Auditorium High-Density Design Under Real Load
A university is designing Wi-Fi for a 1,200-seat auditorium with a 14 m ceiling. Peak lecture load is 1,800 associated devices (laptops and phones), mostly 5 GHz capable. The current design uses 24 ceiling APs, 40 MHz channels, and all legacy rates enabled. During a pilot, the 5 GHz channel utilization reached 78%, retry rate 34%, and users reported video stalls even within 10 m of an AP. The room has no cabling below the ceiling, and APs must be mounted above the seating. The customer wants a design that supports 1,800 devices with 2 Mbps average throughput per device and room for 20% growth.
1. Determine an AP count and channel-width plan that meets the 1,800-device load with 20% growth. State assumptions and show calculations.
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Assumptions: Design for 2,160 devices (1,800 x 1.20). Use 5 GHz as the primary band and 20 MHz channels. A realistic high-density design target is 50-60 clients per radio, because airtime and management traffic matter more than the theoretical association limit.
AP count: 2,160 / 60 = 36 APs; 2,160 / 50 = 44 APs. Choose 40-44 APs, with 44 giving headroom for lecture peaks and uneven client distribution. At 44 APs, the average is 49 devices per AP, which is inside the target.
Throughput check: Required aggregate load is 2,160 x 2 Mbps = 4.32 Gbps. Forty-four APs with roughly 250 Mbps of effective 5 GHz capacity each provide about 11 Gbps of nominal capacity, but the real constraint is airtime. A 20 MHz channel and disabled low rates keep each client's airtime short.
Channel plan: Use the largest set of non-overlapping 20 MHz channels allowed in the regulatory domain, with at least 12 distinct channels and no adjacent APs on the same channel. Do not use 40 MHz: it halves the reusable channel set and increases co-channel interference in a single room. Mount APs in a staggered grid above the seating, aiming for roughly 25-30 seats per AP and clear line of sight to client rows.
2. Explain why the pilot showed 78% channel utilization and 34% retries despite strong signal, and list the configuration changes you would make.
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Strong RSSI does not mean usable capacity. The 40 MHz channels reduce the number of non-overlapping channels, so nearby APs share airtime and raise co-channel interference. All legacy rates enabled means distant or poor-performing clients still associate at 1, 2, 5.5, or 11 Mbps in 2.4 GHz and at low 5 GHz rates; each low-rate frame consumes far more airtime, reducing capacity for everyone. The 24-AP design also yields about 75 devices per AP at peak, well above the 50-60 target.
Changes: Move the design to 20 MHz on 5 GHz; disable 802.11b rates and the lowest 5 GHz rates; enable band steering to push capable clients to 5 GHz; set a client limit per radio close to the design target; enable load balancing and multicast/broadcast optimization; and verify that AP transmit power is not so high that clients cling to a distant AP. Re-survey with the actual client mix, not just a clean spectrum scan.
3. Design an acceptance test that proves the design meets the stated requirement before the first lecture.
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Test at three load levels: empty, 50% (900 devices), and 100% (1,800 devices), then repeat a 2,160-device simulation if possible. At each level record per-AP client count, 5 GHz channel utilization, retry rate, association success rate, and average per-client throughput. Use至少 two traffic patterns: many low-rate video streams and a bulk download mix, because video exposes airtime problems that a single iperf flow hides.
Pass criteria: At least 95% of clients achieve 2 Mbps average downstream; channel utilization stays below 60-65% per AP; retry rate below 10%; association success above 99%; and no AP exceeds 60 clients. Walk the seating rows with a test client and confirm that roaming between APs completes without a visible video stall. Validate that low-rate clients are not dragging down nearby APs by checking rate distributions and airtime per client. Document the final channel plan and AP power settings so later changes can be compared against the baseline.