Keynote Night in the Auditorium
A 400-seat auditorium hosts a 1,200-device keynote. Six 4x4 APs are ceiling-mounted, set to 40 MHz on 5 GHz and 20 MHz on 2.4 GHz using channels 1/6/11. About 15% of devices are 2.4 GHz-only badge printers and legacy tablets. Post-event telemetry: 5 GHz channel utilization 85%, 38% average retries, average client RSSI -72 dBm, and many clients at MCS 3 or below. 2.4 GHz utilization was 58% with 200 clients attached. The customer wants the room working for the next event in three weeks and asks whether to move to 80 MHz channels or to buy more APs.
1. Compute the client-to-AP ratio and evaluate the 80 MHz proposal using an airtime argument.
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Ratio. 1,200 devices over 6 APs is 200 clients per radio. Even at a modest 2 Mbps average per client, that is 400 Mbps of aggregate demand per AP, which no single radio delivers once management, control, and contention overhead are counted.
Why 80 MHz makes it worse. Doubling channel width roughly doubles per-frame PHY rate but halves the number of non-overlapping channels available. Six APs in one room would then reuse the same few wide channels, and because 802.11 is listen-before-talk, co-channel neighbors and their clients consume each other's airtime. More retries and more deferral means airtime per successful frame goes up, not down. The 85% utilization and 38% retries are the signature of an airtime shortage, not a bandwidth-per-frame shortage.
Direction. More cells at 20 MHz, lower transmit power to shrink each cell, disabled low data rates to remove the slowest airtime hogs, and band steering to move dual-band clients to 5 GHz. The answer to 'buy more APs or go wider' is more APs, but only if they are planned as capacity cells rather than coverage cells.
2. What design targets would you change for a high-density venue, and how would you validate the design before the event?
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Change from coverage to capacity thinking. Set a per-client throughput target for the actual application mix, then derive AP count from client-to-AP ratio (roughly 1:30 to 1:50 for general data, tighter for high-throughput use) instead of from a coverage contour. Hold design targets of about -65 dBm and 25 dB SNR at the seat, not at the aisle. Use 20 MHz channels, disable 1/2/5.5/11 Mbps and consider dropping 6/9 Mbps as basic rates, cap the SSID count, and use directional or downtilted antennas to keep each cell from spilling across the room.
Validation. A predictive survey cannot see the client mix, so it cannot validate this. Run a validation survey with the real device types, then a load test with representative clients and a throughput tool at several seat blocks, measuring retries and utilization under load, not just RSSI. Include the actual presenter devices and any wireless presentation hardware. Capture a baseline before the next event so the post-event telemetry is comparable.
3. How would you handle the 15% of 2.4 GHz-only devices on the day?
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Recognize the constraint. 180 devices are sharing three channels that are already at 58% utilization with 200 attached clients. That segment is close to saturation and will stay that way regardless of what happens in 5 GHz.
Practical handling. Keep 1/6/11 at 20 MHz, disable 802.11b rates so the slowest frames stop dominating airtime, and give the legacy devices their own SSID and VLAN on dedicated APs or dedicated radios so their contention does not land on the same BSS as anything else. Use a low basic-rate set and, if the platform allows it, limit the number of legacy clients per radio. Confirm the badge printers actually need to be associated during the keynote - if they can be idle, that is 180 fewer clients.
Do not solve this by band-steering dual-band clients down to 2.4 GHz for 'balance'. That adds contention to the segment with the least capacity. Also resist enabling 802.11b rates as a compatibility reflex; find the specific device that needs them first.