This is part two of the article on Implementing Cisco Unified Wireless Networking Essentials (IUWNE).
To go to part 1, please visit: Wireless 101 Terminology (Part 1). Implementing Cisco Unified Wireless Networking Essentials (IUWNE)
In this part we will discuss: Free Path Loss,Absortption, Reflection, Multipath, and Scattering.
When an antenna radiates a signal, it travels out through the air. Radio signals do not travel in a tightly focused beam like a laser; instead the signal spreads as it travels and while the energy contained stays constant. Since it spreads thinner as it travels, the signal will lose strength relative to any receiver because the receiver’s antenna will only capture a part of the wave carrying the radio signal. Therefore, the first effect of wave propagation is an attenuation of the signal strength. When this weakening of the signal happens over free space, or an area with no obstacles, we refer to that attenuation as free path loss.
When a radio signal travels out from the access point it loses energy, not only to free path loss, but also by being absorbed by any obstacles the wave encounters. These obstacles can be walls, windows, people, or any other matter (solid, liquid or gas, all of them can absorb an RF signal). Absorption of the signal takes energy away from the wave; this energy is dissipated into the obstacles as heat. If 100% of the energy is absorbed, then the wave stops. If only a portion of the wave’s energy is absorbed, then the wave will continue with whatever energy it has left. Stated another way, absorption reduces a signal’s amplitude, meaning that while the signal keeps its original wavelength and frequency, its amplitude and therefore its strength are reduced. If enough amplitude is stripped away, then the signal will be unrecognizable and the receiver will only perceive noise. A common example of this can be seen every day in the home. Your spouse sits in another room watching TV at a modest volume while you are trying to focus on reading a book. You may still hear the show well enough to follow what’s happening even though the wall will make the volume sound less. If the wall is thick enough it may even diminish the sound to the point that you can’t understand any words, but still hear some indistinguishable sounds.
When a signal hits an obstacle, the nature of the obstacle determines what happens to the signal. Porous materials absorb signal strength. A flat surface reflects the signal. A rough surface could both reflect and absorb at the same time, meaning that part of the energy could be reflected and part of it could continue in its transmission but along a different pathway. If a signal is reflected, its initial angle, the angle of inflection, will be equal to the angle of reflection. How much of the signal is reflected and how much is absorbed is dependent on how rough the surface is relative to the wave. If all of a signal is reflected, then there is no impact on the signal’s amplitude.
Multipath is a common and destructive phenomenon in wireless networking. Radio signals reflect off of smooth surfaces like metal or glass; some of these reflections will arrive at the receiver at different times and they may be out of phase. These multiple copies of the signal combined out of phase at the receiver can be very destructive. How destructive these multiple copies of the signal are depend on factors like the wavelength, the position of the receiver, and how much out of phase the recombined signals are.
Two signals are said to be in phase when the crests of their cycles coincide. The signals are said to be out of phase if the crests of their cycles do not coincide. If the signals are between 0 and 120° out of phase, they cause up fade of the signal. If the signals are between 121 and 179° out of phase they cause down fade of the signal. Being exactly 180° out of phase will cause the signal to cancel completely. So we see that being out of phase weakens both signals or can cancel them out if the amplitude and wavelength are identical.
Dust, smoke, or micro drops of water in the air can cause parts of a signal wave to reflect. When microparticles cause the wave to deviate in multiple directions we refer to this as scattering. Scattering attenuates the received signal by deflecting it in other directions, thereby affecting signal quality. How the signal is affected depends on the nature of the surface or surfaces that cause the scattering to happen. Scattering can also be caused by broader surfaces such as leaves on a tree or water on a lake or an ocean. Scattering affects shorter wavelengths more than longer ones. Scattering can weaken a signal, or if the scattering is bad enough, block the signal completely.
To go to part 3, please visit: Wireless 101 Terminology (Part 3). Implementing Cisco Unified Wireless Networking Essentials (IUWNE)
Deniz Kaya is a senior security instructor at New Horizons Bulgaria
Deniz has planned, directed, and coordinated multiple projects simultaneously, ensuring goals and objectives were accomplished within time limitations and funding conditions. His core competency lies in areas of penetration testing, security assessments, enterprise network design, capacity planning and vendor evaluation. Equipped with Industry recognized certification under his belt, Deniz has demonstrated his determination to continuously self-improve and make his presence felt.
Deniz currently holds the prestigious industry certifications Microsoft Certified Systems Engineer (MCSE), Certified Cisco Systems Instructor (CCSI #31650), CCIE Security Written, Cisco Certified Network Professional (CCNP), Cisco Certified Security Professional (CCSP), Cisco Ironport Certified Security Instructor (ICSI), Cisco Ironport Certified Security Professional (ICSP), Certified Penetration Testing Specialist (CPTS), Microsoft Certified Trainer (MCT).
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