Environmental Issues; Fresnel Zone - Cisco BR1400 Deployment Manual

perhaps less costly approach might be to find a location that both sites can see, for example, some
communities use their water tower or a nearby mountaintop for this purpose. If you have access to
such a location, you could install another Bridge or Bridge pair using that site as a relay point.
Once you have determined that Line of Sight is possible, you are now ready to determine if the
bridge link is possible given data rate and coverage distance required.
Tip: Topographical maps can be found at airports, flight schools and libraries.

4.2 Environmental Issues

Now that we have discussed how free space path loss (FSP) and line of sight (LOS) can affect the
distance of a bridge link lets examine a few other variables that can degrade a bridge link.
Weather – Rain, Snow, Fog and other weather climates with high humidity can slightly obstruct or
affect our line of sight introducing a small loss (sometimes referred to as rain fade or simply fade
margin) but generally this has little effect on our bridge link. If you have established a good stable
bridge link this is not a problem, however; if the link was poor to begin with then bad weather
could degrade performance or cause loss of link.
Tip: Cisco Outdoor Bridge Range Calculation Utility allows you to choose your Climate and
Terrain allowing the program to compensate for any degradation in weather.

4.3 Fresnel Zone

An imaginary ellipse around the visual line of sight between the transmitter and receiver. As radio
signals travel through free space to their intended target, if the radio waves encounter an obstruction
in the Fresnel area the signal can be attenuated (degraded) sometimes severely. Best performance
and range is attained when there is no obstruction of this Fresnel area. While this is not always
unavoidable, generally links can still be made so long as 60-70 percent of the Fresnel area is
unobstructed.
Fresnel Zone is an ellipse (desired clearance) surrounding the radio path
Figure 19.
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