Case Study: Severe Service Suspension Occurs on STAs Due to Improper Channel Planning

Symptom

Radio channels are improperly planned at a site, causing severe adjacent-channel and co-channel interference. As a result, users suffer from severe service freezing.

Relevant Alarms and Logs

• Co-channel interference alarm: WLAN_1.3.6.1.4.1.2011.6.139.16.1.1.1.5 hwAPCoInterfDetectedTrap
• Adjacent-channel interference alarm: WLAN_1.3.6.1.4.1.2011.6.139.16.1.1.1.7 hwNerborInterfDetectedTrap

Cause Analysis

Check AP neighbor information based on the high channel interference. It is found that 2.4 GHz channels 1, 6, and 11 are allocated to most APs, but some APs use 2.4 GHz channel 5 that is manually configured. Currently, only non-contiguous channels 1, 6, and 11 are non-overlapping and available on the 2.4 GHz frequency band. When the 20 MHz frequency bandwidth is used, channel 5 overlaps by 15 MHz with channel 6 and by 5 MHz with channel 1. As a result, the adjacent-channel interference is severe.

Check the channel, transmit power, and channel utilization of outdoor APs. The check result shows that the 5 GHz radio uses the 80 MHz frequency bandwidth, which results in high channel utilization and severe co-channel interference. The country code of APs used at the site is CN. According to Chinese laws and regulations, only five 20 MHz channels (149–165) can be used in outdoor scenarios. When the 80 MHz frequency bandwidth is configured, four channels need to be bonded. As a result, only channel 149 is available.

Procedure

Adjust the frequency bandwidth and channel of radios. On the 2.4 GHz frequency band, channels 1, 6, and 11 are automatically distributed to minimize adjacent-channel interference. Change the frequency bandwidth of the 5 GHz radio to 20 MHz so that five channels are available for the 5 GHz radio and co-channel interference is reduced.

1. In office scenarios where APs are deployed in a certain order, configure automatic channel and transmit power calibration for AP radios.
   # Enable automatic channel and transmit power selection. By default, automatic channel selection and automatic transmit power selection are enabled.

   <AC> system-view
   [AC] wlan
   [AC-wlan-view] ap-group name ap-group1
   [AC-wlan-ap-group-ap-group1] radio 0
   [AC-wlan-group-radio-ap-group1/0] calibrate auto-channel-select enable
   [AC-wlan-group-radio-ap-group1/0] calibrate auto-txpower-select enable
   [AC-wlan-group-radio-ap-group1/0] quit
   [AC-wlan-ap-group-ap-group1] radio 1
   [AC-wlan-group-radio-ap-group1/1] calibrate auto-channel-select enable
   [AC-wlan-group-radio-ap-group1/1] calibrate auto-txpower-select enable
   [AC-wlan-group-radio-ap-group1/1] quit
   [AC-wlan-ap-group-ap-group1] quit

   # Set the calibration bandwidth of 5 GHz radios to 20 MHz and use the default calibration channels.

   [AC-wlan-view] regulatory-domain-profile name default
   [AC-wlan-regulate-domain-default] dca-channel 5g bandwidth 20mhz

2. Manually trigger radio calibration. By default, the radio calibration mode is auto.

   [AC-wlan-view] calibrate enable auto
   [AC-wlan-view] calibrate manual startup
   Warning: The operation may cause business interruption, continue?[y/n]:y

3. Run the display radio all command on the AC to check the radio calibration effect. The command output shows that the AP channels are staggered.

   [AC-wlan-view] display radio all
   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
   ------------------------------------------------------------------------------------
   1     area_2  0     2.4G  bgn     on      1/20M    28/28   1        10%   normal
   1     area_2  1     5G    an      on      149/20M  29/29   0        15%   normal
   0     area_1  0     2.4G  bgn     on      6/20M    28/28   1        15%   normal
   0     area_1  1     5G    an      on      153/20M  29/29   0        49%   normal
   ------------------------------------------------------------------------------------
   Total:4

Suggestion and Summary

As shown in Figure 6-1, the bandwidth of each 2.4 GHz channel is 20 MHz, and the center frequencies of neighboring channels is only 5 MHz apart. Currently, only channels 1, 6, and 11 are non-overlapping channels. In practice, it is recommended that 2.4 GHz channels 1, 6, and 11 be automatically deployed and 20 MHz channels be strictly used to minimize adjacent-channel interference.

Figure 6-1 Channel distribution on the 2.4 GHz frequency band

As shown in Figure 6-2, each of 5 GHz channels is 20 MHz wide. Currently, there are 25 channels in total on the 5 GHz frequency band, including channel 144 newly added in some countries. Different countries plan different channels for indoor and outdoor APs depending on the country code. For example, 13 indoor channels (36 to 64 and 149 to 165) and five outdoor channels (149 to 165) are available in China. For details, see WLAN Country Codes and Channels Compliance. To improve the user concurrency rate and network performance, the 40 MHz or 80 MHz frequency bandwidth is configured at some sites in indoor scenarios. However, when the 80 MHz frequency bandwidth is used, there are only three channels available in indoor scenarios, including radar channels. Therefore, the expected performance improvement cannot be achieved, and user experience is affected due to high interference.

Figure 6-2 Channel distribution on the 5 GHz frequency band

To sum up, in actual projects, the frequency bandwidth and channel of radios should be properly planned based on the network plan and site requirements. The automatic radio calibration function can be enabled for channel deployment, so that the wireless network has higher performance and co-channel and adjacent-channel interference is minimized.