QUESTION: 17

A manufacturing facility has installed a new automation system which incorporates an 802.11 wireless network. The automation system is controlled from tablet computers connected via the WLAN. However, the automation system has not gone live due to problem with the tablets connecting to the WLAN. The WLAN vendor has been onsite to perform a survey and confirmed good primary and secondary coverage across the facility. As a CWAP you are called in to perform Spectrum Analysis to identify any interference sources. From the spectrum analysis, you did not identify any interference sources but were able to correctly identify the issue.
Which of the following issues did you identify from the spectrum analysis?

The tablets are connecting to the wrong SSID
The tablets are entering power save mode and failing to wake up to receive the access points transmissions
A high noise floor has resulted in a SNR of less than 20dB
There is a power mismatch between the APs and the clients

Answer(s): D

Explanation:

The most likely issue that can be identified from the spectrum analysis is a power mismatch between the APs and the clients. A power mismatch occurs when the APs transmit at a higher power level than the clients, or vice versa. This can cause asymmetric communication, where one side can hear the other, but not vice versa. This can result in poor performance, disconnections, or packet loss. A spectrum analysis can reveal a power mismatch by showing different signal amplitudes or RSSI values for the APs and the clients on the same channel or frequency. The other options are not correct, as they cannot be identified from the spectrum analysis alone. The tablets' SSID, power save mode, and noise floor can be determined by using other tools or methods, such as protocol analysis, site survey, or device configuration.

Reference:

[Wireless Analysis Professional Study Guide CWAP- 404], Chapter 3: Spectrum Analysis, page 79-80

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QUESTION: 18

Finish the statement:
It is possible to distinguish between_______22 MHz transmissions and________20 MHz transmissions when looking at an FFT plot.

HR/DSSS and ERP
OFDM and HT
ERP and VHT
HT and VHT

Answer(s): B

Explanation:

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.

Reference:

[Wireless Analysis Professional Study Guide CWAP-404], Chapter 3: Spectrum Analysis, page 70-71

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QUESTION: 19

What is the function of the PHY layer?

Convert PPDUs to PSDUs for transmissions and PSDUs to PPDUs for receptions
Convert MSDUs to PPDUs for transmissions and PPDUs to MSDUs for receptions
Convert PPDUs to MSDUs for transmissions and MSDUs to PPDUs for receptions
Convert PSDUs to PPDUs for transmissions and PPDUs to PSDUs for receptions

Answer(s): D

Explanation:

The function of the PHY layer is to convert PSDUs to PPDUs for transmissions and PPDUs to PSDUs for receptions. A PSDU (PHY Service Data Unit) is the data unit that is passed from the MAC layer to the PHY layer for transmission, or from the PHY layer to the MAC layer for reception. A PPDU (PHY Protocol Data Unit) is the data unit that is transmitted or received over the wireless medium by the PHY layer. A PPDU consists of a PSDU and a PHY header, which contains information such as modulation, coding, and data rate. The PHY layer adds or removes the PHY header to or from the PSDU during the conversion process.

Reference:

[Wireless Analysis Professional Study Guide CWAP- 404], Chapter 4: 802.11 Physical Layer, page 97-98

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QUESTION: 20

What is the function of the PHY Preamble?

To terminate a conversation between transmitter and receiver
To set the modulation method for the MPDU
Carries the NDP used in Transmit Beamforming and MU-MIMO
Allows the receiver to detect and synchronize with the signal

Answer(s): D

Explanation:

The function of the PHY preamble is to allow the receiver to detect and synchronize with the signal. The PHY preamble is a part of the PPDU that is transmitted before the PHY header and the PSDU. The PHY preamble consists of a series of training fields that help the receiver to adjust its parameters, such as frequency, timing, and gain, to match the incoming signal. The PHY preamble also helps the receiver to estimate the channel conditions and noise level.

Reference:

[Wireless Analysis Professional Study Guide CWAP-404], Chapter 4: 802.11 Physical Layer, page 99-100

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