Aisle-End Throughput Collapse in a Steel-Racking Warehouse
A 180,000-square-foot distribution center with 12 m ceilings and dense steel racking is served by 48 Mist APs. Handheld barcode scanners used by 220 pickers are 2.4 GHz-only 802.11n devices; laptops and tablets use 5 GHz. At aisle ends, scanners report RSSI around -74 dBm, retry rates near 45%, and throughput of 1.5-2 Mbps, while the same scanners at aisle midpoints get 12 Mbps. A site survey shows six APs on channel 6 at -62 dBm or stronger in the same aisle, 2.4 GHz channel utilization at 78%, and 5 GHz limited to channels 36-48 because UNII-2 channels were disabled after an old radar event. The RF template uses 40 MHz on 5 GHz, default basic rates, and auto power. Operations wants a fix within two weeks and cannot replace the scanners this year.
1. What is the most likely cause of the aisle-end throughput collapse, and what would you change first?
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The scanners are not simply out of range. At -74 dBm the link is usable, but a 45% retry rate and 78% channel utilization point to co-channel interference and airtime starvation. Six APs at -62 dBm on channel 6 in one aisle means the scanner hears all six; at the aisle end it still holds a low MCS and burns airtime retrying while its own AP is farther away. Because the scanners are 2.4 GHz-only, 40 MHz on 5 GHz is irrelevant, and DFS will not help them. First change: fix the 2.4 GHz plan. Use only channels 1, 6 and 11, reduce 2.4 GHz transmit power, and stagger AP placement/channels so no aisle sees six strong overlapping BSSs. Then disable 1, 2, 5.5 and 11 Mbps basic rates to shrink the cell and force cleaner roaming. Validate carefully: disabling low rates can create holes at aisle ends, so one or two additional APs or directional antennas may be needed rather than higher power.
2. Would enabling DFS channels and 80 MHz on 5 GHz improve the situation? Justify your answer.
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Not for the scanners. They are 2.4 GHz-only, so any 5 GHz change leaves their airtime problem untouched. DFS can add usable 5 GHz channels for laptops, which may reduce 5 GHz congestion, but radar events can force channel changes and make the RF less stable. The bigger risk is 80 MHz: it consumes four 20 MHz channels per BSS, reducing the number of non-overlapping channels and increasing co-channel interference in a high-density warehouse with 48 APs. The better 5 GHz design here is 20 MHz or selective 40 MHz, with DFS considered only after checking client support and radar history. Band steering also will not help the 2.4 GHz-only scanners. Treat DFS and wider channels as separate capacity decisions for laptop traffic, not as a fix for the scanner symptom.
3. How would you use Mist visibility to prove the change worked?
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Establish a baseline before the change and compare the same client population after it. Tag the scanners as a client group or use their device fingerprint so Coverage, Capacity, Throughput and Time to Connect can be filtered to that group rather than blended with laptops. Use the AP RF neighborhood view to confirm that the number of strong overlapping BSSs on channel 6 has fallen and that channel utilization has dropped. Check retry rate, RSSI and MCS for scanner clients at the same aisle-end locations, not just site averages. Marvis can surface AP or client actions if the change creates coverage holes; watch for a rise in coverage-related SLE failures at aisle ends. Finally, walk the same aisles with a scanner and a survey tool to confirm that the improvement is real and that low-rate disabling did not shrink coverage below the picking aisles.