Signal Interference Occurs (Cloud AP Scenario)

Symptom

In the cloud AP scenario, interference exists on the Wi-Fi network, affecting wireless user experience.

Possible Causes

• On the same or overlapping channel, different STAs interfere with each other (co-channel interference).
• On non-overlapping channels, different STAs interfere with each other (adjacent-channel interference).
• Hidden nodes exist between APs.
• Other Wi-Fi interference exists.
• There are microwave ovens, Bluetooth devices, wireless cameras, or infrared sensors in the surroundings of an AP.

Troubleshooting Procedure

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

      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.

      Log in to iMaster NCE-Campus, choose Design > Device Management, select the AP to be queried, and click the RF tab. The channel utilization values on different frequency bands of the AP are displayed, as shown in the following figure.

   2. Check the noise floor of AP radios on iMaster NCE-Campus.

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

      Click Command Line on the right, wait until the AP is connected, and run the following command in the diagnostic view:

      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.)

         On iMaster NCE-Campus, choose Provision > Site Configuration. In the navigation tree on the left, choose AP > Radio. In the Channel Planning area, select the AP to be modified and click Edit on the right.

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.

   [AP] display ap traffic statistics wireless 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 rate of the radio
    Wireless retransmitted rate of the last 5min(%)       :6  //Packet retransmission rate of the radio in the last statistical period
   ......
   -----------------------------------------------------------------------

   If the packet retransmission rate 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 reduce the probability of air interface collisions.

   [AP] wlan
   [AP-wlan-view] radio-2g-profile name default 
   [AP-wlan-radio-2g-prof-default] rts-cts-mode rts-cts   
   Warning: This action may cause service interruption. Continue?[Y/N]y 
   [AP-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 are difficult to identify. In most cases, spectrum analyzers and dedicated tools (such as Fluke) can be used to identify non-Wi-Fi interference sources.

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