site survey, you deter- mine that four APs will provide plenty of coverage. This is because you cannot see absorption. Nothing causes the issue.

Now look at Figure 3-3, which shows the same office after move-in. Notice that with the furniture, cubicle walls, and other obstacles, the four APs that you originally thought would be sufficient no longer provide the proper coverage because of the signal being ab- sorbed. This is an illustration of absorption.


Reflection
Although absorption causes some problems, it is not the only obstacle that you are going to encounter that will affect your wireless deployments. Another obstacle is reflection. Reflection happens when a signal bounces off of something and travels in a different di- rection. This can be illustrated by shining a flashlight on an angle at a mirror, which causes it to reflect on an opposite wall. The same concept is true with wireless waveforms. You can see this effect in Figure 3-4, where the reflection of the signal is reflected at the same angle that it hits the mirror. You can also relate this to sources of interference in an office environment. Although offices do not usually have mirrors lying around, they do have other objects with similar reflective qualities, such as monitors and framed artwork with glass facing.

Reflection depends on the frequency. You will encounter some frequencies that are not af- fected as much as others. This is because objects that reflect some frequencies might not reflect others.


Multipath

Multipath is what happens when portions of signals are reflected and then arrive out of order at the receiver, as illustrated in Figure 3-5.
One characteristic of multipath is that a receiver might get the same signal several times over. This is dependent on the wavelength and the position of the receiver.

Another characteristic of multipath is that it can cause the signal to become out of phase. When you receive out-of-phase signals, they can cancel each other out, resulting in a null signal.


Scattering

The issue of wireless signals scattering happens when the signal is sent in many different directions. This can be caused by some object that has reflective, yet jagged edges, such as dust particles in the air and water. One way to illustrate the effects would be to consider shining a light onto a pile of broken glass. The light that is reflected shoots off in many different directions. The same is true with wireless, only the pile of glass is replaced with microparticles of dust or water.
On a large scale, imagine that it is raining. Large raindrops have reflective capabilities. When a waveform travels through those microparticles, it is reflected in many directions. This is scattering. To visualize this, notice that Figure 3-6 involves a waveform traveling between two sites on a college campus. During a heavy downpour of rain, the wireless signal would be scattered in transit from one antenna to the next.

Scattering has more of an effect on shorter wavelengths, and the effect depends on fre- quency. The result is that the signal weakens.


Refraction
Refraction is the change in direction of, or the bending of, a waveform as it passes through something that is a different density. This behavior causes some of the signal to be reflected away and part to be bent through the object. To better understand this con- cept, Figure 3-7 demonstrates the effect of refraction. A waveform is being passed through a glass of water. Notice that, because the glass is reflective, some of the light is re- flected, yet some still passes through.

The waveform that is passed through the glass is now at a different angle. Because refraction usually has the most effect on outdoor signals, dryness refracts away from the earth (as seen in dust particles), and humidity refracts toward the earth.


Line of Sight

As an object travels toward a receiver, it might have to deal with various obstructions that are directly in the path. These obstructions in the path cause many of the issues just dis- cussed—absorption, reflection, refraction, scattering. As wireless signals travel farther distances, the signal widens near the midpoint and slims down nearer to the receiver. Figure 3-8 illustrates where two directional antennas are sending a signal between the two points. The fact that it appears to be a straight shot is called visual line of sight (LOS). Although the path has no obvious obstacles, at greater distances the earth itself becomes an obstacle. This means that the curvature of the earth, as well as mountains, trees, and any other environmental obstacles, can actually interfere with the signal.
Even though you see the other endpoint as a direct line, you must remember that the sig- nal does not. The signal in fact widens, as illustrated in Figure 3-9. What was not an obvi- ous obstruction in Figure 3-8 is more clearly highlighted in Figure 3-9.

Determining Signal Strength Influences

The Fresnel Zone

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)