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Troubleshooting Excessive Wireless Interference

 

Problem:

Description: The network is experiencing a high level of interference. Excessive wireless interference on the wireless network has been reported by one or more of these resources:

Symptoms: The symptoms of wireless interference include:

  • Users are experiencing reduced range for your WiFi network, generally much lower than what is stated in the hardware specifications.

  • Users are experiencing sudden drops in transfer speeds, even without much traffic on the network.

  • The wireless signal is dropping out in certain places or at certain times during the day.

  • Wireless signal strength going up and down randomly.

  • Steaming audio, video, or over-network file transfers halt, and then restart

Cause

Leaving the channel number on each radio set to the default value can result in crowding on that channel and high interference levels on the radios.

Overlapping channels can interfere with each other—for more information, see Understanding Wireless Radio Channels.

A high number of CRC errors can indicate a hidden node. Hidden nodes in a wireless network refer to access points that are out of range. Due to the inherent coverage limitations of access points, AP2 may be able to see both AP1 and AP3 and receive data from both access points, but AP1 and AP3 cannot see each other. The problem occurs when both AP1 and AP3 send packets simultaneously to AP2. Since the nodes cannot sense the carrier, Carrier sense multiple access with collision avoidance (CSMA/CA) does not work, and collisions occur, corrupting the data at the access point.

Other wireless products, such as cordless phones, microwave ovens, Bluetooth devices, and test equipment, share the 2.4-gigahertz (GHz) radio frequency bands.

A knowledgeable attacker with the right tools can easily jam the 2.4 GHz frequency in a way that drops the signal to a level where the wireless network can no longer function. For more information, see Monitoring RF Interference Sources For Radios on One Access Point, Monitoring RF Interference Sources on One Radio, and Monitoring RF Interference Sources on Wireless Devices.

Solution

Because RF interference can happen at any time, it is prudent to use any automated responses provided. Automated responses to interference data include:

  • Auto-tune channel switching, which selects channels to minimize co-channel interference between access points and also mitigates severe interference by temporarily tuning to another channel.

    Use auto-tuning to balance traffic on all channels.

  • Rate adaptation, which adjusts the modulation scheme at the access point's transmitter in response to variations in signal-to-noise ratio (SNR) at the receiver. The rate adaptation feature is always enabled to use the best rate to reach associated clients. There are no settings to enable or disable this feature, or change its behavior.

Other methods for mitigating interference include:

  • Assign some individual radios to other specific channels. See Understanding Wireless Radio Channels .

  • Move interfering devices out of range.

  • Choose a cordless phone that uses the 5.8-GHz, 1.9-GHz, or 900-megahertz (MHz) band.

  • Overcome the hidden node CRC error problem, implement handshaking in conjunction with the CSMA/CA scheme.