The main factor that needs to be considered when designing
building-to-building WLANs is radio line of sight. The remote location antenna
should be visible from the main site. There should be no obstructions between
the antennas
. Another
consideration in a building-to-building design is the Fresnel zone, which
relates to line of site. It is more of an ellipse, due to how radio waves
actually propagate. This ellipse must be clear of obstacles year round. The
first key consideration is to ensure that the antennas are mounted high enough
to provide clearance at the mid-point of the Fresnel zone
. As the
distance increases, an additional concern becomes the curvature of the earth.
This must also a consideration when determining the antenna mounting height.
When planning a wireless bridge implementation, it is important to
achieve the optimum balance among cost, reliability, distance, and data rate
. Part of the
cost could entail an antenna tower, which can escalate into a major expense. In
order to determine the antenna mounting height, take the mid-path Fresnel zone
width, at 60 percent for 2.4 GHz, and add it to the curvature of the earth. In
order to get these measurements, refer to Figure
. Links over 40
km (25 miles) in distance are very hard to install and align, so caution must
be taken when recommending these types of configurations.
Path loss refers to the loss incurred by the RF signal mainly due to
dispersion or broadening of the wave front. It is a measurement of loss in
decibel (dB) over a specific distance. The values can be derived from the
theoretical model.
The margin determines how much path interference can
be inserted while still maintaining communications. A 10-dB fade margin is
required for dependable communications in all weather conditions.
If the
bridging scenario pictured in Figure
were being
planned, would the system work as indicated? Using path loss calculations,
antenna gains, and cable lengths, the distances can be theoretically checked.
This allows changes to the design prior to installation, based on these
calculations.
Cisco offers the Cisco Aironet Outdoor Bridge Range
Calculation utility. This utility is a spreadsheet that contains the necessary
formulas to calculate how far a proposed bridge link can go. It can be
downloaded from Cisco Connection Online (CCO). Beyond using the calculations,
or the science, it is somewhat of a trial and error, or art, when implementing
a link across long distances.