It is important to choose a good mounting location for the bridge because this affects the reliability of the wireless link and the maximum data rates it can support. The most important considerations are the distance between bridges and the clearance from obstacles. The mounting location can be the top or side of a building or a tower that provides a clear unobstructed line-of-sight to the remote bridges.
With the integrated antenna bridge configuration, it is necessary to physically place the bridge within the line-of-sight of the remote bridges. The location must allow the bridge to be oriented in the proper direction.
With the external antenna bridge configuration, the external antenna must be located within line-of-sight of the remote bridges. The bridge can be located in a convenient location that minimizes the cable length to the antenna.
Note: An understanding of wireless bridging techniques, antenna alignment and adjustment, and grounding methods is necessary for installation.
Note: To meet regulatory restrictions, the external antenna bridge unit and the external antenna must be professionally installed. The network administrator or other IT professional responsible for installation and configuration of the unit is a suitable professional installer. After installation, access to the unit must be password protected by the network administrator to maintain regulatory compliance.
Choosing a Mounting Location
Choosing a good mounting location for the bridge is important because it affects the reliability of the wireless link and maximum data rates it can support. The most important considerations are distance between bridges and clearance from obstacles.
Signal Path Distance
In an environment without obstacles in the signal path, the maximum distance between bridges depends primarily on the type of antennas and the free space loss between them. Make sure your proposed mounting site is within range of the remote antenna. The bridge supports 54-Mbps data rates at distances of up to 7.8 miles and 6 Mbps at distances up to 15.5 miles when using the integrated 22.5-dBi antenna on both bridges. When using a 9-dBi omni antenna at the hub and an integrated 22.5-dBi antenna remotely, the bridge supports 54-Mbps at distances up to 2 miles. Cisco.com has a range calculation tool for outdoor bridges that helps you estimate the range for your specific installation. To access the tool go this URL: http://www.cisco.com/en/US/prod/collateral/wireless/ps5678/ps458/prod_technical_reference09186a00800a912a.xls.
The integrated antenna radiates and receives polarized radio signals. Polarization helps reduce interference because the antenna tends to reject cross-polarized signals from other sources. Therefore, you can solve some interference problems by changing the antenna polarization. For the link to operate correctly, two antennas at each end of the link must always be set for the same polarization, either vertical or horizontal.
The bridge mounting hardware accommodates either vertical or horizontal antenna polarization. For more information, see the "Assembling the Mounting Hardware" section.
Signal Path Clearance
A radio beam travels from one bridge to another in a straight line. Therefore, the path between the antennas must be free of major obstacles. The effects of obstacles and terrain, both along and near the path, have a significant bearing on the propagation of radio signals and can cause both interference and signal cancellation.
When choosing a site, consider the effects of the following common obstacles:
•Trees and large plants
A tree directly in the path can totally block the signal. With clearance above the trees there are usually no secondary effects, but you should allow for future tree growth.
A large round container such as a gas storage reservoir or water tower that is partially in the path causes some blocking. These obstacles may also reflect some energy, which can interfere with other receivers. Square or rectangular objects in or near the path have rectangular surfaces that can block and diffract signals over and around them.
The earth surface also interferes with signals if the antenna is mounted to low. Mount the antenna just high enough to allow adequate clearance from the ground (see Table 1). Placing the bridge too high makes it susceptible to interference from other systems.
To determine how much clearance to leave around the signal path, use the following clearances as a guide:
Clearance Guidelines for UNII 5.7-GHz Frequencies
Total Path Length (Miles)
Clearance Radius Around Signal Path (feet)
Install the bridge or external antenna where obstacles along the propagation path, including the ground, are no closer than these values. For tower installations, you may need to climb the tower to the proposed mounting location to verify a clear path to the other site. If trees are in the line of signal propagation, leave extra clearance around them for future growth into the signal path and seasonal changes.
Release notes / product overview / data sheet / FAQ
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