This document addresses some of the major issues you encounter when you
try to establish a radio link between elements of a wireless LAN (WLAN). You
can trace problems with the radio frequency (RF) communications between Cisco
Aironet WLAN components to four root causes:
Firmware and driver problems
Software configuration problems
RF impairments that include antenna and cable
There are no specific requirements for this document.
This document is not restricted to specific software and hardware
Refer to the
Technical Tips Conventions for more information on document
Occasionally, you can trace a problem with the radio signal to a
problem in the firmware on the communicating devices.
If you encounter a radio communication problem with your WLAN, ensure
that each component runs the latest revision of its firmware or driver. Use the
most recent version of the driver or firmware with your WLAN products. Use the
(registered customers only)
to obtain updated drivers and
You can find the directions to upgrade firmware at:
When you encounter radio communication problems, the configuration of
the WLAN devices can be the cause of the radio failure. You must configure
certain parameters properly for the devices to communicate successfully. If you
configure the parameters incorrectly, the problem that results appears to be a
problem with the radio. These parameters include the Service Set Identifier,
frequency, data rate, and distance.
Cisco Aironet WLAN devices must be set to the same Service Set
Identifier (SSID) as all the other Cisco Aironet devices on the wireless
infrastructure. Units with different SSIDs fail to communicate directly with
Radio devices are set to automatically find the correct frequency. The
device scans the frequency spectrum, either to listen for an unused frequency
or to listen for transmitted frames that have the same SSID as the device. If
you have not configured the frequency as Automatic, ensure that all the devices
in the WLAN infrastructure are configured with the same frequency.
Data rates affect AP coverage areas. Lower data rates (such as 1 Mbps)
can extend the coverage area farther from the AP than higher data rates. If
WLAN devices are configured for different data rates (expressed in megabits per
second), the devices fail to communicate. Here are some common
Bridges are used to communicate between two buildings. If one bridge
is set at a data rate of 11 Mbps and the other is set at a data rate of 1 Mbps,
If the pair of devices are configured to use the same data rate,
other factors probably prevent them from reaching that rate. As a result,
If one of a pair of bridges has a data rate of 11 Mbps set, and the
other is set to use any rate, then the units communicate at 11 Mbps. But, if
there is some impairment in the communication that requires the units to fall
back to a lower data rate, the unit set for 11 Mbps fails to fall back, and
Cisco recommends that WLAN devices are set to communicate at more than
one data rate.
The radio link between bridges are sometimes very long. Therefore, the
time that the radio signal takes to travel between the radios can become
significant. The Distance parameter adjusts the various timers used in radio
protocol to account for the delay. Enter the parameter only on the root bridge,
which tells the repeaters. The distance of the longest radio link in the set of
bridges is entered in kilometers, not in miles.
Many factors impair the successful transmission or reception of a radio
signal. The most common issues are radio interference, electromagnetic
interference, cable problems, and antenna problems.
You do not require a license to operate radio equipment in the 2.4 GHz
band where the Cisco Aironet WLAN equipment operates. As a result, other
transmitters can broadcast on the same frequency that your WLAN uses.
A spectrum analyzer is the best tool to determine the presence of any
activity on your frequency. The Carrier Busy test available in the Test menus
of Cisco Aironet bridges functions as a substitute for this item. This test
generates a rough display of activity on the different frequencies. If you
suspect radio interference with transmission and reception on your WLAN, turn
off the equipment that operates on the frequency in question and run the test.
The test shows any activity on your frequency and the other frequencies on
which the equipment can operate. You can thus determine if you want to change
Note: High error counters on radio interfaces on the client, the access
point or bridge indicate the effects of RF interference. You can also identify
RF interference through 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 errors and PLCP errors can occur due to RF interference. The higher
the number of radios in a cell (APs, Bridges or Clients), higher are the
chances for the occurrence of these errors. Refer to the
PLCP errors section of
Connectivity Issues in Wireless Bridges for an explanation of how CRC
and PLCP errors affect performance.
Non-radio equipment that operates in close proximity to the Cisco
Aironet WLAN equipment can sometimes generate electromagnetic interference
(EMI). Theoretically, this interference can directly affect the reception and
transmission of signals. However, EMI more likely affects the components of the
transmitter rather than the transmission.
Isolate the radio equipment from potential sources of EMI in order to
minimize the possible effects of EMI. Locate the equipment away from such
sources if possible. Also, supply conditioned power to the WLAN equipment in
order to lessen the effects of EMI generated on the power circuits.
