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introduction

This document covers some of the most important problems you may encounter when establishing a radio link between elements of a wireless LAN. Problems with radio communication between Cisco Aironet WLAN components can be traced back to four causes:

  1. Firmware and driver problems

  2. Software configuration problems

  3. Radio interference, including antenna and cable problems

  4. Client problems

requirements

conditions

There are no special requirements for this document.

Components used

This document is not limited to specific software and hardware versions.

Conventions

For more information about document conventions, see the Cisco Technical Tips Conventions.

Firmware and driver problems

Occasionally, a problem with the radio signal can be traced back to a problem in the firmware on the communicating devices.

If you have a radio communication problem with your wireless network, make sure that each component is running the latest version of their firmware or driver. Use the latest version of the driver or firmware for your wireless products. You can download updated drivers and firmware from Cisco Downloads (registered customers only).

Instructions for updating the firmware can be found at:

Software configuration problems

In the case of radio communication problems, the configuration of the WLAN devices can be the cause of the radio failure. You must configure certain parameters correctly for the devices to communicate successfully. If you configure the parameters incorrectly, the resulting problem with the wireless module appears to be a problem. These parameters include the service set identifier, the frequency, the data rate and the distance.

Service set identifier

Cisco Aironet WLAN devices must be set to the same Service Set Identifier (SSID) as all other Cisco Aironet devices in the wireless infrastructure. Units with different SSIDs cannot communicate directly with each other.

frequency

Radios are set to automatically find the correct frequency. The device scans the frequency spectrum to either monitor unused frequencies or to listen for transmitted frames that have the same SSID as the device. If you have not configured the frequency as Automatic, make sure that all devices in the WLAN infrastructure are configured with the same frequency.

Data rate

The data rates affect the AP coverage areas. Lower data rates (e.g. 1 Mbit / s) can extend the coverage area far from the access point than the higher data rates. If WLAN devices are configured for different data rates (expressed in megabits per second), the devices will not communicate. Here are some common scenarios:

  • Bridges are used for communication between two buildings. If a data rate of 11 Mbit / s is set for one bridge and a data rate of 1 Mbit / s for the other, communication will fail.

  • If the pair of devices is configured to use the same data rate, other factors are likely to prevent them from achieving this speed. As a result, communication fails.

  • If one of the bridges has a data rate of 11 Mbit / s and the other uses a transmission rate, the units communicate at 11 Mbit / s. However, if communication is compromised and the units must fall back to a lower data rate, the unit set for 11 Mbps will not fall back and communication will fail.

Cisco recommends that wireless LAN devices be configured to communicate at more than one data rate.

distance

The radio link between bridges is sometimes very long. Therefore, the time it takes for the radio signal to travel between the radio modules can be significant. The Distance parameter adjusts the various timers used in the radio protocol to account for the delay. Enter the parameter only on the root bridge that shows the repeaters. The distance of the longest radio link in the bridges is entered in kilometers, Not in miles.

Radio interference

Many factors affect the successful transmission or reception of a radio signal. The most common problems are radio interference, electromagnetic interference, cable problems, and antenna problems.

Radio interference

No license is required to operate radio devices in the 2.4 GHz frequency band in which the Cisco Aironet WLAN devices are operated. This means that other stations can be transmitted on the same frequency that your WLAN is using.

A spectrum analyzer is the best tool to determine if there is activity in your frequency. The "Carrier Busy" test available in the test menus of the Cisco Aironet bridges serves as a replacement for this entry. This test gives a rough indication of the activity on the various frequencies. If you suspect that there is radio interference on your wireless network, switch off the device that is operating on that frequency and run the test. The test shows all activity on your frequency and on the other frequencies on which the device can operate. In this way you can determine whether you want to change the frequencies.

Note: High sources of error at radio interfaces on the client, access point or bridge indicate the effects of radio interference. You can also identify radio interference using system messages in the logs of the access point (AP) or bridge. The output looks like this:

May 13 18: 57: 38.208 Information Interface Dot11Radio0, Deauthenticating Station 000e.3550.fa78 Reason: Previous authentication no longer valid
May 13 18: 57: 38.208 Warning Packet to client 000e.3550.fa78 reached max retries, removing the client

CRC, PLCP errors

CRC and PLCP errors can occur due to radio interference. The higher the number of radio modules in a cell (APs, bridges or clients), the higher the chances of these errors occurring. Intermittent Connectivity Issues in Wireless Bridges, see the CRC, PLCP Errors section under Intermittent Connectivity Issues in Wireless Bridges (CRC, PLCP Errors) for an explanation of how CRC and PLCP errors affect performance.

Electromagnetic interference

Devices that do not use radio modules and that operate in close proximity to Cisco Aironet WLAN devices can occasionally cause electromagnetic interference (EMI). In theory, this interference can directly affect the reception and transmission of signals. However, EMI affects the components of the transmitter rather than the transmission.

