Helping Hand Questions for CWAP-404

 In the 2.4 GHz band, probe requests are usually sent at the minimum basic rate from an unassociated STA. A probe request is a type of management frame that is transmitted by a STA to discover available BSSs in its vicinity. A probe request can be sent on one or more channels in either passive or active scanning mode. In passive scanning mode, a STA listens for beacon frames from APs on each channel. In active scanning mode, a STA sends probe requests on each channel and waits for probe responses from APs. A probe request is usually sent at the minimum basic rate, which is the lowest data rate among the supported rates that is required for all STAs to join and communicate with a BSS. The minimum basic rate can vary depending on the configuration of each BSS, but it is typically one of these values: 1 Mbps, 2 Mbps, 5.5 Mbps, or 11 Mbps in the 2.4 GHz band. The other options are not correct, as they do not reflect how probe requests are usually sent in the 2.4 GHz band. MCS 0 is a modulation and coding scheme used by 802.11n/ac devices in either band, but it is not a data rate per se. 6 Mbps is a data rate used by OFDM devices in either band, but it is not usually configured as a minimum basic rate in the 2.4 GHz band. References: [Wireless Analysis Professional Study Guide CWAP-404], Chapter 5: 802.11 MAC Sublayer, page 123-124

It is possible to distinguish between OFDM 20 MHz transmissions and HT 20 MHz transmissions when looking at an FFT plot. OFDM and HT are two different modulation schemes used by 802.11 WLANs. OFDM is used by legacy 802.11a/g devices, while HT is used by newer 802.11n/ac devices. OFDM and HT have different spectral characteristics that can be observed on an FFT plot. OFDM transmissions have a flat spectrum with sharp edges, while HT transmissions have a tapered spectrum with rounded edges. This is because HT uses guard intervals and cyclic prefixes to reduce inter-symbol interference and improve performance. The other options are not correct, as they do not describe different modulation schemes or channel widths that can be distinguished on an FFT plot. References: [Wireless Analysis Professional Study Guide CWAP-404], Chapter 3: Spectrum Analysis, page 70-71

The most likely cause of higher airtime utilization on the first 20 MHz of the 80 MHz channel is non-Wi-Fi interference. Non-Wi-Fi interference can prevent an AP from using its full channel width, as it will degrade the signal quality and increase the noise floor on some parts of the channel. This will force the AP to fall back to a narrower channel width, such as 20 MHz or 40 MHz, to maintain communication with its clients. The waterfall plot can help identify non-Wi-Fi interference by showing spikes or bursts of RF energy on specific frequencies or sub-channels. The other options are not correct, as they do not explain why only the first 20 MHz of the channel has higher airtime utilization. References: [Wireless Analysis Professional Study Guide], Chapter 3: Spectrum Analysis, page 74-75