Bridged Networks

To diagnose problems with bridges can cause issues. The following helps to address those issues.

Line-of-Sight Networks

If you employ a bridging technology to connect two sites, you are likely to use line-of-sight to forge that connection. Line-of-sight is a technology that requires the transmitter and the receiver to point at and have a clear view of each other. Although it's useful to connect two remote sites, wirelessly, several things can cause performance problems or complete outages.

First, line-of-sight antennas are difficult to align. Many bridges (such as the Cisco 1300) come with self-aligning features. For example, the Cisco 1300 uses numerous light emitting diodes (LEDs) to indicate whether the bridge associates to another bridge. If the unit associates to a root bridge, the LED turns amber. If the unit does not associate to a root bridge in the first minute, the LED blinks green.

If you notice performance problems between your remote sites, check the antenna alignment. You might run into trouble with performance, especially on windy days because antenna masts tend to sway. Although the disparity between antennas might be a few inches only, those few inches cause connection problems.


Towers cause problems. Avoid mounting your antennas on poles, which can sway dramatically.

Fresnel Zone

Although antennas that broadcast in a line-of-of sight configuration are best if they point directly at each other, it is also important to consider the Fresnel Zone. The Fresnel Zone is the area around the line-of-sight radio waves that spread out after they leave the antenna. This is illustrated in Figure 14-2. This area must be at least 60 percent clear, or else the signal weakens.

Figure 14-2. The Fresnel Zone

2.4-GHz wireless signals pass through walls, although they have trouble passing through organics, such as trees. This is because walls are dry and trees contain a lot of waterwater absorbs the 2.4-GHz radio waves.


Microwave ovens are a source of interference for WLANs because they cook food on the same 2.4-GHz band. The water in food resonates to cook the food.

In addition to the water content of materials between antennas, the density and composition of other materials between antennas can cause problems. Table 14-4 lists some materials you might encounterindoors or outdoorsand explains what makes it problematic.

Table 14-4. Effect of Various Materials on RF Signal Performance



Paper and vinyl

Negligible impact on signal propagation.

Concrete (solid)

The signal can penetrate one or two walls without serious coverage degradation. However, some walls have steel reinforcement that blocks radio signals.

Concrete (block)

The signal can penetrate three or four walls without serious coverage degradation.


Causes the signal to reflect.


The signal can penetrate five or six walls without serious coverage degradation.

Mesh fences

Fences (such as chain-link) with 1- to 1.5-inch spacing can block a 2.4-GHz signal.

Check to ensure that the Fresnel Zone is not obstructed. If it is, reposition the antenna to clear the Fresnel Zone. Also, if the antennas are more than 6 miles apart, the curvature of the Earth might interfere with the connection.

Performance problems can frustrate the network users, as well as the technology professionals who try to fix the problem. If you examine your equipment and its configuration, you probably can resolve a great deal of your performance issues.

Cisco 802.11 Wireless Networking Quick Reference
Cisco 802.11 Wireless Networking Quick Reference
ISBN: 158705227X
EAN: 2147483647
Year: 2005
Pages: 126

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