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Deal with excessive RF interference

Track phones and microwaves? Take a walk with a sensor.

Article comments

Q: Is there something other than a wireless network that could be interfering at channel 11? We get 25 percent to 30 percent noise detected using AirMagnet. The interference could be at channel 10. This appears to be common at every site, including our corporate building. Any ideas?
- Keith V.

The Wizards gaze into their crystal ball and respond:

Bob Friday, Airespace
Both 802.11b and 802.11g use a mechanism within the 802.11 standard to determine when the air is idle and can be used to send a frame. This mechanism is called Carrier Sense Multiple Access (CSMA). Using this method, 802.11b/g radios can detect other 802.11b/g frames and energy in the channel on which they are operating. Whenever the air is determined to be in use, CSMA prevents 802.11 radios from transmitting. As microwave ovens, Bluetooth devices, and many cordless phones transmit in the 2.4 GHz frequency, these devices will be detected as energy in the channel when they are transmitting, preventing 802.11b/g access points from sending anything until the competing transmission is done.

CSMA is the reason why an 802.11b/g Wireless LAN can share the unlicensed spectrum of 2.4 GHz with devices like microwave ovens, Bluetooth and cordless phones. It is important to note that the 802.11 specification itself does not dictate how interference should be managed in a wireless LAN. This function is an add-on to the specification, typically provided within individual wireless LAN vendors’ systems. In most cases, these will detect interference sources and adjust channels on the affected access points to avoid conflict - both with external interference sources and with each other.

Marcel Wiget, Chantry Networks
There are indeed other devices sharing the 2.4 GHz spectrum used by 802.11b/g. Based on your discovery that the noise level seems to be on a specific channel, I would look for wireless video surveillance cameras and wireless phones. Try to locate them using AirMagnet on channel 10 by walking through the building until you can detect where the noise is emanating from.


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