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FAQ-Where Are Interference Sources in WLAN and How Is the Interference Strength

Publication Date:  2015-06-16 Views:  625 Downloads:  0

Issue Description

Where Are Interference Sources in WLAN and How Is the Interference Strength?


Two frequency bands are available on WLANs: 2.4 GHz and 5 GHz.

The 2.4 GHz frequency band is the Industrial, Scientific, and Medical (ISM) open frequency band. Interference sources in the 2.4 GHz frequency band include cordless phones, baby monitors, microwave ovens, wireless cameras, Bluetooth devices, infrared sensors, and fluorescent light ballasts.

Compared with 2.4 GHz frequency band, 5 GHz frequency band has fewer interference sources and more devices begin to use the 5 GHz frequency band, such as cordless phones, radars, wireless sensors, and digital satellites.

In most cases, microwave ovens work at the frequency band ranging from 2.4 to 2.5 GHz, which overlaps the 2.4 GHz frequency band used by WLAN devices. In addition, the power of microwave ovens ranges between 800 W and 2000 W, which is much higher than the transmit power of APs and STAs. Even though interference shielding is performed, microwave ovens still have severe interference on WLAN devices. Microwave ovens greatly reduce the throughput of WLAN devices if they are within a distance shorter than 8 meters around WLAN devices.

The power of cordless phones is about 3 W, which is higher than the AP's transmit power. According to the test analysis on the interference caused by cordless phones on WLAN devices, when the distance between cordless phones and APs (or STAs) is within 1 meter, interference increases significantly. When the distance is shorter than 0.5 meter, WLAN devices are even offline and the cordless phone voice is not clear. Therefore, you are advised to deploy cordless phones more than 2 meters away from APs or STAs.

The transmit power of wireless cameras ranges from 500 to 1000 MW. In indoor scenarios, wireless cameras may affect the WLAN network but have lighter interference than microwave ovens and cordless phones. Therefore, you are advised to deploy wireless cameras far away from WLAN devices during WLAN planning.

Bluetooth devices use the frequency hopping spread spectrum (FHSS) technology and 1 MHz channel bandwidth. If a Bluetooth device is sending data at the frequency band overlapping with a WLAN channel that is being monitored by a WLAN device, the WLAN device selects a random backoff period. During this period, the Bluetooth device changes to work at a non-overlapping channel, allowing the WLAN device to send data. Therefore, Bluetooth devices have small interference on WLAN devices. This interference can be ignored during WLAN planning.