While an outdated driver or disabled radio are possible, the scenario specifies the client is modern and supports 802.11ac, making a driver issue less likely. A disabled radio would prevent any connection. An incorrect PSK would cause an authentication failure, not a complete inability to see or attempt connection. The most plausible cause is a channel width mismatch. Many client devices, even those supporting 802.11ac, do not support 160 MHz channel widths. If the AP is configured for 160 MHz on a channel the client doesn’t support, the client will not be able to associate.

In a dense, multi-tenant environment, minimizing co-channel interference is critical. While 80 MHz or 160 MHz channels offer more bandwidth, they are far more likely to overlap with neighboring networks, causing constant contention and reducing effective throughput. The 2.4 GHz band is too crowded for reliable high-throughput applications. Using 20 MHz channels in the 5 GHz band, especially in the non-DFS ranges (UNII-1, UNII-3), provides the most non-overlapping channels. This allows for a careful, manual channel plan that avoids neighbors, making it the most effective strategy for stability and performance in this scenario.

A signal strength of -67 dBm is generally adequate for data connectivity. However, a low SNR (12 dB is poor for voice) indicates that the noise floor is very high relative to the signal. SNR is calculated as Signal Strength – Noise Floor. A low SNR with a decent signal strength means the noise floor is high. This is a classic signature of non-802.11 interference (e.g., from microwaves, Bluetooth, cordless phones, video cameras), which elevates the baseline noise and corrupts packets, causing the choppy audio experienced by users.

The key goals are reliability and battery life for legacy 802.11b/g devices. Disabling low 802.11b data rates (like 1 and 2 Mbps) can help reduce airtime consumption and management frame overhead, improving overall efficiency. U-APSD (Unscheduled Automatic Power Save Delivery) is a more efficient power save method than legacy power saving mode (PSM). It allows the scanner to sleep and have its buffered data delivered in bursts when it wakes, significantly extending battery life while maintaining responsiveness for small data transactions like barcode scans.

MIMO operations are negotiated. A 3×3:3 AP can transmit up to three spatial streams, but only if the client can receive them. When associating, the client advertises its capabilities (e.g., a 2×2:2 client). For a downlink transmission to that specific client, the AP’s radio will use a transmission mode compatible with the client, typically sending only two spatial streams. It will not waste energy on a third stream the client cannot process. This dynamic adjustment is a fundamental part of MIMO and beamforming protocols.

WPA3 includes a feature called ‘Transition Mode’ specifically for this scenario. When enabled, the AP broadcasts the same SSID as supporting both WPA3 and WPA2 security suites. Modern WPA3-capable clients will connect using the more secure WPA3, while legacy WPA2-only clients (like the IoT devices) can still connect using WPA2. This allows for a gradual transition without needing separate SSIDs for different security levels. It is more practical than managing multiple SSIDs and more secure than downgrading the entire network or using a less secure pre-shared key method.

A Probe Request frame with a null (or zero-length) SSID field is known as a broadcast Probe Request. The client is not looking for a specific network; instead, it is asking every AP within range to respond and identify themselves. APs that receive this will respond with a Probe Response frame containing their configured SSID and capabilities. This is a standard part of the active scanning process used by clients to build a list of available networks.

The scenario describes a high-density, single-AP environment where the airtime is the limiting factor. With many clients, the traditional Single-User MIMO (SU-MIMO) serves only one client at a time. Downlink Multi-User MIMO (MU-MIMO) allows the AP to transmit simultaneously to multiple clients (a MU-MIMO group) within the same channel, using different spatial streams. This dramatically improves spectral efficiency and aggregate throughput in high-density situations by sharing airtime more effectively. The other options either won’t address the airtime contention (increasing power, band steering) or could harm connectivity (reducing basic rates).

A constant, wide-bandwidth signal that blankets an entire 20 MHz channel is characteristic of a non-802.11 noise source. Microwave ovens operate at 2.45 GHz and emit strong, wideband RF leakage when running, which appears as a continuous, high-energy signal across multiple 2.4 GHz Wi-Fi channels. A Wi-Fi throughput test would show the typical bursty, packet-based structure. Bluetooth is frequency-hopping and lower power, appearing as narrow, hopping spikes. A damaged cable causes issues at Layer 1/2 but wouldn’t create this specific RF signature on a spectrum analyzer.

While an outdated driver or disabled radio are possible, the scenario specifies the client is modern and supports 802.11ac, making a driver issue less likely. A disabled radio would prevent any connection. An incorrect PSK would cause an authentication failure, not a complete inability to see or attempt connection. The most plausible cause is a channel width mismatch. Many client devices, even those supporting 802.11ac, do not support 160 MHz channel widths. If the AP is configured for 160 MHz on a channel the client doesn’t support, the client will not be able to associate.0