Lecture Hall Video Stalls Under High Density
An 800-seat auditorium hosts live-streamed lectures. Eighteen APs are ceiling-mounted at 6 m. 2.4 GHz is enabled with 40 MHz channels; 5 GHz uses 80 MHz; all APs transmit at maximum power. During lectures, 500 students associate to one SSID with WMM enabled. Video stalls and ICMP RTT rises from 10 ms to 300 ms. The AC reports 60% channel utilization on 5 GHz and 85% on 2.4 GHz, with a 45% retry rate. Twelve adjacent classrooms each have one AP. The auditorium access switch has a 1 Gbps uplink to the AC, and the AC is in tunnel forwarding mode. The customer wants 4K video for 50 VIP tablets and best-effort for the remaining 450 students. There are only 9 non-overlapping 20 MHz channels in the 5 GHz UNII bands available for the auditorium.
1. Diagnose the air-interface and forwarding bottlenecks. Which symptoms point to co-channel interference versus wired/uplink congestion?
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The 45% retry rate and 300 ms RTT with 60%/85% utilization point primarily to the air interface, not the wired uplink. 80 MHz in 5 GHz gives very few non-overlapping channels; 18 APs at maximum power in one room create co-channel interference and large overlapping cells. 2.4 GHz at 40 MHz and 85% utilization is worse. A wired/uplink bottleneck would show output drops or queues on the AC uplink and low retry rates; here retries are high, so the medium is contending. However, the 1 Gbps uplink is also mathematically insufficient for 50 VIP 4K streams: 50 × 25 Mbps = 1.25 Gbps before overhead. Tunnel forwarding adds CAPWAP encapsulation and sends all user traffic to the AC, so uplink and AC forwarding capacity are a second bottleneck. The combination explains the stalls: high airtime consumption/retries plus headroom exhaustion.
2. Propose a channel/power, WMM/QoS, and forwarding design for 50 VIP 4K streams plus 450 best-effort users. Include capacity math and tradeoffs.
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Air plan: disable 2.4 GHz in the auditorium or set it to 20 MHz and low power; set 5 GHz to 20 MHz (40 MHz only if the 9-channel plan permits) and reuse the 9 channels across 18 APs with a staggered pattern; lower transmit power so cells are smaller and co-channel overlap is reduced.
QoS: keep WMM enabled, map voice/video to AC_VO/AC_VI, enable admission control for the VIP SSID, and use airtime fairness so 450 best-effort clients do not starve the 50 VIP tablets. Give VIP a separate SSID/VLAN with a per-client rate limit around 25 Mbps and DSCP marking; use local forwarding on the auditorium APs so 4K traffic does not traverse the AC.
Capacity: 50 × 25 Mbps = 1.25 Gbps, so the access switch uplink must be 10 Gbps or the VIP traffic must be distributed/local; 450 students at 2 Mbps = 0.9 Gbps, so the original 1 Gbps uplink has no headroom. Tradeoff: 20 MHz reduces peak PHY rate but is necessary for reuse; local forwarding reduces AC load but moves QoS/ACL enforcement to the AP.