Coverage SLE Drop After Firmware Upgrade in a Lecture Hall
Maple University's main lecture hall seats 600 and is served by 22 Wi-Fi 6 APs. During a 09:00 lecture, 1,800 client devices associate, mostly laptops and phones. After an AP firmware upgrade, the building's Coverage SLE falls from 98% to 82%, while Capacity, Roaming, and Time to Connect remain above 95%. Channel utilization on 5 GHz is 65-80%, and 2.4 GHz is 45%. About 300 clients are on 2.4 GHz with RSSI -72 dBm or weaker, even though 5 GHz RSSI is -60 dBm at the same seats. Marvis reports 'clients experiencing poor coverage in Main Lecture Hall' and recommends checking transmit power. The change window also enabled a new 80 MHz channel plan and increased minimum basic rates. Other buildings are unaffected. The network team wants to know whether this is a real coverage problem, a capacity problem, or a client behavior issue.
1. Interpret the SLE data and Marvis finding. Is the Coverage SLE drop sufficient evidence of an RF coverage hole? Explain what else you would check.
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The Coverage SLE drop is a signal, not proof of an RF hole. Coverage SLE is computed from client experience and classifiers such as weak signal/SNR, sticky clients, and failed connections; a wide 80 MHz channel can lower SNR and raise the noise floor without changing the AP's physical coverage. The fact that 300 clients sit on 2.4 GHz at -72 dBm while 5 GHz is -60 dBm at the same seats points to band steering or client behavior, not lack of 5 GHz signal. Check per-classifier impact, affected client list, AP transmit power, channel width, minimum basic rates, and whether the firmware reset RF profiles. Also compare a before/after channel plan and neighbor report. Marvis's 'poor coverage' is a starting point; validate with client details and a spot survey before adding APs.
2. Identify the most likely root cause and the sequence of validation steps. Include specific settings and metrics.
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Most likely root cause is the 80 MHz plan plus a power/rate change, causing co-channel interference and sticky 2.4 GHz clients. Validate in order: (1) filter Coverage SLE by building and classifier; (2) open 10-20 affected clients and compare RSSI, SNR, band, retries, and AP; (3) check AP RF profiles for channel width, tx power, and minimum basic rates; (4) review 5 GHz channel reuse and neighbor count; (5) confirm band steering is enabled and check 2.4 GHz minimum RSSI. If APs are at high power and 80 MHz, clients may see strong signal but poor SNR. The increased minimum basic rates can also make 2.4 GHz clients fail or roam oddly. Do not conclude 'add APs' until you prove a coverage hole with a survey.
3. Recommend a remediation plan and how to verify it in Mist.
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Remediate by moving the hall to 40 MHz (or 20 MHz where reuse is tight), reducing tx power to create a more uniform cell, enabling band steering to 5 GHz, and restoring minimum basic rates to a sane set. Set a minimum RSSI/roam threshold so clients at -72 dBm on 2.4 GHz are nudged to 5 GHz. Re-check the 5 GHz channel plan for reuse and disable 80 MHz in the auditorium. Verify after the next lecture: Coverage SLE by classifier, percentage of clients on 5 GHz, retry rate, SNR distribution, and per-client throughput. Use Marvis to re-query the same client group. If Coverage recovers but Capacity drops, revisit AP density; if clients still stick, adjust client drivers or band-steering aggressiveness.