Interference sources on WLAN networks

Two frequency bands are available in WLAN: 2.4 GHz and 5 GHz.

The 2.4 GHz frequency band is the industrial, scientific, and medical (ISM) band and is open to cordless phones, baby monitors, microwave ovens, wireless cameras, Bluetooth devices, infrared sensors, and fluorescent light ballasts. These devices are all interference sources.

Compared with the 2.4 GHz frequency band, the 5 GHz frequency band has fewer interference sources. However, 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 frequency bands ranging from 2.4 GHz to 2.5 GHz, which overlap 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 from WLAN devices.

The power of cordless phones is about 3 W, which is higher than APs' 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 meters, WLAN devices may go offline and the cordless phone voice is not clear. Therefore, you are advised to use cordless phones more than 2 meters away from APs or STAs.

The transmit power of wireless cameras ranges from 500 mW 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 APs far away from wireless cameras during WLAN planning.

Bluetooth devices adopt frequency hopping spread spectrum technology, and each Bluetooth channel has a bandwidth of 1 MHz 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.

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