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Site Preparation and Planning
Determining Line of Sight
When determining the suitability of a successful link, you need to define how far the link is expected to
transmit and at what radio data rate. Very close links, one kilometer or less, are fairly easy to achieve
assuming there is clear line of sight (LOS)–a path with no obstructions.
Since mesh radio waves have very high frequency in the 5 GHz band, the radio wavelength is small;
therefore, the radio waves do not travel as far as radio waves on lower frequencies, given the same
amount of power. This higher frequency range makes the mesh ideal for unlicensed use because the radio
waves do not travel far unless a high-gain antenna is used to tightly focus the radio waves in a given
direction.
This high gain antenna configuration is recommended only for connecting RAPs to the MAPs. To
optimize mesh behavior omni-directional antennas are used, because mesh links are limited to one mile
(1.6 km). The curvature of the earth does not impact line-of-sight calculations because the curvature of
the earth changes every six miles (9.6 km).
Weather
In addition to free space path loss and line of sight, weather can also degrade a mesh link. Rain, snow,
fog, and any high humidity condition can slightly obstruct or affect line of sight, introducing a small loss
(sometimes referred to as rain fade or fade margin), which has little effect on the mesh link. If you have
established a stable mesh link, weather should not be a problem; however, if the link is poor to begin
with, bad weather can degrade performance or cause loss of link.
Ideally you need line of sight, a white-out snow storm does not allow line of sight. Also while storms
may make the rain or snow itself appear to be the problem, many times it is might be additional
conditions caused by the adverse weather. For example, perhaps the antenna is on a mast pipe and the
storm is blowing the mast pipe or antenna structure and that movement is causing the link to come and
go, or you might have a large build-up of ice or snow on the antenna.
Fresnel Zone
A Fresnel zone is 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, they could encounter an
obstruction in the Fresnel area, degrading the signal. Best performance and range is attained when there
is no obstruction of this Fresnel area. Fresnel zone, free space loss, antenna gain, cable loss, data rate,
link distance, transmitter power, receiver sensitivity, and other variables play a role in determining how
far your mesh link goes. Links can still occur as long as 60–70 percent of the Fresnel area is
unobstructed, as illustrated in
Figure 22
Cisco Aironet 1520, 1130, 1240 Series Wireless Mesh Access Points, Design and Deployment Guide, Release 6.0
40
Figure
illustrates an obstructed Fresnel zone.
21.
OL-20213-01
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