How to detect interference on WLAN networks

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You can use Wirelessmon, NetStumbler, or InSSIDer (downloaded for free on the Internet) to check whether there is strong co-channel or adjacent-channel interference near the AP. In general, the interference signal strength should be lower than �?0 dBm. If multiple co-channel or adjacent-channel interference signals exist and the strength is strong, perform network optimization to prevent channel conflicts.

If interference sources cannot be determined using a scanning tool,

- Check whether wireless devices such as Bluetooth devices, infrared devices, and microwave ovens exist, because non-WLAN devices may cause severe interference to WLAN networks.

- Use the interference detection function to further determine the interference source.

Radio channels are often affected by the radio environment, and the service quality is therefore degraded. Interference detection allows APs to send alarms to an AC when co-channel, adjacent-channel, or STA interference exceeds the alarm threshold.

Interference detection can detect AP co-channel interference, AP adjacent-channel interference, and STA interference.

AP co-channel interference: Two APs working at the same frequency band interfere with each other. For example, on a large-scale WLAN (a university campus network), different APs often use work in the same channel. When coverage areas of the APs overlap, co-channel interference exists, degrading network performance.

AP adjacent-channel interference: Two APs with different center frequencies have overlapping areas, resulting in adjacent-channel interference. Therefore, if APs are placed too close to each other or they have strong signals, bigger noise will be produced, degrading network performance.

STA interference: If there are many STAs that are managed by other APs around an AP, services of the STAs managed by the local AP may be affected.

The following are alarms for co-channel interference and STA interference.

V200R005C10:

WLAN/4/AP_COMMON_FREQUENCY_DISTURB: OID [oid] The co-channel interference exceeds the alarm threshold. (APID=[INTEGER], APID=[INTEGER], RadioID=[INTEGER], APMAC=[OPAQUE], APChannel=[INTEGER], BssId=[OPAQUE], RSSI threshold percent=[INTEGER])//AP_COMMON_FREQUENCY_DISTURB indicates that co-channel interference exists.

WLAN/4/STATION_DISTURB: OID [oid] Jam of station is detected notify.(APID=[INTEGER], APID=[INTEGER], RadioID=[INTEGER], APMAC=[OPAQUE], APChannel=[INTEGER], StatMACList=[STRING])//STATION_DISTURB indicates that STA interference exists.

V200R006:

WLAN/4/AP_COMMON_FREQUENCY_DISTURB: OID [oid] The co-channel interference exceeds the alarm threshold.(APMAC=[OPAQUE], RadioID=[INTEGER], ApName=[OCTET], APChannel=[INTEGER], BssId=[OPAQUE], RSSI threshold percent=[INTEGER]%, APID=[INTEGER])//AP_COMMON_FREQUENCY_DISTURB indicates that co-channel interference exists.

WLAN/4/STATION_DISTURB: OID [oid] Jam of station is detected notify.(APMAC=[OPAQUE], RadioID=[INTEGER], ApName=[OCTET], APChannel=[INTEGER], StatMACList=[STRING], APID=[INTEGER])//STATION_DISTURB indicates that STA interference exists.

Other related questions:
How to configure interference detection on a WLAN device
For details about how to configure interference detection on an AC, see V200R005: Configuring Interference Detection V200R006: Configuring Interference Detection

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.

Where are interference sources on a 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 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 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.

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