The cables that connect antennas to Cisco Aironet WLAN devices are a
possible source of radio communication difficulties.
If you set up bridges to communicate over a long distance, ensure that
the antenna cables are not longer than is necessary. The longer a cable, the
more is the signal attenuation, which results in lower signal strength and
consequently, a lower range. A tool is available which you can use to calculate
the maximum distance over which two bridges can communicate based on the
antenna and cable combinations in use. Download this tool from the
spreadsheet (Microsoft Excel format).
Like any other network cables, you must properly install the antenna
cables to ensure that the signal carried is clean and free from interference.
In order to ensure that the cables perform to their specifications, avoid
Loose connections—Loose connectors on either end
of the cable result in poor electrical contact and degrade the signal
Damaged cables—Antenna cables with obvious
physical damage do not perform to specification. For instance, damage sometimes
results in induced reflection of the signal within the cable.
Cable runs shared with power cables—The EMI that
power cables produce can affect the signal on the antenna cable.
Use the antennae calculation
spreadsheet (Microsoft Excel format) to calculate the maximum distance
two bridges can communicate based on the antenna and cable combinations in
In many instances Line of Sight (LOS) is not seen as a problem,
particularly for WLAN devices that communicate over short distances. Due to the
nature of radio wave propagation, devices with omni-directional antennae often
communicate successfully from room to room. The density of the materials used
in the construction of a building determine the number of walls the RF signal
can pass through and still maintain adequate coverage. Here is a list of
material impact on signal penetration:
Paper and vinyl walls have little effect on signal penetration.
Solid and pre-cast concrete walls limit signal penetration to one or
two walls without degrading coverage.
Concrete and concrete block walls limit signal penetration to three
or four walls.
Wood or drywall allows for adequate signal penetration for five or
A thick metal wall causes signals to reflect off. This results in
poor signal penetration.
Chain link fence, wire mesh with 1 - 1 1/2" spacing acts as a 1/2"
wave that blocks a 2.4 GHz signal.
When you connect two points together (for example, an Ethernet bridge),
you must consider the distance, obstructions, and antenna location. If you can
mount the antennas indoors and the distance is short—several hundred feet—you
can use the standard dipole or magnetic mount 5.2 dBi omni-directional or Yagi
For long distances of ½ mile or more, use directional high gain
antennas. These antennas must be as high as possible, and above obstructions
such as trees and buildings. If you use directional antennas, ensure that you
align them such that you direct their main radiated power lobes at each other.
With a line of sight configuration and the Yagi antennas, distances of up to 25
miles at 2.4 GHz are reachable with the help of Parabolic Dish Antennas,
provided a clear line of site is maintained.
Note: The Federal Communications Commission (FCC) requires professional
installation of high gain directional antennas for systems that must operate
solely as point-to-point systems and have total power which exceeds the +36 dBm
Effective Isotropic Radiated Power (EIRP). The EIRP is the apparent power
transmitted towards the receiver. The installer and the end user must ensure
that the high power systems are operated strictly as a point-to-point system.
Client Issues in the Cisco Unified Wireless Network explains various
issues that you can encounter when you connect a wireless client in a Cisco
Unified Wireless environment, as well as the steps to be taken to troubleshoot
and resolve these issues.
Even if there is a clear LOS or no fresnel blockage between wireless
links, you might still receive a low signal strength. There can be several
reasons for this problem.
One possible reason might be the radiation pattern of the antennas
used. In many cases, a higher gain omni has a pattern that resembles a
champagne glass. Lower gain omni-directional antennas resemble a doughnut or a
frisbee, centered around the long axis of the stick.
The way to check this is to look at the radiation pattern diagrams
that accompany most, if not all, antennas. There are usally two diagrams. One
shows the pattern from the side (important for an omni), and the other shows
the pattern from the top (important for directionals, Yagis, dishes, and
panels). There is a good chance that the transmitted signal goes over the head
of your receiving antenna.
Check whether the devices are properly grounded. The grounding is
very important, if only for the safety aspects. Lightning Arrestors do not stop
lightning. These arrestors bleed off static electricity and (tend to) reduce
the space charge that can accumulate on exposed elements.
Also, it is always a good idea to put a segment of fiber between the
APs and the wired network to prevent the zap from killing the rest of the
Check the coax for kinks or places that were kinked, sharp bends,
broken jacket, etc. At Gigaplus frequencies, any malformed section of cabling
can have a significant impact on the propagation of the signal.