Connect802 is a nationwide wireless data equipment reseller providing system design consulting, equipment configuration, and installation services.


CSS Mega Menu Css3Menu.com

 

 

July 1, 2005

Essential Wi-Fi:
For those who are new to Wi-Fi networking...
 
Technology and Engineering:
For the engineer and Wi-Fi network administrator...
 
To Infinity... and Beyond!
News from the wireless marketplace...
 

Essential Wi-Fi

Horizontal and Vertical Beamwidth
The signal is strongest in a plane perpendicular to the axis of a dipole antenna. Above, the green volume represents dense (strong) RF signal energy.

 

One of the keys to designing and troubleshooting an 802.11 network is understanding how RF signals propagate and how they can change as they do. This month, we'll look at various ways in which an RF signal can be transformed as it travels through the air, and what this means for network engineers.

Attenuation is the most basic way in which an 802.11 signal changes as it is transmitted. As the signal travels away from the antenna, its wavefront expands and its energy is spread out over a larger and larger area. As a result, the amount of energy in any given area decreases. As an analogy for this, think of a balloon being blown up. The surface of the balloon represents the RF wavefront. As the balloon expands, it gets thinner. This is analogous to the RF energy being weaker the further you get from the antenna--it's not that there's really less energy, but that the energy that's there is spread "thinner". It's important to keep in mind that this type of attenuation is inherent to signal transmission and would occur even if the signal were transmitted in deep space, where nothing else could interact with it. This type of attenuation is referred to as Free Space Path Loss, or FSPL.

Absorption

Absorption refers to attenuation of the signal as it passes through matter. Some amount of absorption happens as the signal travels through air molecules, but the most significant RF absorption occurs when the signal passes through metal objects or objects containing lots of water (water is highly absorptive to 2.4 GHz RF energy). Examples of metal objects include sheet metal walls, metal stairwells, metal studs in walls, and the metal reinforcement in concrete rebar construction. Examples of water-containing objects include aquariums (obviously), deciduous trees and other broad-leafed plants, paper (when stacked such as in a paper mill or in filing cabinets), and last but not lest, people. Absorption is usually a function of the type of object and the thickness of the object. The amount of absorption for an object can be measured by placing an access point near the absorbing object and measuring signal strength on both sides. Once you know the absorption of common objects, you can more accurately calculate your system operating margin.

Obstructions will absorb some of the RF signal energy and reduce the strength of the propagation wave

 

Reflection

Reflection refers to a signal bouncing off of an object. We're all familiar with reflection of light from looking in a mirror and anyone who has tried to make a bank shot in a pool game has also dealt with reflection. RF signals experience reflection too, although it's not always obvious what types of objects will absorb the signal and what types of objects will reflect it. In general, large, flat metal objects, such as sheet metal sides of hangars, cause reflection.
Metallic objects, such as tinted glass on skyscrapers, can also cause reflection, and the flat surfaces of still lakes are also well-known as causes of reflection. Reflection is not inherently bad for the signal, and in some cases, it can create coverage in areas that wouldn't otherwise have it (by bouncing the signal around an obstruction), but if reflection creates multipath cancellation, it will do more harm than good. In general, RF network designers should attempt to avoid reflecting objects if at all possible, if only because of their unpredictable effect. In point-to-point links, reflection can be minimized by using high-gain antennas that focus their energy tightly; in indoor environments with omni-directional antennas, reflection really can't be avoided, but it tends to help more than it hurts by creating coverage in areas that don't have line-of-sight to the access point.
The direct path and the reflected path are actually different lengths and this is the basis of "Multipath Fading" - the two signals can cancel each other when they arrive at the receiver.

Attenuation, absorption, and reflection are the most easily-understood types of RF signal distortion. Next month, we'll discuss less intuitive ones: diffraction, refraction, and scattering.


To Infinity... and Beyond!