N10-009 Question 161
Single answerChannels: Channel width, Non-overlapping channels, Regulatory impacts (802.11h)A network administrator is troubleshooting poor Wi-Fi performance in a European office. The 2.4 GHz WLAN uses 40 MHz channel width on multiple APs, and users report frequent interference and inconsistent throughput. The 5 GHz WLAN also experiences occasional channel changes after deployment near a weather radar installation. The administrator wants to improve stability while remaining compliant with regional regulations. Which action is the BEST solution?
- A
Reconfigure the 2.4 GHz radios to use 20 MHz channels on non-overlapping channels and enable 802.11h support on the 5 GHz radios
- B
Keep 40 MHz channels on 2.4 GHz for maximum throughput and disable automatic channel changes on 5 GHz radios
- C
Move all APs to channel 3 in the 2.4 GHz band and increase transmit power to overcome interference
- D
Use 80 MHz channels on 5 GHz and disable radar detection to prevent client disconnects
Show answer and explanation
Correct answer: A
Explanation
The best solution combines proper channel planning with regulatory compliance. In the 2.4 GHz band, wider channels such as 40 MHz are usually a poor choice in business environments because the band has limited spectrum and only a small number of practical non-overlapping 20 MHz channels. Best practice is typically to use 20 MHz channels and plan around non-overlapping channel assignments to minimize adjacent-channel and co-channel interference. In the 5 GHz band, 802.11h introduced mechanisms including DFS and TPC to help WLAN devices comply with regional requirements, especially in areas where radar systems operate. APs changing channels near a weather radar installation is expected behavior when DFS events occur. This aligns with Wi-Fi design guidance from enterprise WLAN vendors and IEEE 802.11 requirements implemented for regulatory domains such as ETSI in Europe.
- A. Correct.
Correct. In 2.4 GHz, 20 MHz channels are preferred in most enterprise deployments because they reduce adjacent-channel interference and allow use of the standard non-overlapping channels (commonly 1, 6, and 11 in many regulatory domains). Using 40 MHz in 2.4 GHz often causes excessive overlap and poor coexistence. On 5 GHz, 802.11h supports mechanisms such as Dynamic Frequency Selection (DFS) and Transmit Power Control (TPC), which are used in many regions to comply with radar-avoidance and power regulations. If APs near radar are changing channels, that behavior is consistent with DFS compliance rather than a fault.
- B. Incorrect.
Incorrect. Although 40 MHz can increase theoretical throughput, using it in the 2.4 GHz band commonly worsens performance in real environments because there are too few non-overlapping channels. Disabling automatic channel changes on 5 GHz would undermine DFS behavior required in channels subject to radar detection and could create regulatory compliance issues.
- C. Incorrect.
Incorrect. Channel 3 overlaps significantly with channels 1 and 6 and is not considered a standard non-overlapping choice in 20 MHz 2.4 GHz planning. Increasing transmit power does not fix channel overlap; it can increase contention and interference, especially in dense deployments. This reflects the common misconception that stronger signal alone resolves RF design problems.
- D. Incorrect.
Incorrect. Using 80 MHz channels on 5 GHz may be appropriate in some low-density environments, but it does not address the 2.4 GHz interference problem and can reduce the number of usable non-overlapping channels in 5 GHz. Disabling radar detection is not a valid or compliant solution where DFS is required. This would violate regulatory rules enforced through 802.11h-related behavior in applicable bands.