Weather; Fresnel Zone - Cisco Mesh Access Points Deployment Manual

Site Preparation and Planning
This high-gain antenna configuration is recommended only for connecting a RAP to the MAP. To optimize
mesh behavior, omnidirectional 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 the 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, the 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 a line of sight; a white-out snow storm does not allow a line of sight. Also, while storms
may make the rain or snow itself appear to be the problem, many times it 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 there might be 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 are 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 percent to 70 percent of the Fresnel area is unobstructed, as illustrated in
24: Point-to-Point Link Fresnel Zone, on page
Figure 24: Point-to-Point Link Fresnel Zone
OL-27593-01
63.
Cisco Mesh Access Points, Design and Deployment Guide, Release 7.3
Weather
Figure
63