80 MHz in DFS and a 40 MHz 2.4 GHz Legacy Trap
During a post-deployment audit of a three-floor office, you find 42 access points: 5 GHz radios are 802.11ax configured for 80 MHz channels 52, 56, 60, and 64; 2.4 GHz radios are 802.11n at 20 MHz except one AP on channel 6 set to 40 MHz. A spectrum analyzer shows intermittent radar-like energy in UNII-2 extended. Users on 802.11n laptops report stalls when roaming between floors, while 802.11ac phones show an 866 Mbps PHY rate but only 90-130 Mbps iperf3 throughput. The customer wants 'more bandwidth' and asks whether to move to 160 MHz. You have a 20 MHz-only legacy barcode scanner on 2.4 GHz and a VoIP handset that requires consistent 20 ms latency. You must judge the standard, channel-width, and band-plan trade-offs without taking the network down during business hours.
1. Explain why the 80 MHz UNII-2 channels create both regulatory and operational risk, and what happens to client traffic when radar is detected.
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80 MHz on channels 52-64 sits entirely inside UNII-2 extended, so the AP is a DFS master. It must complete a channel availability check before transmitting, and if it detects radar it must vacate immediately and announce a channel switch. All associated clients, including the 802.11ac phones, are moved or dropped, so an 80 MHz DFS channel is a single point of failure for that AP. The wider channel also consumes four 20 MHz blocks, reducing the number of non-overlapping 80 MHz channels available for reuse. In a 42-AP, three-floor design this increases co-channel interference and makes a radar event affect more users. The correct judgment is not 'more width is better'; it is to treat DFS width as an availability trade-off and keep a non-DFS fallback.
2. The 2.4 GHz AP on channel 6 uses 40 MHz. What is the practical effect on the legacy scanner and neighboring 2.4 GHz APs, and why is 40 MHz in 2.4 GHz generally discouraged?
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A 40 MHz 2.4 GHz channel on channel 6 occupies 6 and 10 (or 2), consuming most of the band. The legacy barcode scanner is a 20 MHz 802.11b/g device; it will see the AP's wider energy as adjacent-channel interference, causing retries and missed scans. Neighboring APs on 1 or 11 are also overlapped. In 2.4 GHz there are only three non-overlapping 20 MHz channels; using 40 MHz leaves at most one clean 20 MHz channel and breaks any reuse plan. The VoIP handset needs low, consistent latency; retries and CCA backoff from a 40 MHz channel increase jitter. Disable 40 MHz on 2.4 GHz and pin the AP to 20 MHz on 1, 6, or 11.
3. Recommend a channel-width and band-steering plan that preserves 802.11ax benefits while meeting the scanner and VoIP constraints.
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Keep 2.4 GHz at 20 MHz on 1/6/11; move the scanner to its own low-duty SSID if possible. On 5 GHz, use 40 MHz in UNII-1/UNII-3 for capacity and 20 MHz in DFS if radar is confirmed; avoid 160 MHz entirely because it spans DFS and reduces reuse. If 80 MHz is required, restrict it to clean UNII-3 channels and provide a 20/40 MHz fallback. Enable 802.11k/v/r for roaming; band-steer 802.11ax/ac clients to 5 GHz while leaving legacy devices on 2.4. Validate with a post-change survey and iperf3/latency tests, not PHY rate alone.