Isolate the radios from potential sources of EMI to minimize the potential impact of EMI. If possible, you should remove the device from these sources. Provide conditioned power to WiFi devices to reduce the effects of EMI on the circuits.

Cable problems

The cables that connect antennas to Cisco Aironet WLAN devices can cause radio communications interference.

Cable selection

When setting up bridges for long distance communication, make sure that the antenna cables are no longer than necessary. The longer a cable, the greater the signal attenuation, which leads to a lower signal strength and thus a lower range. A tool is available to calculate the maximum distance that two bridges can communicate depending on the antenna and cable combination used. Download this tool from the antenna calculation table (Microsoft Excel format).

installation

Like any other network cable, you must properly install the antenna cables to ensure that the transmitted signal is clean and free of interference. To ensure that the cables meet their specifications, avoid:

  • Loose links - Loose plugs at both ends of the cable lead to bad contacts in the power supply and impair the signal quality.

  • Damaged cables - Antenna cables with obvious physical damage do not meet specifications. Damage, for example, sometimes leads to an induced reflection of the signal in the cable.

  • Cable guides together with power cords - The EMI that power cables produce can affect the signal on the antenna cable.

Antenna problems

Communication area

Use the Antenna Calculation Table (Microsoft Excel format) to calculate the maximum distance two bridges can transmit based on the antenna and cable combinations used.

Positioning the line of sight and antenna

In many cases, Line of Sight (LOS) is not seen as a problem, especially with WiFi devices that communicate over short distances. Due to the transmission of radio waves, devices with omnidirectional antennas often communicate successfully from room to room. The density of materials used to construct a building will determine the number of walls the radio signal can pass and will still maintain adequate coverage. The following list contains the main effects on signal penetration:

  • Paper and vinyl walls have little impact on signal penetration.

  • Solid and pre-cut concrete walls limit signal penetration to one or two walls without affecting the covering.

  • Concrete block walls limit signal penetration to three or four walls.

  • Wood or plaster of paris allow sufficient signal penetration for five to six walls.

  • A thick metal wall ensures that the signals are reflected off. This leads to poor signal penetration.

  • Chain fence, a wire mesh 1-1 / 2 "apart, acts as a 1/2" wave that blocks a 2.4 GHz signal.

When connecting two points together (e.g. an Ethernet bridge), consider the distance, obstacles and antenna location. If you can mount the antennas indoors and the distance is tight (over several hundred meters), you can use the standard dipole antenna or magnetic mounting with 5.2 dBi omnidirectional antenna or yagi antenna.

For long distances of at least ½ mile, use high gain omnidirectional antennas. These antennas need to be as tall as possible and overcome obstacles such as trees and buildings. If you are using directional antennas, be sure to orient them so that you line up the main utility poles with each other. With a line of sight and the Yagi antennas, distances of up to 40 km at 2.4 GHz can be achieved using parabolic dish antennas, provided a clear line is maintained from the side.

Note: The Federal Communications Commission (FCC) requires professional installation of directional antennas with high gain for systems that are operated exclusively as point-to-point systems and have a total output of more than +36 dBm. The EIRP is the apparent power transmitted to the receiver. The installer and the end user must ensure that the high-performance systems are operated strictly as a point-to-point system.

Client problems

The Troubleshooting Cisco Unified Wireless Network Client Problems document explains various problems you might encounter when connecting a wireless client in a Cisco Unified Wireless environment and the steps you need to take to troubleshoot and resolve those problems.

Other reasons for a reduced signal strength

Even if there is a definite LOS or no Fresnel blocker between wireless links, you can still get low signal strength. There could be several reasons for this problem.

  • One possible reason could be the radiation pattern of the antennas used. In many cases, a higher win omni has a pattern that resembles a champagne glass. Lower gain omnidirectional antennas are similar to a donuss or frisbee centered around the long axis of the stick.

    The way to check this is to look at the radiation patterns that accompany most, if not all, of the antennas. There are usually two charts. One shows the pattern from the side (important for an omni), the other shows the pattern from the top (important for directionals, yagis, dishes and panels). There is a good chance the transmitted signal will be transmitted over the head of your receiving antenna.

  • Verify that the equipment is properly grounded. Grounding is very important, if only for safety reasons. Flash arresters don't stop the flash. These closures ignited static electricity and (tended to) reduced the amount of space pollution that can build up with exposed elements.

  • Also, it is always a good idea to place a fiber optic segment between the APs and the wired network to prevent the dead spots from destroying the rest of the network.

  • Check the coaxial cables for bruised or bent areas, sharp bends, broken jackets, etc. At Gigaplus frequencies, every faulty cable section can have a significant impact on the signal transmission.

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