Signal Interference Occurs

Context

The air interface environment is affected by various types of interference that severely affects user experience, including Wi-Fi and non-Wi-Fi interference. Wi-Fi interference includes co-channel and adjacent-channel interference between APs and interference from other Wi-Fi networks. Non-Wi-Fi interference sources include microwave ovens, Bluetooth devices, wireless audio and video devices, baby monitors, wireless cameras, infrared sensors, and cordless phones.

Symptom

Interference exists on the Wi-Fi network, affecting the normal use of the Wi-Fi network.

Possible Causes

• Channels of neighboring APs are not proper, generating co-channel or adjacent-channel interference.
• Hidden nodes exist between APs.
• Interference from other Wi-Fi networks exists.
• Interference sources exist around the AP, such as microwave ovens, Bluetooth devices, wireless audio and video devices, baby monitors, wireless cameras, infrared sensors, and cordless phones.

Troubleshooting Procedure

1. Check whether co-channel and adjacent-channel interference exists.
   1. Check the AP's channel utilization on the AC.

      Channel utilization is a key factor representing the air interface status. If the service volume on an AP is small but the channel utilization is high, interference is severe on the air interface.

      <AC> display radio ap-id 0
      CH/BW:Channel/Bandwidth  
      CE:Current EIRP (dBm)  
      ME:Max EIRP (dBm)  
      CU:Channel utilization  
      ST:Status  
      WM:Working Mode (normal/monitor/monitor dual-band-scan/monitor proxy dual-band-scan)  
      ------------------------------------------------------------------------------------  
      AP ID Name            RfID  Band  Type    ST  CH/BW    CE/ME   STA     CU    WM  
      ------------------------------------------------------------------------------------  
      0     60de-4474-9640  0     2.4G  bgn     on  6/20M    24/24   0       55%   normal   
      0     60de-4474-9640  1     5G    an      on  56/20M   25/25   0       3%    normal  
      ------------------------------------------------------------------------------------  
      Total:2

   2. Check the noise floor of AP radios on the AC.

      <AC> display ap traffic statistics wireless ap-id 0 radio 0
      ...
      Wireless noise(dBm)                                   :-92
      ...

   3. Log in to the AP to check the interference rate.

      In V200R019C00 and earlier versions:

      [AP-diagnose] display wifi base-info radio 0
      ------------------------------------
      ......                  
      CoChanInterferenceRate(%) = 46
      ChanMismatchProbeReq      = 110
      ------------------------------------ 

      In V200R019C10:

      [AP-diagnose] display lmac base-info radio 1
      ......
       ChannelUtilizationRate(%)  : 72
       CoChanInterferenceRate(%)  : 63

   If the channel utilization exceeds 60% and the interference rate is high, interference is severe on the air interface. Address this issue by adjusting the AP's channel and transmit power.

   If the noise floor is higher than -80 dBm, you are advised to switch the AP to a channel with a lower noise floor. Consider the channel utilization and interference rate when switching the AP channel.

   Adjust the AP's channel and transmit power as follows:

   1. Use inSSIDer to scan surrounding signals for a channel with less interference.
   2. Switch the AP's channel and lower the transmit power. (Multiple attempts can be made to set the transmit power properly.)

      [AC-wlan-view] ap-id 0
      [AC-wlan-ap-18] radio 0
      [AC-wlan-radio-0/0] channel 20mhz 6
      Warning: This action may cause service interruption. Continue?[Y/N]y
      [AC-wlan-radio-0/0] eirp 10
      Info: The EIRP value takes effect only when automatic transmit power selection is disabled, and the value depends on the AP specifications and local laws and regulations.

2. Check whether hidden nodes exist between APs.

   If co-channel APs are invisible to each other but have overlapping coverage areas, STAs in the overlapping coverage areas do not send data in a certain period and become hidden nodes. Hidden nodes may lead to signal conflicts, thereby causing data transmission failures.

   [AC-master-wlan-view] display ap traffic statistics wireless ap-id 0 radio 0
   -----------------------------------------------------------------------
   ......
   Wireless retry frames: 3 //Number of frames retransmitted by the radio
   Wireless PER(%)                                       :0
   Wireless PER of the last 5min(%)                      :0
   Wireless port drop rate(%)                            :0
   Wireless port drop rate of the last 5min(%)           :0
   Wireless retransmitted rate(%) :0 //Packet retransmission ratio of the radio
   Wireless retransmitted rate of the last 5min(%) :6 //Packet retransmission ratio of the radio in the last statistical period
   ......
   -----------------------------------------------------------------------

   If the packet retransmission ratio exceeds 20%, there is a high probability of air interface collisions. It is recommended that the RTS-CTS mode be set to rts-cts to lower the probability of air interface collisions.

   [AC-wlan-view] radio-2g-profile name default
   [AC-wlan-radio-2g-prof-default] rts-cts-mode rts-cts  
   Warning: This action may cause service interruption. Continue?[Y/N]y
   [AC-wlan-radio-2g-prof-default] rts-cts-threshold 1400
   Warning: This action may cause service interruption. Continue?[Y/N]y

   Set the RTS/CTS threshold based on the actual service scenario. Before sending a packet whose length exceeds the RTS/CTS threshold, an AP sends an RTS frame to clear channels. For packets whose length is less than the RTS/CTS threshold, APs do not use this mechanism. This mechanism aims to prevent packet collisions caused by hidden STAs. The recommended RTS/CTS threshold is 1400 bytes to protect large packets. This is because large packets require a long transmission period and are vulnerable to interference.

   If too many STAs or low-rate STAs are associated with the AP, interference may occur on the air interface. In this case, you can enable smart roaming to force low-rate STAs to go offline to ensure network experience of other STAs.

3. Check whether other Wi-Fi interference exists.

   After intra-AP interference is eliminated, check for other Wi-Fi interference in the surroundings by scanning the air interface environment using a scanning tool such as WirelessMon, inSSIDer, or Network Stumbler. WiFi Analyzer or CloudCampus APP on Android mobile phones is also a useful tool.

   The scanning result shows whether other Wi-Fi signals exist on the AP's working channel. If other Wi-Fi signals exist, you can manually eliminate these signal interferences or switch the AP's working channel to another with less interference.

4. Check whether non-Wi-Fi interference exists.

   Non-Wi-Fi interference is generated by equipment that works on the same or adjacent frequency bands as the AP's operating band (2.4 GHz or 5 GHz band).

   • The 2.4 GHz frequency band is the Industrial, Scientific, and Medical (ISM) open frequency band. Interference sources on the 2.4 GHz frequency band include cordless phones, baby monitors, microwave ovens, wireless cameras, Bluetooth devices, infrared sensors, and fluorescent light ballasts.
   • The 5 GHz frequency band has fewer interference sources than 2.4 GHz frequency band. More and more devices begin to use the 5 GHz frequency band, such as cordless phones, radars, wireless sensors, and digital satellites.

   These non-Wi-Fi interference sources have great impact on AP services, and it is difficult to identify them. In most cases, spectrum analyzers and dedicated tools (such as Fluke) can be used to identify non-Wi-Fi interference sources. The following figure shows how to determine non-Wi-Fi interference through spectrum analysis (on the web system).

   If any of the preceding non-Wi-Fi interference sources is detected, eliminate the interference source to prevent interference.