Auditorium Wi-Fi Capacity Collapse With No DFS Available
An auditorium 60 m by 40 m uses 12 FortiAP 431F units mounted at 5 m. One SSID serves 600 attendees with WPA3-SAE transition and 802.11ax. All APs use 5 GHz 80 MHz channels in UNII-1 (36, 40, 44, 48) and 2.4 GHz at maximum power on channels 1, 6, and 11. Users report video calls freeze when the room fills; FortiGate shows 5 GHz channel utilization 78-85%, retry rate 30%, and many clients at 2.4 GHz. The venue cannot use DFS because legacy wireless microphones operate there. The customer refuses to disable 2.4 GHz because some IoT sensors are 2.4-only. Two SSIDs are also broadcast: corporate and guest.
1. Is the primary problem coverage or capacity? Use the numbers to support your answer.
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The primary problem is capacity and channel reuse, not coverage. Twelve APs in a 60 m by 40 m room at 5 m height is already dense; with 600 clients, the airtime is the constraint. The 5 GHz utilization of 78-85% and 30% retry rate show contention, not a weak signal. All APs are confined to UNII-1 channels 36-48 at 80 MHz. That range provides only one 80 MHz channel, so all 12 radios effectively share one wide channel and take turns. Wider channels reduce the number of available channels and increase the airtime cost of each transmission, making co-channel interference and retries worse.
The many clients on 2.4 GHz also point to steering and power problems. Maximum 2.4 GHz power extends the cell and invites clients to stay on the slower band. Coverage is likely adequate or excessive; adding APs or increasing power would increase contention. The fix is a narrower-channel, lower-power, higher-reuse design with band steering, not more coverage.
2. Propose a 5 GHz and 2.4 GHz plan that respects the no-DFS constraint and keeps the IoT sensors. Include channel width, channels, power, and SSID policy.
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For 5 GHz, move to 20 MHz channels in the available non-DFS ranges. Depending on the regulatory domain, use UNII-1 36, 40, 44, 48 and UNII-3 149, 153, 157, 161, 165. That gives up to nine 20 MHz channels, enough to stagger 12 APs with reuse. Avoid 80 MHz entirely in this room; 40 MHz is acceptable only after a validation survey shows low utilization. Lower transmit power to shrink cells, roughly in the low-to-medium range such as 8-11 dBm, and stagger channels so adjacent APs do not reuse the same channel. Enable band steering to 5 GHz and consider a 5 GHz-only SSID for corporate and guest clients.
For 2.4 GHz, keep 20 MHz on channels 1, 6, and 11, reduce power, and disable the 1, 2, 5.5, and 11 Mb/s legacy rates. Put the IoT sensors on a dedicated 2.4 GHz SSID with only the rates they need, and keep corporate and guest off 2.4 GHz where possible. If the IoT devices are fixed and low-throughput, consider limiting that SSID to specific APs rather than broadcasting it from all 12. Do not enable DFS. Validate the plan with a spectrum survey and per-radio utilization before the event.
3. What would you measure before and after the change, and what client behavior would still limit the design?
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Measure per-radio 5 GHz and 2.4 GHz channel utilization, retry rate, client count per band and per AP, airtime per SSID, throughput, roaming events, and DHCP or authentication failures. Use the FortiGate wireless dashboard, FortiAP spectrum analysis, and a site survey before and after. A load test with representative clients is better than an empty-room survey because the failure appears only when the room fills.
Client behavior can still limit the design. 2.4-only IoT sensors cannot move to 5 GHz, so they need their own low-rate, low-power SSID and should not share the corporate SSID. Some clients ignore band steering or remain sticky to a distant AP; a 5 GHz-only SSID for high-density users and a minimum RSSI or roaming policy can help. WPA3-SAE transition mode may slow some older clients, and 80 MHz-capable clients may still request wide channels. The AP profile should enforce 20 MHz in this room, and the validation should confirm that the client mix, not just the AP configuration, meets the target.